People often ask why stainless steel, being stainless, can still rust. What is the mechanism behind its rust resistance? Is it safe to use as long as the material composition is correct? Let's delve into these questions one by one. While some advantages of stainless steel can become disadvantages over time, some obvious advantages can deteriorate rapidly when they start to turn negative. For instance, stainless steel has excellent corrosion resistance due to its high chromium content (over 10.5%). This is because the presence of chromium forms a thin, nanometer-thick passive film on the surface, significantly enhancing the material's corrosion resistance. However, if the chromium content is insufficient, it can affect the stainless steel's resistance to pitting corrosion. Poor solid solution treatment can lead to the formation of carbides at grain boundaries, reducing the stainless steel's resistance to intergranular corrosion and making it more susceptible to stress corrosion cracking.

1.Factors Affecting the Pitting Corrosion Performance of Austenitic Stainless Steel Some stainless steel products exhibit localized pitting corrosion during storage or long-term maritime transport, which can severely impact shipping and customer receipt schedules. The locations where pitting corrosion occurs are similar to those of fatigue sources, typically starting from chemical or physical discontinuities in the material, such as non-metallic inclusions, second-phase particles, and surface damage. Therefore, preventing pitting corrosion involves reducing or avoiding these discontinuity defects. In terms of chemical composition, the PREN (Pitting Resistance Equivalent Number) is a key indicator of a material's resistance to pitting corrosion. The formula for PREN is generally PREN=Cr%+3.3Mo%+16N%, indicating that the three elements—chromium (Cr), molybdenum (Mo), and nitrogen (N)—play a significant role in the pitting corrosion resistance of stainless steel. Controlling the content of these elements is crucial. For example, in 316 austenitic stainless steel, the typical elemental requirements are Cr at least 16%, Mo at least 2%, and N at most 0.1%. To control costs, manufacturers usually add alloy elements at the minimum levels. However, for higher corrosion resistance, the content of Cr and Mo can be increased, such as reaching 17.5% for Cr and 2.8% for Mo, significantly enhancing the material's pitting corrosion resistance.

2. The factors affecting the intergranular corrosion resistance of austenitic stainless steel, based on extensive analysis, reveal that when corrosion failure occurs in austenitic stainless steel, examining the material's metallographic structure often reveals carbides precipitating along grain boundaries. The formation and presence of carbides occupy more Cr atoms, causing the surrounding Cr to diffuse into the carbide formation area. This results in a thin Cr-deficient layer around the carbides, with the Cr content below the minimum requirement of 10.5% for stainless steel. This layer becomes a weak point for corrosion, making it highly susceptible to corrosion along the Cr-deficient areas, typically along the grain boundaries, known as intergranular corrosion. Furthermore, based on this grain boundary corrosion, stress corrosion cracking may develop and lead to further failure.

The presence of chromium-rich regions around the grain boundary carbides is caused by their precipitation

Cracks along austenite grain boundaries are primarily due to the formation of intergranular carbides. The main reason for this is that austenitic stainless steel products do not cool quickly enough during solution heat treatment, leading to a prolonged residence time in the 425℃-850℃ temperature range, where intergranular carbides precipitate, with the fastest precipitation rate at 675℃. Understanding the failure mechanism can help identify solutions. Knowing that carbides are the key and the stage of their formation, rapid cooling through the 425℃-850℃ temperature range during solution heat treatment can prevent this issue. Typically, manufacturers focus on component testing to ensure the material meets specifications like 304 or 316, but for critical stainless steel parts subjected to stress, adding a metallographic test to check the effectiveness of solution heat treatment, particularly whether intergranular carbides have precipitated, is essential. This can reduce or even eliminate the risk of intergranular corrosion and stress corrosion in subsequent materials. Adding a test item ensures product reliability and maintains the company's reputation.

3.Cracks along austenite grain boundaries are primarily due to the formation of intergranular carbides. The main reason for this is that austenitic stainless steel products do not cool quickly enough during solution heat treatment, leading to a prolonged residence time in the 425℃-850℃ temperature range, where intergranular carbides precipitate, with the fastest precipitation rate at 675℃. Understanding the failure mechanism can help identify solutions. Knowing that carbides are the key and the stage of their formation, rapid cooling through the 425℃-850℃ temperature range during solution heat treatment can prevent this issue. Typically, manufacturers focus on component testing to ensure the material meets specifications like 304 or 316, but for critical stainless steel parts subjected to stress, adding a metallographic test to check the effectiveness of solution heat treatment, particularly whether intergranular carbides have precipitated, is essential. This can reduce or even eliminate the risk of intergranular corrosion and stress corrosion in subsequent materials. Adding a test item ensures product reliability and maintains the company's reputation.