Aluminum Oxide Impregnation

Usually when someone says that he doesn’t want any aluminum oxide used on his parts, he is concerned about aluminum oxide impregnation in the metal surface or aluminum oxide grit left on the surface, since this can be carried into service with the part. This can come from mass finishing, grinding, lapping, brushing or other surface conditioning operations.

First, it is important to know that there are two basic types of aluminum oxide involved in this discussion: 1) Porcelain is fired alumina that is used for dinnerware, toilet bowls, porcelain sinks, etc. Porcelain is used to make most of the ceramic tumbling media used in mass finishing. It is the white matrix of the media; 2) Fused aluminum oxide is made by fusing iron filings with alumina in an electric arc furnace. This is the black granular stuff used for roofing and floor grit, grinding wheels, and, you guessed it, the little black specks you see in tumbling media. It gives the cutting power to the media. Fused aluminum oxide nuggets are used as tumbling media in some plants, and this is the pure stuff.

The fused aluminum oxide can impregnate the surface of a part. It is very tough and harder than steel. It can be ground into the surface of a part. Also, the loose particles may be present in “media dust” that can later contaminate a compressor or other sensitive, close tolerance assemblies. If parts are impregnated with fused aluminum oxide, and those parts are to be welded, the weld will be only about 60% as strong as it should be. The same applies to brazing.

In most cases, when people ask about aluminum oxide, they are really asking about fused aluminum oxide. The porcelain type is too friable (brittle) to be ground into a steel surface. Media dust is usually a mixture of both types of aluminum oxide. If media dust gets into an assembly, it can be a problem.

It is necessary to know what the specific concern is. Will the part be welded or brazed? Will the part be assembled in a close tolerance mechanism such as an engine or compressor? Is the part a bearing surface? Is surface cleanliness the only concern or is impregnated fused aluminum oxide the concern?

In a majority of cases, the concern is about free abrasive dust on the parts. For those cases, a good wash after mass finishing is generally acceptable. There may be an actual cleanliness standard established by a laboratory or defined macro inspection.

If the parts are to be welded or brazed, the concern is about fused aluminum oxide impregnation. One answer is to change to silicon carbide media. Silicon carbide media is still porcelain media, but the added abrasive grain is silicon carbide in place of the fused aluminum oxide. Sometimes, media abrasion is not required, and you can use the polishing compositions that are still made from porcelain, but there is no abrasive added. In extreme cases, you don’t use porcelain at all. This can call for steel media or even one of the plastic media. We have been successful with stainless steel screws as media. You still get some deburring, and there is no media contamination of any kind.

Aluminum oxide impregnation results from the pounding and grinding of the media against the part surface. It is more likely to occur in tumbling barrels and high-energy processes such as centrifugal barrels and centrifugal disks. It can be minimized in bowl-type vibratory finishers with adequate flow through of a good cleaning compound.

One company, a manufacturer of refrigerators, has been using ceramic polishing media since the mid-1970’s. All internal parts are mass finished in that media. External parts, such as brackets, are processed in conventional abrasive ceramic media. Many of the internal parts are aluminum that is more subject to impregnation than steel. Before changing to the polishing ceramic media, it had a disturbing number of warranty claims traced to imbedded aluminum oxide. In all these years since then, it has never identified any warranty problem as having come from the vibratory finishing, and almost all of its parts go through that process.

If mass finishing is the source of the aluminum oxide problem, look into the rinsing and compound systems. Investigate a change in the media. Don’t panic and change without all the answers.