THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Atmospheric corrosion of Mg and MgAl alloys–characterization and mechanisms

The atmospheric corrosion of Mg and MgAl alloys was investigated. Corrosion tests were performed in the laboratory under controlled environment. CP Mg, AM50 and AZ91 samples were exposed at 95% RH and 22°C in the presence and in the absence of 400 ppm CO2 for 24 h to 672 h. The passive film was characterized by means of XPS, FTIR, AES and XRD. The film consisted of MgO/Mg(OH)2 with carbonate on top in the presence of CO2. In addition, Al was present in the film formed on the alloys. The thickening of the surface film was described in terms of a hydration mechanism. Also, a dissolution-precipitation mechanism was proposed for the break-down of the passive film in humid air. FIB cross-sectioning revealed that, in the presence of CO2, dissolution of the passive film increased compared to CO2-free exposures. The NaCl–induced atmospheric corrosion of Mg and MgAl alloys was studied in the same environment. Brucite (Mg(OH)2) was the main corrosion product in the absence of CO2. In the case of the alloys, meixnerite (Mg6Al2(OH)18·4.5H2O) was also detected. Magnesium hydroxy carbonates were the dominating corrosion products in the presence of CO2. All tested materials exhibited higher corrosion rates in the absence of CO2 compared to exposures with CO2. The electrochemical corrosion cells were more extensive in the absence of CO2 compared to when 400 ppm CO2 was present. EDX analysis revealed that η-phase particles were more efficient cathodes compared to the β-phase and the interdendritic areas. The FIB and BIB techniques combined with SEM-EDX revealed that Cl accumulated at the bottom of the pits. The localized corrosion in the presence of NaCl (aq) was investigated in 2D and 3D. Plan view characterization was performed by means of SE and BSE imaging in SEM. Interference microscopy was employed to study the distribution of the pits after corrosion product removal. SEM-3D imaging of pitted regions was performed using a FIB-SEM system. It was shown that the β-phase acted as a barrier against corrosion, especially in the case of alloy AZ91. In the absence of CO2, the corrosion pits tended to interconnect below the metal surface, forming severely pitted regions. In the presence of CO2, the pits were isolated and shallower. The beneficial effect of CO2 on corrosion is attributed to its acidity that caused neutralization of the catholyte.