The final operation after fabrication or heat treatmentis cleaning to remove surface contamination and restore corrosion resistance ofthe exposed surfaces. Degreasing to remove cutting oils, grease, crayonmarkings, fingerprints, dirt, grime and other organic residues is the firststep.
Non-chlorinated solvents should be used in order to avoidleaving residues of chloride ions in crevices and other locations where theycan initiate crevice attack, pitting, and/or stress corrosion later on when theequipment is placed in service.
After degreasing, machined components are sometimes“passivated” in 10% nitric acid. Nitric acid enhances the natural oxide surfacefilm.
After degreasing, metallic surface contaminants such asiron embedded in fabrication shop forming and handling, weld splatter, heattint, inclusions and other metallic particles must be removed in order torestore the inherent corrosion resistance of the stainless steel surface.
Nitric-HF pickling, (10% HNO3, 2% HF at 49C to 60C (120to 140F), is the most widely used and effective method removing metallicsurface contamination. Pickling may be done by immersion or locally using apickling paste.
Electro polishing is using oxalic or phosphoric acid forthe electrolyte; a copper bar or plate for the cathode can be equallyeffective. Electro-polishing may be done locally to remove heat tint alongsideof welds or over the whole surface.
Both pickling and electro polishing remove a layer severalatoms deep from the surface. Removal of the surface layer has the furtherbenefit of removing surface layers that may have become somewhat impoverishedin chromium during the final heat treatment operation.
Glass Bead or Walnut Shell Blasting:
Glass bead or walnut shell blasting are very effective inremoving metallic surface contamination without damaging the surface. It issometimes necessary to resort to blasting with clean sand to restore heavilycontaminated surfaces such as tank bottoms, but care must be taken to becertain the sand is truly clean, is not recycled and does not roughen thesurface. Steel shot blasting should not be used as it will contaminate thestainless steel with an iron deposit.
Stainless steel wire brushing or light grinding withclean aluminum oxide abrasive discs or flapper wheels are helpful. Grinding orpolishing with grinding wheels or continuous belt sanders tend to overheat thesurface layers to the point where resistance cannot be fully restored even withsubsequent pickling.
More information on cleaning and finishing may be found in:
Penn Stainless Products is providing this guide on aninformational basis only. We strongly recommend that you work with an expert inthe field of stainless steel maintenance and follow all directions supplied bythe manufacturer. For more information on caring for stainless steels, pleasevisit our web: www.topsinosteel.com