Atmospheric corrosion effects of HNO3—method development and results on laboratory-exposed copper

The effects of the atmospheric pollutant nitric acid (HNO3) on materials compared to other corrosive gases, e.g. sulphur dioxide (SO2), nitrogen dioxide (NO2) or ozone (O3), have so far received little or no attention. The high sticking coefficient of HNO3 makes this gas one of the most difficult gases to work with. A new apparatus has now successfully been developed for studying the atmospheric corrosion effects of HNO3 on materials. HNO3 concentration measurements up to 1080 μg m−3 (420 ppb) were performed by dissolving the gas in water and analysing the nitrate concentration with ion chromatography (IC). Small changes in relative humidity (RH) largely affect the concentration of this pollutant in the exposure chamber and the high sticking coefficient of this gas on copper and quartz glass has been shown. The quartz glass surface, however, became saturated after a certain time of exposure and at 82% RH, the number of monolayers on the surface was estimated to be 10–13. Initial results of copper samples exposed to HNO3 show that at 63% RH and 25 °C, the deposition of HNO3 on copper is slightly lower than on a perfect absorber. The loss of HNO3 during exposure of the samples showed good agreement with the amount of nitrates dissolved from surfaces of the samples after exposure. FT–IR, XRD and IC analyses of copper exposed to HNO3 and mass loss and mass gain analyses confirmed cuprite (Cu2O) and the basic copper nitrate, gerhardtite, as the main corrosion products. Deposition, as well as the corrosion effect, of HNO3 on copper appeared to be greater than that of any of the other above-mentioned pollutants.