Hard Chromium plating:

Extremely hard and corrosion resistant. The process is used for applications where excellent wear and/or corrosion resistance is required. Chrome plating may be either a thick layer of chrome (generally known as hard chrome) that offers exceptional hardness,wear-resistance, and oil-holding capability, but limited corrosion resistance;

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.

The average life of a chromium-plated ring is approximately five times that of an unplated ring made of the same base metal.

Nickel Plating

The Nickel plating process is used extensively for decorative, engineering, and electroforming purposes, because the appearance and other properties of electrodeposited nickel can be varied over wide ranges by controlling the composition and the operating parameters of the plating solution. Decorative applications account for about 80% of the nickel consumed in plating; 20% is consumed for engineering and electroforming purposes.

These deposits may be specified to improve corrosion and wear resistance, to salvage or build up worn or undersized parts, to modify magnetic properties, electrolytic nickel plating is decorative, functional, and corrosion resistant but it tarnishes and it is not as hard as chromium.

Nickel-chrome:

Nickel plating followed by a flash of chrome for tarnish resistance and extra corrosion resistance.

Industrial (Hard) Chromium Plating

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.

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 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). 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.

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

The fact that a chromium deposit can significantly reduce fatigue strength must be considered in determining whether chromium plating can be safely used.

TThe excellent wearing qualities and low coefficient of friction of chromium are highly advantageous.

Selection Factors

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

The inherent hardness and wear resistance of electrodeposited chromium

The thickness of chromium required

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

The type of metal from which the part is made

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

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

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.

Engineering Plating

The engineering applications of nickel plating include those where a fully bright appearance is not required. Engineering nickel deposits are usually sulfur-free and matte in appearance. These deposits may be specified to improve corrosion and wear resistance, to salvage or build up worn or undersized parts, to modify magnetic properties, to prepare surfaces for enameling or for organic coating, to function as diffusion barriers in electronic applications and for other purposes. Engineering applications exist in the chemical, nuclear, telecommunications, consumer electronics, and computer industries.

Engineering and Electroforming Processes

Electrodeposited nickel coatings are applied in engineering applications to modify or improve surface properties, such as corrosion resistance, hardness, wear, and magnetic properties. Although the appearance of the coating is important and the plated surface should be defect-free, the lustrous, mirror-like deposits described in previous sections are not usually required. Nickel electroforming is the specialized use of the nickel plating process to produce or reproduce articles by electroplating onto a mandrel that is subsequently separated from the deposit.

Chromium Alloy Plating

For both chromium and stainless steel types of alloys, the most important properties to develop in a coating are corrosion resistance, abrasion and wear resistance, hardness, surface texture, and luster.

Conventional electrolytic nickel plating is decorative, functional, and corrosion resistant but it tarnishes and it is not as hard as chromium.

Electroless nickel plating (actually autocatalytic nickel plating) is a glassy nickel-phosphorous coating that is very corrosion resistant and quite hard, and often used on rough-service applications like down-hole applications.

Chrome plating may be either a thick layer of chrome (generally known as hard chrome) that offers exceptional hardness,wear-resistance, and oil-holding capability, but limited corrosion resistance; or it may be nickel-chrome (nickel plating followed by a flash of chrome for tarnish resistance and extra corrosion resistance, as is employed on truck bumpers and automotive brightwork).