Carbon Dioxide: The Unknown Factor in the Atmospheric Corrosion of Light Metals. A Laboratory Study
The atmospheric corrosion of Al and Mg-Al alloys was investigated using controlled air in the laboratory. The effects of CO2, temperature, NaCl, relative humidity, SO2, NO2 and O3 on corrosion rate and corrosion product composition were examined. Initial stages of corrosion were studied in situ using Scanning Kelvin Probe Force Microscopy (SKPFM), which monitors the Volta potential and the topography at sub-micrometer resolution. The corrosion products were analyzed by techniques including IC, GI-XRD, ESEM/EDX, SKP, FIB/SEM and AES.
The main finding is that ambient concentrations of CO2 (350ppm) inhibit the NaCl-induced atmospheric corrosion of Al and Mg-Al alloys. In the absence of CO2, high pH regions form in the surface electrolyte due to the cathodic reduction of oxygen. The high pH electrolyte dissolves the alumina-containing passive film, resulting in rapid corrosion. When CO2 is present, it forms carbonic acid that neutralizes the hydroxide formed in the cathodic vicinity, which slows the corrosion. On aluminum-based alloys NaCl produces general corrosion in humid air without CO2, while pitting is observed at ambient CO2 levels. The temperature-dependence of the NaCl induced atmospheric corrosion of aluminum alloys is strongly non-linear. With increasing temperature, the inhibitive effect of CO2 declines due to the decreasing solubility of CO2 in water. On Mg-Al alloys, corrosion attack is localized in the absence of CO2, while general corrosion prevails in its presence. The inhibitive effect of CO2 is relevant to occluded areas, where the supply of CO2 is limited, for example in crevice corrosion and corrosion beneath organic coatings.
In humid air, SO2 accelerates the atmospheric corrosion of Al and Mg-Al alloy. The surface electrolyte is acidified by sulfuric acid formation, destabilizing the passive film. However, for aluminum alloys, SO2 inhibits the NaCl-induced pitting corrosion. The precipitation of aluminum hydroxy sulfates was believed to retard pit propagation. The deposition rate of SO2 on Al is slow and almost independent of the SO2 concentration. In contrast, SO2 deposition on Mg-Al alloys is transport limited at high humidity. In the presence of SO2, Mg-Al alloys suffer pitting corrosion that preferentially attacks the a phase.