Corrosion of carbon steel in stainless steel equipment?

I’m referring to the corrosion of carbon steel that occurs when it comes into contact with stainless steel; I really want to learn more about it*.

It is a type of potential corrosion, caused by iron ions

Reply to 3# penryn: Could you explain it in more detail?

Reply to 4# mafan123: Due to different corrosion potentials, local corrosion occurs at the contact points between different metals in the same medium; this is known as galvanic corrosion, also referred to as contact corrosion or bimetallic corrosion. These two metals form a galvanic cell, generating an electromotive force that increases the dissolution rate of the metal with the lower potential (the anode) and decreases the dissolution rate of the metal with the higher potential (the cathode). Therefore, the cathode is protected by the anode. An increase in the anode-to-cathode area ratio and a decrease in dielectric conductivity both exacerbate anode corrosion. In their natural state, stainless steel has a more positive potential than carbon steel; as a result, carbon steel acts as the anode and gets corroded. Of course, if the area of carbon steel is larger than that of stainless steel, it is a large anode, and there’s no major problem. If the area of stainless steel is large, carbon steel will also corrode. Additionally, iron ion contamination occurs when iron ions displace Cr and Ni on the surface of stainless steel. There is a theory of carburization: stainless steel contains less carbon while carbon steel contains more; carbon will move from areas with higher carbon content to areas with lower carbon content. So both stainless steel and carbon steel corrode. I’m not familiar with this concept, so I can’t explain it clearly; I’m also confused about what the cathodes and anodes are. I’ve almost forgotten all my chemical knowledge: lol. Try to figure it out on your own and let me know then{:1_90:} 　

Electrochemical corrosion: This type of corrosion can occur when two metals with a large difference in electrochemical potential come into contact with each other. If water vapor connects these two metals, an electrical circuit is formed, and the resulting current will significantly increase the corrosion rate of the metal that is prone to chemical reactions. The potential energy difference between any two stainless steels is not sufficient to cause such corrosion; it only has a slight effect and does not lead to a significant increase in corrosion. However, when carbon steel and large areas of stainless steel are combined, the carbon steel will corrode rapidly; therefore, where different metals are joined together, it is necessary to prevent water vapor from accumulating in those areas. If it is impossible to avoid water vapor, these two metals must be electrically insulated from each other.

Yangyang, Jiangsu – supplies glass-lined equipment

This post was last edited by snifflily on 2010-12-10 08:58. What medium and under what conditions it occurs requires specific analysis; it cannot be simply assumed to be galvanic corrosion – it might be crevice corrosion or something else

I’ve learned a lot; this question is very valuable.