Aluminum is very reactive, far from noble. It immediately forms a layer of oxide approximately 0.1 µm in thickness when exposed to air. This amorphous oxide layer offers corrosion protection to the metal under most circumstances. However, the color this oxide layer develops, particularly when combined with various pollutants from the environment, is not always pleasing. Depending on the surface finish, blotchiness, dark stains, and fuzzy deposits can develop. A maintenance program that periodically removes deposits from the surface can reduce these occurrences.
If the aluminum surface is well drained and kept free of deposits from other metals, it will develop an even gray-white color. The surface will still retain a metallic appearance dulled slightly from the oxide growth on the surface.
Finishes on Aluminum
Nearly all uses of aluminum require some degree of additional cost through surface finish. Aluminum will oxidize if left unfinished, becoming dull and prone to dirt and scratches. Options including anodizing, polishing, coating, or patinating, each with benefits and cautions.
Mirror-polish on aluminum (or near-mirror polish) can be achieved for both a protective and visually intriguing effect. Applying a high-polish on aluminum surface has the added advantage of protecting the metal from corrosion. Highly polished aluminum will provide a very smooth and non-porous surface it will drain and resist deposits from collecting. When left to weather, these surfaces will perform well if used where they are naturally washed or when they are maintained by a periodic scrubbing to remove the particles that do collect on its surface.
Anodized aluminum used on exteriors will have a satin texture, and is an effective method for protection from light handling. Because the aluminum substrate is porous, anodization offers an option to fill these pores with metallic dyes to achieve a variety of colors, all-the-while retaining the distinctive metallic feature of aluminum.
Blackened aluminum will exhibit a course surface on the metal. This is a less common means of imparting color to aluminum. The blackening agent produces a mottled flat black patina on its surface. It may be possible to develop additional tones including dull gray and dark reddish-black tones on the metal through chemical patina. These processes combine aluminum's reactive nature with its stable oxide growth, and are somewhat difficult to control.
Different alloys of aluminum can be coated onto the surface of aluminum sheet and plate to develop specific properties. Known as alclad, this process coats one alloy of aluminum with another, usually a higher purity aluminum alloy. The higher-purity alloy protects the stronger, less-pure alloying material from atmospheric attack. In this manner, you can use a higher-strength alloy aluminum with poor corrosion resistance. The pure sheet provides excellent barrier coating and a decent galvanic protection.
When the proper pretreatment techniques are used, painted aluminum surfaces offer durable and long-term color solutions to the designer. Aluminum will receive paint finishes readily after it has gone through pretreatment. Baked-on finishes and urethanes are excellent coatings for exterior aluminum.