The process of hard chrome plating has been used since the early part of the 20th century. It is a process that is used in nearly every aspect of manufacturing where wear resistance and lubricity are critical. Typically coating thickness deposited are between 8 to 250 mm (0.00032”-.0098”) and hardness between 800-1000HV. High hardness and resistance to abrasion are retained unchanged up to 200o C. Early processes were standard chromic acid and sulfate processes. The late 1940’s gave to processes that used anions such as fluoride to improve plating rates and finish quality. Later evolution in the late 1970’s combined the use of specific organic acids that produced enhanced cathode efficiencies and higher hardness. This serves as a brief outline of the hard chrome process and preferred areas of application still used today.
Standard Chromic acid / sulfate processes are used widely today. This process has a typical cathode efficiency of 10% to 12%. In other words for every 100 amps supplied only 10% to 12% of that current is actually depositing chromium at the cathode surface. Typical deposit rates are in the range of 0.0005” to 0.0006” per hour.(12.5-15mm) Typically the current density is in the 1.5 to 2.0 amps per square inch (144 to 216 amp / square foot). Higher amperages may produce rough or pitted deposits requiring more post plating grinding and finishing. Although the process may be slow in comparison to other processes it is used when higher deposits are necessary (i.e. deposits in excess of 0.012”) .
Fluoride Catalyzed process still use sulfate but the fluoride ion produces better cathode efficiencies, better micro cracking, and higher hardness. Typically the deposit from these processes is brighter and smoother requiring less secondary machining. Typical cathode efficiencies are in the range of 12% to 15% and produce deposit rates of 0.0008” to 0.001” per hour. Higher current densities can be achieved in the range of 2.0 to 2.5 amps per square inch (288 to 360 amps / square foot). As you can see due to the faster deposit rates the overall throughput is better with an incremental increase in energy consumption. Due to the fluoride in the process it can be very hard on tooling and fixtures and frequent inspection of tooling is necessary to optimize the process.
High Speed Chrome plating is just that, FAST. In high speed processes a specific family of organic acids is used in conjunction with chromic acid and sulfate to produce cathode efficiencies over 20% and deposit rates exceeding 0.002” hour. The deposit from the high speed baths is very bright and very hard. The unique micro cracking that occurs enhances corrosion resistance and improves the lubricity of the surface. The high speed processes are excellent where post plate grinding is minimized and thicknesses of less than 0.006” are used. Areas of application for high speed chrome are hydraulics, pumps, and molds. Typically the process chemistry is more expensive but where high throughput is necessary the costs are neutralized.
Hard chromium deposits can be further enhanced by combining an underlying coating of electroless nickel followed by a topcoat of hard chromium. This combination has both good corrosion protection and wear resistance.
All of these processes are still employed in the 21st century and replacements, although being actively pursued are lacking in economic feasibility. Environmental pressures continue to mount on a domestic and foreign front but it is still the process where all potential replacements will be forever measured.
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