Etch Damage to Automotive Coatings

By K. Henderson, R. Hunt, K. Spitler, J. Boisseau

When automobile manufacturers began using clear coat systems on vehicles in the 1980s, both dealerships and buyers complained about the occurrence of ring, or “water spot,” etches on the horizontal surfaces of vehicles. This type of paint damage was found predominately in locations with certain environmental conditions; specifically, locations with more humidity, acid rain, and higher temperatures.

Normal rain is slightly acidic because of dissolved carbon dioxide; however, acid rain is defined differently and is known to cause more severe damage to coatings. Acid rain (acid deposition) occurs in highly industrialized region of the United States and results when sulfur dioxide (SO2) and nitrogen oxide (NOx) gases react m the atmosphere with water and oxygen to form mild solutions of sulfuric and nitric acid. An additional contribution in the South arises from the fact that both sunlight and high temperatures increase the rate of most of these reactions.

Both sulfur dioxide and oxides of nitrogen are by-products of transportation and industrial processes such as those used in utility plants and manufacturing. Acid deposition occurs in three forms: (1) wet deposition ( dew, rain, snow, sleet, and hail); ( 2) dry deposition (particles and gases); and (3) vapor deposition (clouds or fog occurring in coastal areas).