Non-destructive methods of control of corrosion-resistant coatings. Capillary method for non-destructive testing of welded seams. Capillary flaw detection of welded joints Color method of non-destructive testing

COMPLETED BY: LOPATINA OKSANA

Capillary flaw detection - a flaw detection method based on the penetration of certain liquid substances into surface defects of the product under the action of capillary pressure, as a result of which the light and color contrast of the defective area increases relative to the intact one.

Capillary flaw detection (capillary inspection) designed to detect invisible or weakly visible to the naked eye surface and through defects (cracks, pores, cavities, lack of penetration, intercrystalline corrosion, fistulas, etc.) in objects of control, determining their location, length and orientation along the surface.

Indicator liquid(penetrant) is a colored liquid designed to fill open surface defects and the subsequent formation of an indicator pattern. A liquid is a solution or suspension of a dye in a mixture of organic solvents, kerosene, oils with additives of surfactants (surfactants), which reduce the surface tension of water in the cavities of defects and improve the penetration of penetrants into these cavities. Penetrants contain dyes (color method) or luminescent additives (luminescent method), or a combination of both.

Purifier- serves for preliminary cleaning of the surface and removal of excess penetrant

Developer is a defectoscopic material designed to extract a penetrant from a capillary discontinuity in order to form a clear indicator pattern and create a background contrasting with it. There are five main types of developers used with penetrants:

Dry powder; - aqueous suspension; - suspension in a solvent; - solution in water; - plastic film.

Devices and equipment for capillary control:

Materials for color flaw detection, Luminescent materials

Kits for capillary flaw detection (cleaners, developers, penetrants)

Spray guns, Pneumatic hydraulic guns

Sources of ultraviolet lighting (ultraviolet lights, illuminators).

Test panels (test panel)

Control samples for color flaw detection.

The capillary control process consists of 5 stages:

1 - preliminary cleaning of the surface. In order for the dye to penetrate into defects on the surface, it must first be cleaned with water or an organic cleaner. All contaminants (oils, rust, etc.) and any coatings (paintwork, metallization) must be removed from the controlled area. After that, the surface is dried so that no water or cleaner remains inside the defect.

2 - application of the penetrant. Penetrant, usually red in color, is applied to the surface by spraying, brushing or immersing the test object in a bath for good impregnation and full coverage with the penetrant. As a rule, at a temperature of 5 ... 50 ° C, for a period of 5 ... 30 minutes.

3 - removal of excess penetrant. Excess penetrant is removed by wiping with a tissue, rinsing with water, or the same cleaner as in the pre-cleaning stage. In this case, the penetrant must be removed only from the test surface, but not from the defect cavity. Then the surface is dried with a lint-free cloth or a jet of air.

4 - application of the developer. After drying, the developer (usually white) is immediately applied to the test surface with a thin even layer.

5 - control. Identification of existing defects begins immediately after the end of the development process. During the control, indicator traces are identified and recorded. The intensity of the color of which indicates the depth and width of the opening of the defect, the paler the color, the smaller the defect. Deep cracks are intensely colored. After testing, the developer is removed with water or a cleaner.

To the disadvantages capillary control should be attributed to its high labor intensity in the absence of mechanization, the long duration of the control process (from 0.5 to 1.5 h), as well as the complexity of mechanization and automation of the control process; decrease in the reliability of results at negative temperatures; subjectivity of control - the dependence of the reliability of the results on the professionalism of the operator; limited shelf life of flaw detection materials, the dependence of their properties on storage conditions.

The advantages of capillary control are: simplicity of control operations, simplicity of equipment, applicability to a wide range of materials, including non-magnetic metals. The main advantage of capillary flaw detection is that it can not only detect surface and through defects, but also obtain valuable information about the nature of the defect and even some of its causes (stress concentration, non-observance technologies, etc.).

Flaw detection materials for color flaw detection are selected depending on the requirements for the inspected object, its condition and inspection conditions. As a parameter of the defect size, the transverse size of the defect on the surface of the test object is taken - the so-called defect opening width. The minimum amount of disclosure of detected defects is called the lower threshold of sensitivity and is limited by the fact that a very small amount of penetrant retained in the cavity of a small defect turns out to be insufficient to obtain a contrast indication for a given layer thickness of the developing substance. There is also an upper threshold of sensitivity, which is determined by the fact that from wide, but shallow defects, the penetrant is washed out when removing the excess penetrant on the surface. The detection of indicator traces corresponding to the above main features serves as the basis for an analysis of the admissibility of a defect in terms of its size, nature, position. GOST 18442-80 established 5 classes of sensitivity (at the lower threshold), depending on the size of the defects

The blades of turbojet engines, sealing surfaces of valves and their seats, metal sealing gaskets of flanges, etc. (detected cracks and pores up to tenths of a micron in size) are monitored with a sensitivity of class 1. For class 2, the vessels and corrosion-resistant surfacing of reactors, the base metal and welded joints of pipelines, bearing parts (detected cracks and pores up to several microns in size) are checked. For class 3, the fasteners of a number of objects are checked, with the possibility of detecting defects with an opening of up to 100 microns, for class 4 - thick-walled casting.

Capillary methods, depending on the method of identifying the indicator pattern, are divided into:

· Luminescent method based on the registration of the contrast of a luminescent at long-wavelength ultraviolet radiation visible indicator pattern on the background of the surface of the test object;

· contrast (color) method, based on the registration of the contrast of a colored indicator pattern in visible radiation against the background of the surface of the test object.

· luminescent color method based on the registration of the contrast of a color or luminescent indicator pattern against the background of the surface of the test object in visible or long-wave ultraviolet radiation;

· brightness method based on the registration of the contrast in the visible radiation of the achromatic pattern against the background of the object surface.

COMPLETED BY: VALYUH ALEXANDER

Capillary control

Capillary method non-destructive testing

CapillI amflaw detectorandI am - a flaw detection method based on the penetration of certain liquid substances into surface defects of the product under the action of capillary pressure, as a result of which the light and color contrast of the defective area increases relative to the intact one.

Distinguish between fluorescent and color methods of capillary flaw detection.

In most cases, according to technical requirements, it is necessary to identify defects so small that they can be noticed when visual control with the naked eye is almost impossible. The use of optical measuring instruments, for example, a magnifying glass or a microscope, does not allow detecting surface defects due to insufficient contrast of the image of the defect against a metal background and a small field of view at high magnifications. In such cases, the capillary control method is used.

During capillary inspection, indicator liquids penetrate into the cavities of surface and through discontinuities of the material of the objects of control, and the resulting indicator traces are recorded visually or using a transducer.

Capillary testing is carried out in accordance with GOST 18442-80 “Non-destructive testing. Capillary methods... General requirements."

Capillary methods are divided into basic, using capillary phenomena, and combined, based on a combination of two or more non-destructive testing methods that are different in their physical essence, one of which is capillary testing (capillary flaw detection).

Purpose of capillary inspection (capillary flaw detection)

Capillary flaw detection (capillary inspection) designed to detect invisible or weakly visible to the naked eye surface and through defects (cracks, pores, cavities, lack of penetration, intercrystalline corrosion, fistulas, etc.) in objects of control, determining their location, length and orientation along the surface.

Capillary methods of non-destructive testing are based on the capillary penetration of indicator liquids (penetrants) into the cavities of surface and through discontinuities of the material of the test object and registration of the resulting indicator traces by a visual method or using a transducer.

Application of capillary non-destructive testing

The capillary control method is used to inspect objects of any size and shape, made of ferrous and non-ferrous metals, alloy steels, cast iron, metal coatings, plastics, glass and ceramics in power engineering, aviation, rocketry, shipbuilding, chemical industry, metallurgy, and in the construction of nuclear reactors, in the automotive industry, electrical engineering, mechanical engineering, foundry, stamping, instrument making, medicine and other industries. For some materials and products, this method is the only one for determining the suitability of parts or installations for work.

Capillary flaw detection is also used for non-destructive testing of objects made of ferromagnetic materials, if their magnetic properties, shape, type and location of defects do not allow reaching the sensitivity required by GOST 21105-87 by the magnetic particle method and the magnetic particle control method is not allowed to be used according to the operating conditions of the object.

A necessary condition for detecting defects such as material discontinuity by capillary methods is the presence of cavities free from contamination and other substances that have an exit to the surface of objects and a depth of propagation that significantly exceeds the width of their opening.

Capillary control is also used for leak detection and, in conjunction with other methods, for monitoring critical facilities and facilities during operation.

The advantages of capillary flaw detection methods are: simplicity of control operations, simplicity of equipment, applicability to a wide range of materials, including non-magnetic metals.

The advantage of capillary flaw detection is that it can be used not only to detect surface and through defects, but also to obtain valuable information about the nature of the defect and even some reasons for its occurrence (stress concentration, non-compliance with technology, etc.) by their location, length, shape and orientation on the surface. ).

Organic phosphors are used as indicator liquids - substances that give a bright self-glow under the influence of ultraviolet rays, as well as various dyes. Surface defects are detected with the help of means that allow removing indicator substances from the cavity of defects and detecting their presence on the surface of the tested product.

Capillary (crack), which emerges on the surface of the test object from only one side, is called a surface discontinuity, and the one connecting the opposite walls of the test object is called a through one. If surface and through discontinuities are defects, then the terms "surface defect" and "through defect" may be used instead. The image formed by the penetrant at the location of the discontinuity and similar to the shape of the section at the exit to the surface of the test object is called an indicator pattern, or indication.

For a single crack type discontinuity, the term “indicator trace” may be used instead of the term “indication”. Discontinuity depth - the size of the discontinuity in the direction towards the inside of the test object from its surface. Discontinuity length - the longitudinal dimension of a discontinuity on the surface of an object. Discontinuity opening - the transverse dimension of the discontinuity at its exit to the surface of the test object.

A prerequisite for reliable detection by the capillary method of defects that have an exit to the surface of the object is their relative uncontamination by foreign substances, as well as a depth of propagation that significantly exceeds the width of their opening (at least 10/1). A cleaner is used to clean the surface before applying the penetrant.

Capillary flaw detection methods are subdivided on the main, using capillary phenomena, and combined, based on a combination of two or more methods of non-destructive testing that are different in physical essence, one of which is capillary.

Capillary inspection of welded joints is used to detect external (surface and through) and. This method of checking allows you to identify defects such as hot and, lack of penetration, pores, sinks and some others.

With the help of capillary flaw detection, it is possible to determine the location and size of the defect, as well as its orientation along the metal surface. This method applies both and. It is also used for welding plastics, glass, ceramics and other materials.

The essence of the capillary control method is the ability of special indicator fluids to penetrate into the cavities of seam defects. Filling defects, indicator fluids form indicator traces, which are recorded by visual inspection or using a transducer. The procedure for capillary control is determined by standards such as GOST 18442 and EN 1289.

Classification of methods of capillary flaw detection

Capillary inspection methods are divided into basic and combined. The main ones imply only capillary control by penetrating substances. Combined are based on the combined use of two or more, one of which is capillary control.

Basic control methods

The main control methods are subdivided:

1. Depending on the type of penetrant:

• penetrant test
• check with filter suspensions

1. Depending on the method of reading information:

• luminance (achromatic)
• color (chromatic)
• luminescent
• luminescent color.

Combined methods of capillary control

Combined methods are subdivided depending on the nature and method of exposure to the tested surface. And they are:

1. Capillary electrostatic
2. Capillary-electric induction
3. Capillary-magnetic
4. Capillary Radiation Absorption Method
5. Capillary radiation method of radiation.

Capillary flaw detection technology

Before capillary inspection, the surface to be tested must be cleaned and dried. After that, an indicator liquid - panetrant is applied to the surface. This liquid penetrates into the surface defects of the seams and, after some time, an intermediate cleaning is carried out, during which the excess indicator liquid is removed. Next, a developer is applied to the surface, which begins to draw the indicator liquid from the welded defects. Thus, defect patterns appear on the controlled surface, visible to the naked eye, or with the help of special developers.

Capillary control stages

The capillary inspection process can be divided into the following stages:

1. Preparation and pre-cleaning
2. Intermediate cleaning
3. Manifestation process
4. Identification of welding defects
5. Drawing up a protocol in accordance with the test results
6. Final surface cleaning

Capillary control materials

The list of materials required for capillary flaw detection is given in the table:

Preparation and preliminary cleaning of the tested surface

If necessary, contaminants such as scale, rust, oil stains, paint, etc. are removed from the controlled surface of the weld seam. These contaminants are removed using mechanical or chemical cleaning, or a combination of these methods.

Mechanical cleaning is recommended only in exceptional cases, if there is a loose oxide film on the controlled surface or there are sharp drops between the beads of the seam, deep undercuts. Limited use I received mechanical cleaning due to the fact that during its implementation, surface defects often turn out to be closed as a result of rubbing, and they are not detected during inspection.

Chemical cleaning is carried out with the use of various chemical cleaning agents that remove contaminants such as paint, oil stains, etc. from the tested surface. Residuals of chemical reagents can react with indicator fluids and affect the accuracy of control. Therefore, the chemicals after preliminary cleaning must be washed off the surface with water or other means.

After preliminary cleaning of the surface, it must be dried. Drying is necessary so that no water, solvent, or any other substances remain on the outer surface of the tested joint.

Application of indicator liquid

The application of indicator liquids to the test surface can be performed in the following ways:

1. By capillary method. In this case, filling of welded defects occurs spontaneously. The liquid is applied by wetting, dipping, jetting or spraying with compressed air or inert gas.
2. Vacuum method. With this method, a rarefied atmosphere is created in the cavities of defects and the pressure in them becomes less than atmospheric, i.e. a kind of vacuum in the cavities is obtained, which sucks in the indicator liquid.
3. Compression method. This method is the opposite of the vacuum method. The filling of defects occurs under the influence on the indicator fluid of a pressure exceeding Atmosphere pressure... Under great pressure liquid fills in defects, displacing air from them.
4. Ultrasonic method. Defect cavities are filled in an ultrasonic field and using the ultrasonic capillary effect.
5. Deformation method. The cavities of defects are filled under the action of elastic vibrations of a sound wave on the indicator fluid or under static loading, which increases the minimum size of defects.

For better penetration of the indicator liquid into the cavities of defects, the surface temperature should be in the range of 10-50 ° C.

Intermediate surface cleaning

Apply intermediate surface cleaning agents in such a way that the indicator liquid is not removed from surface defects.

Water cleaning

Excess indicator liquid can be removed by spraying or wiping damp cloth... At the same time, mechanical impact on the controlled surface should be avoided. The water temperature should not exceed 50 ° C.

Solvent cleaning

First, remove excess liquid with a clean, lint-free cloth. After that, the surface is cleaned with a cloth soaked in solvent.

Cleaning with emulsifiers

Water-sensitive emulsifiers or oil-based emulsifiers are used to remove indicator fluids. Before applying the emulsifier, wash off the excess indicator liquid with water and apply the emulsifier immediately after that. After emulsification, it is necessary to rinse the metal surface with water.

Combined cleaning with water and solvent

With this method of cleaning, first, the excess indicator liquid is washed off from the controlled surface with water, and then the surface is cleaned with a lint-free cloth moistened with solvent.

Drying after intermediate cleaning

There are several ways to dry the surface after intermediate cleaning:

• wiping with a clean, dry, lint-free cloth
• evaporation at ambient temperature
• drying at elevated temperature
• air-drying
• by combining the above drying methods.

The drying process must be carried out in such a way that the indicator liquid does not dry out in the cavities of defects. For this, drying is performed at a temperature not exceeding 50 ° C.

The process of manifestation of surface defects in a weld

The developer is applied to the surface to be inspected in an even thin layer. The development process should be started as soon as possible after intermediate cleaning.

Dry developer

Dry developer can only be used with fluorescent indicator fluids. Applied dry developer by spraying or electrostatic spraying. The controlled areas should be covered uniformly, evenly. Local buildup of developer is unacceptable.

Liquid developer based on aqueous suspension

The developer is applied uniformly by dipping the controlled compound into it or by spraying it with an apparatus. When using the dive method, for best results, the duration of the dive should be as short as possible. Thereafter, the controlled compound must be dried by evaporation or blowing in an oven.

Liquid solvent based developer

The developer is sprayed onto the surface to be inspected so that the surface is evenly wetted and a thin and uniform film is formed on it.

Liquid developer in the form of an aqueous solution

Uniform application of such a developer is achieved by immersing the controlled surfaces in it, or by spraying with special devices. The immersion should be short-lived, in which case the best test result is achieved. Thereafter, the test surfaces are dried by evaporation or blowing in an oven.

Duration of the manifestation process

The duration of the manifestation process lasts, as a rule, for 10-30 minutes. In some cases, an increase in the duration of manifestation is allowed. The countdown of the development time begins: for a dry developer immediately after its application, and for a liquid developer - immediately after the end of the surface drying.

Detection of welding defects as a result of capillary flaw detection

If possible, the inspection of the surface to be inspected begins immediately after the application of the developer or after it has dried. But the final control comes after the completion of the manifestation process. Magnifying glasses or glasses with magnifying lenses are used as auxiliary devices for optical control.

When using fluorescent indicator fluids

The use of photochromatic glasses is inadmissible. It is necessary for the controller's eyes to adapt to the darkness in the test booth for at least 5 minutes.

Ultraviolet radiation must not enter the controller's eyes. All surfaces to be inspected must not fluoresce (reflect light). Also, objects that reflect light under the influence of ultraviolet rays should not fall into the field of view of the controller. General UV illumination may be used to enable the inspector to move freely around the test chamber.

When using colored indicator fluids

All controlled surfaces are inspected in daylight or artificial light. Illumination on the surface to be checked must be at least 500 lux. At the same time, there should be no glare on the surface due to light reflection.

Repeated capillary control

If there is a need for repeated control, then the whole process of capillary flaw detection is repeated, starting with the preliminary cleaning process. For this, it is necessary, if possible, to provide more favorable conditions for control.

For repeated control, it is allowed to use only the same indicator fluids, of the same manufacturer, as in the first control. Using other fluids, or the same fluids, but different manufacturers, not allowed. In this case, it is necessary to thoroughly clean the surface so that no traces of the previous check remain on it.

According to EN571-1, the main stages of capillary control are shown in the diagram:

Video on the topic: "Capillary flaw detection of welded seams"

Capillary inspection (capillary / fluorescent / color flaw detection, penetrant inspection)

Capillary inspection, capillary flaw detection, fluorescent / color flaw detection- these are the most common names among specialists for the method of non-destructive testing by penetrating substances, - penetrants.

Capillary control method- the best way to detect defects that emerge on the surface of products. Practice shows the high economic efficiency of capillary flaw detection, the possibility of its use in a wide variety of forms and controlled objects, from metals to plastics.

With a relatively low cost of consumables, equipment for fluorescence and color flaw detection is simpler and less expensive than most other methods of non-destructive testing.

Capillary control kits

Color flaw detection kits based on red penetrants and white developers

Standard set for operation in the temperature range -10 ° C ... + 100 ° C

High temperature set for operation in the range of 0 ° C ... + 200 ° C

Kits for capillary flaw detection based on fluorescent penetrants

Standard set for operation in the temperature range -10 ° C ... + 100 ° C in visible and UV light

High temperature set for operation in the range 0 ° C ... + 150 ° C using UV lamp λ = 365 nm.

A set for the control of particularly critical products in the range of 0 ° C ... + 100 ° C using a UV lamp λ = 365 nm.

Capillary Flaw Detection - Overview

Historical reference

Method for studying the surface of an object penetrating penetrants which is also known as capillary flaw detection(capillary control), appeared in our country in the 40s of the last century. Capillary inspection was first used in aircraft construction. Its simple and straightforward principles have remained unchanged to this day.

Abroad, at about the same time, a red-and-white method for detecting surface defects was proposed and soon patented. Subsequently, it received the name - Liquid penetrant testing. In the second half of the 50s of the last century, materials for capillary flaw detection were described in the US military specification (MIL-1-25135).

Penetrant quality control

The ability to control the quality of products, parts and assemblies by penetrating substances - penetrants exists due to such a physical phenomenon as wetting. A non-destructive liquid (penetrant) wets the surface, fills the mouth of the capillary, thereby creating conditions for the appearance of the capillary effect.

Penetration is a complex property of liquids. This phenomenon is the basis of capillary control. Penetration depends on the following factors:

• properties of the investigated surface and the degree of its cleaning from contamination;
• physical and chemical properties of the material of the control object;
• properties penetrant(wettability, viscosity, surface tension);
• temperature of the test object (affects the viscosity of the penetrant and wettability)

Among other types of non-destructive testing (NDT), the capillary method plays a special role. First, in terms of the totality of qualities, it is perfect way control of the surface for the presence of microscopic discontinuities invisible to the eye. It is favorably distinguished from other types of NDT by portability and mobility, the cost of controlling a unit area of ​​the product, and the relative ease of implementation without the use of complex equipment. Secondly, capillary control is more versatile. If, for example, it is used only to control ferromagnetic materials with a relative magnetic permeability of more than 40, then capillary flaw detection is applicable to products of almost any shape and material, where the geometry of the object and the direction of defects do not play a special role.

Development of capillary testing as a method of non-destructive testing

The development of surface defectoscopy methods as one of the areas of non-destructive testing is directly related to scientific and technological progress. Manufacturers industrial equipment have always been concerned with saving materials and manpower. At the same time, the operation of the equipment is often associated with increased mechanical loads on some of its elements. As an example, let us take turbine blades of aircraft engines. In the regime of intense loads, it is the cracks on the surface of the blades that pose a known hazard.

In this particular case, as in many others, capillary control turned out to be very useful. Manufacturers quickly appreciated it, it was adopted and received a stable vector of development. The capillary method has proven to be one of the most sensitive and demanded non-destructive testing methods in many industries. Mainly in mechanical engineering, serial and small-scale production.

Currently, the improvement of capillary control methods is carried out in four directions:

• improving the quality of flaw detection materials aimed at expanding the range of sensitivity;
• decline harmful effects materials on environment and a person;
• the use of electrostatic spraying systems of penetrants and developers for a more uniform and economical application of them to the controlled parts;
• introduction of automation schemes into the multi-operation process of surface diagnostics in production.

Organization of a color (luminescent) flaw detection section

The organization of the site for color (luminescent) flaw detection is carried out in accordance with industry recommendations and standards of enterprises: RD-13-06-2006. The site is assigned to the non-destructive testing laboratory of the enterprise, which is certified in accordance with the Certification Rules and the basic requirements for non-destructive testing laboratories PB 03-372-00.

Both in our country and abroad, the use of color flaw detection methods at large enterprises is described in internal standards, which are completely based on national ones. Color flaw detection is described in the standards of Pratt & Whitney, Rolls-Royce, General Electric, Aerospatiale and others.

Capillary control - pros and cons

Advantages of the capillary method

1. Low costs for expendable materials.
2. High objectivity of control results.
3. It can be used for almost all solid materials (metals, ceramics, plastics, etc.), with the exception of porous ones.
4. In most cases, capillary inspection does not require the use of technologically sophisticated equipment.
5. Implementation of control in any place under any conditions, including stationary, using appropriate equipment.
6. Due to the high inspection performance, it is possible to quickly inspect large objects with large area investigated surface. When using this method in enterprises with a continuous production cycle, in-line control of products is possible.
7. The capillary method is ideal for detecting all types of surface cracks, providing a clear visualization of defects (when inspected properly).
8. Ideal for inspecting complex geometries, light metal parts such as turbine blades in the aerospace and power industries, engine parts in the automotive industry.
9. Under certain circumstances, the method can be applied for tightness tests. For this, the penetrant is applied to one side of the surface, and the developer to the other. At the point of leakage, the penetrant is drawn to the surface by the developer. Leakage control to detect and locate leaks is extremely important for products such as tanks, vessels, radiators, hydraulic systems, etc.
10. Unlike X-ray inspection, capillary flaw detection does not require special safety measures, such as the use of radiation protection equipment. During the research, the operator only needs to exercise elementary care when working with consumables and use a respirator.
11. No special requirements regarding the knowledge and qualifications of the operator.

Limitations for color flaw detection

1. The main limitation of the capillary inspection method is the ability to detect only those defects that are open to the surface.
2. The factor that reduces the efficiency of capillary testing is the roughness of the object under investigation - the porous structure of the surface leads to false readings.
3. The special cases, although quite rare, should be attributed to the low surface wettability of some materials by penetrants, both water-based and based on organic solvents.
4. In some cases, the disadvantages of the method include the complexity of performing preparatory operations associated with the removal of paint and varnish coatings, oxide films and drying of parts.

Capillary inspection - terms and definitions

Capillary non-destructive testing

Capillary non-destructive testing is based on the penetration of penetrants into cavities that form defects on the surface of products. Penetrant is a dye... Its trace, after appropriate surface treatment, is recorded visually or with the help of instruments.

In capillary control apply different ways testing based on the use of penetrants, surface preparation materials, developers and for capillary studies. There are currently a sufficient number of capillary inspection consumables on the market that allow selection and development of techniques to meet essentially any sensitivity, compatibility and environmental requirements.

Physical foundations of capillary flaw detection

The basis of capillary flaw detection is a capillary effect, as a physical phenomenon and a penetrant, as a substance with certain properties. The capillary effect is influenced by such phenomena as surface tension, wetting, diffusion, dissolution, emulsification. But in order for these phenomena to work for the result, the surface of the test object must be well cleaned and degreased.

If the surface is properly prepared, a drop of penetrant that falls on it quickly spreads, forming a stain. This indicates good wetting. Wetting (adhesion to a surface) is understood as the ability of a liquid body to form a stable interface at the interface with a solid. If the forces of interaction between the molecules of a liquid and a solid exceed the forces of interaction between molecules inside the liquid, then the surface of the solid is wetted.

Pigment particles penetrant, is many times smaller in size than the width of the opening of microcracks and other damage to the surface of the research object. In addition, the most important physical property of penetrants is low surface tension. Due to this parameter, penetrants have sufficient penetrating ability and wet various types of surfaces well - from metals to plastic.

Penetration of a penetrant into a discontinuity (cavity) of defects and the subsequent extraction of the penetrant during the development process occurs under the action of capillary forces. And deciphering the defect becomes possible due to the difference in color (color flaw detection) or glow (luminescent flaw detection) between the background and the surface area above the defect.

Thus, under normal conditions, very small defects on the surface of the test object are not visible to the human eye. In the process of stage-by-stage surface treatment with special compounds, on which capillary flaw detection is based, an easily readable, contrasting indicator pattern is formed above the defects.

In color flaw detection Due to the action of the penetrant developer, which "pulls" the penetrant to the surface by diffusion forces, the size of the indication usually turns out to be significantly larger than the size of the defect itself. The size of the indicator pattern as a whole, subject to the control technology, depends on the volume of the penetrant absorbed by the discontinuity. When evaluating the control results, one can draw some analogy with the physics of the "amplification effect" of signals. In our case, the "output signal" is a contrast indicator pattern, which can be several times larger in size than the "input signal" - an image of a discontinuity (defect) that cannot be read by the eye.

Non-destructive materials

Non-destructive materials for capillary inspection, these are the means that are used in the control of a liquid (penetration control) that penetrates into the surface discontinuities of the tested products.

Penetrant

Penetrant is an indicator liquid, a penetrating substance (from the English penetrate - to penetrate) .

Penetrants are a capillary defectoscopic material that is capable of penetrating the surface discontinuities of the inspected object. Penetration of the penetrant into the damage cavity occurs under the action of capillary forces. As a result of low surface tension and the action of wetting forces, the penetrant fills the defect void through the mouth open to the surface, thus forming a concave meniscus.

Penetrant is the main consumable for capillary flaw detection. Penetrants are distinguished by the method of visualization into contrast (color) and luminescent (fluorescent), by the method of removal from the surface to water-washable and removed by a cleaner (post-emulsifiable), by sensitivity into classes (in descending order - I, II, III and IV classes according to GOST 18442-80)

Foreign standards MIL-I-25135E and AMS-2644, in contrast to GOST 18442-80, divide the sensitivity levels of penetrants into classes in ascending order: 1/2 - ultra-low sensitivity, 1 - low, 2 - medium, 3 - high, 4 - ultra-high ...

A number of requirements are imposed on penetrants, the main of which is good wettability. The next parameter that is important for penetrants is viscosity. The lower it is, the less time it takes to completely saturate the surface of the test object. Capillary control takes into account such properties of penetrants as:

• wettability;
• viscosity;
• surface tension;
• volatility;
• flash point (flash point);
• specific gravity;
• solubility;
• sensitivity to pollution;
• toxicity;
• smell;
• inertia.

The composition of the penetrant usually includes high-boiling solvents, pigment-based dyes (phosphors) or soluble, surfactants, corrosion inhibitors, binders. Penetrants are available in aerosol cans (the most suitable form of release for fieldwork), plastic cans and barrels.

Developer

Developer is a material for capillary non-destructive testing, which, due to its properties, extracts the penetrant in the defect cavity to the surface.

The penetrant developer is usually white and acts as a contrasting background for the indicator image.

The developer is applied to the surface of the test object with a thin, uniform layer after it has been cleaned (intermediate cleaning) from the penetrant. After the intermediate cleaning procedure, a certain amount of penetrant remains in the defect area. The developer, under the action of the forces of adsorption, absorption or diffusion (depending on the type of action) "pulls" the penetrant remaining in the capillaries of defects to the surface.

Thus, under the action of the developer, the penetrant "tints" the surface areas above the defect, forming a clear defectogram - an indicator pattern that repeats the location of the defects on the surface.

According to the type of action, developers are divided into sorption (powders and suspensions) and diffusion (paints, varnishes and films). Most often, developers are chemically neutral sorbents made of silicon compounds, white. Such developers, when covering the surface, create a layer having a microporous structure, into which, under the action of capillary forces, the coloring penetrant can easily penetrate. In this case, the developer layer above the defect is painted in the color of the dye (color method), or is moistened with a liquid with the addition of a phosphor, which begins to fluoresce in ultraviolet light (luminescent method). In the latter case, the use of a developer is not necessary - it only increases the sensitivity of the control.

The correct choice of developer should provide an even coating of the surface. The higher the sorption properties of the developer, the better it "pulls" the penetrant from the capillaries during development. These are the most important properties of the developer that determine its quality.

Capillary inspection involves the use of dry and wet developers. In the first case, we are talking about powder developers, in the second about water-based developers (water, water-washable), or based on organic solvents (not water-based).

The developer in the flaw detection system, like the rest of the materials of this system, is selected based on the sensitivity requirements. For example, to detect a defect with an opening width of up to 1 micron, in accordance with the American standard AMS-2644, a powder developer and a luminescent penetrant should be used to diagnose moving parts of a gas turbine unit.

Powder developers have good dispersion and are applied to the surface using an electrostatic or vortex method, with the formation of a thin and uniform layer, which is necessary for guaranteed drawing of a small volume of penetrant from the cavities of microcracks.

Water-based developers do not always produce a thin and uniform layer. In this case, if there is minor defects, the penetrant does not always come to the surface. A too thick developer layer can mask the defect.

Developers can chemically interact with indicator penetrants. By the nature of this interaction, developers are divided into chemically active and chemically passive. The latter are the most widespread. Chemically active developers react with the penetrant. Detection of defects, in this case, is carried out by the presence of reaction products. Chemically passive developers act only as a sorbent.

Penetrant developers are available in aerosol cans (the most suitable form for fieldwork), plastic cans and drums.

Penetrant emulsifier

An emulsifier (penetrant extinguisher according to GOST 18442-80) is a non-destructive material for capillary control used for intermediate surface cleaning when using a post-emulsified penetrant.

In the process of emulsification, the penetrant remaining on the surface interacts with the emulsifier. Subsequently, the resulting mixture is removed with water. The purpose of the procedure is to remove excess penetrant from the surface.

The emulsification process can have a significant impact on the quality of visualization of defects, especially when inspecting objects with a rough surface. This is expressed in obtaining a contrasting background of the required purity. To obtain a well-readable indicator pattern, the brightness of the background should not exceed the brightness of the indication.

In capillary control, lipophilic and hydrophilic emulsifiers are used. Lipophilic emulsifier - oil-based, hydrophilic - water-based. They differ in the mechanism of action.

The lipophilic emulsifier, covering the surface of the product, passes into the remaining penetrant under the action of diffusion forces. The resulting mixture is easily removed from the surface with water.

The hydrophilic emulsifier acts differently on the penetrant. When exposed to it, the penetrant is divided into many particles of a smaller volume. As a result, an emulsion is formed, and the penetrant loses its properties to wet the surface of the test object. The resulting emulsion is removed mechanically (washed off with water). The basis of hydrophilic emulsifiers is a solvent and surfactants.

Penetrant cleaner(surface)

Capillary control cleaner is an organic solvent for removing excess penetrant (intermediate cleaning), cleaning and degreasing the surface (preliminary cleaning).

A significant influence on the wetting of the surface is exerted by its microrelief and the degree of cleaning from oils, fats and other contaminants. In order for the penetrant to penetrate even the smallest pores, in most cases, mechanical cleaning is not enough. Therefore, before the inspection, the surface of the part is treated with special cleaners made on the basis of high-boiling solvents.

The most important properties of modern surface cleaners for capillary control are:

• ability to degrease;
• absence of non-volatile impurities (the ability to evaporate from the surface without leaving traces);
• the minimum content of harmful substances that have an impact on humans and the environment;
• Operating temperature range.

Capillary Consumables Compatibility

Defectoscopic materials for capillary inspection by physical and chemical properties must be compatible both with each other and with the material of the test object. The components of penetrants, cleaning agents and developers should not lead to a loss of performance properties of the controlled products and damage to equipment.

Compatibility table of Elitest consumables for capillary control:

⚫ - recommended to use; ∨ - can be used; × - can not use
Download the compatibility table for capillary and magnetic particle inspection consumables:

Capillary control equipment

Equipment used for capillary control:

• reference (control) samples for capillary flaw detection;
• sources of ultraviolet lighting (UV lights and lamps);
• test panels (test panel);
• pneumohydro pistols;
• pulverizers;
• capillary control chambers;
• systems for electrostatic deposition of flaw detection materials;
• water purification systems;
• drying cabinets;
• tanks for immersion application of penetrants.

Defects detected

Capillary flaw detection methods allow detecting defects that emerge on the surface of the product: cracks, pores, cavities, lack of penetration, intergranular corrosion and other discontinuities with an opening width of less than 0.5 mm.

Control samples for capillary flaw detection

Control (standard, reference, test) samples for capillary control are metal plates with artificial cracks (defects) of a certain size applied to them. The surface of the control samples may be rough.

Control samples are made according to foreign standards, in accordance with European and American standards EN ISO 3452-3, AMS 2644C, Pratt & Whitney Aircraft TAM 1460 40 (standard of the enterprise - the largest American manufacturer of aircraft engines).

Control samples are used:

• to determine the sensitivity of test systems based on different flaw detection materials (penetrant, developer, cleaner);
• for comparison of penetrants, one of which can be taken as an exemplary;
• to assess the quality of rinsing of fluorescent (fluorescent) and contrast (color) penetrants in accordance with AMS 2644C;
• for a general assessment of the quality of capillary control.

The use of control samples for capillary control is not regulated by the Russian GOST 18442-80. Nevertheless, in our country, control samples are actively used in accordance with GOST R ISO 3452-2-2009 and the norms of enterprises (for example, PNAEG-7-018-89) to assess the suitability of flaw detection materials.

Capillary control techniques

To date, quite a lot of experience has been accumulated in the application of capillary methods for the purposes of operational control of products, assemblies and mechanisms. However, the development of a working procedure for capillary inspection often has to be done on a case-by-case basis. This takes into account factors such as:

1. sensitivity requirements;
2. the state of the object;
3. the nature of the interaction of flaw detection materials with the controlled surface;
4. consumable compatibility;
5. technical capabilities and conditions of work performance;
6. the nature of the expected defects;
7. other factors affecting the effectiveness of capillary control.

GOST 18442-80 defines the classification of the main capillary control methods depending on the type of penetrating substance - penetrant (solution or suspension of pigment particles) and depending on the method of obtaining primary information:

1. luminance (achromatic);
2. color (chromatic);
3. luminescent (fluorescent);
4. luminescent color.

Standards GOST R ISO 3452-2-2009 and AMS 2644 describe six main methods of capillary control by type and group:

Type 1. Fluorescent (luminescent) methods:

• method A: water washable (Group 4);
• method B: post-emulsification (Groups 5 and 6);
• Method C: Soluble (Group 7).

Type 2. Color methods:

• method A: water washable (Group 3);
• method B: subsequent emulsification (Group 2);
• Method C: Soluble (Group 1).

Capillary flaw detection

Capillary control

Capillary non-destructive testing

CapillI am flaw detectorand I am - a flaw detection method based on the penetration of certain liquid substances into surface defects of the product under the action of capillary pressure, as a result of which the light and color contrast of the defective area increases relative to the intact one.

Distinguish between fluorescent and color methods of capillary flaw detection.

In most cases, technical requirements it is necessary to identify defects so small that they can be noticed when visual control with the naked eye is almost impossible. The use of optical measuring devices, for example, a magnifying glass or a microscope, does not allow detecting surface defects due to insufficient contrast of the image of the defect against a metal background and a small field of view at high magnifications. In such cases, the capillary control method is used.

During capillary inspection, indicator liquids penetrate into the cavities of surface and through discontinuities of the material of the objects of control, and the resulting indicator traces are recorded visually or using a transducer.

Capillary testing is carried out in accordance with GOST 18442-80 “Non-destructive testing. Capillary methods. General requirements."

Capillary methods are divided into basic, using capillary phenomena, and combined, based on a combination of two or more non-destructive testing methods that are different in their physical essence, one of which is capillary testing (capillary flaw detection).

Purpose of capillary inspection (capillary flaw detection)

Capillary flaw detection (capillary inspection) designed to detect invisible or weakly visible to the naked eye surface and through defects (cracks, pores, cavities, lack of penetration, intercrystalline corrosion, fistulas, etc.) in objects of control, determining their location, length and orientation along the surface.

Capillary methods of non-destructive testing are based on the capillary penetration of indicator liquids (penetrants) into the cavities of surface and through discontinuities of the material of the test object and registration of the resulting indicator traces by a visual method or using a transducer.

Application of capillary non-destructive testing

The capillary control method is used to control objects of any size and shape, made of ferrous and non-ferrous metals, alloy steels, cast iron, metal coatings, plastics, glass and ceramics in power engineering, aviation, rocketry, shipbuilding, chemical industry, metallurgy, in the construction of nuclear reactors, in the automotive industry, electrical engineering, mechanical engineering, foundry, stamping, instrument making, medicine and other industries. For some materials and products, this method is the only one for determining the suitability of parts or installations for work.

Capillary flaw detection is also used for non-destructive testing of objects made of ferromagnetic materials, if their magnetic properties, shape, type and location of defects do not allow reaching the sensitivity required by GOST 21105-87 by the magnetic particle method and the magnetic particle control method is not allowed to be used according to the operating conditions of the object.

A necessary condition for detecting defects such as material discontinuity by capillary methods is the presence of cavities free from contamination and other substances that have an exit to the surface of objects and a depth of propagation that significantly exceeds the width of their opening.

Capillary control is also used for leak detection and, in conjunction with other methods, for monitoring critical facilities and facilities during operation.

The advantages of capillary flaw detection methods are: simplicity of control operations, simplicity of equipment, applicability to a wide range of materials, including non-magnetic metals.

The advantage of capillary flaw detection is that it can be used not only to detect surface and through defects, but also to obtain valuable information about the nature of the defect and even some reasons for its occurrence (stress concentration, non-compliance with technology, etc.) by their location, length, shape and orientation on the surface. ).

Organic phosphors are used as indicator liquids - substances that give a bright self-glow under the influence of ultraviolet rays, as well as various dyes. Surface defects are detected with the help of means that allow removing indicator substances from the cavity of defects and detecting their presence on the surface of the tested product.

Capillary (crack), which emerges on the surface of the test object from only one side, is called a surface discontinuity, and the one connecting the opposite walls of the test object is called a through one. If surface and through discontinuities are defects, then the terms "surface defect" and "through defect" may be used instead. The image formed by the penetrant at the location of the discontinuity and similar to the shape of the section at the exit to the surface of the test object is called an indicator pattern, or indication.

For a single crack type discontinuity, the term “indicator trace” may be used instead of the term “indication”. Discontinuity depth - the size of the discontinuity in the direction towards the inside of the test object from its surface. Discontinuity length - the longitudinal dimension of a discontinuity on the surface of an object. Discontinuity opening - the transverse dimension of the discontinuity at its exit to the surface of the test object.

A prerequisite for reliable detection by the capillary method of defects that have an exit to the surface of the object is their relative uncontamination by foreign substances, as well as a depth of propagation that significantly exceeds the width of their opening (at least 10/1). A cleaner is used to clean the surface before applying the penetrant.

Capillary flaw detection methods are subdivided on the main, using capillary phenomena, and combined, based on a combination of two or more methods of non-destructive testing that are different in physical essence, one of which is capillary.

Instruments and equipment for capillary control:

• Kits for capillary flaw detection (cleaners, developers, penetrants)
• Atomizers
• Pneumatic hydraulic guns
• Sources of ultraviolet lighting (ultraviolet lights, illuminators)
• Test panels (test panel)

Test pieces for color flaw detection

Sensitivity of the capillary flaw detection method

Capillary control sensitivity- the ability to detect discontinuities of a given size with a given probability using a specific method, control technology and penetrant system. According to GOST 18442-80 the class of control sensitivity is determined depending on minimum size identified defects with a transverse size of 0.1 - 500 microns.

Detection of defects with an opening width of more than 0.5 mm by capillary control methods is not guaranteed.

With a sensitivity of class 1, using capillary flaw detection, the blades of turbojet engines, sealing surfaces of valves and their seats, metal gaskets of flanges, etc. (detected cracks and pores up to tenths of a micron in size) are monitored. For class 2, the vessels and corrosion-resistant surfacing of reactors, the base metal and welded joints of pipelines, bearing parts (detected cracks and pores up to several microns in size) are checked.

The sensitivity of flaw detection materials, the quality of intermediate cleaning and control of the entire capillary process are determined on control samples (standards for color flaw detection of CD), i.e. on metal of a certain roughness with normalized artificial cracks (defects) applied to them.

The control sensitivity class is determined depending on the minimum size of the detected defects. The perceived sensitivity, if necessary, is determined on natural objects or artificial samples with natural or simulated defects, the dimensions of which are specified by metallographic or other methods of analysis.

According to GOST 18442-80, the control sensitivity class is determined depending on the size of the detected defects. As a parameter of the defect size, the transverse size of the defect on the surface of the test object is taken - the so-called defect opening width. Since the depth and length of the defect also have a significant effect on the possibility of its detection (in particular, the depth should be significantly greater than the opening), these parameters are considered stable. The lower threshold of sensitivity, i.e. the minimum amount of disclosure of detected defects is limited by the fact that there is a very small amount of penetrant; lingering in the cavity of a small defect turns out to be insufficient to obtain a contrast indication for a given layer thickness of the developing substance. There is also an upper threshold of sensitivity, which is determined by the fact that from wide, but shallow defects, the penetrant is washed out when removing the excess penetrant on the surface.

There are 5 classes of sensitivity (according to the lower threshold), depending on the size of the defects:

Physical foundations and technique of the capillary control method

Capillary non-destructive testing (GOST 18442-80) It is based on capillary penetration into the defect of the indicator liquid and is designed to detect defects that have an exit to the surface of the test object. This method is suitable for detecting discontinuities with a transverse size of 0.1 - 500 microns, including through, on the surface of ferrous and non-ferrous metals, alloys, ceramics, glass, etc. It is widely used to control the integrity of the weld.

A colored or dye penetrant is applied to the surface of the test object. Due to the special qualities that are provided by the selection of certain physical properties penetrant: surface tension, viscosity, density, it, under the action of capillary forces, penetrates into the smallest defects that have an exit to the surface of the test object

The developer applied to the surface of the test object some time after careful removal from the surface of the penetrant dissolves the dye inside the defect and, due to diffusion, “pulls” the penetrant remaining in the defect onto the surface of the test object.

The existing defects are visible in sufficient contrast. Indicator marks in the form of lines indicate cracks or scratches, individual dots indicate pores.

The process of detecting defects by the capillary method is divided into 5 stages (carrying out capillary control):

1. Pre-cleaning the surface (use a cleaner)

2. Application of the penetrant

3. Removal of excess penetrant

4. Application of the developer

5. Control

Pre-cleaning the surface. In order for the dye to penetrate into defects on the surface, it must first be cleaned with water or an organic cleaner. All contaminants (oils, rust, etc.) and any coatings (paintwork, metallization) must be removed from the controlled area. After that, the surface is dried so that no water or cleaner remains inside the defect.

Penetrant application. Penetrant, usually red in color, is applied to the surface by spraying, brushing or dipping OK into a bath for good impregnation and full coverage penetrant. As a rule, at a temperature of 5-50 0 С, for a period of 5-30 minutes.

Removal of excess penetrant. Excess penetrant is removed by wiping with a napkin, rinsing with water. Or with the same cleaner as in the pre-cleaning stage. In this case, the penetrant must be removed from the surface, but not from the cavity of the defect. The surface is then dried with a lint-free cloth or a jet of air. When using a cleaner, there is a risk that the penetrant will wash out and display incorrectly.

Developer application. After drying, a developer, usually white, is immediately applied to the OK in a thin even layer.

Control. OK inspection begins immediately after the end of the development process and ends according to different standards in no more than 30 minutes. The intensity of the color indicates the depth of the defect; the paler the color, the smaller the defect. Deep cracks are intensely colored. After testing, the developer is removed with water or a cleaner.
The coloring penetrant is applied to the surface of the control object (OC). Due to the special qualities that are provided by the selection of certain physical properties of the penetrant: surface tension, viscosity, density, it, under the action of capillary forces, penetrates into the smallest defects that have an exit to the surface of the test object. The developer applied to the surface of the test object some time after careful removal from the surface of the penetrant dissolves the dye inside the defect and, due to diffusion, “pulls” the penetrant remaining in the defect onto the surface of the test object. The existing defects are visible in sufficient contrast. Indicator marks in the form of lines indicate cracks or scratches, individual dots indicate pores.

Sprayers, such as aerosol cans, are most convenient. Can be applied with developer and dipping. Dry developers are applied in a vortex chamber or electrostatically. After applying the developer, you should wait from 5 minutes for large defects, up to 1 hour for small defects. Defects will appear as red marks on a white background.

Through cracks on thin-walled products can be detected by applying developer and penetrant with different sides products. The dye that has passed through will be clearly visible in the developer layer.

Penetrant (penetrant from the English penetrate - to penetrate) is called a capillary flaw detection material that has the ability to penetrate the discontinuities of the test object and indicate these discontinuities. Penetrants contain dyes (color method) or luminescent additives (luminescent method), or a combination of both. The additives make it possible to distinguish the region of the developer layer above the crack impregnated with these substances from the main (most often white) solid object material without defects (background).

Developer (developer) is a defectoscopic material designed to extract a penetrant from a capillary discontinuity in order to form a clear indicator pattern and create a background contrasting with it. Thus, the role of the developer in capillary control is, on the one hand, to extract the penetrant from defects due to capillary forces, on the other hand, the developer must create a contrasting background on the surface of the controlled object in order to reliably identify colored or luminescent indicator traces of defects. At the right technology the width of the trace in 10 ... 20 and more times may exceed the width of the defect, and the brightness contrast increases by 30 ... 50%. This magnifying effect allows experienced technicians to detect very small cracks even with the naked eye.

Sequence of operations for capillary control:

The main capillary methods of non-destructive testing are subdivided, depending on the type of penetrating substance, into the following:

· Method of penetrating solutions - liquid method of capillary non-destructive testing, based on the use of a liquid indicator solution as a penetrating substance.

· The method of filterable suspensions is a liquid method of capillary non-destructive testing, based on the use of an indicator suspension as a liquid penetrating substance, which forms an indicator pattern from filtered particles of the dispersed phase.

Capillary methods, depending on the method of identifying the indicator pattern, are divided into:

· Luminescent method based on the registration of the contrast of a visible indicator pattern luminescent in long-wave ultraviolet radiation against the background of the surface of the test object;

· contrast (color) method, based on the registration of the contrast of a colored indicator pattern in visible radiation against the background of the surface of the test object.

· luminescent color method based on the registration of the contrast of a color or luminescent indicator pattern against the background of the surface of the test object in visible or long-wave ultraviolet radiation;

· brightness method based on the registration of the contrast in the visible radiation of the achromatic pattern against the background of the surface of the test object.

Physical foundations of capillary flaw detection. Luminescent flaw detection (LD). Color flaw detection (CD).

There are two ways to change the contrast ratio between the defect image and the background. The first method consists in polishing the surface of the item to be inspected, followed by etching it with acids. With this treatment, the defect becomes clogged with corrosion products, turns black and becomes noticeable against the light background of the polished material. This method has a number of limitations. In particular, under production conditions it is completely unprofitable to polish the surface of the product, especially the welded seams. In addition, the method is not applicable when inspecting precision polished parts or non-metallic materials. The etching method is more often used to control some local suspicious areas of metal products.

The second method consists in changing the light output of defects by filling them from the surface with special light and color-contrast indicator fluids - penetrants. If the penetrant contains luminescent substances, that is, substances that give a bright glow when irradiated with ultraviolet light, then such liquids are called luminescent, and the control method, respectively, is luminescent (luminescent flaw detection - LD). If the basis of the penetrant is dyes visible in daylight, then the control method is called color (color flaw detection - CD). In color flaw detection, dyes of bright red color are used.

The essence of capillary flaw detection is as follows. The surface of the product is cleaned of dirt, dust, grease, flux residues, paint and varnish coatings, etc. After cleaning, a penetrant layer is applied to the surface of the prepared product and held for some time so that the liquid can penetrate into the open cavities of defects. Then the surface is cleaned of liquid, part of which remains in the cavities of defects.

In the case of fluorescent flaw detection the product is illuminated with ultraviolet light (ultraviolet illuminator) in a darkened room and examined. Defects are clearly visible in the form of brightly luminous stripes, dots, etc.

With color flaw detection, it is not possible to identify defects at this stage, since the resolution of the eye is too low. To increase the detectability of defects, after removing the penetrant from the surface of the product, a special developing material is applied in the form of a quickly drying suspension (for example, kaolin, collodion) or varnish coatings... The developing material (usually white) pulls the penetrant out of the defect cavity, which leads to the formation of indicator traces on the developer. Indicator traces completely repeat the configuration of defects in plan, but they are larger in size. Such indicator traces are easily distinguishable by the eye, even without the use of optical means. The increase in the size of the indicator trace is the greater, the deeper the defects, i.e. the greater the volume of the penetrant filling the defect, and the more time has passed since the application of the developing layer.

The physical basis of capillary flaw detection methods is the phenomenon of capillary activity, i.e. the ability of the liquid to be drawn into the smallest through holes and channels open at one end.

Capillary activity depends on wetting ability solid liquid. In any body, molecular cohesion forces act on each molecule from other molecules. They are larger in a solid than in a liquid. Therefore, liquids, unlike solids, do not have shape elasticity, but they have high volumetric elasticity. The molecules on the surface of the body interact both with the molecules of the same name in the body, striving to draw them into the volume, and with the molecules of the environment surrounding the body, and have the greatest potential energy. For this reason, an uncompensated force, called the surface tension force, arises perpendicular to the boundary in the direction of the body. The surface tension forces are proportional to the length of the wetting contour and, naturally, tend to decrease it. The liquid on the metal, depending on the ratio of intermolecular forces, will spread over the metal or collect in a drop. A liquid wets a solid if the forces of interaction (attraction) of the liquid with the molecules of the solid are greater than the forces of surface tension. In this case, the liquid will spread over the solid. If the forces of surface tension are greater than the forces of interaction with the molecules of the solid, then the liquid will be collected in a drop.

When liquid enters the capillary channel, its surface is curved, forming the so-called meniscus. The forces of surface tension tend to reduce the value of the free boundary of the meniscus, and an additional force begins to act in the capillary, leading to the absorption of the wetting liquid. The depth to which the liquid penetrates into the capillary is directly proportional to the surface tension of the liquid and inversely proportional to the radius of the capillary. In other words, the smaller the radius of the capillary (defect) and the better the wettability of the material, the faster and deeper the liquid penetrates into the capillary.

You can buy materials for capillary inspection (color flaw detection) from us at a low price from a warehouse in Moscow: penetrant, developer, cleaner Sherwin, capillary systemsHelling, Magnaflux, ultraviolet lights, ultraviolet lamps, ultraviolet illuminators, ultraviolet illuminators and control samples (standards) for color flaw detection of CD.

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Non-destructive testing becomes important when the development of the coating has already ended and it is possible to move on to its industrial application. Before the coated product is put into service, it is checked for strength, the absence of cracks, discontinuities, pores or other defects that can cause destruction. The more complex the object to be coated, the greater the likelihood of defects being present. Table 1 presents and below describes the existing non-destructive methods for determining the quality of coatings.

Table 1. Non-destructive methods quality control of coatings before use.

VISUAL INSPECTION

The simplest quality assessment is an external examination of the coated product. This control is relatively simple and becomes especially effective in good lighting conditions using a magnifying glass. As a rule, the external examination should be carried out by qualified personnel and in combination with other methods.

SPRAYING WITH PAINT

Cracks and depressions on the surface of the coating are revealed by the absorption of paint. The test surface is sprayed with paint. Then it is thoroughly wiped off and the indicator is sprayed on it. After a minute, paint emerges from cracks and other minor defects and stains the indicator, thus revealing the outline of the crack.

FLUORESCENT CONTROL

This method is similar to the paint soaking method. The sample to be tested is immersed in a solution containing fluorescent paint, which penetrates all cracks. After cleaning the surface, the sample is covered with a new solution. If the coating has any defects, the fluorescent paint in this place will be visible under ultraviolet light.

Both methods, based on absorption, are used only for detecting surface defects. In this case, internal defects are not detected. Defects lying on the surface itself are difficult to identify, since when wiping the surface before applying the indicator, the paint is removed from them.

RADIOGRAPHIC CONTROL

Penetrating radiation monitoring is used to detect pores, cracks and cavities within the coating. X-rays and gamma rays pass through the test material and hit the film. The intensity of X-rays and gamma rays changes as they pass through the material. Any pores, cracks or changes in thickness will be recorded on the photographic film, and with the appropriate decoding of the film, the position of all internal defects can be established.

Radiographic monitoring is relatively expensive and slow. Radiation protection of the operator is required. It is difficult to analyze products with complex shapes. Defects are identified when their size is more than 2% of the total coating thickness. Consequently, the radiographic technique is unsuitable for detecting small defects in large structures of complex shape; it gives good results on less complex products.

VORTEX CONTROL

Surface and internal defects can be determined using eddy currents induced in the product by introducing it into the electromagnetic field of the inductor. When the part is moved in the inductor, or the inductor relative to the part, the induced eddy currents interact with the inductor and change its impedance. The induced current in the sample depends on the presence of defects in the conductivity of the sample, as well as its hardness and size.

Defects can be detected by using appropriate inductances and frequencies, or a combination of both. Eddy current control is impractical if the product configuration is complex. This type of inspection is unsuitable for detecting defects on edges and corners; in some cases, the signals from an uneven surface can be the same as from a defect.

ULTRASONIC CONTROL

In ultrasonic testing, ultrasound is passed through the material and changes in the sound field caused by defects in the material are measured. The energy reflected from defects in the sample is received by a transducer, which converts it into an electrical signal and is fed to the oscilloscope.

Depending on the size and shape of the sample, longitudinal, transverse or surface waves are used for ultrasonic testing. Longitudinal waves propagate in a test material in a straight line until they meet a boundary or discontinuity. The first boundary that the incoming wave meets is the boundary between the transducer and the product. Part of the energy is reflected from the boundary, and a primary pulse appears on the oscilloscope screen. The rest of the energy passes through the material until it meets the defect or the opposite surface, the position of the defect is determined by measuring the distance between the signal from the defect and from the front and rear surfaces.

Discontinuities can be located so that they can be detected by directing radiation perpendicular to the surface. In this case, the sound beam is injected at an angle to the surface of the material to create shear waves. If the entry angle is increased enough, surface waves are formed. These waves travel along the contour of the sample and can detect defects near its surface.

There are two main types of ultrasonic testing machines. The resonance test uses radiation with a variable frequency. Upon reaching the natural frequency corresponding to the thickness of the material, the amplitude of the oscillations increases sharply, which is reflected on the oscilloscope screen. The resonant method is mainly used for measuring thickness.

With the pulsed echo method, pulses of a constant frequency with a duration of a fraction of a second are injected into the material. The wave travels through the material and the energy reflected from the defect or back surface is incident on the transducer. Then the transducer sends another pulse and receives the reflected one.

The transmission method is also used to detect defects in the coating and to determine the bond strength between the coating and the substrate. In some coating systems, measuring the reflected energy does not adequately identify the defect. This is due to the fact that the boundary between the coating and the substrate is characterized by such a high reflectivity that the presence of defects does not change the total reflectivity.

The use of ultrasonic testing is limited. This can be seen from the following examples. If the material has a rough surface, sound waves dissipate so much that the test is meaningless. For testing objects of complex shape, converters are required that repeat the contour of the object; surface irregularities cause bursts on the oscilloscope display, making it difficult to identify defects. Grain boundaries in metal act similarly to defects and scatter sound waves. Defects located at an angle to the beam are difficult to detect, since the reflection occurs mainly not towards the transducer, but at an angle to it. It is often difficult to distinguish discontinuities close to one another. In addition, only those defects are detected, the sizes of which are comparable to the length of the sound wave.

Conclusion

Screening tests are carried out during the initial stage of coating development. Since the number of different samples is very large during the search for the optimal mode, a combination of test methods is used to weed out unsatisfactory samples. This screening program usually consists of several types of oxidation testing, metallographic testing, flame testing, and tensile testing. Coatings that have successfully passed the screening tests are tested under conditions similar to those in use.

Once it is determined that a particular coating system has passed the field test, it can be applied to protect the actual product. It is necessary to develop a technique for non-destructive testing of the final product before putting it into operation. Non-destructive techniques can be used to detect surface and internal holes, cracks and discontinuities, as well as poor adhesion of the coating to the substrate.