Non-uniform corrosion resistance on castings?

[kclim]

Is the subject possible?

Reason I ask is that we have in our possession an impeller cast from martensitic stainless steel (CA 6NM), that has been extensively pitted on the vanes, but not on the shroud material between the vanes. The vanes themselves are fairly thin sectioned, with the non-corroded parts being reasonably thick. Thought it might be possible for casting segregation to produce a non-uniform corrosion resistance, although whether segregation is occurring in our case is difficult to determine.

Note: cavitation damage has more or less been ruled out as a damage mechanism - pitting appears on both sides of vanes, in addition to other impeller surfaces, and over the whole vane surface area.

 

[gieter]

Thinner sections are more prone to gas (mainly hydrogen) related problems like blow holes and surface porosity, especialy in stainless steel.

I can't imagine that segreggation is the cause. Seggregation is something you will see on the inside of a casting. At the surface the metal solidifies verry fast, freezing the "nominal" analysis. Seggregation only occurs on slow cooling and even then differences in analysis will be minor.

 

[metengr]

We have a number of BFP and BWCP impellers that are supplied as CA 6NM (410 NiMo ss) and CA 15 (410 ss), and we have seen this as well. The concave and convex surfaces of the vanes can sometimes contain minor porosity or roughness as a result of being "as-cast" surfaces. Impellers are tricky castings to produce because of the complex geometry with the vane passages. The shrouds or side plates are typically machined or turned on the exposed exterior surfaces and, as such, have the as-cast surface porosity removed.

 

[EdStainless]

What heat treatment were these given?
You should look at some microstructures and see if they are getting the same structure in the vanes and shrouds.

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Corrosion never sleeps, but it can be managed.

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