"Stainless Castings in Salt Spray Environment"

Question:

"I would like to get a better understanding of the various types of Austenitic Stainless steel casting materials and their relationship to traditional 300 series designations such as 316 or 316l. We currently have a casting which is made from CF-8M per ASTM-A-743 which is showing signs of rust and corrosion on a 'North Sea' oil platform. The material is also showing much stronger evidence of being magnetic, which is not typical of wrought or forged 316. I believe pasivating the casting will help with the corrosion resistance, but I would like to know if there is a different (but economical) material available."

Answer:

CF-8M is the casting equivalent of 316 stainless steel. Its chemistry has been adjusted (particularly silicon) to improve casting properties, especially fluidity. Its increased magnetic response is a result of its greater ferrite content vs. 316 stainless steel and should not adversely affect corrosion resistance. If the rusting is due to embedded iron contamination, then passivating the castings may eliminate the unsightly rust. If the rusting results from pitting due to exposure to salt water, then any decision regarding upgrading metallurgy should be made considering whether the rusting is a cosmetic nuisance or if it affects structural integrity. We cannot provide alternate materials of construction without knowing specifics of the service. However, as a general rule increased resistance to chloride pitting is afforded by increasing alloying content, especially chromium and molybdenum.

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