Cast aluminum does not rust because rusting is specific to iron-containing metals. Rust is iron oxide, a reddish-brown compound that forms when iron or steel reacts with oxygen and moisture. Aluminum, lacking iron, undergoes oxidation or corrosion when exposed to the environment. This distinction makes cast aluminum ideal for outdoor and marine applications, though it is not immune to surface degradation.
The Chemical Distinction Between Rust and Aluminum Oxidation
The fundamental difference between rust and aluminum corrosion lies in the resulting chemical compound and its physical properties. Rust (hydrated iron oxide) forms a flaky, porous structure that flakes away easily. This constantly exposes the underlying metal to air and moisture, allowing the destructive process to continue unchecked.
Aluminum reacts with atmospheric oxygen to form aluminum oxide. This oxide forms an extremely thin, dense, and tough layer on the metal’s surface almost instantly upon exposure to air. This layer is known as a passive film, and its formation is a self-limiting reaction.
The aluminum oxide film adheres strongly to the underlying cast aluminum, acting as a barrier that seals the metal from further contact with oxygen and water. This protective mechanism halts the corrosion process. This self-repairing oxide layer is only a few nanometers thick, yet its stability provides remarkable protection and superior resistance compared to iron-based metals.
Appearance and Effects of Aluminum Corrosion
When the protective aluminum oxide layer is compromised, visible corrosion occurs, looking distinctly different from reddish-brown rust. Aluminum corrosion typically results in a dulling of the surface followed by the appearance of a white or gray powdery residue. This residue is the aluminum oxide itself, which may build up where the protective film has been repeatedly broken down.
Unlike rust, this white powder usually remains bonded to the surface and does not readily flake off. This uniform corrosion is generally superficial and does not compromise structural integrity, though it affects the metal’s appearance.
A more concerning form of localized corrosion is pitting, which appears as small depressions or pinholes on the surface. Pitting occurs when aluminum is exposed to aggressive environments, particularly those containing chloride ions like road salt or saltwater spray. These compounds locally break down the passive film, concentrating the corrosive attack. Severe pitting can eventually weaken thinner cast aluminum sections.
Preventative Measures and Cleaning Cast Aluminum
Maintaining the integrity of the protective layer is the most effective way to prevent corrosion on cast aluminum. Preventative steps include regular cleaning with mild soap and water to remove dirt, debris, and especially salt residues. After washing, thoroughly drying the object minimizes the time moisture interacts with the surface.
Applying a clear protective coating, such as high-quality car wax, a clear sealant, or paint, creates a sacrificial barrier over the oxide layer. This barrier repels moisture and environmental contaminants. For outdoor pieces, re-applying this protective layer periodically is simple maintenance.
When the characteristic white, powdery corrosion appears, it must be removed carefully. For light corrosion, a soft brush or a non-abrasive pad with a gentle cleaning solution is often sufficient. More tenacious buildup can be addressed using fine-grade synthetic steel wool or a specialized aluminum cleaner.
Some household acidic cleaners, like white vinegar, can dissolve the oxide, but they must be used sparingly and rinsed immediately to prevent etching. Harsh abrasives or strong alkaline chemicals should be avoided, as they rapidly strip away the thin, naturally protective oxide layer.