Removal of Chromium Plate

Most manufacturers require salvage of misplated parts because of their high value. Further, in the aircraft industry, business machine industry, and plastic mold industry, significant numbers of parts are run for the life of the deposit and then overhauled by stripping worn deposits and replating.

Methods of Stripping. Chemical, electrochemical, or mechanical methods are used to remove hard chromium deposits. When the base material is steel, brass, copper, or nickel, hydrochloric acid at any concentration over 10 vol% and at room temperature or above removes chromium. In some operations, inhibitors are added to the acid solution to minimize attack on the steel substrate.

Chromium is removed electrochemically from steel or nickel by the use of any convenient heavy-duty alkaline cleaner at room temperature or above, at 5 to 6 V with anodic current. This method is unsatisfactory for nickel-base alloys, which should be stripped chemically in hydrochloric acid. Chromium may be stripped from aluminum by making the part the anode in a cold chromium (nonfluoride) plating solution or in conventional chromic acid or sulfuric acid anodizing solutions. Aluminum alloys with a high alloy content and alloys subjected to various heat treatments all react differently in stripping solutions, so precautions must be taken to prevent attack on the base metal. Anodic stripping operations result in formation of oxide films on the base metal. These films should be removed by one of the conventional deoxidizing processes prior to replating.

Stripping of chromium deposits from high-strength steel must be performed electrochemically in an alkaline solution. The parts are then stress relieved at 190 °C (375 °F) for a minimum of 3 h. The following solutions and operating conditions are recommended for removing chromium deposits from the materials indicated. Proprietary formulations having a longer operational life are also available.

Removal from steel or nickel-plated steel

• Sodium hydroxide, 45 to 320 g/L (6 to 30 oz/gal); anodic treatment at 3 to 8 A/dm2) (0.2 to 0.5 A/in2); solution temperature, 21 to 71 °C (70 to 160 °F)

• Anhydrous sodium carbonate, 45 to 60 g/L (6 to 8 oz/gal); anodic treatment at 2.5 to 5.5 A/dm2 (0.15 to 0.35 A/in2); solution temperature, 21 to 66 °C (70 to 150 °F). Use 2.3 A/dm2 (0.15 A/in2) with solution temperature of 66 °C (150 °F) to reduce possibility of pitting alloy steel.

• Sodium hydroxide, 52 g/L (7 oz/gal); sodium carbonate, 30 g/L (4 oz/gal); anodic treatment at 8 A/dm2 (0.5 A/in.2)

• Concentrated hydrochloric acid at room temperature

• Hydrochloric acid, 50 vol%, at room temperature

Removal from aluminum and aluminum alloys

• Sulfuric acid, 67 vol%; glycerin, 5 vol%; anodic treatment at 1 to 3 A/dm2 (0.1 to 0.2 A/in2); solution temperature, 21 to 27 °C (70 to 80 °F)

Removal from magnesium and magnesium alloys

• Anhydrous sodium carbonate, 50 g/L (6.5 oz/gal); anodic treatment at 2 to 5 A/dm2 (0.15 to 0.30 A/in2); solution temperature, 21 to 27 °C (70 to 80 °F)

Grinding is used occasionally to remove heavy chromium deposits. Most defective chromium deposits are observed during subsequent grinding for finishing, so it is sometimes expedient to continue grinding to remove all of the plate and then replate. In the grinding of heavy deposits for the removal of several thousandths of an inch of chromium to attain required dimensions or surface finish, the most important requisites for successful results are:

• A soft grinding wheel

• A sufficient amount of coolant

• Correct peripheral speed

• Freedom from vibration

• Frequent wheel dressing

Because chromium is hard and brittle, a soft grinding wheel is essential. A hard wheel forms a glazed surface, which results in a temperature rise that causes the chromium to crack. A soft wheel breaks down rapidly enough to prevent formation of a glaze; however, too soft a wheel is not economical because of rapid wheel wear. Good performance can be obtained with an aluminum oxide resin-bonded wheel of about 60 grit and H-grade (hardness).

To prevent or minimize glazing, the contact area should be flooded with a coolant. Usually, the coolant is water with a small amount of soluble oil. Because of its hardness, excess chromium cannot be removed as rapidly as when grinding most other materials. The maximum thickness of metal removed should not exceed 5 pm (0.2 mil) per pass, and this amount should be reduced if there is any evidence of cracking. The optimum grinding speed is about 20.4 m/s (4000 sfm).

Effective grinding requires a rigid machine. Any appreciable vibration can cause cracking of chromium because of uneven contact pressure, and it also results in a wavy surface. Factors essential for a rigid machine include a well-fitting spindle bearing, a balanced wheel, a heavy bed, and a well-supported workpiece. Whenever there is the least indication of glazing or nonuniform wheel surface, the wheel should be dressed with a diamond point. Adherence to the preceding recommendations will result in a good surface with a finish of 0.35 to 0.5 pm (14 to 20 pin.). Subsequent lapping (240 grit) will produce a finish of 0.1 to 0.3 pm (5 to 10 pin.).

Special care should be taken when grinding chromium-plated parts made from high-strength steel (steel with an ultimate tensile strength of 1240 MPa, or 180 ksi, and above) that are to be used in stressed applications. Numerous failures have occurred due to formation of untempered martensite caused by the heat of the grinding operation. For information and guidelines on grinding chromium-plated high-strength steel parts, see military specification MIL-STD-866B.

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