Protect aluminum finishes

Typically, two choices in aluminum finishes exist in the glass business: painted material and anodized material. Paint is applied on top of the metal and seals it from oxidation. As long as the paint does not chip, the metal is protected from oxidation indefinitely. Paint manufacturers now have a “coastal” specification designed to be especially resistant to environments with an abundance of salt. This is the best means of protecting the aluminum from oxidation.

Often the glazier doesn’t have a choice. Either an anodized finish is specified or required to offer a competitive price. This does not mean that all is lost when it comes to the performance of the finish. With the right decisions, anodizing can be almost as durable as a Kynar-painted finish.

Anodizing accelerates the oxidation of the aluminum by passing an electric charge through it while immersed in an acid electrolyte bath. This controlled oxidation forms the protective anodic coating. Once the desired coating thickness is acquired, determined by the amount of time in the bath, the anodic coating is sealed so that no further oxidation occurs. The thicker the anodized coating, the more durable the finish will be. Anodized aluminum has three general coating thickness specifications.

The least expensive anodized finish is coil-coated material. A coil of mill-finished material is run through the acid electrolyte bath for a minimal amount of time in a continuous process. The finish on this material might not hold up to an outdoor environment and is not recommended for outside use. The anodic coating on coil anodized material is typically between 0.2 and 0.3 mils. How long it lasts will be determined by the elements at the site. A better alternative to coil-coated material is called Class II anodizing, 0.4 to 0.6 mils. Class II anodic coatings are two to three times thicker than coil-anodized coatings. It is thus a better anodic coating than coil-anodized aluminum.

The best anodized finish if the aluminum will be exposed to a harsh outdoor environment is Class I anodizing of 0.7-plus mils. The coating thickness on this material is at least four times thicker than coil-anodized material. Class I anodizing should generally withstand being sprayed with salt water for 1,000 hours in accordance with ASTM B117, which applies the standards for testing with a salt spray. The anodic coating will eventually break down, but it can be minimized. Little can be done once the anodic coating is compromised. The only way to completely restore the material is to remove and send it to an anodizer for refinishing. At this point, many believe that it is better to replace the material.

A few steps can be taken to maintain the best finish on material already installed. Always test a small area before cleaning to help determine the effects of the cleanser. Do not clean the anodized finish with products such as steel wool or a steel brush. A scratch in the anodic coating will lead to unwanted oxidation of the aluminum. This will definitely damage the anodizing. The best technique for cleaning an anodized finish is by using an abrasive cleaning sponge with a mild dishwashing soap. Do not use solvents on the anodizing as the solvents tend to stain the finish. Do not use harsh acidic or alkaline products as they tend to eat away at the coating. Numerous cleaning products are available at hardware stores that are made for cleaning metal. You will have to determine how well they work in your particular environment. Distilled water might also sufficiently clean the anodizing. Most importantly, do not scratch the finish. The AAMA 609 specification contains the guideline for cleaning and maintenance of anodized architectural aluminum.

Salt is the enemy of any anodized finish, especially in coastal communities and in locations where salt is used on the roads and sidewalks during winter. The best way to cope with the salt accelerating the oxidation is by using either painted material or by using Class I anodized material. If the material is already installed, the life of the anodized coating can be lengthened by cleaning the material regularly as per AAMA 609. Again, do no allow the metal to be scratched and use a mild soap. Once the salt has compromised the finish, nothing can be done on site to stop the corrosion. The only options are to anodize the material again or replace it. The material will continue to oxidize and will spread. Pitting will occur and the material will continue to get worse.

The Aluminum Anodizers Council of Wauconda, Ill., has an abundance of information at www.anodizing.org and the Aluminum Association Inc. of Arlington, Va., has a book called “Care of Aluminum” available at www.aluminum.org.