Originally Posted by GaryHLucas
Interesting little story about stainless steels. This week we got back a wastewater RO filtration unit from a customer. All the stainless on this unit was 316. The sch 40 piping was welded and not subsequently finished in any way. It was all quite corroded at all the welds. The filter canister was a bead blasted finish, and eaten completely through in several places! The high pressure Can pump had an electropolish finish, and was still bright and shiny everywhere. Interestingly, the hot dip galvanized frame still looked great! I don't know what they were processing, but it sure raised hell with stainless steel.
Gary H. Lucas
Not always is stainless welded with the exact same material as the base metal. Close to the same material is not good enough. Polishing the welds can deter external corrosion but not internal and as we know some steel boats rust from the inside out. Aluminum and stainless steel fuel, water and holding tanks rust and corrode from the inside out. Most commonly this rust and corrosion appears at the welded joints and especially where these tanks come in contact with foam insulation or rest on their mounts. In your customer's situation I would consider not just these conditions but also the dissimilar metals and especially if they are in contact with the galvanized mild steel framework. On my water maker I replaced every item I could with PVC, even making the filter chambers from 4" PVC pipe with end caps and reducers thus eliminating 99% of the corrosion problems with no adverse effects to the system.