The ACM (Atmospheric Corrosion Monitor) sensors each of which consists of a Fe/Ag-galvanic couple were exposed to marine atmosphere for one year, for the purpose of evaluating the corrosivity of the site. The outputs of the sensors set up at Shimizu in Shizuoka Prefecture, which is regarded as the most moderate of the four exposure sites, were obtained every 10 minutes with temperature and relative humidity, RH. The sensor outputs were analyzed by distinguishing the time for the durations of dew, rain and dry periods in a day. The time durations for dew period and rain period at Shimizu were found to be 3454h and 1705h respectively throughout the year from 1992.7 to 1993.6. Critical humidity for dew, RH*, decreased exclusively from 80% to 50 % with the increase of amount of deposited sea salt, WS . It is thus suggested that the ISO recommendation method is not adequate by which the time of wetness is defined as the time when RH value is at and above 80%. The frequency data of dew period and dry+dew period against RH measured by the ACM sensor will be available to study the rusting for stainless steels. It is considered that corrosivity of each site can be estimated effectively by analyzing the above mentioned factors, especially amount of deposited sea salt, obtained by this ACM sensor.
Y he mechanism of transgranular stress corrosion cracking of pure copper in 0.05M NH4OH solution at 70C was investigated by means of stress corrosion tests both under the constant strain rates from 1.0x10-4min-1 to 5.0x10-4min-1, and under potentiostatic anodic polarization in the range of-90.0mV to-10.0mV (SCE) at a constant strain rate of 1.0x104min-2. Stress corrosion cracking can be observed only at lower strain rates than 1.0x10-4min-1 which is slower than rate of tarnish film growth. Good adherent and protective film is formed over the potential range of-90.0mV to-10.0mV. The susceptibility to stress corrosion cracking is increased by anodic polarization in the potential range of-90.0mV to-50.0mV in which thick and black tarnish film is formed, but is decreased by anodic polarization in the potential range of-30.0mV to-10.0mV in which thin and dark brown film is formed. Weight losses of specimens both on the surface produced by straining and on the surface of static specimens were measured over the potential range of-70.0 mV to-10.0mV. These weight losses decrease with the increase of applied potential, and the ratio of these weight losses is nearly constant and independent of applied potential and susceptibility to cracking. These results support the theory that crack propagates with the repetition of formation and rupture of the brittle tarnish films, and the distance of propagation equals the film thickness formed after each rupture.
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