<p>The role of nitric acid (HNO<sub>3</sub>) on the atmospheric corrosion of metals has so far received little or no attention. However, the last decades of decreasing sulphur dioxide (SO<sub>2</sub>) levels and unchanged HNO<sub>3</sub> levels in many industrialized countries have resulted in an increased interest in possible HNO<sub>3</sub>-induced atmospheric corrosion effects. In this study a new method was developed for studying the corrosion effects of HNO<sub>3</sub> on metals at well-defined laboratory exposure conditions. The method has enabled studies to be performed on the influence of individual exposure parameters, namely HNO<sub>3</sub>-concentration, air velocity, temperature and relative humidity, as well as comparisons with newly generated field exposure data.</p><p>The corrosion rate and deposition rate of HNO<sub>3</sub> on copper was shown to follow a linear increase with HNO3 concentration. The deposition velocity (Vd) of HNO<sub>3</sub> increased up to an air velocity of 11.8 cm s<sup>-1</sup>. Only at a higher air velocity (35.4 cm s<sup>-1</sup>) the Vd on copper was lower than the Vd on an ideal absorbent, implying the Vd of HNO3 at lower air velocities to be mass-transport limited.</p><p>Within the investigated temperature range of 15 to 35 ºC only a minor decrease in the HNO<sub>3</sub>-induced copper corrosion rate could be observed. The effect of relative humidity (RH) was more evident. Already at 20 % RH a significant corrosion rate could be measured and at 65 % RH the Vd of HNO<sub>3</sub> on copper, zinc and carbon steel reached maximum and nearly ideal absorption conditions.</p><p>During identical exposure conditions in HNO<sub>3-</sub>containing atmosphere, the corrosion rate of carbon steel was nearly three times higher than that of copper and zinc. The HNO<sub>3</sub>-induced corrosion effect of copper, zinc and steel turned out to be significantly higher than that induced by SO2 alone or in combination with either NO<sub>2</sub> or O<sub>3</sub>. This is mainly attributed to the much higher water solubility and reactivity of HNO3 compared to SO<sub>2</sub>, NO<sub>2</sub> and O<sub>3.</sub> Relative to SO<sub>2</sub>, zinc exhibits the highest sensitivity to HNO<sub>3</sub>, followed by copper, and carbon steel with the lowest sensitivity.</p><p>Extrapolation of laboratory data to an assumed average outdoor wind velocity of 3.6 m s-1 enabled a good comparison with field data. Despite the fact that ambient SO<sub>2</sub> levels are still much higher than HNO<sub>3</sub> levels, the results show that HNO<sub>3</sub> plays a significant role for the atmospheric corrosion of copper and zinc, but not for carbon steel. The results generated within this doctoral study emphasize the importance of further research on the influence of HNO<sub>3</sub> on degradation of other materials, e.g. stone and glass, as well as of other metals. </p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:kth-4194 |
| Date | January 2006 |
| Creators | Samie, Farid |
| Publisher | KTH, Chemistry, Stockholm : Kemi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
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