Green Cars
Vehicle Assembly: Anticorrosion Operation
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The anticorrosion operation pre-treats metal body parts, preparing them for subsequent operations (i.e., Priming), while applying a protective coating against corrosion. Three steps are included in the anticorrosion operation: aqueous washing, zinc phosphating, and a pacifying rinse. A water rinse step follows the zinc phosphating and pacifying rinse steps. |
Anticorrosion Process Description
After assembly, the vehicle body (structural frame, door and side panels, hood, trunk, and roof) is cleaned in an aqueous wash. In a process similar to that of a dishwasher, a water-based detergent solution is sprayed over the vehicle body to remove dirt, oils, and any film that may have formed on the parts. If the surface of the vehicle body is not adequately cleaned, the quality of subsequent operations may be diminished, resulting in an inferior final product. This process is also called an alkaline wash, since the water solution is basic (i.e., high pH) in chemical composition.
The phosphating process then applies a corrosion-resistant crystalline coating of zinc phosphate to the vehicle body using either a spray or an immersion system. Like the operation of a dishwasher, a spray system douses the vehicle body with the chemical solution, thus coating its surface with zinc phosphate. An immersion system works like a washing machine for clothes: the vehicle body is submerged in a circulating bath of chemicals to achieve the desired coating. A water rinse following the phosphating process removes excess chemicals.
A chrome-free acid rinse (also called a pacifying rinse) follows the phosphating process and further enhances the anticorrosion properties of the zinc-phosphate coating. Again, a water rinse follows to remove excess pacifying solution from the parts.
Primary Wastes: hazardous/solid waste, wastewater
Likely TRI Chemicals:
Anticorrosion Pollution Prevention Options
Large quantities of water are used in the anticorrosion operation for cleaning and rinsing vehicle body parts and in coating formulations. To minimize the consumption of water and the generation of contaminated wastewater, purification processes (e.g., ultrafiltration) can be employed to remove contaminants and allow rinse water and bath chemicals to be recycled back into the process.
Reducing or eliminating the need for toxic metals in the anticorrosion operation has been the focus of many pollution prevention initiatives. Most assembly plants have eliminated chromium from the pacifying rinse step, replacing it with a chrome-free solution; other initiatives may eliminate the need for zinc. The elimination of chromium and zinc from anticorrosion operations has positive benefits for
Pollution Control
While the elimination of chromium and zinc from anticorrosion operations is the preferred pollution prevention option, waste recycling also should be considered if complete elimination is not possible at this point. Recycling creates a product from materials that otherwise would be considered waste and may minimize the quantity of waste that requires disposal. At this time, zinc recycling is the only economically feasible option for the wastes originating from the anticorrosion operation. See the waste hierarchy for more information about waste recycling, treatment, and control technologies.
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