What causes this?
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As stated in the title: material mixing blades. The material contains no chloride ions; it is alkaline before mixing, containing OH- ions, and becomes neutral after neutralization with sulfuric acid.
For clarification, the material is a suspension with a solid content of around 25%
Sulfate ions are also corrosive; the impeller is made of carbon steel and is a moving component
Judging from its appearance, it seems to be made of carbon steel, with surface corrosion. You mentioned that it’s in contact with alkaline substances; under normal circumstances, carbon steel does not corrode in such an environment. However, since sulfuric acid is also present, carbon steel cannot be used. Stainless steel would be a better choice, and the reason is definitely corrosion. The specific principles behind this can be discussed in more detail.
Carbon steel isn’t suitable for this; applying PTFE or enamel should solve the problem
Corrosion caused by the remaining sulfuric acid?
By \"neutral\" it is meant the state after mixing, but in reality, local over-acidity may occur during the stirring process. PS. By \"neutral\" it is meant a ratio of H+ to OH- of 1:1, but as some have said before, sulfate ions also possess corrosive properties