Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D
2007 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 49, no 3, 1540-1558 p.Article in journal (Refereed) Published
Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 mu g/cm-2 NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg-5 (CO3)(4) (OH)(2)4H(2)O) and nesquehonite (MgCO3 3H(2)O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH2)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO2 and O-2 to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na+ and Cl- ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.
Place, publisher, year, edition, pages
2007. Vol. 49, no 3, 1540-1558 p.
Magnesium; IR spectroscopy; Atmospheric corrosion; Atmospheric corrosion; Atmospheric humidity; Electrolytes; Infrared spectroscopy; Sodium chloride; X ray diffraction analysis; Corrosion product formation; Corrosion sites; Exposure times; Magnesium alloys; Atmospheric corrosion; Atmospheric humidity; Electrolytes; Infrared spectroscopy; Magnesium alloys; Sodium chloride; X ray diffraction analysis
Metallurgy and Metallic Materials
IdentifiersURN: urn:nbn:se:kth:diva-7682DOI: 10.1016/j.corsci.2006.08.004ISI: 000245420400038OAI: oai:DiVA.org:kth-7682DiVA: diva2:12781
QC 201008022007-11-212007-11-212010-08-02Bibliographically approved