Introduction to hexavalent chromium plating

- Aug 13, 2018 -

Hexavalent chrome plating, also known as hexachrome, Cr +6 and chromium (VI) coatings, uses chromium trioxide (also known as chromic anhydride) as the main component. Hexavalent chromium plating solutions are used for decorative and hard plating, as well as bright impregnation of copper alloys, chromic anodizing and chromate conversion coatings.

Typical hexavalent chromium plating processes are: (1) activation bath, (2) chromium bath, (3) rinse, and (4) rinse. The activation bath is typically a chromic acid tank with reverse current. This etches the surface of the workpiece and removes any proportions. In some cases, the activation step is done in a chromium bath. The chromium bath is a mixture of chromium trioxide (CrO 3 ) and sulfuric acid (sulfate, SO 4 ), the ratio of which varies greatly from 75:1 to 250:1. This results in an extremely acidic bath (pH 0). Temperature and current density affect brightness and final coverage in the bathtub. For decorative coatings, the temperature range is 35 to 45°C (100 to 110°F), but for hard coatings it ranges from 50 to 65°C (120 to150°F). The temperature also depends on the current density, as higher current densities require higher temperatures. Finally, the entire bath was stirred to keep the temperature stable and achieve uniform deposition.

Disadvantage: One of the functional disadvantages of hexavalent chromium plating is the low cathode efficiency, which results in poor polishing power. This means it will leave an uneven coating with more edges and fewer internal corners and holes. To overcome this problem, the component can be overplated and ground to size, or an auxiliary anode can be used around the hard plate area.

From a health point of view, hexavalent chromium is the most toxic chromium. In the United States, the Environmental Protection Agency has carried out extensive supervision. The US Environmental Protection Agency lists hexavalent chromium as a harmful air pollutant because it is a human carcinogen, a "priority pollutant" under the Clean Water Act, and a "hazard component" under the Resource Conservation and Recovery Act. ". Due to its low cathode efficiency and high solution viscosity, toxic water and hexavalent chromium mist are released from the bath. Wet scrubbers are used to control these emissions. The effluent from the wet scrubber is treated to precipitate chromium in the solution as it cannot remain in the wastewater.

Maintaining a bath surface tension of less than 35 dynes/cm requires frequent circulation of the bath with a wetting agent and confirms the effect on surface tension. Traditionally, surface tension has been measured using a stalagmite meter. However, this method is cumbersome and inaccurate (errors of up to 22 dynes/cm have been reported) and depend on the user's experience and ability.

The additional toxic waste produced by the hexavalent chromium bath includes lead chromate, which is formed in the bath because of the use of lead anodes. Niobium is also used to control the concentration of sulphate, which leads to the formation of barium sulphate (BaSO 4 ), which is a hazardous waste.

Previous: Introduction to hard chrome Next: Introduction to trivalent chromium plating

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