Anodizing Discolorization "Leopard Spots"

A. The blemishes were caused by the irregular agitation pattern. Re-position the agitation spargers so that the solution moves uniformly without pounding in one particular spot.

When you strip and re-process, be sure the silicon carbide sandpaper has not ben used on any other metal (and thus imbedding iron particles).

A. Pretty sure this same defect has been discussed here a number of times. On thread 15288 and 55451 you can see the same spotting, but there have been more posts about this.

I've seen this on parts made from extrusions when minimum amount of material has been removed from the extrusion; for instance, your extrusion is 20x100 mm cross section, and you make 19x90 mm part, this defect will show on the long side faces where only 0.5 mm has been milled off, you can sometimes even see the changing grain structure by looking at the part from extrusion direction, it may be only 0.5-0.7 deep, but may be deeper closer to corners.

This is random, but regular, occurrence with the material we are able to buy around here (mostly 6082 T6), sometimes there will be no issues at all, sometimes up to 1/3 of the parts will have it (and other "extrusion bar end" related defects). A practice has been adopted to try and plan the machining in a way that surfaces requiring perfect cosmetic looks will have most material removed, or buy a larger stock...

A. It is clearly a material issue. The microstructure is visible and granular in structure, probably coarse-grained. In the case of extruded material, the problem is often only on the surface.

A. Maybe rinsing water quality and good rinsing.

Good luck

A. I believe what you are referring to is what I call "mottling" which normally shows up after the caustic etch. You are revealing the grain structure of the aluminum. I wish you could have included a picture, for me to be sure. Take a look at topic 30013, and see if this looks like your problem. If so, follow the advice in that letter. If not, please send a picture, or a more detailed description.

A. Could be peripheral coarse grain which forms during the extrusion process (I'm assuming the part is extruded). To eliminate this as a possibility do anodizing with no etch as as sample or as minimum etch as possible if the problem persists then it's probably as per above reply. See other letters on 'galvanized finish on aluminum parts.'