Corrosion and accelerated testing

Corrosion and accelerated testing

Corrosive environment, corrosion types, methods!

General provisions

The destruction of the construction materials and equipment from corrosion leads to significant economic losses. These losses are inevitable, but they can be minimized by knowing the operating conditions (type and extent of aggressive environments), and applying the best in these conditions materials, types of protective coatings and methods of their application. According to [1], the total annual cost of corrosion protection in developed countries is estimated at 2-4% of gross national product. Figure 1 [1] shows the structure of costs for protection of steel structures and equipment from corrosion. The loss caused by corrosion of the US economy, according to [2], is estimated at 300 bln. Dollars. Per year, per capita income is $ 1,200. Per year.

It should be noted that the corrosion of the exposed structure not only metal, but also reinforced. For example, in Moscow, a number of road flyovers, metro bridge in the Luzhniki Stadium had deteriorated due to the aggressive impact of environment on the rebar in reinforced concrete. Therefore, all steel products and structures, including fittings and inserts in reinforced concrete, operated under the influence of aggressive media, should be adequately protected against corrosion. The choice of the corrosion protection is determined by the conditions of use of the product, given life and economic considerations. It is not always the best in terms of protection from corrosion will be cost-effective option. In the design of the corrosion protection of structures, components and equipment necessary to take into account the results of the calculation of the cost-effectiveness of measures to protect against corrosion.

Cost-effectiveness of corrosion protection is calculated from the design life of the object, the cost of protection against corrosion during manufacturing, the number of planned work on the restoration of corrosion protection for the entire period of operation and cost.

The National Association of Corrosion Engineers USA (NACE) has developed a standard RP 0272 «Direct calculation of the cost-effectiveness of measures to protect against corrosion.» The method proposed in this standard, allows you to quickly calculate the cost of protection against corrosion, periodic restoration costs and facilitates the underlying economics of the various options of protection [1].

Corrosive environments

The following types of corrosion of steel products, construction and equipment: high-temperature oxidation, exposure to liquid corrosion, corrosion in soils and atmospheric corrosion. In practice, for each kind of aggressive environments, optimal from the point of view of economy and durability, materials and methods of protection. Most of the designs, products and equipment are exposed to atmospheric corrosion.

Atmosphere industrial areas contains corrosive gases such as ammonia, sulfur dioxide, hydrogen sulfide, nitrogen oxides, chlorine, hydrogen chloride, and water-soluble solids and aerosols: chlorides, sulfates, phosphates, oxides and hydroxides. The aggressiveness of the atmosphere due to the presence of sea spray and spray it with dissolved chlorides.

The nature and rate of atmospheric corrosion is largely determined by the presence and thickness on the surface of steel structures and parts of the film of moisture and the amount of dissolved gases and there is corrosive salts. The thickness of the electrolyte film on the surface of the structure also determines the degree of corrosion attack. The greatest impact on the metal phase has a film of moisture, which is formed at its condensation from the atmosphere or from rainfall. The thickness of the phase moisture film allows oxygen to easily reach korrodiruemoy surface and has little ohmic resistance to flow there electrochemical processes.

On the metal surface, nestled against precipitation or located in a relatively dry atmosphere as possible hydration and, consequently, the occurrence of corrosion processes. In this case, corrosion processes take place under the action of moisture present in the atmosphere (humidity) of fog due to corrosion products hygroscopic (moisture adsorption) due to moisture condensation of the film (due to temperature changes the metal and air). Condensation of moisture on the surface of steel products, operated in a dry atmosphere, due to sudden changes in day and night temperatures leads to corrosive atmosphere that in these conditions is comparable with the aggressiveness humid conditions.

Atmospheric corrosion problem has recently become a large enough value. It is associated with significant air pollution industrial products, chemical plant protection and fertilizers, automobile engine exhaust. One of the major factors affecting the increase in the degree of exposure to aggressive industrial atmosphere for the metal, is the concentration of sulfur dioxide (S02). The presence in the atmosphere of water-soluble salts (particularly chlorides) also affects the acceleration of corrosion processes under moisture phase film. It should be noted that when equal concentrations of sulfur dioxide has a greater influence on the corrosion of steel products than chlorides.

However, this is not applicable to marine corrosion in the variable wetting when exposed to splash or mist [3].

Classification of atmospheric effects on steel structures, industrial and agricultural companies set SNP 1-85 2.03.1 «Protection of building constructions against corrosion.» According to the degree of aggressiveness of the atmosphere divided into four groups: non-corrosive, slightly aggressive, moderately aggressive, silnoagressivnyh. In Table 1, the estimation of the rate of corrosion and loss of the bearing capacity of steel and concrete structures without protective coatings under various operating conditions. [4]

Share distribution of media on the degree of aggressiveness on the industrial and agricultural production

Buildings industrial production

1 — metallurgy

2 — chemical and petrochemical industry;

3 — mechanical engineering

4 — pulp and paper industry;

5 — food and light industry

Agricultural buildings

6 — cattle farms

7 — pig farm

8 — poultry farms

9 — storage of fertilizers

10 — Database reconstruction

Share distribution of media on the degree of aggressiveness on the industrial and agricultural purposes is shown in Fig. 2. As seen from the material [4], the most aggressive impact on the construction, equipment and products have chemical and petrochemical plants, warehouses fertilizers, pig and fattening farms.

It should be noted that the industrial atmosphere mainly characterized as aggressive and slightly aggressive, and in conditions of agricultural production atmosphere mainly characterized as mild and aggressive environments.

International standardization in the field of corrosion and corrosion protection establishes five categories of aggressive atmosphere: very low, low, medium, high, very high. This classification is made based on the following:

— the duration of wetting of the surface by precipitation, dew and other forms of moisture;

— the degree of atmospheric pollution corrosive gases (SO2 atmosphere of Commerce and Industry, CI-Navy atmosphere);

— the degree of air pollution particles suspended salts, the salts are distinguished by their solubility in water;

— the results of measuring the speed of corrosion failure of standard samples in the atmospheres of different classes of aggressiveness.

International Standard ISO

9223 identifies categories corrosivity atmospheres when exposed to products of carbon steel, zinc, aluminum and their alloys. ISO

9224 and ISO 9223 establish the basic parameters of corrosion of these materials. These standards define the relative useful life of structural materials and coatings in different atmospheric conditions and contribute to the definition of aggressive cost-effective protection of steel structures and products from corrosion.

Various metals and alloys react differently to environmental influences. This should be considered when selecting the material of construction and form a protective coating. For example, in industrial or marine atmosphere aluminum coatings are more durable in comparison with zinc. However, to prevent corrosion of embedded products and fixtures in concrete products most preferred zinc coatings. This is due to the fact that at steaming concrete products in the autoclave to valve moist alkaline environment. Aluminum is superior zinc corrosion resistance in acidic environments and inferior in alkali.

Corrosion products can shield a metal surface from exposure to the environment if they are insoluble in water, and may accelerate the corrosion processes, if they are hygroscopic.

The rate of corrosion of zinc coatings slow down over time, it is connected with the formation of dense enough insoluble corrosion products of zinc. Weathering steel such as «Corten» (alloyed with copper) in a non-aggressive or slightly aggressive atmosphere (SNIP 2.03.11-85 «protection of structures from corrosion») are protected from the effects of the environment dense corrosion products. However, if the aggressive influence of the atmosphere on separate portions of the surface of articles made of these steels friable corrosion products are formed, which cause the formation there of local effect types.

In addition to general corrosion, which provides a uniform destruction of the product under the influence of aggressive environment, there is a localized corrosion resulting in local lesions, mainly related to the heterogeneity of the environment and the heterogeneity of the corroding surface. The most dangerous manifestations of localized corrosion are intergranular corrosion, stress corrosion cracking, contact corrosion, crevice corrosion, pitting corrosion. Generally, localized corrosion is a result of the flow of anodic processes at selected sites or structural components of metal, while the rest of the surface of the cathode processes proceed advantageously.

The reasons for localization processes at the anode surface may be corroding its structural heterogeneity, heterogeneity of the properties of the electrolyte, the conditions of exposure to the environment, the local destruction of protective coatings, particularly metal structure, instability of the physical properties of corrosion products [5].

An example of localized corrosion associated with the structural heterogeneity is intergranular corrosion of stainless steels and aluminum alloys. Selective ionization of the metal along the grain boundaries of the intermetallic compounds or leads to the destruction of these sites, this weakens the bond between the crystallites and leads to the destruction of the material.

In addition, the appearance of localized corrosion results in the presence of lattice defects, the local concentration of internal stress, uneven temperature field.

An example of localized corrosion associated with the heterogeneity of the properties of the electrolyte, is the formation of pitting due to inhomogeneous aeration of the metal surface. Pitting under a layer of corrosion products in the pipeline is a consequence of changes in the composition of the medium.

An example of localized corrosion associated with the conditions of exposure to the environment is corrosive destruction along the waterline elements of marine structures, inserts for concrete products in the area of ​​concrete output, operating in conditions of high corrosive environments.

Local failure cathode coatings (e.g., electroplated nickel) lead to localized corrosion of a basic defect in a coating. This is the most dangerous in the case of cathodic galvanic coatings in environments of increased aggressiveness. In areas with a defective coating intensive flow anodic processes leading to quite serious destruction of the local base.

The design of the product should not be used nodes and gateway elements that contribute to the appearance there of local failure. For example, when interfacing structural elements is not allowed formation of cracks, which delayed the electrolyte. The presence of cracks leads to the manifestation of the above-mentioned factors contributing to the formation of local lesions.

Illiterate designed or adapted welded or bolted connection (for example, a dual area) having a dangerous gap between the structural elements, leading to its destruction. This results in a gap intensive corrosion (crevice corrosion). The volume of corrosion products is greater than the volume of metal poshedshego their formation. In this connection, with good access to the gap electrolytic corrosion products increase in volume.

Association of sellers and manufacturers of metalware "RosMetie *

Voronezhprommetiz dealer and a representative of the DMZ CHSPZ gut deformed welded or bolted connection or destroy it.

The accelerated corrosion tests

For right choice of material and protection of steel structures and products from corrosion using accelerated testing samples of materials and coatings. The intensity of exposure to atmospheric corrosion and product structure defined by a plurality of concurrent factors: air temperature and humidity, concentration of corrosive gas and slurry residence time of the film of moisture.

The concept of accelerated tests is the understanding that the corrosion resistance is not only the property of the material, but is largely determined by the nature of the corrosive environment. A variety of operating conditions designs and products leads to the need to use different materials and coatings, so it is not possible to create a single system of protection against corrosion and universal method of accelerated tests.

Developed and standardized by the various methods of accelerated corrosion tests. These tests include determination of corrosion resistance of structural materials and coatings in various aggressive environments, as well as their tendency to various types of corrosion damage.

Acceleration of corrosion processes is achieved by intensifying factors such as temperature, humidity, condensation, corrosive gases, salts or aerosols.

Terms of accelerated corrosion tests must meet the following requirements [6]:

— accelerate the corrosion process should not be the result of changes in its mechanism, accelerate corrosion processes can only be due to changes in the kinetics of the cathodic process;

— modes of accelerated corrosion tests should take into account the composition and properties of the corrosive environment in natural conditions;

— materials and coatings must be grouped according to the degree of exposure to a variety of corrosive media;

— It should produce the correct choice of the different types of corrosion index for each type of corrosive media;

— Results of accelerated corrosion tests should mainly be used as a comparative evaluation of materials and coatings, they can not be used to calculate their durability.

General requirements for models, equipment and methods of accelerated corrosion tests established by ISO 7384.

The results of the accelerated corrosion tests in an environment where not only reproduce the aggressive action of the mechanism, but also accelerate it possible to predict the durability and susceptibility to certain types of corrosion test materials and coatings.

Selecting a corrosive environment, accelerating corrosion manifestation of the results allows for short tests only to compare the corrosion resistance of different materials and coatings.

Selecting koorozivnoy protection of special accelerated corrosion, allows rapid control, detect defects and weaknesses in corrosion protection products and designs.

Tests in humid atmosphere

The presence of water vapor in the atmosphere is a necessary and sufficient condition for the occurrence of corrosion processes on the surface of structures and products. Moisture adsorption film (without condensation) on the surface of the metal promotes the anodic processes.

Ambient temperature and humidity are the main criteria corrosivity humid atmosphere. The purpose of the accelerated environmental tests is to establish the most stringent in terms of corrosion conditions throughout the test period.

There are several ways of humidifying the atmosphere in the climate chamber: aqueous aerosol spray generated moisture and disperse using steam in this case, there should be no condensation. Modes such tests are standardized to IEC 60068-2-3 (CA test), the basic modes are shown in Table. 2 [1].

Tests under a layer of condensation

More intense corrosive to metal phase has a film of moisture that forms on the surface of the product resulting from the condensation at a sufficiently rapid change in temperature. To speed up the process of anode in tests conducted on a periodic drying of the sample surface.

Temperature, Relative Humidity,% Duration of exposure, h.

The most widely used method for standardized IEC 60068-2-30 (DB test). The main characteristics of this test method are given in the Table. 3 [1].

Salt spray tests

This method consists of spraying a salt solution (sodium chloride) in the test chamber. When on the surface of the test samples of sodium chloride particles fall which are centers of condensation. Condensation and precipitation of salt accelerate corrosion of samples without changing the mechanism of action in the field conditions.

According to [6], the concentration of sodium chloride sprays should not exceed 3 — 5% increase in solubility reduces the concentration of oxygen in the solution and accordingly the intensity of corrosive action.

Test Method spraying a 5% solution of sodium chloride was designed to evaluate the corrosion properties of the galvanic coating is standardized in many countries and eventually became used as a universal test for corrosion resistance regardless of the type of materials and coatings, as well as their operating conditions. With this method allowed a comparative evaluation of corrosion resistance of new materials and coatings.

The test method is standardized salt spray IEC 60068-2-11 (Ka test). To intensify the corrosive effects of sodium chloride solution acidified with glacial acetic acid to pH 3.1 — 3.3.

Table. 4 [1] shows the comparative characteristics of the test methods in the salt spray.

More intense local impact on the metal during testing have salt drops (SD test). Use a solution simulating chemical composition seawater tests 23’S conducted at a temperature and relative humidity of 90%.

Tests under the influence of sulfur dioxide

Tests in humidity chamber in the presence of sulfur dioxide are well mimic the industrial atmosphere. The concentration of sulfur dioxide is determined by objective tests, the material structure and protective coating. These tests can tighten condensation of moisture on the surface of samples. The test conditions are standardized MEK2-42 (Ks test).

Comparative characteristics of methods of corrosion tests in a humidity chamber with sulfur dioxide are given in Table. 5 [1].

Also sulphurous gas atmosphere to simulate an industrial-use corro- sive such gases as hydrogen sulfide (H2S), chloro (CI), nitrogen oxide (NO2). In the humidity chamber in the presence of corrosive gases in the condensed moisture on the surface of the film samples, a solution of the corresponding acids. The concentration of acid in the moisture phase film determined by the temperature and the concentration in the chamber there corrosivity gas or mixture of gases.

These tests make it possible to simulate any industrial environment.

Cyclic Test

Developed and widely used in industry accelerated testing of materials and coatings are generally used as reference.

However, in the design of equipment and structures is often required to have data on the intended durability of materials and coatings used in these conditions. For this developed and widely used cyclical accelerated corrosion tests.

The ISO 14993 is summarized many years of experience and field of accelerated corrosion testing of materials and coatings in various corrosive atmospheres.

Cyclic testing is usually carried out when exposed to salt fog patterns or korozivnyh gas humidity chamber followed by dipping them in alternating electrolyte drying.

Table. 6 [1] as an example, the basic parameters of the conditions of cyclic tests, standardized ISO 14993 compared to GOST 9.409.

These tests are quite lengthy. But to assess the durability of metallic coatings, for example, diffusion, durability in harsh conditions, which should reach dozens of years, it is quite acceptable.

Corrosion failure samples were evaluated qualitatively and quantitatively the appearance of weight loss. For this purpose, after removing the corrosion products from the metal-coated samples was weighed and compared to determine the initial weight change of the coating thickness (weight loss per unit surface area, for example g / m2).

When choosing a method of accelerated tests should take into account the kind of coverage, and class of its durability, the degree of aggressiveness of natural environments designed products or structural steel.

To evaluate the corrosion resistance of the metal coating to diffusion bond to the substrate or incorrect use of short-term rapid methods of corrosion testing. This is mainly due to the inhomogeneity of chemical composition of coating thickness and hence the difference in the corrosion properties of the individual layers.

To illustrate this can be mentioned, for example, measurements of the electrode potential of the various layers of the aluminum coating (refractive reactivity).

The electrode potential of the intermetallic layer is -0,261V and aluminum layer -0,515V. Alloying of the molten silicon or manganese leads to a greater or lesser degree, to a change in the electrode potential of the coating layers.

If at the express-analysis to evaluate the corrosion properties of the coating its surface layer and carried out the extrapolation of the results to cover a whole, these tests do not give an objective assessment of the durability of coating under these conditions.

Thus, if a rapid test surface zone homogeneous materials allow to assess their corrosion properties, these test layered composite materials, among which may include metal diffusion coating, do not permit.

In addition, increased corrosive environments by increasing the concentration corro—invasive agent during the rapid test often leads to corrosion and distortion of the mechanism, as a consequence, to change the physical properties of corrosion products on the surface of the sample. This can have a significant effect on the kinetics of corrosion damage, especially laminate.

Analysis of the above types of accelerated corrosion test shows that the test Test diffusion of metallic coatings when exposed to salt fog due to their short duration do not give an objective evaluation.

Experience accelerated and in-situ corrosion tests protective metal coatings (zinc and aluminum melt) showed that the most objective evaluation of the corrosion properties and durability of these coatings can be obtained by cyclic tests at variable wetting of the electrolyte solution in a humidity chamber with corrosive gases, salts and aerosols. In this case, for the duration of objective results of tests of samples with metal coatings diffusion type (number of cycles) until the basics of corrosion products will not be hours and days and months.

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