Corrosion of 304 stainless steel
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The root part of the mixing teeth in one of our tooth-mixing granulators is made of 304 material. During a recent inspection, it was found that although the appearance of the root part was intact, it broke immediately when struck with a hammer; its strength was very low, and its cross-section did not appear fresh, looking rather like honeycomb. The operating environment is a wet, water-containing environment; what type of corrosion is this caused by?
304 belongs to the 18-8 type of stainless steel, and it is highly susceptible to intergranular corrosion; based on your description, it is likely caused by intergranular corrosion.
Analysis above the branch level. This is typical intergranular corrosion; based on an analysis of the material properties, it is necessary to select a different material
I’m sorry, there’s a slight mistake; this could also be pitting corrosion.
What is the difference between pitting corrosion and intergranular corrosion?
It might be possible to find relevant online materials to distinguish between pitting corrosion and intergranular corrosion
Let me share my own perspective. Based on the information provided above, this should not be considered pitting corrosion; it is almost certainly intergranular corrosion or stress corrosion. I. Pitting corrosion affects only the outer surface of metals; its effect is to create small holes or pits, resulting in irregularly distributed pitted corrosion on the material’s surface. Destroy the oxide layer of the object! II. Intergranular corrosion. It may be that improper heat treatment leads to intergranular corrosion in certain environments. It’s possible that the material processing was not done properly. Thirdly, stress corrosion could be taken into consideration if possible. Of course, this corrosion depends on how your components are used; stress corrosion cracking is the result of the combined effect of static stress and corrosion that leads to metal embrittlement. I suspect it might be stress corrosion.
I’m not sure what exactly this wet environment with water involves; are there any other substances present, and what is being stirred? Corrosion due to pitting can be ruled out – what was said above about pitting is correct; it won’t result in a honeycomb pattern. It’s likely intergranular corrosion, as this type of corrosion destroys the bonds between metal grains, thereby reducing the mechanical strength of the metal. Moreover, after corrosion occurs, the metal surface still retains a certain degree of metallic luster with no signs of damage visible; however, the bonding strength between the grains is significantly reduced, leading to a deterioration in mechanical properties such that the material cannot withstand impacts. External stresses during mixing are also one of the factors that accelerate corrosion.
In many cases, corrosion cracking is the result of the combined effect of various forms of corrosion. Personally, I think, as mentioned above, stress corrosion should be the main factor. At the same time, due to certain types of local corrosion (such as pitting and intergranular corrosion), cracks or fractures can easily occur. It is recommended to check whether the gear teeth are under tensile stress in one direction at all times (usually in the direction opposite to that of the crack). If there is, try to eliminate it. Check again whether the chloride level at the root of the stirring tooth is above the limit (for stainless steel of 304L grade, it is generally recommended that the chloride level not exceed 500 ppm). When in use, the water at the root of the teeth is often stagnant, and local corrosion can easily occur at this time as well. It is recommended to ensure that water flows in this area as much as possible.