Nitric Acid is used in anodizing to remove heavy metals. Most notably Copper and Zinc but more so copper. Aluminum always has different elements alloyed with it to give strength and in anodizing we do not want these contaminants.
Anodized metals that are alloyed with copper give a great anodize and also gives a hard glossy shell like appearance if done properly. The problem is too much copper is usually left after phosphoric de-smut. Too much copper left can give black streaks and spots. The parts to the right have copper content (Not the 6061). Note the glossy finish but no blackening on the gold part. The 7075 parts feel slick to the touch. this is assisted by a 10% nitric acid dip for about 5 minutes, followed by a hot water blast and re-dip. The hot water helps wash the surface.
Nitric Acid is the only acid that will aggressively attack copper and the noble metals. It's not so aggressive on aluminum so can give a shine boost if done properly. It will also aggressively attack eyes, skin and clothes much worse that sulfuric. It also fumes badly if heated and is a requirement in brite dip. The fumes on reaction with copper is strong and produces Nitrogen Dioxide or Mustard Gas...not good. For this reason we only sell reduced to 35% and has to be shipped surface hazard material. At this level it wont fume out and greatly reduces risk.