Salt spray test
Corrosion is the damage or deterioration of materials or their properties caused by the environment. Most of the corrosion occurs in the atmosphere, which contains corrosive components and factors such as oxygen, humidity, temperature changes, and pollutants.
Salt spray corrosion is a common and most destructive atmospheric corrosion. The corrosion of salt spray on the surface of metal materials is caused by the electrochemical reaction between the contained chloride ions penetrating the oxide layer and the protective layer of the metal surface and the internal metal. At the same time, chloride ions contain a certain amount of hydration energy and are easily displaced by the pores and cracks adsorbed on the metal surface to replace the oxygen in the oxide layer, turning insoluble oxides into soluble chlorides, and turning the passivated surface into an active surface.
A salt spray test is an environmental test that mainly uses artificially simulated salt spray environmental conditions created by salt spray test equipment to assess the corrosion resistance of products or metal materials. It is divided into two categories, one is the natural environment exposure test, and the other is the artificially accelerated simulated salt spray environment test.
The artificial simulation salt spray environment test is to use test equipment with a certain volume space – the salt spray test chamber (as shown in the figure), and uses artificial methods in its volume space to create a salt spray environment to test the salt spray corrosion resistance of the product. performance quality assessment.
Compared with the natural environment, the salt concentration of chloride in the salt spray environment can be several times or dozens of times that of the general natural environment salt spray content, which greatly increases the corrosion rate. The salt spray test is carried out on the product and the results are obtained. The time is also greatly shortened. For example, if a product sample is tested in a natural exposure environment, it may take 1 year for it to corrode, but it only takes 24 hours to obtain similar results in an artificially simulated salt spray environment.
Laboratory-simulated salt spray can be divided into four categories:
⑴Neutral salt spray test (NSS test) is an accelerated corrosion test method that appeared the earliest and is currently the most widely used. It uses a 5% sodium chloride saline solution, and the pH value of the solution is adjusted in the neutral range (6.5-7.2) as a solution for spraying. The test temperature is 35°C, and the sedimentation rate of salt mist is required to be 1-2ml/80cm/h.
⑵Acetic salt spray test (ASS test) is developed based on a neutral salt spray test. It is to add some glacial acetic acid to the 5% sodium chloride solution so that the pH value of the solution is reduced to about 3, the solution becomes acidic, and the final salt spray is also changed from neutral salt spray to acidic. Its corrosion rate is about 3 times faster than that of the NSS test.
(3) Copper salt accelerated acetic acid salt spray test (CASS test) is a rapid salt spray corrosion test recently developed abroad. 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.
⑷Alternating salt spray test is a comprehensive salt spray test, which is a neutral salt spray test plus a constant damp heat test. It is mainly used for cavity-type complete machine products. Through the penetration of the humid environment, salt spray corrosion not only occurs on the surface of the product but also inside the product. It is to alternately switch the product under two environmental conditions of salt spray and humid heat, and finally assess whether the electrical and mechanical properties of the whole product have changed.
Result judgment
The test results of the salt spray test are generally given in qualitative rather than quantitative form. There are four specific judgment methods.
① The rating judgment method is to divide the percentage of the corrosion area to the total area into several grades according to a certain method and use a certain grade as the basis for qualified judgment, which is suitable for the evaluation of flat samples.
②Weighing judgment method is to judge the corrosion resistance quality of the sample by calculating the weight of the corrosion loss by weighing the weight of the sample before and after the corrosion test. It is especially suitable for evaluating the corrosion resistance quality of a certain metal.
③Corrosion appearance judgment method is a qualitative judgment method. It judges the samples by whether the product has a corrosion phenomenon after the salt spray corrosion test. Most of the general product standards adopt this method.
④ Statistical analysis of corrosion data provides a method for designing corrosion tests, analyzing corrosion data, and determining the confidence level of corrosion data. It is mainly used for analysis and statistics of corrosion conditions, rather than for quality judgment of a specific product.
Salt spray test of stainless steel
Invented in the early 20th century, the salt spray test is the longest-used “corrosion test”. It is favored by users of corrosion-resistant materials and has become a “universal” test. The main reasons are 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 last? Practitioners must be familiar with this issue.
Material manufacturers usually use methods such as passivation treatment or improving the surface polishing level to increase the salt spray test time of stainless steel. But the most critical determining factor is the composition of the stainless steel itself, namely the content of chromium, molybdenum, and nickel.
The higher the content of both chromium and molybdenum, the greater the corrosion properties required for resistance to pitting and crevice corrosion to begin to occur. This corrosion resistance is expressed by the so-called pitting resistance equivalent (PRE) value: PRE=%Cr+3.3×%Mo.
Nickel does not increase the resistance of steel to pitting and crevice corrosion, but it can effectively slow down the corrosion process once it has begun. Therefore, nickel-containing austenitic stainless steel tends to perform better in the salt spray test, and its corrosion phenomenon is much lighter than that of low-nickel ferritic stainless steel with a similar pitting resistance equivalent.
Tips: 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 pointed out that the salt spray test has major flaws in testing the performance of stainless steel. In the salt spray test, the chloride content of the salt spray is extremely high, far exceeding the real environment, so the stainless steel that can resist corrosion in the actual application environment with a very low chloride content will also be corroded in the salt spray test.
The salt spray test changes the corrosion behavior of stainless steel, and it can neither be regarded as an accelerated test nor a simulated experiment. The results are one-sided and have no equivalent relationship with 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 rate the material. When specifically selecting stainless steel materials, salt spray testing alone is often not informative, as we do not have sufficient knowledge of the relationship between test conditions and the actual application environment.
For the same reason, it is also impossible to estimate the service life of a product based solely on the salt spray test of a stainless steel sample.
In addition, different types of steel cannot be compared. For example, we cannot compare stainless steel with 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 either.
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