Anodizing is an electrochemical process that deposits a protective layer on the surface of aluminum. In addition to corrosion and wear protection, it provides an aesthetic finish.
Anodizing involves cleaning the aluminum part (typically a sheet or an extrusion) and placing it in a tank of acid. An electrical current is passed through the aluminum and an aluminum oxide layer forms on the surface of the part.
When selecting an adhesive to use on an anodized surface, it’s important to not only understand the physical characteristics of the surface itself, but the additional treatments that are typically performed. Variables are introduced during these processes that can affect bond performance.
There are three basic types of anodizing:
Type I uses chromic acid. Because of environmental concerns associated with chromium, it is no longer commonly used.
Type II uses sulfuric acid. This type is by far the most common; architectural finishes are among the surfaces created using Type II.
Type III, often referred to as black or hardcoat anodizing, also uses sulfuric acid. However, a Type III surface has a much thicker oxide layer.
In addition to these types, other anodizing processes have been developed. For example, thin-film anodizing is a boric/sulfuric acid process developed by Boeing. Phosphoric acid anodizing is used for pre-paint and pre-bonding surface preparation.
Colorants, which may be inorganic or organic dyes or metal salts, may be used. They fill the pores of the aluminum oxide layer and may be sealed in using chemical washes, steam, hot water, epoxy or polyurethane coatings, and more.
What Does This Mean for Adhesive Bonding?
LORD acrylic adhesives designed for metal bonding, along with epoxies, are good first choices for bonding anodized surfaces. LORD 850/25GB, an acrylic metal-bonding adhesive, and either LORD 310 A/B or LORD 309-1/2 epoxies are good starting choices for bonding to anodized surfaces.
Adhesives that easily “wet out,” or spread, can flow into the pores of an anodized surface, create a mechanical keying and bond quite well. However, an adhesive bond is only as good as the weakest link in the “chain” of bonded surfaces. If the aluminum oxide’s bond to the base is not strong, or the dye/coating’s adhesion to the aluminum oxide pores is not strong, the strength of the adhesive bond to the overall part will be compromised.
The best bonding surfaces are generally created by steam or hot water sealing or by unsealed, undyed anodized finishes. Clear finishes or colored/dyed finishes should be checked for adhesive wet out and—most importantly—bond strength. A good mitigating step is to have the anodizer perform a final step of hot water cleaning/sealing of the surface. (Many types of chemical sealers use surfactants to reduce smut buildup during the anodizing process and these can be an impediment to bonding.) When working with Type III surfaces, specify a hot, deionized water seal in lieu of nickel acetate sealing or use a sodium hydroxide etching of the final surface.
A good practice is to have the adhesive supplier conduct testing on a representative sample of the specified surface. This should include testing for adhesion to the anodized surface as well as for the effect of environmental conditions.
When you select an anodized surface, there may be challenges associated with finishing treatments or with the anodizing process itself. Be sure you understand what coatings and chemicals have been used on the aluminum part in question.
We can collaborate with you to identify the best adhesive solution for your anodized aluminum application. Talk to one of our experts today.