Painting Stainless Steel

QUESTION:

We have a painted steel structure that has exhibited repeated corrosion problems despite all of our efforts in trying to find a paint that will successfully protect the structure. We have decided to resolve this by making the structure out of T316 stainless steel, but we would like to continue to paint it for esthetic reasons. Is it possible that a coating of paint could defeat the mechanism by which stainless steel becomes stainless? Could the paint actually create a greater corrosion problem than if the stainless was electropolished instead? Thanks. J.A.

ANSWER:

It is unfortunate that you are experiencing corrosion problems after painting your steel structure. Choosing the right paints for the finish system is the easy part, since there is a wealth of corrosion resistant materials on the market. For example, a two-component epoxy or even an alkyd primer containing corrosion-inhibiting pigments will provide excellent corrosion protection to steel. The hard part is pretreating the metal and applying them. The greatest single cause of paint failure (loss of adhesion leading to corrosion) is improper surface preparation (cleaning and pretreatment). Surfaces to be painted must be free from oily soils, rust and corrosion products, and particulates.

Having said that, let's talk about stainless steel. It is relatively inert, and I can't imagine that painting would defeat the corrosion-resistance mechanism. Since stainless steel is relatively inert, paint adhesion could be a problem. Paint adhesion to stainless steel can be enhanced by using pretreatments such as acid etching, abrasion, or wash primers. It is important to note that the first step in any pretreatment is surface degreasing. Acid etches must always be used under controlled conditions. A safer pretreatment is to abrade the surface. This can be done by sanding, wire brushing, or media blasting. After degreasing and abrading, apply the organic coating as soon as possible. Since you are painting stainless steel and corrosion is not a major issue, you will not need a corrosion-resistant primer. Instead, the issue is adhesion and the ability of your chosen coating to adhere to stainless steel.