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March 3 - March 7, 2014

Which single type of paint system can be used to protect both stainless steel and carbon steel welded together?

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Selected Answers

From William Slama of International Paint/Ceilcote Products on March 6, 2014:
This combination is an example of the basic galvanic driver of corrosion. The anode can be sacrificed if current flow is available. That depends on the conductivity of the electrolyte (the fluid in the vessel) and the geometry (amount of cathode area vs. anode area and the distance between). A typical situation presents when the vessel has components that are carbon steel and other electrically bonded (welded) components of a more noble alloy. The carbon steel surfaces need to be coated/lined to protect against corrosion. Corrosion engineers learned long ago that the significant area of exposed alloy would cause high current flow to a single imperfection (i.e. pinhole) in the steel coating. The result would be a very rapid corrosion, literally “drilling” a hole through the steel because of the larger area of exposed alloy and small area of exposed carbon steel. For that reason, the alloy surfaces are also coated to reduce or eliminate the potential galvanic current flow. However, experience with high build linings has not shown this to be a problem as could occur with thinner coating systems, and it is very common to line only the carbon steel surfaces and just lap the lining a few inches onto the alloy. In some cases, vessels are specified and constructed of alloy believed to be suitable for the intended chemical environment, only to learn later that the selected alloy was not able to withstand the actual chemical conditions. In those cases, the entire alloy vessel would then be lined with a suitable lining (epoxy, vinyl ester, etc.). As Mr. Albertoni indicated, it is important to achieve the correct surface prep on the alloy surface, just as it is necessary for carbon steel surfaces. The blast media and pressure may need to be selected for best results. The typical requirement is a minimum 3 mil sharp profile, SSPC-SP 5 or SP 10.

From James Albertoni of CA Department of Water Resources on March 5, 2014:
I am assuming this is for a submerged application, or else there would be no reason to coat the stainless other than aesthetics. We typically use epoxies for this application with decent results. Generally, the stainless steel needs a fairly deep profile, and you can expect the coating to fail on the stainless quicker than the carbon steel. The reason we do this is to prevent accelerated corrosion of the carbon steel and to minimize the current drain the bare stainless steel would provide.

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Tagged categories: Carbon Steel; Stainless steel

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