Chromate vs. Phosphate

Question:

I know iron phosphate works great on steel, but it is not as effective on aluminum. Chromate is excellent on aluminum, but can it also be used on steel? If so, is it as good as iron phosphate? D.Z.

Answer:

Iron phosphate is only effective on ferrous alloys, unless it is specifically formulated to be for multiple metals. The iron phosphate coating works by the free acid in the bath dissolving the base metal. This causes an increase in pH immediately at the surface of the part. This local increase in pH makes the phosphate salt in solution less soluble. It then "precipitates" out on the part, forming an integral part of the surface. Depending on the type of phosphate used (three stage, five stage, zinc, iron, etc.), you can expect to see at least a few advantages. The primary benefit of the iron phosphate process is the increased paint adhesion while corrosion protection may be helped only slightly.

Iron phosphates can be formed on other metals including aluminum but need to be formulated to do so. The typical ingredient present in a formulation for aluminum would be a fluoride activator (present in the bath as a fluoride salt). The occurrence of this salt can probably be found on the material safety data sheet. If your pretreatment system needs to coat other metals, it is important that your chemical supplier know this requirement and recommend a product that is capable of achieving this.

While you are correct about the performance of chromate conversion coatings on aluminum, they cannot be used on steel. Chromates tend to passivate steel and will not form a good coating on it. Because of this, they are effective inhibitors in boiler treatment chemical formulations (or any closed loop, flowing water system) but cannot maintain a coating on the steel. Additionally, with the current state of environmental and safety standards, it would be easiest for you to avoid the use of chromates wherever possible. In short, you can phosphate your steel and aluminum parts, but you cannot chromate the same things.