Chrome plating

One functional disadvantage of hexavalent chromium plating is low cathode efficiency, which results in bad throwing power. This means it leaves a non-uniform coating, with more on edges and less in inside corners and holes. To overcome this problem the part may be over-plated and ground to size, or auxiliary anodes may be used around the hard-to-plate areas.[1]

From a health standpoint, hexavalent chromium is the most toxic form of chromium. In the U.S., the Environmental Protection Agency regulates it heavily. The EPA lists hexavalent chromium as a hazardous air pollutant because it is a human carcinogen, a "priority pollutant" under the Clean Water Act, and a "hazardous constituent" under the Resource Conservation and Recovery Act. Due to its low cathodic efficiency and high solution viscosity, a toxic mist of water and hexavalent chromium is released from the bath. Wet scrubbers are used to control these emissions. The discharge from the wet scrubbers is treated to precipitate the chromium from the solution because it cannot remain in the waste water.[1]

Maintaining a bath surface tension less than 35 dynes/cm requires a frequent cycle of treating the bath with a wetting agent and confirming the effect on surface tension.[2] Traditionally, surface tension is measured with a stalagmometer. This method is, however, tedious and suffers from inaccuracy (errors up to 22 dynes/cm have been reported), and is dependent on the user's experience and capabilities.[3]

Additional toxic waste created from hexavalent chromium baths include lead chromates, which form in the bath because lead anodes are used. Barium is also used to control the sulfate concentration, which leads to the formation of barium sulfate (BaSO₄).[1]

The functional advantages of trivalent chromium are higher cathode efficiency and better throwing power. Better throwing power means better production rates. Less energy is required because of the lower current densities required. The process is more robust than hexavalent chromium because it can withstand current interruptions.[1]

From a health standpoint, trivalent chromium is intrinsically less toxic than hexavalent chromium. Because of the lower toxicity it is not regulated as strictly, which reduces overhead costs. Other health advantages include higher cathode efficiencies, which lead to less chromium air emissions; lower concentration levels, resulting in less chromium waste and anodes that do not decompose.[1]

One of the disadvantages when the process was first introduced was that decorative customers disapproved of the color differences. Companies now use additives to adjust the color. In hard coating applications, the corrosion resistance of thicker coatings is not quite as good as it is with hexavalent chromium. The cost of the chemicals is greater, but this is usually offset by greater production rates and lower overhead costs. In general, the process must be controlled more closely than in hexavalent chromium plating, especially with respect to metallic impurities. This means processes that are hard to control, such as barrel plating, are much more difficult using a trivalent chromium bath.[1]

Art Deco portfolio with chrome-plated cover, ca 1925

Decorative chrome is designed to be aesthetically pleasing and durable. Thicknesses range from 0.002 to 0.02 mils (0.05 to 0.5 μm), however, they are usually between 0.005 and 0.01 mils (0.13 and 0.25 μm). The chromium plating is usually applied over bright nickel plating. Typical base materials include steel, aluminium, plastic, copper alloys, and zinc alloys.[1] Decorative chrome plating is also very corrosion resistant and is often used on car parts, tools and kitchen utensils.

Hard chrome plating

Hard chrome, also known as industrial chrome or engineered chrome, is used to reduce friction, improve durability through abrasion tolerance and wear resistance in general, minimize galling or seizing of parts, expand chemical inertness to include a broader set of conditions (such as oxidation resistance), and bulking material for worn parts to restore their original dimensions.[4] It is very hard, measuring between 65 and 69 HRC (also based on the base metal's hardness). Hard chrome tends to be thicker than decorative chrome, with standard thicknesses in nonsalvage applications ranging from 0.02 to 0.04 mm (20 to 40 μm),[5] but it can be an order of magnitude thicker for extreme wear resistance requirements, in such cases 0.1 mm (100 μm) or thicker provides optimal results. Unfortunately, such thicknesses emphasize the limitations of the process, which are overcome by plating extra thickness then grinding down and lapping to meet requirements or to improve the overall aesthetics of the "chromed" piece.[1] Increasing plating thickness amplifies surface defects and roughness in proportional severity, because hard chrome does not have a leveling effect.[6] Pieces that are not ideally shaped in reference to electric field geometries (nearly every piece sent in for plating, except spheres and egg shaped objects) require even thicker plating to compensate for non-uniform deposition, and much of it is wasted when grinding the piece back to desired dimensions.

Modern "engineered coatings" do not suffer such drawbacks, which often price hard chrome out due to labor costs alone. Hard chrome replacement technologies outperform hard chrome in wear resistance, corrosion resistance, and cost. Rockwell hardness 80 is not extraordinary for such materials. Using spray deposition, uniform thickness that often requires no further polishing or machining is a standard feature of modern engineered coatings. These coatings are often composites of polymers, metals, and ceramic powders or fibers as proprietary embodiments protected by patents or as trade secrets, and thus are usually known by brand names.[7]

Hard chromium plating is subject to different types of quality requirements depending on the application; for instance, the plating on hydraulic piston rods are tested for corrosion resistance with a salt spray test.