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How to prevent "rusty" appearance of cast CA6NM?

How to prevent "rusty" appearance of cast CA6NM?

(OP)
We procure pumps for our customer with casings made of CA6NM, similar to 400-series stainless steel that develops a thin rust layer on as-cast surfaces. Our customer objects to the rusty look, but would also like to avoid the lifetime maintenance burden of painting the pumps.

My understanding of the rust layer on as-cast surfaces of this material is that it is purely cosmetic and results from the heat treatment that creates a thin chromium depletion zone on the surface, which subsequently oxidizes from moisture in the air. Any machining or grounding on the as-cast surface strips away this chromium depletion zone, leaving those surfaces largely "rust" free.

So my question is, what is the best, most cost-effective method of preventing this rusty appearance on non-machined, as-cast surfaces that we can require of the pump/casting supplier? Changing to a different material is not an option. Is passivation a good option for CA6NM? Would simply wire-brushing the entire surface to remove the chromium depletion zone maintain a rust-free appearance?

RE: How to prevent "rusty" appearance of cast CA6NM?

The best solution is either to have them heat treated in hydrogen (bright heat treatment) or to have them acid cleaned after HT.
The process is usually to abrasive blast them parts and then give them a quick pickle.
These alloys pickle in an acid solution to remove the residual oxide.
There are a number of solutions available to to this.

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P.E. Metallurgy, Plymouth Tube

RE: How to prevent "rusty" appearance of cast CA6NM?

(OP)
Ed, thank you for the response. Let me ask for a clarification...where you say "The process is usually to abrasive blast the parts and then give them a quick pickle.", is that intended to apply to the option "...to have them acid cleaned after HT". I.e., is the quick pickle the same as the acid clean, and to be done after HT and blasting?

The material spec, ASTM-A487, requires the heat treatment to be performed in "air or liquid", so we would need specific approval from our customer to heat treat in hydrogen, with technical justification for the change. Are there any technical differences with heat treatment in hydrogen other than it giving the desired rust-free appearance? Any effect on mechanical properties would be problematic for us.

RE: How to prevent "rusty" appearance of cast CA6NM?

Yes, it is usually easier to HT in air and then remove the oxide, that is where the blast and pickle come in.
There are two acid cleaning operations, pickle (which is more aggressive) can actually remove oxides, and passivation (usually just straight nitric acid) will just remove free Fe from the surface.
Though even with good cleaning these may rust, CA is barely stainless and in a humid environment some blushing may still occur.

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P.E. Metallurgy, Plymouth Tube

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