Pretreatment of Multiple Substrates

Q. We are currently coating various substrates for aerospace products with various coatings. We are using a Mil-T-81772 thinner via a wipe method to achieve pretreatment for coating. Our coatings are Chromate primers, TT-E-489 enamel, Epoxy primer and Mil-C-22750 Epoxy topcoat. Substrates are SS 300 series steel, alodined aluminum, anodized aluminum, titanium and magnesium. We are experiencing problems passing dry and wet tape tests. And I have been tasked to get us up to a NADCAP audit ready status. Is there any one method and chemical/solvent that could be used to clean all these situations? I like the idea of an ultrasonic cleaner as most of our parts are small with irregular features but would not like to have to maintain various different baths. J.R.

 

A. I can’t say that I am knowledgeable in the military paint standards, but in general, I think what you are trying to do may be difficult with a single cleaning process. Let’s go through the reasons.

First, it is important to identify the contaminants you are trying to remove and the processing done to the metal components prior to cleaning. How is the condition of the parts as received? What do you do with them after receiving? Are you adding lubricants or contaminants with your processing? What type of contamination is on the part, either from you or your supplier?

It will be important to answer some or all of these questions before deciding on the cleaning process. In general, except for being labor intensive, a wipe cleaning with some sort of solvent (thinner mentioned above) can be a reasonable, overall cleaning method that can remove particulate and mineral-oil based lubricants and rust inhibitors.

Unfortunately, it can be difficult to completely remove all contaminants in this manner. Water-based lubricants and coolants will not respond well to a solvent-based cleaning due to the two being immiscible. Additionally, crevices, blind holes and complex shapes can be difficult to access. Finally, changing rags or wiping pads will be necessary and builds up quite a bit of residue, so frequent changing is a necessity.

However, even with those disadvantages, it could still be preferable to setting up an aqueous cleaning system with ultrasonics. Not knowing more about your soils, it is likely that they would not all be able to be removed in the same bath, with the same cleaner. For instance, the only possibility of cleaning both aluminum and magnesium in the same tank would be to use a high pH silicated cleaner that would eliminate the caustic etch on the aluminum, and still have an alkaline enough pH to not attack magnesium. That will significantly limit the choice of cleaners you have available to you and you could be stuck with a residue problem if the silicate is not properly rinsed from the parts. This could cause more problems than you currently have.

An ultrasonic cleaning system sounds like something that could work with your parts, however, you will likely need two cleaning tanks with common rinsing to make the process effective. You need to determine if is worth the capital investment in equipment to implement a change like this.

Also, I found it strange that you would be cleaning parts that you describe as alodined and anodized aluminum. The alodine is a trade name for a chromate conversion coating process done to aluminum for corrosion protection. This is not applied until the surface has first been thoroughly cleaned. Additionally, some portion of the chromate coating will come off in the hot water from the cleaner and rinse you are contemplating using (you will see a gradual build up of the gold chromate color in these solutions). The same comment applies to the anodized aluminum part. Prior to anodizing, it has to go through a fairly rigorous cleaning process before it is ready to be anodized. Once anodized, it should be fairly clean and ready for service (generally specified and used without a final painting).