Study On The Effects Of SO₂ And NaCl On Metals Corrosion In The Simulated Atmospheres

Electrochemical corrosion occurred on the surface of metals covered with a thin electrolyte layer is the essential of atmospheric corrosion. Electrochemical method is an effective way to study atmospheric corrosion. There are very few electrochemical studies on atmospheric corrosion because measurement under the thin electrolyte layer is technically difficult. Errors in the electrochemical measurement of corrosion rates under thin electrolyte layers may arise not only from ohmic drop between the reference and working electrodes, but also because of uneven current distribution over the working electrodes. Recently,these problems have been solved to a certain degree by using the electrochemical impedance spectroscopy(EIS). In this paper, the effects of SO₂ and Cl^- on atmospheric corrosion of copper and Q235 steel were studied by using EIS technique, simultaneously corrosion morphologies and corrosive products were analyzed by SEM/EDS and XRD.Solution resistance (Rs) is associated with relative humidity (RH), temperature, gaseous pollutants and particles. The wetness of the electrode surface was monitored from the Rs. Rs^-1 is regarded as both indices of wetness of the metals surface and the amount of deposited salts. The reciprocal of the polarization resistance (Rp^-1) was normalized to the area of one electrode and used as an index of the corrosion rate. Therefore, Rs^-1 and Rp^-1 were the important parameters in this paper.Changing the velocity of the airflow to gain different atmospheric environments with different concentration of SO₂ (0,150ppb, 1ppm). The corrosion rates of copper and Q235 steel were studied in these environments that simulated atmospheres, room temperature, stable RH. The results show that SO₂ accelerated the corrosion rates of copper and Q235 steel when other environmental factors were stable. Because of the difference of chemical and physical characters between copper and Q235 steel, the effects of SO₂ on copper atmospheric corrosion were different from that of Q235 steel.Cl^- is one of the most important factors that affect atmospheric corrosion. In the atmosphere with 85%RH and stable concentration of SO₂, the processes of wet and dry deposition of NaCl were simulated to research the influence the atmospheric corrosion of copper and Q235 steel deposited NaCl. The amounts of wet deposition were 3.75μg/cm² and 438μg/cm² and that of dry deposition was a NaCl particle which was 0.3mm×0.3mm. The corrosion rates of Copper and Q235 steel were increased with the increasing of the concentration of Cl^-. The influences of NaCl on metal corrosion morphologies were very serious. The surface of metal was destroyed completely. The dry deposition of NaCl was different from the process of wet deposition. In the case of dry deposition, the surfaces of copper and Q235 steel were not symmetrical. It was difficult to form compact and symmetrical rust layer, and the phenomena of localized corrosion was obvious which correspond to the circular area of corrosion (CAC). The CAC of copper was bigger than that of Q235 steel. The borderline of the CAC of copper was not evident, but that of Q235 steel was opposite.