Ask the Expert Question-and-Answer Archive

by Ron Joseph

August, 2004

Stainless Steel vs. Alumina (Primer needed?)

Q. We join stainless steel (type 304) and aluminum (AlMgSi0,5) in electronic devices for rail cars. Is a zinc chromate primer needed for this application? Can you recommend a certain primer if needed?

Thank you for your e-mail. Since you did not provide details of how the aluminum will come into contact with the stainless steel it is difficult to answer your question in detail. However, provided that the aluminum is properly pretreated a zinc chromate primer may well provide galvanic protection against corrosion due to its contact with the stainless steel. Please be aware that any electronic contact between the aluminum and stainless steel, such as with rivets or bolts should be isolated either with washers or other electrically insulating materials.

Depending on the configuration of the aluminum the surface preparation should preferably comprise a chromate conversion coating such as Alodine (by Henkel), or equivalent. If that is not possible then it may be prudent to apply a vinyl butyral wash primer which can be purchased from the major paint manufacturers. If you choose to apply the wash primer, you will need to ensure that the concentration of phosphoric acid in the product is specifically formulated for application to aluminum. When wash primers are applied to mild steel substrates a more acidic concentration is preferred. I strongly suggest that you follow the instructions carefully to ensure that excess acid does not remain on the aluminum serface prior to applying the zinc chromate is a primer.

I hope this answer satisfies your requirements.

Best wishes,

Ron Joseph