Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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surface treatments BLACKENING OF FERROUS METALS BY ROBERT W. FARRELL, JR. HUBBARD-HALL INC., WATERBURY, CONN.; www.hubbardhall.com The commercial application of black conversion coatings to iron, steel, and cast iron has been well established for over 75 years. The three most economically viable methods of blackening ferrous alloys—hot alkaline black oxiding, room temperature black oxiding, and black zinc phosphate—are chosen for a number of reasons. These are: 1. They produce an attractive black finish that enhances the saleability of the articles thus coated. 2.They offer an economical means of imparting moderate corrosion resis- tance to the articles thus coated. 3.They yield a finish that when properly sealed resists galling and enhances lubricity. 4.Their application is economical as far as cost per square foot. 5.These finishes yield little or no dimensional change. HOT ALKALINE BLACK OXIDIZING Hot alkaline nitrate black oxide was originally developed in Germany as a two- bath system during the early 1900s. The modern single bath oxidizing solu- tions became commercially prevalent during the later 1930s and have remained so through today. The modern hot black oxidizing solutions are proprietary blends of sodium hydroxide, sodium nitrate, sodium nitrite, wetting agents, and unique rectifiers supplied as powdered compositions or ready-to-use liquid formulations. The black finish developed by the hot alkaline nitrate black oxidizing solution is a true conversion coating converting iron to the naturally occurring black iron oxide compound called magnetite with chemical formula of Fe3 O4 is produced by immersing the steel parts to be blackened in the hot alkaline nitrate solution operating at a boiling point of between 285 and 295O . The magnetite F. Dwell times are typically from 5 to 20 minutes depending on such parameters as alloy, surface hardness, and nature of the heat-treated surface. Powdered and liquid formulations are available. Powdered formulations are used in water at concentrations of from 5 to 6 lb/gal depending on the propri- etary formula used. The liquid solutions are used as received and typically have boiling points of 280 to 285O F. The solution must be boiling to achieve the prop- er blackening. The boiling point is a function of the salt concentration and is maintained by the automatic addition of water to replace that which is contin- uously boiled away. As water is boiled off the boiling point of the solution rises. Although it is often stated that hot alkaline nitrate oxidizing produces a black oxide finish with no dimensional change, this is not, in reality, the case. The actual dimensional change that does occur has been measured and is approxi- mately 5 millionths of an inch. It is rare, however, to find instances in manu- facturing where this minimal dimension change is objectionable. The process used to blacken steel using the hot alkaline nitrate solution in its 394

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