Corrosion is the destruction or deterioration of a material or its properties caused by the environment. Most of the corrosion occurs in the atmosphere, which contains corrosive components and corrosive factors such as oxygen, humidity, temperature changes, and pollutants.

Salt spray corrosion is a common and most destructive atmospheric corrosion. The corrosion of the surface of the metal material by the salt spray is caused by the electrochemical reaction between the oxide layer containing the chloride ions penetrating the metal surface and the protective layer and the internal metal. At the same time, chloride ions contain a certain hydration energy, which is easily adsorbed on the pores of the metal surface, cracks and replaces the oxygen in the oxide layer, turns the insoluble oxide into a soluble chloride, and turns the passivated surface into a lively surface. .

The salt spray test is an environmental test that uses artificial simulated salt spray environmental conditions created by salt spray test equipment to evaluate the corrosion resistance of products or metal materials. It is divided into two categories, one is the natural environmental exposure test, and the other is the artificial accelerated simulated salt spray environment test.

The artificial simulated salt spray environment test uses a test equipment with a certain volume space - salt spray test chamber (pictured), artificially in its volume space, causing salt spray environment to resist salt spray corrosion of the product. Performance quality is assessed.

Compared with the natural environment, the salt concentration of the chloride in the salt spray environment can be several times or several times higher than the salt spray content of the general natural environment, so that the corrosion rate is greatly improved, and the product is subjected to a salt spray test to obtain a result. The time is also greatly shortened. If a product sample is tested in a natural exposure environment, it may take 1 year to corrode, and in a simulated simulated salt spray environment, as long as 24 hours, similar results can be obtained.

Laboratory simulation of salt spray can be divided into four categories:

(1) Neutral salt spray test (NSS test) is one of the earliest applications in the field of accelerated corrosion test. It uses a 5% aqueous solution of sodium chloride solution, and the pH of the solution is adjusted to a neutral range (6.5 to 7.2) as a solution for spraying. The test temperature is taken at 35 ° C, and the sedimentation rate of the salt spray is required to be 1 to 2 ml / 80 cm / h.

(2) The acetate spray test (ASS test) was developed on the basis of a neutral salt spray test. It is to add some glacial acetic acid to the 5% sodium chloride solution, the pH value of the solution is reduced to about 3, the solution becomes acidic, and the salt spray finally formed is also changed from neutral salt spray to acid. Its corrosion rate is about 3 times faster than the NSS test.

(3) Copper salt accelerated acetate spray test (CASS test) is a rapid salt spray corrosion test developed recently in foreign countries. The test temperature is 50 °C, and a small amount of copper salt-copper chloride is added to the salt solution to strongly induce corrosion. Its corrosion rate is about 8 times that of the NSS test.

(4) The alternating salt spray test is a comprehensive salt spray test, which is actually a neutral salt spray test plus a constant damp heat test. It is mainly used for cavity-type machine products. Through the penetration of the tidal environment, salt spray corrosion is generated not only on the surface of the product but also inside the product. It converts the product alternately under the two conditions of salt spray and hot and humid, and finally checks whether the electrical and mechanical properties of the whole product are changed.

Result determination

The test results of the salt spray test are generally given in qualitative form rather than quantitative form. There are four specific methods of determination.

The 1 rating method is to divide the percentage of the ratio of the corrosion area to the total area into several levels according to a certain method, and use one level as the basis for the qualification judgment, which is suitable for the evaluation of the flat sample.

2 Weighing determination method is to evaluate the corrosion resistance quality of the sample by weighing the weight of the sample before and after the corrosion test, and calculating the corrosion loss quality. It is especially suitable for assessing the corrosion resistance quality of a certain metal.

3 Corrosion occurrence determination method is a qualitative determination method. It determines the sample after the salt spray corrosion test, whether the product is corroded or not. This method is mostly used in general product standards.

4 Corrosion data statistical analysis method provides a method to design corrosion test, analyze corrosion data, and determine the confidence of corrosion data. It is mainly used for analysis and statistical corrosion, rather than specific quality judgment for a specific product.

Stainless steel salt spray test

The salt spray test was invented in the early 20th century and is the longest-used "corrosion test". It has become a "universal" test for users of corrosion-resistant materials. The reasons are mainly as follows: 1 save time; 2 low cost; 3 can test a variety of materials; 4 the results are simple and clear, which is conducive to the resolution of commercial disputes.

In practical applications, the salt spray test of stainless steel is the most widely known – how many hours can the salt spray test of this material reach? Practitioners are no strangers to this problem.

Material manufacturers often use methods such as passivation or improved surface finish to improve the salt spray test time of stainless steel. But the most critical determinant is the composition of the stainless steel itself, namely chromium, molybdenum and nickel.

The higher the content of both chromium and molybdenum, the stronger the corrosion resistance required to resist pitting and crevice corrosion. This corrosion resistance is expressed by the so-called pitting resistance equivalent (PRE) value: PRE = % Cr + 3.3 × % Mo.

Although nickel does not increase the resistance of steel to pitting and crevice corrosion, it can effectively slow the corrosion rate after the corrosion process begins. Therefore, nickel-containing austenitic stainless steels tend to perform better in salt spray tests, and their corrosion is much less severe than low-nickel ferritic stainless steels with similar pitting resistance.

Tip: Standard 304, neutral salt spray is generally between 48 and 72 hours; standard 316, neutral salt spray is generally 72 to 120 hours.

It should be noted that the salt spray test has major drawbacks in detecting the performance of stainless steel. The salt spray has a very high chloride content in the salt spray test, far exceeding the real environment, so stainless steel that can resist corrosion in practical applications where the chloride content is low is also corroded in the salt spray test.

The salt spray test changes the corrosion behavior of stainless steel. It can neither be regarded as an accelerated test nor as a simulation experiment. The result is one-sided and has no equivalent relationship to the actual performance of the stainless steel that is finally put into use.

So we can use the salt spray test to compare the corrosion resistance of different types of stainless steel, but this test can only give the material a rating. When selecting stainless steel materials, the salt spray test alone does not usually provide sufficient information because we do not have sufficient knowledge of the link between the test conditions and the actual application environment.

For the same reason, it is not possible to estimate the service life of a product based solely on the salt spray test of stainless steel samples.

In addition, there is no comparison between different types of steel. For example, we cannot compare stainless steel and coated carbon steel because the corrosion mechanisms of the two materials used in the test are completely different, and the test results are related to the final actual use environment. Sex is not the same.

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