General Conditions For Operating Heatbath Black Oxide Solutions
Information on Delsudging Included
To begin, cleaning is very important to ensure that a receptive surface is produced so that the blackening solution can oxidize the whole surface uniformly and in the shortest possible time. Cleaning covers the removal of oil, grease, shop soils, oxides, scale and rust. Mechanical cleaning will, in some cases, suffice but usually this leaves some oil or physical deposit on the surface that can interfere with efficient blackening. Generally, a good alkaline cleaner, followed in some cases by an acid dip or alkaline deruster, will produce the most satisfactory results.
Thorough rinsing between all operations is important to prevent carry-over of one solution to the next. Cross contamination may cause harmful effects or shorten the life of solutions. Cold running water rinses are preferred between operations prior to blackening. If warm or hot rinses are used, some rusting of the cleaned parts could occur, causing a red or brown smut to develop on the surface of the work during blackening.
The blackening solution should be operated at a slow rolling boil to ensure that all salts are in solution and that the proper operating temperature is maintained. The normal operating temperature will vary somewhat, depending upon the type of steel being processed. Usually, the best temperature is a few degrees above or below 290°F, using a range of 285° – 292°F as the optimum. If the temperature of the solution is too low, the result will be a slow blackening and a light color will be evident on the work. The temperature can be raised by allowing excess water to boil off, or additions of blackening salts can be made to increase the solution concentration and boiling point. If the temperature is too high, the parts will blacken but a superficial red, brown or green film will form on top of the black. If the film is light, it can be wiped off. If it is heavy, it will be most difficult to remove. Parts with a heavy rusty brown or red film would have to be stripped and reprocessed. Too high a temperature can be corrected by adding water to reduce the solution concentration and boiling point. The solution boiling point and concentration should be automatically controlled to allow water additions when required.
The blackening solution is a mixture of water and blackening salts and is super saturated at operating temperature. When the solution is allowed to cool to room temperature, some of the salts will precipitate and form a heavy sludge on the bottom of the tank. A cold tank should always be stirred prior to heating to prevent a hot pocket under the sludge that could overheat and cause an eruption or boil over of the solution. Water should not be added to a cold bath because it will not mix readily and could cause boil over upon heat up.
The blackening solution should be kept clean for efficient and economical operation. The surface of the solution should be skimmed periodically, when at operating temperature, to remove any scum or foam. Periodic desludging should be carried out when the solution has cooled to approximately 230°F, after the heat has been lowered or turned off. The insoluble materials will settle out at this temperature and can be removed with a hoe-like tool. Do not desludge the tank while it is cold, because good blackening salt will be lost by being carried out with the sludge. Periodic decanting of the solution will also help to keep the bath clean. Allow the solution to cool down to about 230°F and bail or pump the top three quarters of the solution into an empty rinse tank or spare tank. Dispose of the bottom quarter of the solution left in the processing tank and clean the tank out thoroughly. Return the good solution to the tank and build up the working level with water and new blackening salts. Approximately 6 ½ pounds of blackening salt will make a gallon of blackening solution that will boil at about 285°F.
Immersion time in the blackening solution should be sufficiently long to produce a deep black color. Usually, an average time is from 10 to 15 minutes. Some thin, mild steel parts can be done in 5 minutes but hardened, alloyed or carburized steels may take up to 30 minutes. Agitation of small parts will ensure complete blackening and elimination of contact spots or marks. Flat parts may require racking to prevent nesting.
Transfer of blackened parts from the blackening solution to the cold water rinse tank should be fast enough to ensure that the solution does not dry on the surface of the work. If parts are allowed to dry in the transfer, a brownish film will form on the surface of the work which will be difficult to rub off. If a hoist is used, vertical travel of 28 feet per minute speed is suggested.
A final finish of hot or cold oil should be applied over the blackened parts. If deep blind holes, recesses or assembled parts are treated, it will be necessary to use a neutralizing dip, such as Rinse Kleen 1020 or No Bleed II to remove the trapped alkali salts prior to oiling. Powdered metal parts can be successfully blackened, however, processing through No Bleed II will be necessary to neutralize and displace entrapped salts. If salts are allowed to remain in or on the finished work, blooming or flowering of white salts may result.
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