Etching Prior to Chromate Conversion Coatings

Question:

We are planning to install a chromate conversion coating line using the standard hex chrome process. The alloys we will be treating are primarily 6061-T6 and 2024-T3.We are not particularly concerned about selecting a conversion coating product that is on the QPL since we are a captive operation making our own product. It is important, of course, that our chromate coating is able to pass the salt spray testing in accordance with your specifications. We are trying to decide what steps our cleaning process should include. We plan on using a non-etch, alkaline cleaner as the first step. Our indecision comes with the next step, that is, whether to include an alkaline etch, or not, followed by a desmutting/deoxidizing step and then chromating. The stuff I’ve read about whether or not to include a chemical etch seems to be a bit unclear on this. We would like to hear your opinion about what would be the best way to go—alkaline etch or no etch. P.S.

Answer:

Good question, and it can be controversial at times. I can answer that question pretty easily by just saying that both MIL-C-5541 and the Boeing spec, BAC 5719, state that the first choice would be to use a non-etching cleaning process. The simple reason is that testing has shown that aluminum products that have been etched, either alkaline or acid, exhibit a higher rate of failure when subjected to salt spray testing in accordance with ASTM B117. This is generally thought to be the result of exposing more of the insoluble alloying elements in the aluminum by etching, thus providing more potential sites for corrosion to appear. This may be particularly true for high copper alloys but can also be the case with other alloys not associated with high copper.

Both of the above specifications, however, provide some leeway for the processor as regards to alkaline or acid etching being incorporated into the process. For instance, in cases where fairly heavy heat treat scale is present on the parts, it may be difficult to adequately clean the metal surface without some etching from either a longer exposure to the non-etching, or etching, cleaner and to some degree of alkaline etching in particular. Also, in the case where a customer may desire a certain “look” to his product that is best achieved by etching, this processing step may be included in the chromating process as long as the parts pass the testing that is required for the performance of the product. This demands excellent rinsing, desmutting and chromating.

As far as the chromating product not being on the QPL, there are many excellent proprietary conversion coating products that are not listed. This does not mean that the resulting coating will not pass the required testing procedures. If the chromating process your company is starting is for its own product then you should feel free to use the products that you feel will work the best for your parts and that will consistently pass the testing you require.