Salt spray testing of stainless steel
Corrosion is the destruction or deterioration of a material or its properties under the action of the environment. Most corrosion occurs in the atmospheric environment, which contains corrosive components and corrosion factors such as oxygen, humidity, temperature changes and pollutants.
Salt spray corrosion is one of the most common and destructive atmospheric corrosion. The corrosion of salt spray on the surface of metal materials is caused by the electrochemical reaction between the chloride ions and the inner metal through the oxide layer and the protective layer of the metal surface. At the same time, chloride ions contain a certain hydration energy, which is easy to be adsorbed in the pores and cracks of the metal surface to exclude and replace the oxygen in the oxide layer, turn the insoluble oxide into a soluble chloride, and turn the passivated surface into an active surface.
Salt spray test is a kind of environmental test which mainly uses artificial 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 environmental exposure test, the other is the artificial accelerated simulated salt spray environmental test.
The salt spray test was invented in the early 20th century and is the longest used "corrosion test", which 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 result is simple and clear, which is conducive to the settlement of commercial disputes.
In practical applications, the salt spray test of stainless steel is the most well-known - how many hours can this material salt spray test? Practitioners are no strangers to this problem.
Material manufacturers usually use methods such as passivation treatment or improving the surface polishing grade to improve the salt spray test time of stainless steel. But the most critical determinant is the composition of the stainless steel itself, namely the content of chromium, molybdenum and nickel.
The higher the content of chromium and molybdenum, the stronger the corrosion resistance required for pitting and crevice corrosion to begin. This corrosion resistance can be 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 down the corrosion rate after the corrosion process begins. Therefore, austenitic stainless steels containing nickel often have better performance in salt spray tests, and the corrosion phenomenon is much less than that of low-nickel ferritic stainless steels with similar pitting resistance.
Trivia: 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 defects in detecting the properties 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 practical application environment with a very low chloride content will also be corroded in the salt spray test.
Salt spray test changes the corrosion behavior of stainless steel, which can neither be regarded as a kind of accelerated test nor as a kind of simulation experiment. The results are one-sided and have no equivalent relationship with the actual properties of the stainless steel eventually put into use.
So we can use a salt spray test to compare the corrosion resistance of different types of stainless steel, but this test is only able to grade the material. When it comes to the specific selection of stainless steel materials, salt spray testing alone usually does not provide sufficient information, because we do not have enough understanding of the connection 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 a stainless steel sample.
In addition, it is not possible to compare different types of steel, such as stainless steel and coated carbon steel, because the corrosion mechanism of the two materials used in the test is completely different, and the correlation between the test results and the final actual use environment is not the same.
