Question

"Area Effects in Galvanic Corrosion" A large ratio of cathode-to-anode surface area is to be avoided because the galvanic attack is concentrated in small areas, and penetration of the anode thickness is hastened. Large anode surface spreads the attack over a greater area and reduces the rate of penetration. A classic example illustrates the importance of area ratio in design and prevention, Several hundred large storage tanks were installed in a major plant expansion. Older tanks were fabricated of carbon steel, and the inner surfaces were coated entirely with a baked phenolic paint. The old tanks had served well for many years, but coating damage on the bottom caused product contamination. The new tank bottoms were upgraded with stainless-steel-clad carbon steel for better service and reduced maintenance. The same phenolic coating was then applied over the tank wall surfaces as with the old tanks, and over the welded junction with the tank bottom, as shown in Figure 1. The inherently resistance stainless steel bottoms were left uncoated. The plant operators were chagrinned and puzzled when leaks developed in the tank walls just above the welded junction after only a few months of service despite their well-intentioned tank upgrades. Storage Tank "Old" Carbon Steel Coating Welded Joint Stainless Steel Subsequent investigations revealed that localized pinhole corrosion had developed through the coating. All coatings have defects in the form of pinholes, holidays and mechanical damage.

Why did accelerated corrosion took place on carbon steel despite the presence of an inner coating? How could it be avoided by changing the design or the material choice?

Answer 1

The mechanical effect of fluid flow or velocity of a fluid combined with the corrosive action of the fluid causes accelerated loss of metal.

If you choose the correct materials for your product from the beginning, you can prevent damage deriving from corrosion, wear and mechanical impact. In many cases, you may completely avoid corrosion by choosing a resistant material for its actual application environment.
Alternatively, you can prevent corrosion by:1. Use of coating (paint, metallic coating), 2.Cathodic protection, 3.Treating the attacking environment (water treatment, dehumidification).