Metal finishing Q&As since 1989

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Corner Line Shrinkage: rusting despite passivation

Dear sir
we are into investment Casting, we have some problem with rust formation by salt spray test for Stainless steel 304 / 316 grade.

we are observing cornerline shrinkages in our castings (SS-316/SS-304) and these castings we carryout SS-Passivation , after passivation parts will be tested for Salt Spray (5% NaCl) and we are observing rust formation at the Shrinkage Spots.

SS-304 / SS-316 being corrosion resistance material , why it is rusting ? does the microporosity / Cornerline shrinkage observed on the surface of casting makes the part to fail in salt spray test ?

Kindly provide the answer ASAP

Sandeep Achar
investment casting - Tirupathi, Andrapradesh, India
April 1, 2011

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[on eBay or Amazon]

Sandeep,
This is something we are continuously exploring, but one thing is clear, stainless castings do not passivate as well as other stainless parts. It is fact that making castings of alloys results in peculiar microstructures and segregation at the surface, I assume this is why stainless cast surfaces seem to often be not so stainless. I have observed that cast surfaces with higher roughness/porosity/what have you tends to be worse. Bead blasting of the surface tends to help.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

April 5, 2011