The final operations after fabrication or heating
are treatments to remove surface contamination and restore
corrosion resistance of the exposed surfaces. Degreasing to
remove cutting oils, grease, crayon markings, fingerprints,
dirt, grime and other organic residues is the first step.
Machined components: After degreasing, machined components
are sometimes passivated in 10% nitric acid. Nitric acid does
not corrode the surface of stainless steel. Nitric acid does
thicken the natural oxide film and is especially beneficial
to surfaces exposed in machining operations.
Fabrications: After degreasing, metallic surface contaminants
such as the iron embedded during fabrication, inclusions,
weld splatter, heat tint, and other metallic particles must
be removed in order to restore the inherent corrosion resistance
of the stainless steel surface.
Degreasing:
Non-chlorinated solvents should be used in order to avoid
leaving residues of chloride ions in crevices and other locations
where they can initiate crevice attack, pitting, and/or stress
corrosion later on when the equipment is placed in service.
Pickling: Nitric HF pickling, (10% nitric acid, 2% hydrofluoric
acid at 50-60°C), is the most widely used and effective
method removing metallic surface contamination. Pickling may
be done by immersion or locally using a pickling paste.
Both pickling and electropolishing remove a layer several
atoms deep from the surface. Removal of the surface layer
has the further benefit of removing surface layers that may
have become somewhat impoverished in chromium during the final
heat treatment operation or during welding.
Electropolishing: Electropolishing,
using oxalic or phosphoric acid for the electrolyte and a
copper bar or plate for the cathode can be equally effective.
Electro-polishing may be done locally to remove heat tint
alongside of welds or over the whole surface.
Both pickling and electropolishing remove a layer several
atoms deep from the surface. Removal of the surface layer
has the further benefit of removing surface layers that may
have become somewhat impoverished in chromium during the final
heat treatment operation or during welding.
Blasting: Glass bead, walnut shell and CO2 pellet blasting
are very effective in removing metallic surface contamination
without damaging the surface. It is sometimes necessary to
resort to blasting with clean sand to restore heavily contaminated
surfaces such as tank bottoms, but care must be taken to be
certain the sand is truly clean, is not recycled and does
not roughen the surface.
Brushing:
Brushing with stainless steel wire brushes only or light grinding
with clean cutting abrasive discs or flapper wheels is helpful.
Grinding or polishing with grinding wheels or continuous belt
sanders tend to overheat the surface layers to the point where
resistance cannot be fully restored even with subsequent pickling.
Abrasives must not have been used previously on ordinary steels
and other metals. Abrasives should not leave surface laps
or microtears that may become potential sites for crevice
corrosion.
