jimhokie
Mechanical
- Feb 16, 2005
- 50
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?
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?