Atmospheric corrosion effects of HNO 3—Comparison of laboratory-exposed copper, zinc and carbon steel

Abstract

The influence of nitric acid (HNO 3) on the atmospheric corrosion of copper, zinc and carbon steel was investigated in laboratory exposures at 65% relative humidity (RH), 25 °C and 0.03 cm s −1 air velocity. The deposition velocity ( V d) of HNO 3 on the specimens, the corrosion rates and corrosion products were determined by gravimetry, ion chromatography, X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) microspectroscopy. Comparisons were also made with literature data on the corrosion effects of sulfur dioxide (SO 2), nitrogen dioxide (NO 2) and ozone (O 3). At 65% RH, the V d of HNO 3 on all metals was at least 70% of that of an ideal absorbent, i.e., an impregnated filter with perfect absorption for HNO 3. The V d of HNO 3 was much higher than that of SO 2, NO 2 or O 3, which is mainly attributed to the relatively high sticking coefficient, high solubility and high reactivity of HNO 3 compared to the other gases. During identical exposures to HNO 3, the corrosion rate of carbon steel was nearly three times higher than that of copper or zinc. However, when comparing the corrosion effects induced by HNO 3 with those induced by SO 2 alone or in combination with either NO 2 or O 3, HNO 3 turned out to be far more aggressive than SO 2. Relative to SO 2, zinc is the metal most sensitive to HNO 3, followed by copper and with carbon steel least sensitive to HNO 3.