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:
- Leaving the parts unsealed.
- Sealing in near-boiling (205–208°F/96–98°C) deionized (DI) water.
- Sealing in dilute solutions of hexavalent chrome.
- Duplex sealing using straight nickel acetate followed by hot DI water.
For parts left unsealed:
- Dip parts in a hot DI rinse (160°F/72°C) for 60 sec. as the final processing step to help them dry faster. Caution: if hot DI water is also used to rinse parts after a proprietary, mid-temp, nickel-acetate seal, and the parts also require good adhesion characteristics, skip this step. Make sure the parts are dried completely before further handling.
- From the unracking step on, parts should be handled with clean, white, cotton gloves.
- Try to paint or do adhesive bonding on the parts within 24 hours of anodizing to minimize contamination of the anodic coating by dust, finger prints and other handling.
- If the parts are packaged after anodizing, use boxes with no paper interleaving or interleaved with an acid-free paper. Some plastic interleaving contains releasing agents that can compromise adhesion. Check with your packaging supplier.
- Unsealed parts will slowly seal themselves over time even if they are wrapped and boxed. This happens at different rates as the ambient conditions change; the parts self-seal by adsorbing moisture from the ambient air over time. It is all right to let the parts “self-seal” if there are several days or even months between the time they are packaged and the time they are painted. Self-sealed anodized parts don’t contain surfactants.
Stored parts may need to be wiped with solvent right before painting or adhesive bonding.
For parts sealed in hot DI water:
- Use high-quality DI water, which typically is defined as <5 micro-Seimens specific conductance (or 200,000 Ohms resistance). If your specification defines high-quality DI water as better than this value, follow that.
- Be aware that as contaminates are dragged into the sealing bath over time, the likelihood of developing sealing smut increases, and this also can also be detrimental to adhesion. The best procedure in this case is to keep a very clean sealing bath and not “over seal.”
- Add approximately ½ lb (225 g) of sodium acetate or ammonium acetate per 100 gallons (378 L) of sealing bath volume to act as a pH buffer, making the bath pH much easier to control. Use no other additives.
- Handle sealed parts with clean, white, cotton gloves.
- Do the post-anodize processes within 24 hrs., if possible.
One straightforward duplex sealing method is:
- Use a concentration of about 5 g/L nickel acetate in DI water with no other additives at 195–205°F (90–96°C) for 5 min. After rinsing, the parts are sealed in hot DI water, as above, for 5 to 10 min. or longer.
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:
- High-quality DI water.
- Bath temperature of 190–200°F (88–94°C).
- Hexavalent chromium concentration of 45–100 ppm (0.045–0.100 g/L).
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.