Common Defects: Coating Adhesion

Coating Adhesion problems range from white spots (small popped blisters) to "feathering" at cut edges, to massive loss of coating. Many shops see this effect on heat treatable alloys; 7075 and to a lesser extent 6061. It can occur on Type II and Type III finishes. The root source is likely metallurgical with current theory suggesting an accumulation of intermetallic particles with possible coprecipitated hydrogen occurring at the aluminium/oxide interface. Unfortunately for the anodizer, a universal processing solution has yet to present itself. Some have suggested anodizing with low frequency pulsed power where, during the recovery phase, the barrier oxide can more easily accommodate these unwieldy particles. Others have found that their rectifier had unexpectedly high levels of ripple, and by addressing this they eliminated their problem. Others suggest that extending the voltage ramp for unusually long periods of time will reduce the unwanted effect.

Blistering can occur for other reasons as well. Anodizing stops and re-starts can be at fault for massive coating loss. A host of poor extrusion practices can cause subsurface pockets or blisters that are uncovered during anodizing. Unusually large unwanted iron phase particles (see photo) can fracture the coating. And for parts that are colored using the so called "2-Step" method, an adjustment to the coloring power will usually remedy the problem.

Example of blistering

coating adhesion 

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