Corrosion test includes laboratory corrosion test and field corrosion test. Laboratory corrosion test can be divided into room temperature or heated liquid immersion corrosion, gas corrosion (including high-temperature oxidation) and salt spray test. Field corrosion tests are also varied, such as atmospheric corrosion, seawater and fresh water immersion, soil corrosion, coupon test, etc. According to the nature of corrosion medium, it can be divided into acid, alkaline, neutral salt corrosion and other types.

The corrosion test conditions shall be determined according to the technical standards of the test coating materials or the service conditions of the coating materials. The pH value of the solution, the amount of dissolved gas, the concentration of dissolved ions, the temperature and flow rate of the solution will directly affect the corrosion reaction of the coating. It is generally believed that the dissolved gas in the solution, especially the oxygen content in the neutral solution, has a great influence on the cathodic reaction during the corrosion process, while the liquid temperature not only affects the reaction speed, but also has a certain influence on the reaction type.

1. Liquid immersion method
The liquid immersion corrosion of the sprayed coating can be carried out simultaneously with the standard sample (generally the sprayed base material), or with the sample of other materials with known corrosion resistance for comparison. As there are many pores in the spraying layer, the sealing treatment shall be carried out first, and then the corrosion test shall be carried out. Liquid immersion corrosion can be divided into static full immersion, partial immersion, interactive immersion, cavitation and other liquid immersion.

(1) Static full immersion type
Static full immersion is a test method in which the sample is completely immersed in the solution. This method is simple, reproducible and easy to control test factors. The test method and requirements are specified in the national standard GB 10124-88 Test Method for Uniform Corrosion of Metallic Materials by Full Immersion in Laboratory. During the test, the concentration of solution shall be kept consistent no matter under normal temperature or boiling. The evaporation loss of solution can be controlled by a constant level device or the solution can be added regularly, so that the fluctuation of solution volume does not exceed ± 1%. Sometimes reflux condensation is also used. In order to prevent the accumulation of corrosion products from affecting the corrosion law, the ratio of medium capacity to sample surface area is generally controlled at 20-200mL/cm3. It is generally required that only one coating is immersed in each test device to avoid the interference of corrosion products of one coating to the corrosion law of another coating.

Use water bath or oil bath to control the temperature of the test solution. Unless otherwise specified, the solution shall not be inflated during the corrosion test. The test container can be sealed or exposed. The test conducted at boiling point temperature shall be heated slowly, and a layer of glass (or ceramic) chips shall be laid at the bottom of the container to avoid excessive boiling and the impact of bubbles. The support and container of the sample shall be inert in the test solution, so as to ensure that they will not be corroded, damaged or contaminated with the test solution.

(2) Partially impregnated
The partial immersion type is to immerse the sample partially in the solution. Because the waterline at the gas/liquid interface is kept at the same fixed position on the coating surface for a long time, serious local corrosion damage is caused. The partial immersion test is also called the waterline corrosion test. This kind of waterline damage can occur on the inner wall of the container for liquid storage and on the coating components partially immersed in seawater. Aluminum coatings and some other metal coatings often have concentration corrosion at the waterline, and partial immersion test conditions provide a very suitable accelerated test method.

The partial immersion test method, requirements, condition control and test device are basically the same as the static full immersion test. The key is how to provide a suitable sample support method to keep the waterline at a fixed position and ensure that the area ratio of the sample above and below the liquid surface is constant. The repeatability of the test results can be significantly improved by using an automatic constant liquid level device.

(3) Interactive immersion type
The cross immersion test, also known as the inter immersion test, refers to alternately immersing the sample with metal coating into the liquid corrosion medium and exposing it to the air. The interactive immersion test is a simulation test and an accelerated test. This interactive immersion state provides conditions for accelerating corrosion due to the action of aqueous solution, because the sample surface can be frequently renewed during most exposure time, and the solution film is almost saturated with oxygen. In addition, during the dry wet alternation process, the corrosive components in the solution are concentrated due to the evaporation of water. In addition, the complete drying of the sample surface often reduces the corrosion rate, but the cracking of the corrosion product film plays an opposite role.

The results of the interactive immersion test are closely related to the frequency of dry and wet changes, the temperature and humidity of the environment. Therefore, the dry and wet change cycle must be reasonably designed and kept unchanged in continuous tests. Similarly, the ambient temperature and humidity should be reasonably controlled to ensure a constant drying rate of the sample during atmospheric exposure. According to different test requirements, sometimes the intermediate immersion test is carried out in a closed device with relatively high humidity to keep the sample moist after leaving the solution; Sometimes, when exposed in the air, dry hot air is used for blowing (or irradiation heating) to accelerate drying.

The selection of the conditions for the interactive immersion test is related to the working principle of the test device used. In order to realize the alternation of dry and wet, the sample can be alternately immersed in and taken out of the solution; The sample can also be fixed and the solution can rise and fall relative to the sample. It is usually easier to move the specimen than to move the corrosive medium. The exposure time ratio of solution to air shall be determined according to the specific requirements of the test, generally 1:1-1:10. The total time of a cycle is usually 1-60min, sometimes up to 24h.

2. Neutral salt spray test

The neutral salt spray test is a commonly used artificial accelerated corrosion test. It simulates the coastal atmospheric environment, where the warm sea surface evaporates to the cold air and the small liquid droplets containing chlorine ions are splashed into the space under the impact of the sea waves to form the corrosion conditions of the metal by the fine mist like medium. The sodium chloride solution of a certain concentration is used and sprayed in the form of fine mist under pressure. As the mist particles uniformly fall on the surface of the sample, and the liquid film is constantly renewed, Therefore, the corrosion of the coating conforms to the basic principle of atmospheric corrosion, and the accelerated corrosion effect of the coating can be measured.

After all, neutral salt spray test can not perfectly simulate the corrosion of metal coatings under atmospheric conditions. In addition, there are many factors that affect the corrosion, making the test conditions difficult to control, etc., and the reproducibility is often poor, so it can only be used as a comparison of the relative corrosion resistance of the same type of coating under this condition. Practice shows that it has a good corrosion resistance effect on anodic coatings (such as zinc layer and cadmium layer on steel substrate), while for cathodic coatings, it can only reveal the degree of pores and defects of the coating, indirectly reflecting the corrosion resistance of the coating.

The equipment, reagents and methods used for neutral salt spray test are specified in the national standard GB/T 6458-86 Metal Coatings Neutral Salt Spray Test.
The number of samples shall be in accordance with the specific provisions of the test. Generally, 3 samples shall be taken from each batch. During the test, the sample is placed in the salt spray test chamber, the test solution is 5% NaCl (mass percent) solution, the pH value is 6.5-7.2, the temperature in the spray chamber is 35 ± 2 ℃, and the relative humidity is greater than 95%. The test surface is upward, let the salt mist settle freely on the tested surface. After 24h spray, the solution collected by each collector should be 1-2mL in terms of 80cm2, and the tested surface should be 15-30 ° from the vertical direction. It cannot be sprayed directly on the tested surface. The test pieces shall not touch the box, nor shall they touch each other. The distance between the samples shall not affect the free falling of the salt spray on the tested surface, and the droplets on the samples shall not fall on other samples. The test time shall be determined according to the requirements of the tested coating or product standard. If there is no standard, the recommended time is: 2, 6, 16, 24, 48, 96, 240, 480, 720h. The sample is sealed in the box and cooled naturally. Take out the sample after the test. In order to reduce the falling off of corrosion products, the samples shall be dried indoors for 0.5-1h before cleaning. Then use clean flowing water with temperature no higher than 40 ℃ to gently clean, remove the residue of salt spray solution on the surface of the sample, and conduct appearance inspection and rating after drying. According to JB/Z 88-66 assessment standard for tropical electrical products in China, the corrosion degree after salt spray test is Grade 4: Grade I is good, Grade 2 is acceptable, and Grade 3-4 is unacceptable. In each level, as long as one item is met, it will be evaluated according to that level.
3. Copper salt accelerated acetate spray test (CASS method)
Copper salt accelerated acetate spray test is an improved method of neutral saline spray test. Because it is acidified with acetic acid in sodium chloride solution, and copper chloride is added to improve the corrosion rate and effect on some metal coatings, good results are obtained that the corrosion mode of cathodic coating on steel substrate is close to the actual environment. Compared with neutral salt water spray, CASS method not only has faster corrosion rate, but also has better reproducibility, so it is gradually replacing neutral salt water spray for corrosion resistance test of cathodic coatings. This method is applicable to the corrosion resistance inspection of cathodic coatings (copper, nickel, chromium coatings, etc.) on steel substrates and anodic oxide films on aluminum or aluminum alloys.
The solution composition of copper salt accelerated acetate spray test is: sodium chloride (NaCl) 50 ± 5g/L, copper chloride (CuCl2 · H2O) 0.26 ± 0.02g/L, and pH (adjusted with acetic acid) 3.2 ± 0.1. The test equipment is basically the same as the salt spray test equipment, but the requirements for the high temperature resistance of the box are higher (50 ± 1 ℃). At present, in addition to the equipment specially used for CASS test, there are also equipment common to brine spray test. Except that the test temperature is 50 ± 1 ℃, other conditions are the same as those of brine spray test.
4. Atmospheric exposure corrosion test
The atmospheric exposure corrosion test refers to the actual corrosion test on the sample rack (outdoor or indoor) of the sprayed coating sample under the natural atmospheric conditions, and the protection life is determined by regular observation and measurement of its corrosion process characteristics and corrosion rate. Atmospheric exposure test is an important method to correctly judge the corrosion resistance of coatings, and its evaluation results are usually used as the basis for formulating coating thickness standards. The specific test method can refer to the national standard GB 6464-86 General Rules for Outdoor Exposure Corrosion Test of Metallic and Other Inorganic Coatings under Stationary Conditions.
The test conditions for atmospheric exposure corrosion test shall be the same or similar to the actual conditions used for the workpiece. Generally, according to the environmental conditions of the area where the exposure site is located, the atmospheric conditions are divided into four categories (including indoor and outdoor): industrial atmosphere, marine atmosphere, rural atmosphere and suburban atmosphere. The exposure site shall be set in a fully open place, which can be fully attacked by the atmosphere (air, sunlight, rain, dew, fog, frost, snow, etc.). The surrounding buildings, trees and sample racks shall be spaced at a certain distance to ensure that the shadows of the surrounding buildings, trees, etc. will not cast on the tested samples at any time.
There are three ways to expose the sample to the atmosphere:

1) Expose to the sun in the open air, that is, place the sample directly on the outdoor frame. The frame is made of materials that can withstand corrosion, and the sample faces south on the frame. The height of plants near the frame shall not be greater than 0.2m;
2) Semi enclosed exposure: put the sample under the cover of the ceiling for exposure to prevent direct exposure to solar radiation and atmospheric precipitation, or expose the sample in partially enclosed space, such as a shutter box;
3) In case of "totally enclosed" exposure, the influence of external factors on the sample is very limited.
The sample for atmospheric exposure test shall not be less than 50cm2 (5cm × 10cm), the specially prepared specimen shall be in the shape of a flat plate, and the length of one side shall preferably be an integral multiple of 50mm, and the thickness shall be 1-3mm. The number of the same sample shall not be less than three. Before exposure to the sun, the specimen shall be thoroughly cleaned to remove contaminants from the coating surface. Each sample shall have an indelible number, such as a steel seal or a plastic plate with lettering. Before the test, the special test record card shall be used to record the sample number, coating structure, thickness, appearance gloss, etc., and the test outline (including test purpose, test requirements, inspection cycle, etc.) shall be prepared. 1~3 pieces of each sample shall be kept in the dryer for comparison and observation during the test.
It is recommended to put the sample in April May or September October before the humid and rainy season every year. As the atmospheric corrosion rate is very slow, the best test time is 1-20 years. After the sample is exposed, it shall be inspected frequently in the first three months, generally 2-3 times a month. During inspection, attention shall be paid to the time when corrosion spots begin to appear and detailed records shall be made. The inspection shall be conducted once a month after three months and once every three months after one year. If the samples exposed outdoors in winter encounter snow (refer to the areas with snow in winter), the snow on the surface shall be wiped off regularly. Do not touch the main surface directly by hand when taking the sample. It is not allowed to contact or rub two specimens to prevent man-made mechanical damage or electrochemical corrosion, which may cause test result error.

In addition to referring to the brine spray test method, the evaluation of test results can also be conducted according to the corrosion rate of the sample surface (see the result evaluation).

5. Sea water corrosion test
At present, thermal spraying anti-corrosion technology has been widely used in a large number of offshore engineering facilities, such as offshore oil production drilling platforms, ships and port facilities. Therefore, seawater corrosion test is one of the test contents of thermal spraying coatings.

The marine corrosion environment can be roughly divided into: marine atmospheric area, splash area, tidal range area, total immersion area and seabed mud area. Due to different environments, the corrosion rate of metals varies greatly. The metal components are in the splash zone, alternating between dry and wet, with sufficient oxygen supply and high corrosivity. When the high tide level area is in contact with the oxygenated seawater at high tide, the corrosion is also serious. In addition, relative to the full immersion area below the low tide line, the tidal range area forms an obvious oxygen concentration cell function. The full oxygen supply in the tidal range area is the cathode, while the full immersion area just below the low tide line becomes the anode due to the relatively small oxygen supply, accelerating corrosion. According to the regional characteristics of the marine environment, the seawater corrosion tests of metal protective coatings can be divided into 6 categories.
(1) Marine atmospheric corrosion test.
It can characterize the corrosion of metal protective coatings on ships and in cabins, as well as the corrosion of metal protective coatings on coastal facilities that will not be directly affected by sea water or spray.
(2) Spray splash zone corrosion test.
It indicates that offshore and coastal structures are often subject to spray splash above sea level, but will not be corroded by metal protective coatings soaked in seawater. Generally, the specimen is placed 0.2-2m above the mean high water level.
(3) Sea water immersion test.
It is also called alternate immersion test. Characterize corrosion of metal protective coating between port structure and hull external structure or soaked by seawater.
(4) Seawater semi immersion test.
The seawater water line corrosion test is used to characterize the corrosion of metal protective coatings at the gas/liquid/solid three-phase junction outside any floating structure.
(5) Sea water immersion test.
Characterizing the corrosion of metal protective coating of various underwater facilities, usually the sample is placed 0.2-2m below the lowest tide level.
(6) Marine soil corrosion test
This is a corrosion test of metal protective coatings simulating pipelines, cables and underwater facilities buried in the seabed soil.

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