Thermal Spraying vs. Hard Chrome Plating

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Hard chrome plating has been a reliable industry solution for erosion, wear, corrosion resistance and dimensional reclamation for several years. It can be applied at a practical cost per unit of surface area, but has restrictions on part size, thickness build-up, and in some cases, performance in service.

In the last few years, costs have been progressively escalating due to the increasing environmental pressures and legislation enforced on the chrome plating process, and the disposal of its by-products. It has, thus, become important to the industry to identify alternative processes that offer analogous characteristics to hard chrome plating, but without the resultant hazards.

Thermal spraying technology is gradually providing a feasible alternative to this technology, and could offer the chrome plating industry with complementary processes for protection and reclamation of parts.

Thermal Spraying vs. Hard Chrome Plating Overview

When equating the two processes, a consideration of the economics involved in setting up and preserving both types of facilities can be made. The following parameters make thermal spraying commercially competitive with hard chrome plating.

• Energy cost—For plating, about 15 W of energy is needed per square inch. As part size expands, so do the energy costs. For thermal spraying, part size impacts the coating application time, and subject to the process used, energy costs can be similar.
• Capital cost—The relative capital cost for setting up facilities with the same production capacity is considerably greater for chrome plating than for thermal spraying.
• Materials diversity—A chromium plating facility is a full commitment to one coating, while a thermal spray facility offers the capability of creating a wide range of coatings.
• Waste disposal—Disposing of effluents from the plating process is becoming increasingly expensive. Federal and State guidelines on pollution control necessitate that each facility makes considerable investments to sufficiently provide for waste treatment. Thermal spraying creates lethal waste in the form of metallic dust, whose disposal is moderately easy.
• Space—A thermal spray facility needs considerably less floor space than a similar plating facility.
• Hard chrome plating utilizes more than twice the number of process steps compared to thermal spraying. This means more processing time, and considerably longer turnaround times for chrome plating than thermal spray.

Table 1. Comparison of chrome plating vs thermal spraying

Benefits of Hard Chrome Plating

• History—A popular, proven process with established properties and limitations
• Surface coverage—Not a line of sight process, will function on inner diameters, and on intricate geometry
• Economical for thin deposits—Chrome plating is inexpensive and dependable for very thin deposits, lesser than 25 to 100 µm (0.001–0.004″)
• High hardness—Hardness value of Vickers 700–1000
• Exceptional surface finish—A surface finish, as coated, of ~40 RMS

Limitations of Hard Chrome Plating

• Slow throughput—Usually, plating needs about an hour to deposit a thickness of 25 μm (0.001″) on any size part.
• Adhesion—Two main reasons result in weak adhesion, particularly on iron-based materials such as cast and ductile iron. The first is irregular or poor surface preparation, and the second is extreme micro-cracking throughout the chrome plating. Micro-cracks, which spread from the surface, arise in the plating because of residual stresses. When the micro-cracks spread all the way down to the substrate, the plating may separate.
• Tank contamination—Contaminants, particularly iron, impact the conductivity of the plating solution, and the associated parameters involved in reproducing plating quality can become difficult at times.
• Nodules/uneven buildup—Nodules of excess plating accumulate onto edge and corner areas where existing density is high. This leads to uneven buildup and may induce high residual stresses and thus result in adhesion issues. It will also extend the finishing time.
• Masking—As parts have to be fully submerged, it is hard to mask areas that do not need plating.

Benefits of Thermal Spraying

• No limitation on part size—The thermal spray process has no real restrictions on part size as there is no need for immersion in a plating tank.
• Waste disposal—Wastes from thermal spraying are not harmful, but contain elements necessitating special disposal. Usually, they are dry powders and some can be reactive.
• Low capital investment—in contrast to chrome plating installation, a low capital investment is required for equipment. Furthermore, the installation of a thermal spray system requires considerably less time, and related installation costs are also minimal.
• Competitive application costs—When parts are sufficiently large and the coating thickness need is high, thermal spraying becomes very expensive than chrome plating.
• On-site capability—Thermal spray equipment is portable and can be used on-site.
• Fewer process steps—Thermal spray coatings need fewer process steps to apply than chrome plating.
• Materials choice—This is perhaps the single most significant advantage. Individual applications can be matched to materials based on the precise requirements for corrosion, service temperature, wear, or other factors.
• High deposition rates—A majority of the thermal spray processes have a higher deposition rate when compared to chromium plating.
• Denser coatings—Coatings can be applied with almost theoretical density and without any crack patterns.
• Thicker coatings—Through the thermal spray process, thicker coatings can be economically applied, in certain cases up to 10 mm.
• Uniform coating thickness—The coating buildup with thermal spraying is relatively more even over the whole part, based on automation and part geometry.

Other Benefits Include:

• No post heat treatment—No post heat-treatment or stress relief is needed.
• No fatigue debit—Chrome plating decreases the fatigue life of parts it is used on, while certain thermal spray coatings can increase it.
• No hydrogen embrittlement—No base metal hydrogen embrittlement issues.
• Process time—The reduced number of process steps, and high application rate can greatly minimize turnaround time, which has become an important factor in repair applications.
• Finish with conventional grinding—Thermal spray coatings can be completed with traditional grinding and polishing technologies. A few can even be single-point machined.

Table 2. Performance comparison between Chrome Plate and various thermal spray coatings

Limitations of Thermal Spraying

• Process parameters—A detailed understanding of the thermal spray process is needed to achieve optimum results. As with all technologies, there is a related learning curve.
• Noise—Thermal spray processes differ in their sound output, a few are as high as 135 dB, which requires noise attenuation.
• Line of sight process—The capability to coat inner diameters and challenging geometry can mean substantial automation programming.
• Dust—All thermal spray processes create dust particles as waste or over-spray which has to be collected for disposal, and also as a means of maintaining the spray area clean.

Some Examples from Industry

The bearing and seal ends of the gas turbine shaft were earlier reclaimed by the application of chrome plating. They are currently thermally sprayed with HP/HVOF that offers a coating with excellent wear resistance. The coating can also be applied to the part when it is partly assembled, thus minimizing turnaround time.

The central axle of a Boeing 767 aircraft was sprayed with tungsten carbide, 10% cobalt, and 4% chrome during the HP/HVOF process. Boeing is at the forefront of using thermal spray to replace the chrome plate on crucial components.

Conclusion

Thermal spraying is not about completely replacing chrome plating around the world, but it’s about providing an alternative solution where coating performance, turnaround time, coating thickness, part size, or environmental problems create difficulties for chrome plating.

Different industry sectors are prudently and methodically progressing through the use of thermal spraying in areas where chrome plating was a reliable solution for numerous years. Both processes have their boundaries; therefore, it is very unlikely that this technology will do anything more than supplement chrome plating and other surface treatments in the industry.

This will offer engineers in the industry an extra solution to the various issues experienced, as higher levels of performance are required.

Top airlines in the United States and Europe, and other high-volume, high-profile users of chrome plating are leading in assessing and using thermal spraying on already plated components. This should strengthen the confidence level of other industries to adequately assess what thermal spraying can offer.

The developments in thermal spraying in the last decade, both in relation to process and product, and also in the level of understanding, now positions this technology to move forward on a larger front, as a tested and comprehensively proven industrial tool.