How to Pass Mil-DTL-5541F for Aluminum

Q. We are having issues passing Mil-DTL-5541F, Type II, Class 1A for Aluminum 2024. We are unable to reach the 168 hours without failing the salt spray requirement. What do you recommend to improve the corrosion resistance?

A. There are definitely a couple of tricks to passing Mil-DTL-5541F, Type II, Class 1A for AL 2024, and they apply to Type I, also. First, we highly recommend using an all-purpose, non-etch light-metal cleaner. The key is it should be free of silicates. Often, sodium metasilicate is used as a builder for light-metal cleaners. Silicates, while they are known to protect aluminum, are a hindrance to developing a quality conversion coating, because the silicon to aluminum bond is very strong, so the silicon is not completely removed in the steps prior to the conversion coating. The conversion coating does not adhere well to silicon, leaving voids in the coating and the potential for premature corrosion.

Rinsing, of course, is important in all steps of the conversion coating process and in all metal-finishing processes. We do not recommend using an etchant, because it exposes a high percentage of copper at the surface of the panel or part. Again, the conversion coating chemistry is designed for aluminum, so it does not adhere as well to copper.

The most important factor is to use the correct aluminum deoxidizer for the AL 2000 series aluminum, which seconds as a smut remover. In our experience, there are only a few proprietary mixtures that perform satisfactorily. I highly suggest you have a discussion with your chemical supplier to determine which products would best fit your requirement.

Lastly, our experience has determined that a better Type II conversion coating is developed with no agitation, slightly warmer operating temperatures and a tighter pH range. Following the above tips should make your Mil-DTL-5541F, Type II processing of the AL2000 and AL7000 series of aluminum headache free!

---

Q. We currently provide a mil-spec-approved yellow hexavalent chromate conversion coating on aluminum. Our customer has advised that we now need to provide a yellow non-hexavalent conversion coating that still meets the mil-spec. What product should I use?

A. We address this inquiry quite often, and have some disappointing news for your customer: there are no Mil-DTL-5541F, Type II-approved products that are yellow. At least not yet.

For clarification, let’s first address the military specifications involved. The QPL referenced for Mil-DTL-5541F, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, is QPL-81706. The QPL qualifications are specified in Mil-DTL-81706B, Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys.

Per the updated military specifications, chemical conversion coatings are of two types: Type I contains hexavalent chromium, and Type II does not. The earlier editions of the military specifications only referenced Type I.

The conversion coatings are further classified into performance requirements, referred to as the class. Class 1A provides maximum corrosion protection, and Class 3 provides corrosion protection in which low electrical resistance is required.

The reason I reference the above is because your customer is likely requesting the yellow coating based on the past appearance of hexavalent chromium conversion coatings on aluminum. Your customer needs to be educated on the newer technologies, which are not yellow in color.

When utilizing the Type I hexavalent chromium conversion coating to meet the corrosion protection requirements of Class 1A, a heavy coating is required. Hexavalent chromium is yellow; the heavier the adherent chromate conversion coating, the more yellow the finish and the better the corrosion protection. Therefore, the yellow color the customer requested is likely based on the past history of the Type I, Class 1A conversion coatings appearance.

Between 2004 and 2006, Type II was added to the updated military specifications. This was because of the capabilities of a new technology invented by NAVAIR and developed into viable products by a few chemical manufacturers. The newly developed products, which are more environmentally and user friendly, provided corrosion protection rivaling that of hexavalent chromium conversion coatings. However, this is accomplished through a different mechanism. The coating is thinner, and the non-hexavalent chromium is not yellow in color, but clear to a slight bluish iridescence.

Well, some folks don’t like change. There were, and still are, many drawings and corporate references that still call out yellow. To accommodate the color requirement, much time and work was put into trying to dye the coating yellow. The yellow dyes just wouldn’t take, however, because the Type II conversion coating was so thin, and the yellow dye molecules were too large to completely penetrate and co-deposit in the film to produce a consistent color and adherent film. Dyes of smaller molecular size, most effectively blue, were able to penetrate and co-deposit with the conversion coating. In my opinion, this is fine for parts identification, but if it’s not needed, don’t use dye; it is just an expensive additive to the conversion coating solution. My point is, although there was a genuine effort to accommodate the old standard yellow color, it just isn’t possible presently.

An added bonus to the Type II aluminum conversion coating is that the same process cycle performs well for both Class 1A and 3. With Type I, the heavy coatings required to pass the corrosion performance were too thick to pass the conductivity test required for Class 3. A thinner coating was required to pass the Class 3, thus two different procedures were required.

As of today, no Type II yellow conversion coating is available that is on QPL-81706B. Please educate your customers that yellow is out, clear, or blue, is in.

---

Steve Gilbert is vice president of Luster-On Products. Visit luster-on.com.