A brief discussion is given of recent efforts to use electrochemical techniques for monitoring of atmospheric corrosion phenomena. Problems existing with the interpretation of time-of-wetness measurements are identified, and the finding that electrochemical sensors in their present design and application determine only fractions of the true corrosion rate is discussed. In order to resolve some of these difficulties, a statistically designed experiment is being carried out to evaluate the reproducibility of electrochemical measurements of atmospheric corrosion phenomena and to determine the effects of sensor design on time-of-wetness and cell efficiency. Results are presented for the first phase of this project, in which 15 atmospheric corrosion monitors (ACM) of the copper/steel and 15 ACM's of the steel/steel type, have been fabricated with steel and copper from three different heats. These ACM's have been tested in triplicate runs by exposure to aqueous 1-mM sodium chloride at a relative humidity of 45 percent until the surface had dried out, followed by additional exposure to either a moist air environment at three levels of relative humidity (65, 80 and 95 percent) or a sodium dioxide (SO₂) test at three levels of SO₂ ≈ 0, 0.2, and 1.1 ppm. During each test, steel plates from each heat were exposed to determine weight loss data. At present, only the drying-out data have been analyzed by statistical methods. It has been found both from electrochemical and weight loss data that the heat of the steel plays an important role, with one heat corroding at a higher rate than the other two which have equal corrosion rates. No differences were found between the five sensors of one heat. Additional factors that influence the measurement are day-to-day variations of the environment in the test chamber and to some extent the position of the ACM's in the test chamber. By comparing the electrochemical and weight loss data, a cell efficiency of about 20 percent was found for copper/steel and about 7 percent for steel/steel. This low cell factor is considered to be due mainly to local cell action on individual plates and to uncompensated ohmic drop in the electrolyte.

The copper/steel and steel/steel ACM's are being exposed on the Rockwell International Science Center roof for an aging period of three months, after which another series of laboratory tests will be conducted.