Anodizing is a chemical means of maximizing the layer of aluminum oxide on the surface of the aluminum. In some cases, a specific color is the purpose of anodizing the aluminum. The point is that anodize is aluminum oxide that has a higher melting temperature than the base metal, i.e., 3200 to 3600 degrees vs 1220 to 1400 depending on the alloy.
Aluminum oxide not only melts at a higher temperature, it is also the same density as the base metal, so unlike oxides in carbon steel, it does not float to the surface nor does it easily break down in the arc. It is very difficult to distinguish when x-raying it.
It never ceases to amaze me when welders say that they can weld over the oxide by cranking up the heat and they assume is all is good. Until you have performed the tensile tests and the bend tests, you have no idea of what the real properties are. My experience has been that the weld lack ductility and tensile strength when the oxides are not removed prior to welding. I've had good looking welds that would fracture as if they were glass, i.e., sharp fractures after slight bending took place.
Again, I can only speak from my personal experience, but I've had very good results by simply hand wire brushing the aluminum to remove the anodized layer. A stiff bristled stainless steel brush, pushed toward the weld groove works good. Do not use a push pull technique because it tends to push some oxide into the surface of the aluminum.
Do not use a power brush. It tends to burnish the surface, thereby smearing the oxide into the soft aluminum metal.
Grinding disks, plumbers cloth, scotchbrite, etc. usually use aluminum oxide as the abrasive media. That's exactly what you are trying to remove. You tend to embed the harder abrasive particles into the softer aluminum base metal. Not a desirable outcome it you are trying to optimize the mechanical properties. When ever there is someone's life or safety involved, i.e., safety rails, ladders, etc. it is only reasonable to ensure the weld's properties are optimized. It only takes one accident to wipe out everything you have worked so hard to achieve, including your reputation.
Best regards - Al
