Aluminum Made Harder Than Steel?

By William Gunnar

Anodizing is a common process in the finishing industry for electrolytic treatment of metals to form stable films or coatings on the metal's surface. Anodized aluminum or magnesium, for example, are typically associated with functional coatings like hard anodize, also known as 'hardcoat'.

In this process, unlike electroplating, the work is made the anode, and its surface is converted to a form of its oxide that is integral with the metal substrate.

Generally, it's agreed that the ceramic oxide coating consists of two layers:

The initial layer, which forms right at the surface, is called the barrier layer. It is a comparatively thin, dense, and nonporous form of aluminum oxide.

Next form the outer, heavier layers of the anodic coating. They are more porous and are stacked somewhat like parallel tubes extending through the layer, from the outermost surface down nearly to the barrier layer.

It's important to recognize that, unlike plating, whose thickness accumulates by depositing more and more onto the outermost surface, the anodic layers form by oxygen transfer from the underlying aluminum surface and force existing layers outward.

This means that oxides newest forming are always located between the metal surface and the last, most recently formed alumina oxide. Consequently, the greater the thickness, the lower the density of coating at the outermost surfaces, based on longer solvent action of the electrolyte. Consequently, for maximum wear resistance, more is not always better.

A factor to consider in your design, such as close tolerance bores, bearing diameters, dowel holes or threads, is that the resultant growth portion of the coating is a fractional percentage of the total coating thickness. Hard anodize, for example, will typically result in 50% penetration and 50% buildup.

Keep in mind that buildup is normal or perpendicular to the surface and sharp corners should be rounded to avoid chipping.

To anodize aluminum, one of the most important factors influencing oxide formation is composition of alloy. Reaction of all the various possible alloying or impurity constituents can result in coating voids or disruptions, while other constituents may themselves oxidize in the conditions of anodizing and lessen the intended properties.

Depending on electrolyte, a wide range of thickness can be obtained. Coatings produced in sulfuric acid electrolyte, for example, can be as low as 0.0001 inch (2.5 um) to as high as 0.003 inch (75 um).

Anodic coatings have a definite cellular structure. Imagine individual cells with pores down their centers totaling millions per square inch. This makes for excellent dyeing and sealing. Sealing processes make the coatings non-absorptive and include, immersion in boiling de-ionized water, steam, or nickel acetate.

De-ionized water is often preferred as a sealing solution for its ability to react with anhydrous aluminum in the outer layers of the film. A mono-hydrate of the oxide is formed, which occupies greater volume than the alumina from which it was formed. The result is a reaction to close down and plug the pore structure.

Structures of hard anodizing can be supplemented with a variety of materials, including Teflon (PTFE) waxes, oils and other compounds, to lower friction or add release (nonstick). And because these compounds penetrate the ceramic, their added surface growth is oftentimes negligible.

Want to make a better product? Or simply, make it faster, more reliably, with less scrap? Where ever your surface engineering takes you William Gunnar shares your goal in surface performance!

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