Black Oxide Coatings: Common Problems and Remedies

07 December 2018

A problem-solving guide for black oxide coatings, that’s the direction this article is taking. The finish should be oily black. Putting a tick in that checkbox, the coating should also be moderately corrosion resistant. Only, the inspector’s pen is wavering. There’s an issue with the ebony-hued conversion coating. Spotting problems are apparent, as are strange material discontinuities. Taking the problem head-on, let’s discuss common coating causes.

A Black Oxide Coatings Review

There’s no need to call up the information from another source, not when the principle is described right here. To keep the explanation short, think of the metal blackening technique as a surface conversion technique. A chemical reaction takes place when a salting bath converts the surface of a ferrous-rich metal into iron oxide. No, this isn’t corrosion, not exactly, it’s iron’s magnetite form. The iron oxide converts to Fe3O4, which isn’t rust. The post-treatment work converts the surface into a corrosion resistant blue-black finish. Different than paints and supplementary coatings, there’s no applied electroplating work or substrates involved.

Troubleshooting Coating Discontinuities

Before getting underway, the process team should check out the post-processing room for problems. Unlike a conventional heat treatment system, chemical reactions rule this zone. If those chemicals are impeded, then the process will produce substandard results. Dirt and oil are an issue, as are oxides and material scale. Sandblasting machines, handheld buffers, and chemical cleaners are responsible for cleaning grimy surfaces. Solving this issue, a competent team leader audits the parts cleansing stations.

If surface discontinuities are still causing trouble, the technician heads on over to the salting bath. Contaminants have found their way into the pool, as evidenced by a scum-like surface film. The oxidizing salts, thanks to the contamination, need to be replaced before the operation can get underway once more. Cranking the process back to full speed, the oxidizing temperature is uniformly applied so that dangerously thinner coatings don’t develop along a part’s flank. Nesting issues are next. If this is a batch processing operation, every item and every item surface requires space so that the chemical reactions can develop properly.

A sharp pair of eyes and a seasoned mind are essential. The spots and discontinuities are easily detected and corrected when a systematic approach is adopted. Pin up a troubleshooting guide. Memorize common problems, as found in this uniquely chemical-centric heat treatment process. For instance, grey finishes are commonly produced by oxidizing bath temperature glitches. Brown “rub-off,” another frustrating processing snag, can often be traced to high-temperature extremes or to unusually high quantities of colloidal iron, which have perhaps been left behind in the oxidizing bath.

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