TYPE I VERSUS TYPE II ANODIZING

Question:

I operate a small sulfuric acid anodizing line and have a question about chromic acid (Type I) anodizing. When discussing the advantages of Type I anodizing, some say it’s better suited for difficult-to-rinse parts because of the less aggressive attack of chromic acid compared to sulfuric acid on the part. But I always thought hex chrome was a highly corrosive oxidizer, even though chromic acid is a weaker acid than sulfuric. Maybe the hex chrome has a minimal effect on the aluminum oxide or the underlying aluminum. Could you explain? C.S.

Answer:

The use of chromic acid anodizing (Type I) is not determined by how difficult the parts might be to rinse. Dilute chromic acid is no easier, or harder, to rinse than dilute sulfuric acid (Type II). Type I anodizing is used primarily in certain critical applications where exceptional corrosion resistance is required and also as a base for organic coatings (paint). Because the Type I anodic coating is so thin, the process is sometimes used to anodize certain aircraft parts, or any part that will undergo a lot of flexing. The fatigue resistance of chromic acid anodizing is quite high when compared with that of sulfuric acid coatings because chromic acid films are more flexible and they are normally very thin.

Typical coating thicknesses of anodic coatings produced using chromic acid range from 0.00003–0.0001 inch (0.08–0.25 µm). The heavier the anodic coating, the less fatigue resistance the part will have.

Dissolution of a Type I coating during the anodizing cycle is subject to virtually the same factors that affect the dissolution of a typical Type II coating. Basically, these factors have to do with the concentration and temperature of the electrolyte, the anodizing voltage used and time in the anodizing tank.

Oftentimes, if the part being anodized is difficult to rinse, it is because of the part configuration. Sheet metal parts folded back on themselves to form “rounded” edges, small-diameter holes, especially threaded holes, and other areas that tend to trap solution are what create rinsing problems. If parts are etched in caustic soda (sodium hydroxide) as part of the cleaning process prior to anodizing, the caustic can be most difficult to rinse from these areas. If there is caustic soda left in these hard-to-rinse areas when they come in contact with the anodizing bath, be it sulfuric or chromic, the chemical reaction of strong alkaline and strong base can, and will, create a conglomeration ranging from “gooey” to “rock hard” that is impossible to rinse off in the normal process rinsing steps.

Chromic acid is toxic and its waste needs to be treated in a special way and kept separate from other anodizing waste products. Supposedly, chromic acid anodizing has been on its way out for the past 30 years, but it is still widely specified, especially in the aircraft/aerospace industry.

If you are doing general anodizing, stick with sulfuric acid. Chromic acid anodizing would not normally be used as a substitute for sulfuric acid anodic coatings.