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Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D
Corrosion and Metals Research Institute (KIMAB), Stockholm.
Corrosion and Metals Research Institute (KIMAB), Stockholm.
Institut de la Corrosion, Brest.
2007 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 49, no 3, 1540-1558 p.Article in journal (Refereed) Published
Abstract [en]

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.
Keyword [en]
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
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-7682DOI: 10.1016/j.corsci.2006.08.004ISI: 000245420400038OAI: oai:DiVA.org:kth-7682DiVA: diva2:12781
Note
QC 20100802Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2017-12-14Bibliographically approved
In thesis
1. The Atmospheric Corrosion of Magnesium Alloys: Influence of Microstructure and Environments
Open this publication in new window or tab >>The Atmospheric Corrosion of Magnesium Alloys: Influence of Microstructure and Environments
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The low density and high specific strength of magnesium alloys have created a great deal of interest in the use of these alloys in the automotive and aerospace industries and in portable electronics. All of these industries deal with applications in which weight is extremely important. However, an obstacle to overcome when using magnesium alloys in engineering applications are their unsatisfactory corrosion properties. This thesis is devoted to the atmospheric corrosion of the two magnesium alloys AZ91D and AM50, in particular the ways the microstructure and exposure parameters of these alloys influence their corrosion behaviour. The work includes both laboratory and field studies. The results obtained show that the microstructure is of vital importance for the corrosion behaviour under atmospheric conditions.

The microstructure of magnesium-aluminium alloys contains different intermetallic phases, e.g. Al8Mn5 and β-Mg17Al12. The local nobility of these intermetallic phases was measured on a submicron level in an atmospheric environment. It was shown that particles of the Al-Mn type exhibit the highest Volta potential among the microstructure constituents of the AZ91D magnesium alloy. Further, it was shown that the Volta potential was highly dependent on the aluminium content of the magnesiumaluminium phases in the surface layer.

When thin electrolyte layers are present, CO2 diffuses readily to the surface forming magnesium carbonate, hydromagnesite. The CO2 lowers the pH in areas on the surface that are alkaline due to the cathodic reaction. This stabilises the aluminium-containing surface film, the result being increased corrosion protection of phases rich in aluminium. Both in the laboratory and under field conditions the corrosion attack was initiated in large α-phase grains, which is explained by the lower aluminium content in these grains. The thin electrolyte film, which is formed under atmospheric conditions, decreases the possibility of galvanic coupling of alloy constituents located at larger distances from each other. Thus the cathodic process is in most cases located in the eutectic α-/β phase close to the α-phases, instead of in intermetallic Al-Mn particles, even though the driving force for the initiation of the corrosion attack in Al-Mn particles should be high, due to their high nobility.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. 53 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2007:75
Keyword
Materials science, metallurgy
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-4545 (URN)978-91-7178-799-6 (ISBN)
Public defence
2007-12-07, F3, KTH, Lindstedtsvägen 26, Stockholm, 13:00
Opponent
Supervisors
Note

QC 20100802

Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2016-12-16Bibliographically approved

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