When there are different types of metal materials inside a metal container, it is essential to avoid the formation of small anodes and large cathodes in order to prevent metal corrosion

Thread Content

When different types of metal materials are present inside a metal container, care must be taken to prevent the formation of small anodes and large cathodes, thereby avoiding metal corrosion. Example: In one factory, a copper heating pipe was placed inside a steel container. Due to mild corrosion conditions, the steel container only rusted slightly. To control the iron content in the material, the inner walls of the container were later coated with paint, but the container walls corroded and perforated shortly after. Analysis: In this case of corrosion damage, the key issue was the incorrect painting method, which resulted in an unfavorable area ratio between the anode and the cathode. Therefore, painting the cathode metal components as well is not intended to protect the cathode metal, which has better corrosion resistance, but rather to protect the anode metal, which has poorer corrosion resistance, from severe galvanic corrosion. Some also believe that the pores in the coating are closely related to the coating application method. By selecting the right coating and applying it properly in accordance with the specifications, and by conducting pore testing after each application, it is possible to achieve an extremely low porosity. If the anode coating is pore-free, then the cathode does not need to be painted. However, it is very difficult to ensure that the coating has no pores at all and is free from mechanical damage. For heat exchangers, the tube bundle is generally cathodic with respect to the shell and head. Coating the tube bundles will affect heat transfer, especially for stainless steel tube bundles. Cathodic protection is the best method to use for such equipment to prevent galvanic corrosion. The lesson learned from this example is that avoiding a large anode-to-cathode area ratio is very effective in reducing the effect of galvanic corrosion; for instance, fasteners attached to the metal components being joined, as well as welds on the base materials being welded, should all be of the cathodic type. It should be noted that the area mentioned here refers to the actual exposed area.