"Unusual Failure, possibly Galvanic"

Question:

"I have the task of explaning a failure in a cast mild steel coupling. The coupling is threaded on the out-side and connects to another mild steel pipe. Water with dissolved solids of 200 ppm flows inside the piping, The piping and coupling is of the same age. The piping shows no corrosion at all. Inside and out. The coupling shows no corrosion on threads but severe metal wastage on the nut (outside the pipe). On the inside of the coupling deep pits have appeared corresponding to the areas of the nut where metal wastage has been the highest on the outside. No evidence of microbial induced corrosion is present. I am at a loss to explain why this coupling should thin at the thickest point !!! Please help , all comments will be appreciated."

Answer:

You don't state what the O.D. environment is contacting the nut. Component thickness is not usually a factor in corrosion initiation sites. All else being equal, we have observed preferential corrosion of one component electrically coupled to another similar material due to microstructural differences, especially when the anodic-cathodic ratios are favorable for accelerated corrosion of the anodic component, as is in your case.

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