Atmospheric corrosion of Mg and MgAl alloys – characterization and mechanisms

Doctoral thesis, 2015

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. It was shown that the film consisted MgO/Mg(OH)2 with carbonate on top of the film in the presence of CO2. In addition, Al3+ was present in the film formed on the alloys. 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-section revealed that in the presence of CO2 dissolution of the metal substrate increased compared to CO2-free exposures. The NaCl–induced atmospheric corrosion of Mg and MgAl alloys was studied in the same environment. Brucite was the main corrosion product in the absence of CO2. In the case of the alloys, meixnerite was also detected. Magnesium hydroxy carbonates were the dominating corrosion products in the presence of 400 ppm 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. Using the FIB and BIB techniques combined with SEM-EDX it was shown that Cl accumulated at the bottom of the pits. The localized corrosion in the presence of NaCl (aq) and in the presence and absence of CO2 was investigated in 2D and 3D. Plan view characterization was performed by means of SE and BSE imaging in SEM. Interference microscopy was employed in order to study the distribution of the pits after corrosion product removal. SEM-3D imaging was performed using a FIB-SEM system investigating pitted regions. It was showed 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.