More on Type I vs. Type II Anodizing

In a question I addressed back in May, C.S. asked about rinsing Type I (chromic acid anodizing) coatings vs. Type II (sulfuric acid anodizing) coatings. He had heard that type I coatings are easier to rinse than Type II coatings. He wondered if perhaps he should switch to Type I anodizing in order to get a finish that would be more easily rinsed.

In my answer, I said that I thought the degree of difficulty in rinsing these two types of coatings is about equal. I went on to talk about how the two methods of anodizing were different and what types of applications each was used for.

In looking back on my response, I see that I missed making an important point. In sulfuric acid anodizing, if the parts are not thoroughly rinsed concentrated acid may leach out of the hard-to-rinse areas such as small-diameter holes, recesses, folded over sheet metal, etc. This concentrated sulfuric acid will dissolve the anodic coating, or, at least, may badly stain the coating and perhaps destroy the quality of seal in that area if the leaching is left for even a short period of time. This problem can be exacerbated when the acid doesn’t start leaching out until hours or even days later.

With chromic acid anodizing, any leaching out of the hard-to-rinse areas doesn’t damage the anodic coating. In other words, the more concentrated area of leached chromic acid will simply maintain the status quo of this corrosion-resistant anodic coating. This is just one of the reasons chromic acid anodizing is many times preferred over sulfuric acid anodizing on aircraft and aerospace parts.