Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2004

Protecting Aluminum Cladding From Corrosion

Q. I am working in the building curtain wall area. Our aluminum(6061-T6)anchor connected to steel embedded plate with zinc chromate primer is buried under concrete. Acrylic paint(PPG duracron)around aluminum anchor serves as anti-corrosion both to concrete and to steel (galvanic action). Is that good enough comparing other paints such as bituminous paint? Are there more options?

Other question is just between aluminum and steel. We use several ways to avoid galvanic action between dissimilar metals. 1. Hot dipped galvanized steel to touch mill finish aluminum. 2. Zinc chromate primer steel to touch mill finish aluminum. Are these acceptable? I heard that the galvanized layer protects steel only not aluminum. Is that right?

A. Your question seems simple, but in reality it is more complex because the potential for corrosion depends very much on the conditions of the concrete and perhaps even the soil surrounding the concrete. In clay moist soils the potential for corrosion is greater than in dry sandy soils

Zinc chromate is a good corrosion inhibitor and primers made with zinc chromate generally provide corrosion resistance. The resin in which the zinc chromate is incorporated and the film thickness at which the primer is applied will make a difference to the corrosion resistance that you can expect over the long term. I went to your company's website and saw that your products are used on large building projects. Hence, I cannot give you glib, generic recommendations without having many more details on the design of the anchors, surface preparation of the substrates; at what point in the process the are coatings applied; who applies the coatings; what quality control specifications are followed, etc.? All of these parameters are important in selecting the most appropriate coatings.

Now to your second question: Both zinc metal and zinc chromate protect steel and aluminum, although the mechanism by which they protect the substrates differ. Zinc metal protects by means of cathodic protection, while zinc chromate protects by inhibition.

If you would like to consult with us further on the selection of the coatings please let me know. We work on a fee basis, but it is possible that this can be accomplished in a short space of time.