Abstract
Automotive conversion coatings consist of layers of materials that are chemically applied to the body structures of vehicles before painting to improve corrosion protection and paint adhesion. These coatings are a consequence of surface-based chemical reactions and are sandwiched between paint layers and the base metal; the chemical reactions involved distinctly classify conversion coatings from other coating technologies. Although the tri-cationic conversion coating bath chemistry that was developed around the end of the 20th century remains persistent, environmental, health, and cost issues favor a new generation of greener methods and materials such as zirconium. Environmental forces driving lightweight material selection during automobile body design are possibly more influential for transitioning to zirconium than the concerns regarding the body coating process. The chemistry involved in some conversion coatings processing has been known for over 100 years. However, recent advances in chemical processing, changes in the components used for vehicle body structures, environmental considerations and costs have prompted the automobile industry to embrace new conversion coatings technologies. These are discussed herein along with a historical perspective that has led to the use of current conversion coatings technologies. In addition, future directions for automobile body conversion coatings are discussed that may affect conversion coatings in the age of multi-material body structures.
Highlights
In a 1911 US Patent [1], Thomas Coslett described the use of six ounces of zinc, a pint of water, and a pint of phosphoric acid as ingredients for making a conversion coating concentrate
Recent advances in chemical processing, changes in the components used for vehicle body structures, environmental considerations and costs have prompted the automobile industry to embrace new conversion coatings technologies
Its efficacy was demonstrated when iron objects boiled in a water solution made from this concentrate displayed a significant reduction in corrosion as a consequence of a protective zinc phosphate coating that had been formed on the surface of the iron
Summary
In a 1911 US Patent [1], Thomas Coslett described the use of six ounces of zinc, a pint of water, and a pint of phosphoric acid as ingredients for making a conversion coating concentrate. Other organic coatings, have improved adhesion characteristics because of a roughened surface that is created by the conversion coatings; in other words, a conversion coating functions to improve mechanical profile for paint. Themetal canned and beverage industriescoatings have exerted significant influence on the development container pasteurization andfor theconversion adhesion coatings of decorative organic coatings. 21st century, of new metal ion chemistries that have improved coating appearance after environmental concerns weighed in on chromates, heavy organic metal sludge, phosphates, andcentury, energy container pasteurization and the adhesion of decorative coatings. The long-term combination of these forces have nudged conversion environmental concerns weighed in on chromates, heavy metal sludge, phosphates, and coating energy technologies to a trend noted byhave. To Zr-based materials, a trend noted by David Chalk of Dubois Chemical as the “shrinking periodic table” [2]
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