Aging [verb]: the process of growing older. While that definition does apply, in the metals industry “aging” is specific jargon referring to treatments which speed up that process. But why would you choose to age your new metal products? It helps if you remember not to view aging as a negative. In fact, much like wine, the properties of a metal alloy often improve with age.
As metal ages, its base material physically transforms. The interaction of the metal’s atoms with the oxygen in its environment – whether surrounded by air or water – will begin change its surface texture and color. This starts with a basic oxide layer being formed. The oxide then becomes a hydroxide, and the hydroxide layer continues to interact with the atmosphere.
So why is this exposure to the elements considered a desirable result, unlike rust? That’s because iron oxide, or rust, is much more fragile and ultimately destructive when compared to a hydroxide. Exposed iron develops rust which flakes off and forms again, and will continue this cycle until it deteriorates the metal below. Meanwhile, a hydroxide layer actually creates a more stable surface composition. This hydroxide effectively creates an outer shell, which shields the metal below from any further interaction to its environment. The aging process of the metal comes to a near halt, with the hydroxide layer giving it both greater strength and longevity.
In general, there are two types of metal aging:
Natural aging: just as the name suggests, this is letting the metal age with time, in its natural environment. The strengthening benefits of aging will be more gradual but still effective.
Artificial aging: this refers to any method used to artificially accelerate the aging process. This is usually done through heat treatment of the metal alloys.
Both types do carry a risk of over-aging. This happens when the aging process pushes the metal past the point of strengthening into stressing and deteriorating it. As you might expect, this is more likely to occur with artificial aging: either because the metal has already undergone the aging process, or the heat applied is too intense or prolonged. However, when properly carried out, metal aging is a great benefit to the finished product.