Stainless Steel Preparation—Follow-Up
The following was submitted by a reader as a follow-up to the October 2010 column regarding a question pertaining to stainless steel pretreatment prior to painting or powder coating. The column recommended nitric/hydrofluoric acid etch, sand or other mass finishing means, possibly followed by an acid wash primer.
Comment: David, glass beads do not “profile” the stainless steel surface, it radiuses (leaves round impressions) on the surface and our testing per ASTM-D3359, Method B failed with epoxy primer.
Likewise, treatment with wash primer also failed the same test. Note that stainless steel (most alloys) is below hydrogen in the electromotive series and thus will not react with acid. Unreacted phosphoric acid does not provide a good paint base. Sand or aluminum oxide blasting with 80–120 grit at 60 psi should provide about 0.5 mil blast profile on stainless surfaces.
This cleans the scale and provides the necessary profile for good epoxy paint adhesion. J.M.
Reply: I appreciate the follow up and agree with the comments on mass finishing, but have to provide some correction regarding the statement specific to stainless steel and other alloys being unreactive with acids since they fall below hydrogen in the electromotive force.
The fact that most metals fall below hydrogen on the electromotive series (or standard reduction potentials) is the major contributing factor as to why most actually are attacked by acid. The electrochemical reduction potentials are expressed as a voltage relative to hydrogen.
For instance, Fe2+ + 2e- = Fe has a potential of -0.44V (meaning it is 0.44 volts below hydrogen). That essentially means that you have to apply 0.44V to make that reaction to occur.
Said another way, the opposite reaction (Fe = Fe2+ + 2e-) occurs spontaneously, which is the case for all reactions with reduction potentials below hydrogen (as is the case with most metals).
Both phosphate-based and zirconium coatings have their advantages, but zirconium is fast becoming the pretreatment of choice.
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.
E-coat can produce uniform finishes with excellent coverage and outstanding corrosion resistance.