Salt Crystals on the Paint Film

Q. I am the quality control manager for a company who makes aluminum equipment enclosures used outdoors. These enclosures are pretreated with a chemical conversion coating in accordance with MIL-C-5541, Class 1A. It is applied at a coating weight of 35 - 40 mg/sq ft. After this pretreatment, we apply an epoxy primer to a dry film thickness of ½ mil. We then topcoat with an acrylic enamel to a dry film thickness of 1½ - 2 mils.

The problem is that we get pinholes and small blisters in the paint film. There are also, on occasion, table-salt-size crystals on the surface that can be brushed off. An analysis of the crystals and the liquid in the blisters indicate they are sodium nitrate.

Since we are a small company, all finishing is done by vendors. Can you tell me what is going on and how durable the finish system will be outdoors? What questions should we be asking the vendors? So far all we have had is a finger-pointing exercise. M.H.

A. As far as the durability of your finish system, any paint coating having pinholes and blisters will not provide sufficient corrosion protection to any metal outdoors. Furthermore, coatings should never be applied over surfaces having loose contaminants.

Although Mil-C-5541 describes a chromate conversion coating on aluminum and its alloys, it does not prescribe the coating procedure. It is a performance specification and not a process specification. The sodium nitrate crystals on the surface and the liquid sodium nitrate in the blisters are the residue from the chemical conversion coating applied by an incorrect process procedure. To me, this indicates poor rinsing or a lack of rinsing in the procedure prior to painting. In your next finger-pointing session ask your vendor how thoroughly the parts are rinsed after the chemical coating stage and prior to painting.