Chromium Plating

HCP Problems and Corrective Procedures

Faulty operation of a chromium-plating solution can result in slow plating speed or deposits with undesirable characteristics. The problems encountered in hard chromium plating are similar for all three chemistries. Possible causes and suggested corrective procedures include:

Poor coverage

• Low chromic acid content
• Low ratio of chromic acid content to total catalyst content. Correct by adding chromic acid or by precipitating sulfate, if too high, with barium carbonate.
• Temperature too high
• Current density too low
• Passive or scaled anodes. Correct by cleaning and reactivating anodes, using high current density until uniform gassing is obtained, and checking for good anode contact.
• Rack contacts too heavily built up with metal, causing rack to rob plate
• Thieves too large or too close
• Open holes preventing uniform plate in adjacent areas. Correct by using nonconducting plugs in holes.
• Gas entrapment preventing plating solution from reaching some areas. Correct by positioning parts in solution so that all gases can escape or by agitating parts while they are plating.
  

Burnt deposits

• Ratio of chromic acid to total catalysts too high. Correct by adding necessary catalyst or by lowering chromic acid content.
• Current density too high
• Temperature too low
• Large parts were colder than solution temperature when plating began.
• Some parts, in a load of different parts, receive too much current. Correct by adjusting anodes and contacts to ensure that each part receives correct current density.
• Excessive amount of anode within a given area for the part being plated. Correct by eliminating all excess anodes in the tank, designing the shape of conforming anodes to minimize current density at high-density areas, and, if necessary, using nonconducting shields at these areas.
  

Slow plating speed

• Chromic acid content too high
• Ratio of chromic acid to total catalyst too high
• Temperature too high
• Current density too low
• Scaled anodes
• Insufficient or inadequate sizes of conductors in anode or cathode circuits
• Thief obtains too much current. Correct by redesigning thief or by removing nodules from thief.
• Single phasing of rectifier caused by partial burnout. Correct by repairing rectifier.
• High contact resistance on busbars, racks, or jigs. Correct by cleaning contacts to lower contact resistance. Do not always rely on voltage for control, because it does not indicate the conditions present on the part; instead, control by amperage.
• Large variety of parts in same tank prohibiting proper current density for each part
• Tank overcrowded with parts
• High content of metallic impurities. Correct by discarding a portion of solution; readjust after dilution.
  

Nodular deposits

• Insufficient etching before plating in relation to thickness of deposit. Correct by increasing etching time.
• Rough surface before plating
• Chromic acid content too high. Correct by removing portion of solution; dilute the remainder and adjust.
• Low temperature
• Low sulfate content
• Current density too high
  

Pitted deposits

• Marking dye not completely removed
• Material suspended in solution. Correct by filtering out suspended material.
• Surface-activating agents caused deep pits when plating thick deposits. Correct by discarding as much of the solution as necessary to eliminate pits and replace with fresh solution. Prevent by discontinuing use of mist suppressors.
• Plastic parts that decompose, such as floats, can form decomposition products that also create pits.
• Gas bubbles adhering to part. Correct by improving surface finish before plating and by agitating part occasionally during plating.
• Part is magnetized. Correct by demagnetizing.
• Magnetic particles in the solution. Correct by removing particles with magnet.
• Insufficient cleaning prior to plating
• Particles falling on work from anodes or thieves. Correct by improving design of anodes and thieves and by cleaning both regularly to remove loose particles.
• Carbon smut on surface. Correct by scrubbing before plating.
• Excessively etched surface during reverse-etch or stripping operation
  

Poor adhesion

• Insufficient or no etching before plating
• Contaminants not completely removed from surfaces during cleaning
• Excessive grinding rate at edges or sharp projections, where base metal fractures and it may appear that plated material did not adhere
• Single phasing of rectifier caused by partial burnout. Correct by repairing rectifier.
• Current interruption during plating
• Cold solution
  

Macrocracks

• Highly stressed base metal; cracks are visible during grinding or when heat is applied. Correct by relieving stresses in base metal.
• Grinding at too fast a rate (heat checks)
  

HCP Problems and Corrective Procedures

Faulty operation of a chromium-plating solution can result in slow plating speed or deposits with undesirable characteristics. The problems encountered in hard chromium plating are similar for all three chemistries. Possible causes and suggested corrective procedures include:

Poor coverage

• Low chromic acid content
• Low ratio of chromic acid content to total catalyst content. Correct by adding chromic acid or by precipitating sulfate, if too high, with barium carbonate.
• Temperature too high
• Current density too low
• Passive or scaled anodes. Correct by cleaning and reactivating anodes, using high current density until uniform gassing is obtained, and checking for good anode contact.
• Rack contacts too heavily built up with metal, causing rack to rob plate
• Thieves too large or too close
• Open holes preventing uniform plate in adjacent areas. Correct by using nonconducting plugs in holes.
• Gas entrapment preventing plating solution from reaching some areas. Correct by positioning parts in solution so that all gases can escape or by agitating parts while they are plating.
  

Burnt deposits

• Ratio of chromic acid to total catalysts too high. Correct by adding necessary catalyst or by lowering chromic acid content.
• Current density too high
• Temperature too low
• Large parts were colder than solution temperature when plating began.
• Some parts, in a load of different parts, receive too much current. Correct by adjusting anodes and contacts to ensure that each part receives correct current density.
• Excessive amount of anode within a given area for the part being plated. Correct by eliminating all excess anodes in the tank, designing the shape of conforming anodes to minimize current density at high-density areas, and, if necessary, using nonconducting shields at these areas.
  

Slow plating speed

• Chromic acid content too high
• Ratio of chromic acid to total catalyst too high
• Temperature too high
• Current density too low
• Scaled anodes
• Insufficient or inadequate sizes of conductors in anode or cathode circuits
• Thief obtains too much current. Correct by redesigning thief or by removing nodules from thief.
• Single phasing of rectifier caused by partial burnout. Correct by repairing rectifier.
• High contact resistance on busbars, racks, or jigs. Correct by cleaning contacts to lower contact resistance. Do not always rely on voltage for control, because it does not indicate the conditions present on the part; instead, control by amperage.
• Large variety of parts in same tank prohibiting proper current density for each part
• Tank overcrowded with parts
• High content of metallic impurities. Correct by discarding a portion of solution; readjust after dilution.
  

Nodular deposits

• Insufficient etching before plating in relation to thickness of deposit. Correct by increasing etching time.
• Rough surface before plating
• Chromic acid content too high. Correct by removing portion of solution; dilute the remainder and adjust.
• Low temperature
• Low sulfate content
• Current density too high
  

Pitted deposits

• Marking dye not completely removed
• Material suspended in solution. Correct by filtering out suspended material.
• Surface-activating agents caused deep pits when plating thick deposits. Correct by discarding as much of the solution as necessary to eliminate pits and replace with fresh solution. Prevent by discontinuing use of mist suppressors.
• Plastic parts that decompose, such as floats, can form decomposition products that also create pits.
• Gas bubbles adhering to part. Correct by improving surface finish before plating and by agitating part occasionally during plating.
• Part is magnetized. Correct by demagnetizing.
• Magnetic particles in the solution. Correct by removing particles with magnet.
• Insufficient cleaning prior to plating
• Particles falling on work from anodes or thieves. Correct by improving design of anodes and thieves and by cleaning both regularly to remove loose particles.
• Carbon smut on surface. Correct by scrubbing before plating.
• Excessively etched surface during reverse-etch or stripping operation
  

Poor adhesion

• Insufficient or no etching before plating
• Contaminants not completely removed from surfaces during cleaning
• Excessive grinding rate at edges or sharp projections, where base metal fractures and it may appear that plated material did not adhere
• Single phasing of rectifier caused by partial burnout. Correct by repairing rectifier.
• Current interruption during plating
• Cold solution
  

Macrocracks

• Highly stressed base metal; cracks are visible during grinding or when heat is applied. Correct by relieving stresses in base metal.
• Grinding at too fast a rate (heat checks)
  

The quality of hard chromium plate is evaluated chiefly from the standpoint of thickness and thickness distribution, appearance, crack pattern, crack size, porosity, roughness, and adhesion of the plate to the base metal.

Surface Cracks

During the deposition of chromium deposits, microcracks form to relieve the internal stress. These cracks subsequently fill in with plated chromium. The chromium in these former cracks is more easily etched and has a different refractive index than the surrounding deposits. For this reason, with the use of a microscope one can determine where and how many microcracks were present at one time on the surface of the deposit. The pattern usually consists of crack-free areas and plateaus completely surrounded by crack boundaries. The plateaus from an average conventional sulfate solution are 2 to 3 times larger than those from a mixed-catalyst solution; that is, there are more cracks per inch in a deposit from the average mixed-catalyst solution. The fluoride-free chemistry gives yet an additional factor of 2 to 3 times more microcracks.

Interestingly, the more microcracks present, the shorter the penetration into the deposit of each one (i.e., with the conventional solution, a microcrack can extend all the way through a 1 mil thick deposit, while in the fluoride-free solution deposit they extend very short distances). Since these (former) cracks provide the pathway for corrosion, it is desirable to have as many discontinuities as possible as one progresses through the deposit to the substrate. It is for this reason that in corrosion applications, the solution that provides the greatest amount of microcracking possible is used.

Crack-Free Deposits

It is possible to plate chromium with virtually no microcracks. This is done by altering the current density and temperature, altering the catalyst concentrations, or using pulsed-current or periodic-reverse plating. Crack-free deposits are gray and have very poor wear properties. The corrosion resistance is also very poor due to residual stresses in the deposit, which eventually (in a few days or months) cause large cracks that extend through the entire deposit.

Some specialized applications for this type of deposit include broaches, cams, dies for metal forming, metalworking rolls, and stamping dies for embossing silverware. Complicated shapes create a large range of current densities and are difficult to plate with a crack-free surface. Corners, edges, or other high-current-density areas are most likely to crack during plating.

Porous Chromium

Although the cracks or porosity that characterize chromium deposits are not desirable for resistance to corrosion, a porous structure can be advantageous in wear applications in which lubrication is required, because it promotes wetting action and provides oil retention after initial lubrication. Engine cylinders are the outstanding application.

Most chromium-plated cylinder surfaces consist of some form of interrupted surface, generally porous chromium. An interrupted surface may be obtained by electrolytic or chemical etching of chromium after it is plated on a smoothly honed bore, as with porous chromium, or by preroughening the bore by shot blasting, knurling, or tooling and then reproducing this roughness in the final chromium plate or by machining in roughness after plating.

Two distinct types of porous chromium are produced. One has pinpoint porosity with many microscopic depressions in a honed chromium surface. This has been used in all types of engine cylinders except aircraft. The other type is also finish honed but is broken by randomly connected channels, leaving isolated bearing plateaus. For both types, the percentage of porosity is generally controlled between 20 and 50% of the total area. Average plateau diameter is further controlled between 0.25 to 0.75 mm (0.010 to 0.030 in.) with the channel type of porosity. Porosity as low as 5% approaches dense chromium and is susceptible to scoring because of sparse oil distribution. High porosity, such as 75%, may cause high initial ring wear and high oil consumption. In normal engine service, cylinders coated with chromium of optimum porosity give wear rates one-third to one-tenth better than those of uncoated cast iron or steel, hardened or unhardened.

Several methods--electrochemical, mechanical, and combinations of both--have been developed to provide controlled porosity in heavy chromium deposits. Mechanical methods entail either severe grit blasting of the surface to be plated or roughening of the surface with a fine knurling tool. The roughened surface is reproduced by the deposit. Using a patterned mask, the surface can also be roughened by chemical or electrochemical means before plating The most widely used techniques, however, involve chemical or electrochemical etching of the chromium deposit after plating. Note that the pattern or crack density and the size of the plateaus are largely determined by the composition (ratio) of the solution, and the plating temperature.

Etching is performed on plated thicknesses ranging from 120 to 180 m (5 to 7 mils). Porosity is developed after plating by electrochemically etching anodically in chromic acid solution. The etched surface is finished by honing, polishing, or lapping. Metal removal that exceeds the depth of porosity must be avoided. To avoid accelerated wear in service, finished surfaces must be thoroughly cleaned of abrasive and chromium particles.

Quality Control Tests

Usually, visual examination is sufficient for determining appearances and roughness of the surface of hard chromium plate. Magnetic particle inspection can be used to examine chromium plate up to 100 m (4 mils) thick for cracks after grinding. The as-plated deposit prior to postfinishing should be as smooth as the base metal before plating and should be free of pits and nodules. The deposit should not exhibit excessive thickness variation. Particularly, deposits with dendritic growths (trees) should be rejected. Adequate plating control requires that such dendritic deposits occur on thieves rather than in functional areas.

For process development and quality verification, destructive testing may be used to determine the crack pattern and bond between the plate and base metal. The crack pattern can be developed by etchants such as a hot 50 vol% hydrochloric acid aqueous solution, or by short etching in a chromium plating solution.

The quality of the bond can be determined by punch testing, bend testing, examining the bond line metallographically, or judging of ground or hammered samples. Well-bonded chromium, because of its low ductility, does not fail by pulling away from the bond line; however, it fails by cracking and spalling if it is subjected to excessive stress or distortion in 45° diagonal tension.

Excessive porosity of thin (less than 25 m, or 1 mil, thick) chromium plate on steel can be determined by applying an acidified copper sulfate solution to the plated areas. The pores permit the solution to copper coat steel by displacement, and the degree of copper coating thus indicates the degree of porosity. Porosity can also be determined by the ferroxyl test described in Metal Finishing Guidebook, 1982.

The mandrel test can also be used in quality control. If a portion of the chromium plate is made anodic for 3 min at 15 A/dm2 (1 A/in2) in a solution containing 250 g/L (33 oz/gal) chromic acid at 60 °C (140 °F), the crack pattern is developed. Counting the crack density under the microscope is an excellent procedure for noting the constancy of the composition (mainly ratio) and the temperature of the solution.

## ASM HANDBOOK VOL 5 ##

The quality of hard chromium plate is evaluated chiefly from the standpoint of thickness and thickness distribution, appearance, crack pattern, crack size, porosity, roughness, and adhesion of the plate to the base metal.

Surface Cracks

During the deposition of chromium deposits, microcracks form to relieve the internal stress. These cracks subsequently fill in with plated chromium. The chromium in these former cracks is more easily etched and has a different refractive index than the surrounding deposits. For this reason, with the use of a microscope one can determine where and how many microcracks were present at one time on the surface of the deposit. The pattern usually consists of crack-free areas and plateaus completely surrounded by crack boundaries. The plateaus from an average conventional sulfate solution are 2 to 3 times larger than those from a mixed-catalyst solution; that is, there are more cracks per inch in a deposit from the average mixed-catalyst solution. The fluoride-free chemistry gives yet an additional factor of 2 to 3 times more microcracks.

Interestingly, the more microcracks present, the shorter the penetration into the deposit of each one (i.e., with the conventional solution, a microcrack can extend all the way through a 1 mil thick deposit, while in the fluoride-free solution deposit they extend very short distances). Since these (former) cracks provide the pathway for corrosion, it is desirable to have as many discontinuities as possible as one progresses through the deposit to the substrate. It is for this reason that in corrosion applications, the solution that provides the greatest amount of microcracking possible is used.

Crack-Free Deposits

It is possible to plate chromium with virtually no microcracks. This is done by altering the current density and temperature, altering the catalyst concentrations, or using pulsed-current or periodic-reverse plating. Crack-free deposits are gray and have very poor wear properties. The corrosion resistance is also very poor due to residual stresses in the deposit, which eventually (in a few days or months) cause large cracks that extend through the entire deposit.

Some specialized applications for this type of deposit include broaches, cams, dies for metal forming, metalworking rolls, and stamping dies for embossing silverware. Complicated shapes create a large range of current densities and are difficult to plate with a crack-free surface. Corners, edges, or other high-current-density areas are most likely to crack during plating.

Porous Chromium

Although the cracks or porosity that characterize chromium deposits are not desirable for resistance to corrosion, a porous structure can be advantageous in wear applications in which lubrication is required, because it promotes wetting action and provides oil retention after initial lubrication. Engine cylinders are the outstanding application.

Most chromium-plated cylinder surfaces consist of some form of interrupted surface, generally porous chromium. An interrupted surface may be obtained by electrolytic or chemical etching of chromium after it is plated on a smoothly honed bore, as with porous chromium, or by preroughening the bore by shot blasting, knurling, or tooling and then reproducing this roughness in the final chromium plate or by machining in roughness after plating.

Two distinct types of porous chromium are produced. One has pinpoint porosity with many microscopic depressions in a honed chromium surface. This has been used in all types of engine cylinders except aircraft. The other type is also finish honed but is broken by randomly connected channels, leaving isolated bearing plateaus. For both types, the percentage of porosity is generally controlled between 20 and 50% of the total area. Average plateau diameter is further controlled between 0.25 to 0.75 mm (0.010 to 0.030 in.) with the channel type of porosity. Porosity as low as 5% approaches dense chromium and is susceptible to scoring because of sparse oil distribution. High porosity, such as 75%, may cause high initial ring wear and high oil consumption. In normal engine service, cylinders coated with chromium of optimum porosity give wear rates one-third to one-tenth better than those of uncoated cast iron or steel, hardened or unhardened.

Several methods--electrochemical, mechanical, and combinations of both--have been developed to provide controlled porosity in heavy chromium deposits. Mechanical methods entail either severe grit blasting of the surface to be plated or roughening of the surface with a fine knurling tool. The roughened surface is reproduced by the deposit. Using a patterned mask, the surface can also be roughened by chemical or electrochemical means before plating The most widely used techniques, however, involve chemical or electrochemical etching of the chromium deposit after plating. Note that the pattern or crack density and the size of the plateaus are largely determined by the composition (ratio) of the solution, and the plating temperature.

Etching is performed on plated thicknesses ranging from 120 to 180 m (5 to 7 mils). Porosity is developed after plating by electrochemically etching anodically in chromic acid solution. The etched surface is finished by honing, polishing, or lapping. Metal removal that exceeds the depth of porosity must be avoided. To avoid accelerated wear in service, finished surfaces must be thoroughly cleaned of abrasive and chromium particles.

Quality Control Tests

Usually, visual examination is sufficient for determining appearances and roughness of the surface of hard chromium plate. Magnetic particle inspection can be used to examine chromium plate up to 100 m (4 mils) thick for cracks after grinding. The as-plated deposit prior to postfinishing should be as smooth as the base metal before plating and should be free of pits and nodules. The deposit should not exhibit excessive thickness variation. Particularly, deposits with dendritic growths (trees) should be rejected. Adequate plating control requires that such dendritic deposits occur on thieves rather than in functional areas.

For process development and quality verification, destructive testing may be used to determine the crack pattern and bond between the plate and base metal. The crack pattern can be developed by etchants such as a hot 50 vol% hydrochloric acid aqueous solution, or by short etching in a chromium plating solution.

The quality of the bond can be determined by punch testing, bend testing, examining the bond line metallographically, or judging of ground or hammered samples. Well-bonded chromium, because of its low ductility, does not fail by pulling away from the bond line; however, it fails by cracking and spalling if it is subjected to excessive stress or distortion in 45° diagonal tension.

Excessive porosity of thin (less than 25 m, or 1 mil, thick) chromium plate on steel can be determined by applying an acidified copper sulfate solution to the plated areas. The pores permit the solution to copper coat steel by displacement, and the degree of copper coating thus indicates the degree of porosity. Porosity can also be determined by the ferroxyl test described in Metal Finishing Guidebook, 1982.

The mandrel test can also be used in quality control. If a portion of the chromium plate is made anodic for 3 min at 15 A/dm2 (1 A/in2) in a solution containing 250 g/L (33 oz/gal) chromic acid at 60 °C (140 °F), the crack pattern is developed. Counting the crack density under the microscope is an excellent procedure for noting the constancy of the composition (mainly ratio) and the temperature of the solution.

## ASM HANDBOOK VOL 5 ##

Hard chromium plating is also known as industrial, functional, or engineering chromium plating. It differs from decorative chromium plating in the following ways:

1. Hard chromium deposits are intended primarily to increase the service life of functional parts by providing a surface with a low coefficient of friction that resists galling, abrasive and lubricated wear, and corrosion. Another major purpose is to restore dimensions of undersized parts.
2. Hard chromium normally is deposited to thicknesses ranging from 2.5 to 500 m (0.1 to 20 mils) and for certain applications to considerably greater thicknesses, whereas decorative coatings seldom exceed 1.3 m (0.05 mil).
3. With certain exceptions, hard chromium is applied directly to the base metal; decorative chromium is applied over undercoats of nickel or of copper and nickel.

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
• A light cut
• Correct peripheral speed
• Freedom from vibration
• Frequent wheel dressing

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 m (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 m (14 to 20 in.). Subsequent lapping (240 grit) will produce a finish of 0.1 to 0.3 m (5 to 10 in.).

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 highstrength steel parts, see military specification MIL-STD-866B.

HCP - Hard Chromium Plating Equipment

The discussion of equipment that follows is confined largely to considerations that are specific to chromic acid plating processes. Mixedcatalyst and fluoride-free solutions have essentially the same equipment requirements as conventional sulfate solutions, except that all parts of the electrical system may need to be heavier to accommodate the increased current used.

Tanks and Linings

Most tanks for chromium plating are made of steel and lined with an acid-resisting material. Because of their excellent resistance to corrosion by chromic acid, lead alloys containing antimony or tin may be used as tank linings.

Acid-resistant brick has been used as a lining material. Because of its electrical insulating characteristics, acid-resistant brick lining has the advantage over metal linings of reducing possible current losses or stray currents. Some installations combine a lead lining or plastic sheet lining with an acid-resistant brick facing. With fluoride-containing solutions, a brick lining is suitable only for temporary use.

Almost invariably, plasticized polyvinyl chloride is used for all three types of chromium plating solutions, provided that the solution temperature does not exceed 66 °C (150 °F). Sheets of this plastic are cemented to tank walls and welded at joints and corners. Other plastic materials are equally resistant to chemical attack but are more likely to fail at the welds when exposed to an oxidizing acid. Fiberglass utilizing either polyester or epoxy is unsatisfactory for use in mixed-catalyst solutions, because exposed fiberglass will be attacked by the secondary fluoride catalyst.

Design specifications for low-carbon steel tanks for chromium plating are given in Table A. Lining materials for low-carbon steel tanks are given in Table B. Steel tanks should be supported at least 100 mm (4 in.) from the floor; steel I-beams are used to provide this support and are mandatory when side bracing is required. To provide insulation, reinforced strips of resin-bonded glass fiber can be placed between the floor and the I-beams. Glass brick can be used as insulation between electrodes and the plating tank.

Tabel A. Design specifications for low-carbon steel tanks for hard chromium plating

Tabel B. Lining materials for low-carbon steel tanks for hard chromium plating

1. Antimonial lead, or lead-tin alloy.

2. Plasticized polyvinyl chloride.

3. Acid-resistant brick. For further protection, brick may be backed up with 39 kg/m2 (8 lb/ft2) of antimonial lead or lead-tin alloy, or with plasticized
   polyvinyl chloride sheet.

4. Lining should be 10 mm ( 3/8in.) thick at top to 0.3 m (1 ft) below top of tank.

Heating and Cooling

Steam heating coils and cooling coils can be made of antimonial lead or silver-bearing lead. Titanium coils are preferred for conventional and fluoride-free plating solutions because of their relatively low cost and long life. Tantalum or niobium-clad coils should be used for mixed-catalyst solutions due to the fluoride attack on titanium. These coils are mounted on tank walls behind the anodes. Steel pipes carrying steam and cooling water to the tank must have a nonconducting section in each leg, so that the coils cannot become an electrical ground back through the power plant system.

Electric immersion heaters sheathed in fused quartz are suitable for heating chromic acid solutions. The quartz is fragile and must be handled with care. Similar immersion heaters are sheathed in either tantalum, titanium, or lead alloy. It is sometimes feasible to heat and cool a chromic acid solution by piping the liquid to a tube bundle, concentric, or tube heat exchanger located outside the plating tank. Preferably, heat exchanger tubes should be made of tantalum or titanium. This method has the disadvantage of requiring pumping of the solution.

Temperature-control planning should begin with selection of the volume of solution required in the plating solution. An ideal volume consists of 1 L or more of solution for each 13 W of plating power (1 gal or more of solution for each 50 W of plating power). About 60% of this plating power (30 W) produces heat and maintains the solution at temperature in an uninsulated tank of standard design. Power applications in excess of 13 W/L (50 W/gal) require cooling of the plating solution and cause relatively rapid changes in solution composition.

Agitation

A chromium-plating solution should be agitated periodically, particularly when the solution is being started, to prevent temperature stratification. Air agitation is effective, but oil from an air pump must not be permitted to leak into the air system. Preferably, the air should come from an oil-free low-pressure blower. A perforated pipe of rigid polyvinyl chloride may be used to distribute air in the solution.

Power Sources

Although dynamos or motor-generator sets were once the usual sources of power for low-voltage direct current for plating, rectifiers are now regularly used. In general, use of motor-generator sets is now restricted to larger and more permanent installations. Originally, plating rectifiers were made of copper oxide or magnesium-copper sulfide, but these have been largely replaced by silicon rectifiers. Silicon is favored for plating rectifiers because of its high resistance to thermal overload and small space requirement. Hard chromium platers often start plating on a piece by sweeping up applied voltage and current from very low values to the high values used for plating. Because silicon-controlled rectifiers have high ripple at low output, the output should be filtered. Tap-switch controls, however, produce relatively low ripple over the entire output range.

A 6 V power source can be used for chromium plating, but it is generally desirable or necessary to operate with 9 to 12 V available. Chromium plating requires full-wave rectification with a three-phase input and full control, giving a ripple less than 5% and no current interruptions. If a rectifier becomes partially burned out, it may single phase to some degree, and this can cause dull or laminated, peeling deposits.

Fume Exhaust

A chromium-plating process produces a chromic acid mist, which is toxic. The maximum allowable concentration for 8 h continuous exposure is 0.1 mg of chromic acid mist per cubic meter of air. This concentration value is in accordance with recommendations by the American Conference of Governmental Industrial Hygienists. Because of the extreme toxicity of this mist, it is mandatory to provide adequate facilities for removing it. The minimum ventilation rate should be 60 m3/min per square meter (200 ft3/min per square foot) of solution surface area. (It should be noted that these regulations are presently under revision and are subject to changes.)

Generally, fumes are exhausted from a chromium plating tank by means of lateral exhaust vents along both long sides of the tank. For narrow tanks, up to 600 mm (24 in.) wide, a lateral exhaust on one side of the tank should be adequate unless strong cross-drafts exist. Velocity of the air at the lateral exhaust hood slots should be 600 m/min (2000 ft/min) or more.

In the design of ductwork, condensate duct traps should be included to capture chromic acid solution. Drains from these traps should be directed to a special container and not to the sewer. In this way, chromic acid solutions can be returned to the tank or recovery system or be safely destroyed. A fume scrubber or a demister should also be included in the system to remove most of the chromic acid fumes before exhausted air is emitted to the atmosphere. Many communities have air pollution regulations requiring fume scrubbers. Fume exhaust ductwork may be made of carbon steel and coated with acid-resistant paint. Modern construction uses chlorinated polyvinyl chloride.

Rinse Facilities

Rinsing the work after chromium plating prevents it from becoming stained or discolored. Insufficient rinsing can result in contamination of cleaning solutions during subsequent cycling of racks. Multiple rinsing facilities are recommended. After being plated, parts should be rinsed in a nonrunning reclaim tank, which can be used to recover part of the chromium solution dragout. After they are rinsed in the reclaim tank, plated parts should be rinsed in counterflowing cold water and hot water tanks. Water should cascade from the hot water tank to the cold water tank. A multiple counterflowing arrangement requires much less water than two separate rinsing tanks.

If rinse water is being returned to a chromic acid waste disposal unit, the flow of water into the hot water tank should be controlled automatically by a conductivity-sensing element in the cold water tank. At a predetermined concentration of chromic acid in the cold water, the water inlet to the hot water tank should flow, causing an overflow of cold water to the waste disposal unit. This arrangement decreases the amount of water consumed and minimizes the required capacity of the waste disposal unit.

Cold water rinse tanks may be coated, sprayed, or otherwise lined with plasticized polyvinyl chloride. Hot water rinse tanks may be constructed of types 347, 304, or 316 stainless steel, or they may be made of carbon steel and lined with lead. Reinforced polyester glass fiber also may be used for either hot water or cold water rinse tanks.

Spray rinsing also effectively removes residual chromic acid. Because spraying does not always reach recessed areas, sprays should be positioned above a dip rinse. As parts are removed from the dip rinse, they may be sprayed with clean water, which, in turn, is returned to the dip tank.

(HCP) Hard Chromium Plating Surface Preparation

All soils and passive films must be removed from surfaces of ferrous and nonferrous metals before they are hard chromium plated. In addition to cleaning, certain surface-activating processes are often important in preparing the base metal for hard chromium plating. The processes include etching of steel, preplate machining, and nonferrous metals preparation.

Etching

Etching of steel before plating is needed to ensure adherence of the chromium deposit. Anodic etching is preferred for this purpose. Slight etching by acid immersion may be used for highly finished surfaces, but with possible sacrifice of maximum adherence.

Steel can be etched anodically in the chromium plating solution at its operating temperature for plating. A reversing switch is used so that the steel to be plated can serve as the anode for 10 s to 1 min (usually 30 s to 1 min) at a current density of about 15 to 45 A/dm2 (1 to 3 A/in2). Tank voltage should ordinarily be 4 to 6 V. Because mixed-catalyst solutions chemically attack the steel, causing etching of the surface, shorter electrochemical etching time frequently is required than is the case with conventional or fluoride-free chemistries. This
process has the disadvantage of causing the solution to become contaminated with iron from the work and with copper from the conductors.

As an alternative, steel may be anodically etched in a separate chromic acid solution without sulfate additions and containing 120 to 450 g/L (16 to 60 oz/gal) of chromic acid. Solution temperature may range from room temperature to that of the chromic acid plating solution, or even higher, provided that current density and time of treatment are adjusted to suit the type of work being processed.

A sulfuric acid solution (specific gravity 1.53 to 1.71) may be used for anodic etching, provided that the solution temperature is held below 30 °C (86 °F), and preferably below 25 °C (77 °F). The time of treatment may vary from 30 to 60 s and the current density may vary from about 15 to 45 A/dm2 (1 to 3 A/in2) at tank voltages ordinarily between 4 and 6 V. A lead-lined tank with lead cathodes should be used. With the use of a sulfuric acid solution, however, two difficulties may be encountered. First, if the rinsing following etching is incomplete, the drag-in of sulfuric acid throws the chromium plating solution out of balance with respect to the ratio of chromic acid to sulfate. Second, in handling parts that are difficult to manipulate, there is danger that surfaces exposed to air more than a very short time will rust and that finely finished surfaces will be overetched.

For high-carbon steel, a sulfuric acid solution of 250 to 1000 g/L (33 to 133 oz/gal), used at a temperature of not more than 30 °C (86 °F) and preferably below 25 °C (77 °F), is effective for anodic etching. The addition of 125 g/L (16.6 oz/gal) of sodium sulfate, based on the anhydrous salt, is of benefit for many grades of steel. Anodic treatment in this solution usually does not exceed 1 min at a current density of about 15 A/dm2 (1 A/in2) (range of 15 to 45 A/dm2, or 1 to 3 A/in2). High acid content, high current density, and low temperature (within the ranges specified) minimize the attack on the base metal and produce a smoother surface. This sulfuric acid solution is stable and not appreciably affected by iron buildup.

Preplate Machining

Metal debris on the surface should be removed before etching (an activation procedure). The use of abrasivecoated papers is common, as is the use of successively finer grit stones in honing and grinding. To prepare a sound surface in superfinishing, 600-grit stones may be used. Electropolishing is sometimes used to remove highly stressed metal and metal debris from the surface of cold-worked steel. This process improves bond strength and corrosion resistance of electroplated coatings. It accomplishes this function without formation of smut, which may result from anodic etching. This treatment is not recommended for parts that are subjected to critical fatigue stresses and that are expensive to manufacture.

Preparation of Nonferrous Metals

Aluminum, in common with certain other metals, quickly develops a natural, passive oxide film after exposure to preplating cleaning cycles. This film must be removed before aluminum is plated. The most widely used method of preparing aluminum for plating involves a zincating treatment, which may be followed by a thin 5 m (0.2 mil) copper electrodeposit. However, it is possible to plate chromium directly over the zincate.

Aluminum parts used in hydraulic systems require a nickel undercoat before being plated, to provide corrosion protection to all plated surfaces that are not completely and constantly immersed in hydraulic fluid or similarly protective fluids. A minimum thickness of 10 to 15 micron (0.4 to 0.6 mil) of nickel is usually specified. This undercoat may also be required for steel parts in similar applications.

Titanium and titanium alloys, as well as magnesium, also form a tight, stable oxide coating and are therefore difficult to plate. These metals can be pretreated with an electroless nickel plate or a coating deposited from a high-chloride nickel strike solution.

Introduction

HARD (INDUSTRIAL) CHROMIUM PLATING is produced by electrodeposition from a solution containing chromic acid (CrO3) and a catalytic anion in proper proportion. The metal so produced is extremely hard and corrosion resistant. The process is used for applications where excellent wear and/or corrosion resistance is required. This includes products such as piston rings, shock absorbers, struts, brake pistons, engine
valve stems, cylinder liners, and hydraulic rods. Other applications are for aircraft landing gears, textile and gravure rolls, plastic rolls, and dies and molds. Hard chromium plating is also known as industrial, functional, or engineering chromium plating.

Hard Chromium Plating Principle Uses

The major uses of hard chromium plating are for wear-resistance applications, improvement of tool performance and tool life, and part salvage.

Wear Resistance. Extensive performance data indicate the effectiveness of chromium plate in reducing the wear of piston rings caused by scuffing and abrasion. The average life of a chromium-plated ring is approximately five times that of an unplated ring made of the same base metal. Piston rings for most engines have a chromium plate thickness of 100 to 200 m (4 to 8 mils) on the bearing face, although thicknesses up to 250 m (10 mils) are specified for some heavy-duty engines. In the automotive industry, hard chromium is also applied to shock absorber rods and struts to increase their resistance to wear and corrosion.

Tooling Applications. Various types of tools are plated with chromium to minimize wear, prevent seizing and galling, reduce friction, and/or prevent or minimize corrosion. Steel or beryllium copper dies for molding of plastics are usually plated with chromium, especially when vinyl or other corrosive plastic materials are to be molded.

Part Salvage. Hard chromium plating is sometimes used for restoring mismachined or worn surfaces. Since 1970, the use of this process for part salvage has been frequently replaced by thermal spraying and plasma coatings, which can be applied more quickly. The fact that a chromium deposit can significantly reduce fatigue strength must be considered in determining whether chromium plating can be safely used.

Selection Factors

The decision to use hard chromium plating on a specific part should take into account the following characteristics:

1. The inherent hardness and wear resistance of electrodeposited chromium

2. The thickness of chromium required

3. The shape, size, and construction of the part to be plated

4. The type of metal from which the part is made

5. Masking requirements (for parts that are to be selectively plated)

6. Dimensional requirements (that is, whether or not mechanical finishing is required and can be accomplished in
   accordance with desired tolerances)

The hardness of chromium electrodeposits is a function of the type of chemistry selected and the plating conditions. In general, chromium plated in the bright range is optimally hard. Typically bright chromium deposits from conventional plating solutions have hardness values of 850 to 950 HV; those from mixed-catalyst solutions have values of 900 to 1000 hV; and those from fluoride-free chemistries have values of 950 to 1100 HV or higher.

Size. Frequently, a very large part can be plated in sections or can be rotated so that only a portion of the part is immersed in the plating solution at any given time. The latter method has been used to plate large cylinders up to 4 m (12 ft) in diameter and up to 18 m (60 ft) long. When this technique is used, all of the surface to be plated that is exposed to the atmosphere must remain wet with plating solution.

Base Metal. Most hard chromium deposits are applied to parts made of ferrous alloys; however, numerous aerospace applications require the chromium plating of aluminum and nickel-base alloys. From the standpoint of processing, hard chromium plate may be applied to steels, regardless of their surface hardness or chemical composition, provided that the base metal is hard enough to support the chromium layer in service. Similarly, cast irons can be plated provided that the surface is capable of conducting the required current and is reasonably free of voids, pits, gross silicate inclusions, massive segregation, slivers, and feather edges.