When connecting stainless steel to carbon steel on storage tanks, be careful of corrosion of the carbon steel

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When connecting stainless steel to carbon steel on storage tanks, care must be taken to prevent corrosion of the carbon steel. Example: A factory had several large beer tanks made of carbon steel, with a phenolic enamel coating on their surfaces; these tanks were in use for 20 years. During the expansion, to address the issue of easy damage to the bottom coating layer, the newly built storage tanks used stainless steel plates for their bottoms, while the cylinder sections continued to be made of carbon steel with anti-corrosion treatment. Thinking that stainless steel is completely corrosion-resistant means there is no need to apply coatings. A few months later, the carbon steel tank wall leaked in a narrow strip adjacent to the stainless steel due to numerous corrosion holes. Analysis: The carbon steel tank wall and the stainless steel tank bottom form a galvanic corrosion cell, with carbon steel acting as the anode, which may lead to accelerated corrosion and damage. The mistake here is that the surface of the carbon steel tank wall was coated with paint, while the surface of the stainless steel tank bottom was not coated. If the bottom of the stainless steel tank had also been painted, the carbon steel tank walls would not have suffered such rapid corrosion and damage. In the minds of most people, since stainless steel is corrosion-resistant, painting its surface is completely unnecessary. To such an extent that after the storage tank developed perforations and leaks, the factory initially refused to consider the idea of painting the stainless steel surface, believing instead that the problem was caused by poor quality of the coating on the carbon steel surface. Due to its thinness, it is difficult to avoid pores in the coating layer. The carbon steel exposed in the pores becomes a small anode area, while the stainless steel at the bottom of the tank serves as a large cathode area. Based on the area ratio of the cathode to the anode, the corrosion rate of carbon steel within the pores is very high; it’s no wonder that the carbon steel tank walls were \"drilled\" with numerous small holes within just a few months.