White Specs on Sharp Edges of Type II & III Aluminum Anodizing

Posted May 26, 2015
By Jack Tetrault

What to Know About Knurls and Graining

You’ve made your parts and shipped them off for finishing. Seemingly an easy request. Then you get your part back from the anodizing company and you see these whitish looking specs along the edge of your part, or maybe in the knurl, or just randomly scattered along a grained finish.

You know the condition is unacceptable and your immediate thought is that the aluminum finishing is inferior or the anodizing company did not do the job correctly. Are those correct assumptions?

It’s Common, Simple … and Complicated

White Specs on Sharp Edges of Type II & III Aluminum Anodizing Sharp edges and the treatment of them is a common problem faced by both customers and their anodizing service. While this seems like a simple issue, it can be quite vexing. The problem is, indeed, simple. Anodic coatings cannot form where there is no aluminum. Aluminum oxide forms perpendicular to the surface. A round or radius object is easily anodized because there is a continual supply of aluminum to convert into oxide. However, a sharp edge, 90-degree bend, a knurl or top of a grain line are not conducive to the formation of aluminum oxide. Since the coating does grow perpendicular, a sharp edge simply cannot supply the aluminum it takes to grow the coating. And the thicker the coating requirement, the more difficult the problem. As the coating forms, it needs a steady supply of aluminum to continue thickness growth. If aluminum is not present, the coating will stop, but worse, it will deteriorate and/or break off and separate. The result is an area with voids or “white spots.” This, of course, is exacerbated if the coating is dyed—these spots become more visible set against a colored backdrop.

The Fix

The best remedy to correct this condition is to round any edges or at least remove any hanging sharp edges or burrs. On knurls, this can be done by a polishing, buffing, abrasive wheel bead blasting or tumbling. Edges should be given a radius whenever possible and formulas exist for coating thickness vs. radius, and graining should be followed by a light surface buff to remove the sharpness, or simply use a less aggressive coarseness belt or paper.

The anodizing company can use chemical means to etch the edges, but remember that ALL dimensions will be affected by the removal of aluminum and it can result in dimensionally out-of-spec parts. Therein lies the rub for the anodizing service. If they treat the part to mitigate the condition, they may ruin the part dimensionally. When given this choice, the aluminum finishing company simply must process the part hoping that the sharp edges aren’t too sharp. Unfortunately, this isn’t known until the parts are completed—and it may prove to be too late.

The best result is obtained when the manufacturer understands this and mitigates these conditions by supplying a part that is anodize-ready.

DCHN can help you understand more about sharp edges and their effect on quality. We know that many questions arise when considering aluminum anodizing, hardcoat, and other metal finishing jobs. Our white paper, “12 Proven Tips to Save Time & Money for Aluminum Anodizing, Hardcoat, and Other Metal Finishing Services,” is a guide full of great tips to help you save time and money. 

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