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Project Status: complete

Title: Effects of temperature, humidity, nitrogen dioxide and nitric acid gases on carbon-steel, galvanized, and painted steel

Principal Investigator / Author(s): Whitbeck, M. R.

Contractor: Desert Research Institute, University of Nevada

Contract Number: A4-109-32


Topic Areas: Acid Deposition


Abstract:

The objective of this study is to investigate the potential effect of nitrogen dioxide and nitric acid on carbon-steel, galvanized and painted steel under initial exposure conditions. This study attempts to measure initial rates of corrosion at realistically short times using techniques less ambiguous than weight loss/gain methods employed with longer exposure times. Surface electrode microcells were developed to directly show the effects of humidity and pollutant nitrogen dioxide and nitric acid gases on metals without requiring wetted surfaces. Corrosion is an electrochemical phenomenon and these miniature electrochemical cells measure corrosion rate more directly and more sensitively than can be done with bulk weight change. The technique used in this study is called the galvanostatic polarization method and has been used to study the corrosion of wetted metal surfaces, but this is the first application to metal corrosion by gases. Fourier transform infrared spectroscopy was used to identify surface corrosion products. Coated galvanized steel sheet was selected for study because it is commonly used for exterior siding in residential and commercial buildings throughout the United States. Bare steel samples were produced by pickling galvanized coupons in concentrated nitric acid followed by surface abrasion. Nitrogen dioxide is only a mild corrodant of steel and zinc under the conditions studied. The initial rate of surface degradation is linear in nitrogen dioxide concentration. At the earliest exposure times nitrogen dioxide may weakly passivate (inhibit) corrosion by water vapor. The corrosion products included the metal oxides and nitrate ion. Exposure of the zinc surface to nitrogen dioxide also produced ammonium ion; consistent with the strong reducing power of zinc. Nitrogen dioxide caused no perceptible damage to the polyester/epoxy painted surfaces under the initial exposure conditions of this study. Nitric acid gas rapidly attacked both steel and zinc surfaces producing nitrate ions, nitrite ions, and for the zinc surface ammonium ions were also formed. There was slight degradation of the polyester/epoxy paint surface by nitric acid producing nitrate ester. Infrared analysis of the surface corrosion products illustrates the nature of the surface alterations. For the pollutants used in this study, ammonium salts were formed which exhibit hygroscopic properties. Hygroscopic salts, which take up water vapor from air, increase the time of wetness of exposed metal surfaces. The mild damage to the polyester paint by nitric acid may soften the coating lowering its scratch resistance. This, in turn, would promote undercutting by corrosion. It is recommended that similar studies to these be undertaken using a matrix of humidity, nitrogen dioxide, and sulfur dioxide exposure conditions. The objectives of such studies should be: 1) to determine the nature and extent of the synergism between nitrogen dioxide and sulfur dioxide; 2) to establish the conditions for extending the use of the surface microcells to field studies: 3) to determine the effects of alternate wetting and 4) to determine


 

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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