I have received many questions regarding problems with adhesion of various organic coatings such as epoxies and polyurethanes applied to anodized aluminum substrates. In virtually every case, the problem involves organic coatings that have been brushed, dipped or sprayed (electrostatically and non-electrostatically) over Type II and Type III anodic coatings with nickel-acetate seals. The following question is an amalgamation of many different questions from folks who are baffled and frustrated by this adhesion problem on their anodized parts.
Q. We send many parts to our outside anodizer for Type II and Type III anodize, clear or dyed. Some of the parts require painting in our facility after anodizing, including with epoxy, polyurethane, alkyd enamel or phenolic insulating varnish. When we do adhesion testing, we find that many of the parts fail, and sometimes the organic coating will start to chip off voluntarily whether we have tested it or not.
We have tried many techniques to try to maximize adhesion, including duplex sealing and/or cleaning the parts in various organic solvents prior to painting. Nothing seems to improve adhesion of the “paint” to the anodized part. Is anodizing simply not a good finish for aluminum parts that are to be painted? Is there something that can be done in the anodizing process to ensure better adhesion?
A. Most nickel-acetate sealing products used to seal Type II and Type III anodic coatings in the United States are what are generally referred to as “proprietary, mid-temperature, nickel-acetate seals.” It is not advisable to use this type of product to seal the anodic coating if it is to be used as a substrate for any type of adhesive bonding, including most paints, epoxies and adhesives. These proprietary seals contain surfactants (wetting agents) that are added in small amounts to reduce surface tension and help prevent sealing smut. This can be a good thing if the parts are not to be painted or bonded because parts without sealing smut have a better appearance, but it also makes it difficult for organic coatings to adhere to the anodic coating.
So, if the anodic coating is to serve as a paint base, no products containing surfactants should be used to seal the parts. Fortunately, there are other viable options to choose from. Some common and acceptable approaches are:
For parts left unsealed:
Stored parts may need to be wiped with solvent right before painting or adhesive bonding.
For parts sealed in hot DI water:
One straightforward duplex sealing method is:
This method could produce sealing smut, and if it does, the parts can be hand-wiped with a hydrocarbon-based solvent such as acetone or lacquer just before painting. This also could be done regardless, unless the specification forbids it.
Sealing in a dilute chromate bath is another excellent method. Some aerospace/aircraft anodizing specs include a procedure for this type of sealing something like this:
There are many possibilities using chromates and dichromates, some of which are less-known and infrequently used. Some methods of hex-chrome sealing involve the dichromate ion using, for example, a mixture of potassium dichromate and sodium carbonate.
If chromate sealing is being used, it is best to follow the methods contained in the current industry specifications and to do the post-anodize processing as soon as possible.
The most important point to remember is not to seal in a bath containing surfactants if the anodic coating must function as a base for good adhesion.