Smut and Desmutting

Question:

I am new to this industry and have heard about smut and desmutting operations. Can you please give me a definition of smut? How is smut generated in the process of anodizing? What is the desmutting operation? I would appreciate any references for further technical reading. C.M.

Answer:

There is a long answer and a short answer to your question. I will give you the short one, as the long one could be the subject of a lengthy paper. There are two different conditions that are commonly referred to as "smut." One is produced as a part of the cleaning phase of anodizing (alkaline etching, sometimes chemical brightening and electrolytic brightening). The other can occur in the sealing stage of anodizing as well as from natural weathering in the field. This latter condition is also known as "bloom." I will deal only with the cleaning phase smut is involved in alkaline etching.

Alkaline etch treatments for aluminum, such as caustic soda (NaOH), react vigorously with the aluminum to generate free hydrogen, heat and complex oxides and hydroxides of aluminum. Some of these reaction products, including intermetallic compounds, are insoluble in the etch solution. These remain on the surface of the aluminum parts in the form of "smut."

Smut can be fairly readily removed in a desmutting or deoxidizing acid bath. This can be any one of numerous compounds, or mixtures, including nitric acid, sulfuric acid, chromic acid and more. There are also mixtures of ferric sulfate, hydrogen peroxide and other chemicals that are used effectively in desmutting baths.

Anodizing or other operations to finish the aluminum can't go forward very successfully with heavy smut present. Thus, the desmutting operation must be extremely effective and efficient in order to clean the parts in the last step before anodizing. Virtually all desmutters used today are free of chromates. This is unfortunate from a processing standpoint because mixtures of chromic acid and sulfuric acid or chromic acid and phosphoric acid were the best desmutters in years past. Chrome can be a waste treatment (disposal) problem, so it is generally not used today. Nitric acid is a pretty good desmutter, but its use has declined in recent years because it can be nasty to handle and it is an active oxidizer when in contact with things other than aluminum and stainless steel. Besides, the EPA has made noises recently about nitrates. As of this writing, however, I understand that EPA has, or is about to, abandon their stance on nitrates.

A popular "proprietary" desmutter today is ferric sulfate to which may be added small amounts of sulfuric acid. Since ferric sulfate tends to be reduced to "ferrous" in the process, additions of an oxidizer, such as hydrogen peroxide, are sometimes made to the bath.

There are many good references and technical papers that have addressed these concerns over the years. Two of the best are "The Technology of Anodizing Aluminum," by A.W. Brace and P.G. Sheasby, and "The Surface Treatment and Finishing of Aluminum and It's Alloys," by Wernick, Pinner and Sheasby.