Introduction

HARD 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. The rebuilding of mismachined or worn parts comprises large segments of the industry. One specialized application is a thin chromium layer used as a lacquer adhesive layer in the manufacture of "tin" cans.

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

• 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.

• Hard chromium normally is deposited to thicknesses ranging from 2.5 to 500 pm (0.1 to 20 mils) and for certain applications to considerably greater thicknesses, whereas decorative coatings seldom exceed 1.3 pm (0.05 mil).

• 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. Principal Uses

The major uses of hard chromium plating are for wear-resistance applications, improvement of tool performance and tool life, and part salvage. Table 1 lists parts to which hard chromium plate is applied and representative data regarding plate thickness and plating times. Plating times can be reduced by using high-efficiency or mixed-catalyst solutions.

Table 1 Typical thicknesses and plating times for selected applications of hard chromium plating

Part

Base metal

Thickness of plate

Plating time(a)

fim

mils

Computer printer type

Carbon steel

25

1

60 min

Face seals

Steel or copper

75-180

3-7

10 h

Aircraft engine parts

Nickel-based alloys, high strength steel

75-180

3-7

10 h

Plastic molds

Tool steel

5-13

0.2-0.5

30 min

Textile guides

Steel

5-100

0.2-4

20-240 min

Piston rings

Steel or cast iron

150-255

6-10

8 h

Balls for ball valves

Brass or steel

7.5-13

0.3-0.5

20 min

Micrometers

Steel

7.5-13

0.3-0.5

20 min

Golf ball molds

Brass or steel

7.5-25

0.3-1

20-60 min

Lock cases

Brass

5-7.5

0.2-0.3

20 min

Cylinder

Cast iron

255

10

300 min

Bushing

1018 carburized, 56 HRC

25

1

45 min

Crankshafts

Steel

255-3800

10-150

Cutting tools

Tool steel

1.3

0.05

5 min

Forming and drawing dies

Steel

25

1

60 min

Part

Base metal

Thickness of plate

Plating time(a)

fim

mils

Gage

Steel

125

5

150 min

Gun barrels, 30 caliber(b)

Steel

25

1

40 min

Hydraulic cylinder

1045 steel

13

0.5

40 min

Pin

Steel

13

0.5

30 min

Pin

1045 steel, 60 HRC

125

5

40 min

Plug gage

1040 steel, 55 HRC

125

5

150 min

Relief-valve plunger

1113 steel, soft

100

4

60 min

Ring gage

Steel

205

8

240 min

Rolls

Steel

13-255

0.5-10

20-300 mm

(a) Times shown are for conventional plating solutions; plating times for the proprietary fluoride-free solution are half of those shown.

(a) Times shown are for conventional plating solutions; plating times for the proprietary fluoride-free solution are half of those shown.

(b) M-16 rifle, barrel and chamber

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 pm (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. Valve stems are plated with a flash coating (about 2.5 pm, or 0.1 mil) to reduce wear. Hydraulic shafts for all kinds of equipment are plated with 20 to 30 pm (0.8 to 1.2 mil) of hard chromium to increase service life.

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. Plating thicknesses of 2.5 to 125 pm (0.1 to 5 mils) usually are recommended for preventing wear in parts sticking in molds and for reducing frequency of polishing when plastics that attack steel or beryllium copper are being molded. Chromium-plated dies should not be used when plastics containing fire-retardant chlorides are molded.

The service life of plug gages and other types of gages may be prolonged by hard chromium plating. Most gage manufacturers provide chromium-plated gages. Records in one plant indicate that plug gages made from hardened O1 tool steel wore 0.0025 mm (0.0001 in.) after gaging 5000 cast iron parts. Hard chromium plating of these gages allowed the gaging of 40,000 parts per 0.0025 mm (0.0001 in.) of wear.

Worn gages can be salvaged by being built up with hard chromium plate. Also, chromium plate provides steel gages with good protection against rusting in normal exposure and handling. Chromium plating is not recommended, however, for gages that are subjected to impact at exposed edges during operation.

Deep drawing tools often are plated with chromium, in thicknesses up to 100 pm (4 mils), for improvement of tool performance and/or building up of worn areas. The life of draw rings and punches may be prolonged by plating. In addition, plating reduces frictional force on punches and facilitates removal of workpieces from punches in instances where sticking is encountered with plain steel surfaces. If deep drawing tools are chromium plated, the base metal should be harder than 50 HRC. Steel dies used for drawing bars and tubes are often plated with relatively heavy thicknesses (up to 250 pm, or 10 mils) of chromium to minimize die wear, reduce friction, and prevent seizing and galling.

The service life of cutting tools is often extended by chromium plate, in thicknesses ranging from less than 2.5 to 13 pm (0.1 to 0.5 mil). Taps and reamers are examples of tools on which chromium plate has proved advantageous. In one case, a flash plate on taps used to thread cold-worked 1010 steel improved tap life from 250 (for unplated taps) to 6000 parts per tap. The poor tool life of the unplated taps was caused by buildup of metal on the cutting edges. Hard chromium plating is not recommended for cold extrusion tools for severe applications where extreme heat and pressure are generated, because the plate is likely to crack and spall and may be incompatible with phosphate-soap lubricants.

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.

Hard chromium plating is used to restore to original dimensions the worn surfaces of large crankshafts for diesel and gas engines and for compressors. In these applications, in which coating thicknesses usually range from 125 to 1250 pm (5 to 50 mils), the excellent wearing qualities and low coefficient of friction of chromium are highly advantageous. The plate is prevented from depositing in fillet areas as a precaution against fatigue failure.

The extremely close dimensional tolerances specified for components of compressors for jet aircraft engines are not always correctly met in machining. Hard chromium plating is sometimes used to salvage mismachined parts. Most frequently mismachined are the diameters of rotor disks and spacers. The maximum thickness of plate on these components, which are made of 4130 and 4340 steels, generally does not exceed 380 pm (15 mils).

Other Applications. Hard chromium plate is applied to printing plates and stereotypes, especially to those intended for long runs, because compared to other materials or coatings used for this application, it wipes cleaner, provides sharper reproduction, and increases the length of press runs. It is used on press rams because of its excellent resistance to corrosion, seizing, galling, and other forms of wear.

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