Introduction

CHROMIUM ALLOYS, which are relatively difficult to deposit, yield alloy coatings with properties that range from completely satisfactory to marginally acceptable, depending on the end use. The idea of depositing a thin, stainless steel type of coating has been receiving renewed interest since the 1970s. Stainless steel coatings would conserve strategic metals, facilitate the manufacturing process, and lower cost if they could be applied to strip or sheet, or to a component that has been fabricated to the required size and shape. This would also mean that a component could be made from a material that is more easily worked than stainless steel.

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. Thickness requirements can range from a few microns up to a few hundred microns. Thicker coatings are rarely specified, primarily because the relatively poor throwing power of the plating solution gives an uneven surface coverage. Further, the low plating efficiencies can lead to high internal stresses from hydrogen uptake. The resulting microcracking has a detrimental effect on coating properties.

It is difficult to deposit chromium from aqueous solutions because of its low hydrogen overvoltage. As a result, cathodic coulombic efficiency typically ranges from 10 to 20% for commercial processes. In order to deposit chromium-containing alloys, it is usually necessary to implement the approaches summarized below and described more fully in Ref 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11:

• Complexing agents are needed to bring the deposition potentials of the alloying metals (e.g., nickel and iron) closer together.

• The plating solution should contain a greater proportion of chromium salts, relative to the alloy addition, in order to obtain significant amounts of chromium in the deposits.

• Care must be taken to deposit metallic coatings rather than hydrated, chromium-containing salts, especially if chloride-base solutions are used.

• Close control of temperature, agitation, current density, and pH is necessary with most of the reported solution chemistries, in order to obtain reproducible alloy compositions and properties, as well as reasonable plating rates.

• Trivalent chromium-base solution chemistries provide better prospects for depositing satisfactory alloy coatings from aqueous solutions. An additional benefit is that waste-treatment requirements are less stringent than they are for hexavalent chromium-base solutions.

• The chromium concentration in the alloy should be limited in order to avoid microcracking.

• Aqueous solutions with a low pH value (between 1 and 3) seem to produce better deposits than solutions with pH values outside this range.

• Nonaqueous solutions, especially those based on molten salts, can provide useful alloy deposits.

However, these tend to be relatively thick.

• Thin deposits offer the best prospect for providing lustrous finishes.

• Post-plating heat treatments can sometimes improve the mechanical properties and corrosion resistance of the deposits.

References to chromium alloy plating date back to the mid-1950s, when corrosion- and wear-resistant coatings were first developed for the inside of gun barrels used in military applications, and patents date back to circa 1970. This article will emphasize just the work performed since the 1980s. Those chromium alloys that contain nickel or iron or both are discussed first, because of their importance. Other alloying elements that have been deposited with chromium are discussed as a group, in less detail.

The variables and techniques available to apply coatings based on chromium and other alloys are extensive (Table 1). Because of space limitations, many of these variables or options are mentioned only briefly, where appropriate, and the discussion of deposit properties is similarly limited.

Table 1 Technology options for the deposition of chromium-base alloys

Type of bath

Aqueous

Acid Alkaline

Nonaqueous

Organic Molten salt

Applied current

None (electroless) Conventional direct current Pulsed direct current Periodic reversed direct current High-speed direct current High-speed pulsed direct current

Composition

Alloy

Mixture

Amorphous

Structure

Homogeneous

Multilayer

Composite

Dispersed Post treatments

Heat treatment

Diffusion Laser glazing

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