Reducing Crazing in Anodized Aluminum
Photo courtesy of Lorin Industries, Inc.
Anodized aluminum can be formed to minimize crazing – different from cracking – of the protective oxide layer. Anodized aluminum can be used in a variety of applications when proper precautions are taken, in consultation with forming experts or manufacturing partners such as Lorin Industries, Inc.
All anodized aluminum, in fact all metals, can experience crazing if subjected to extreme temperatures, or if the metal sheets or rolls are formed at severe angles. “Crazing” is a term that describes micro fracturing of the anodic layer after anodized aluminum has been subjected to forming, fabrication, or extreme heat or cold. Crazing can sometimes be confused with "cracking" of the aluminum, but the two are not the same.
Anodize crazing does not break through the protective oxide layer and into the base aluminum. Cracking, on the other hand, is considered to have occurred when the oxide layer has been compromised to such an extent that the base aluminum is not only exposed, but also cracked and damaged itself. When this occurs, the oxide layer has been separated from the base aluminum and no longer acts as a protective layer along the crack. Unlike other coatings, with anodized aluminum the corrosion will not creep beyond the point where the cracking occurs. Cracking is less common, but more concerning, than crazing.
Crazing from forming stress, known as “mechanical” crazing, appears frosted or dull in appearance. Crazing from extreme temperature changes, known as “thermal” crazing, resembles a spider web. In either of these cases, crazing viewed through a microscope will show hairline surface cracks on the top of the oxide layer only, with no breakthrough to the base aluminum.
While mechanical crazing at the microscopic level can never be eliminated entirely (not even with pre-painted), visual mechanical crazing can be if the fabricated part or sheet is not subjected to any additional forming or extreme temperatures after anodizing.
In a cost-effective, low labor coil anodizing process where parts are formed after the aluminum is anodized, mechanical crazing can be minimized by decreasing the radius of aluminum bends, decreasing the anodize film thickness, or using a colored anodize finish to make crazing less visible.
Given that thermal crazing occurs when temperatures exceed 320° Fahrenheit, the anodized aluminum experts at Lorin suggest that the anodized aluminum part or sheet not exceed 320F and also does not come in contact with other materials at high temperatures that allow the anodized aluminum to reach 320F in order to avoid and/or minimize thermal crazing. This is because aluminum undergoes more rapid thermal expansion than its anodic layer, causing thermal stresses large enough to cause crazing at any temperature above 320F.
Although not roll forming experts per se, Lorin’s experience in working with successful customers can help offer guidance for customers to determine which potential solutions to reduce visible crazing are right for any given application; even though the protective anodic layer is unharmed by crazing, minimizing its visual impact can improve the visual quality of final parts.