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The Initial Steps of Atmospheric Corrosion on Magnesium Alloy AZ91D
Corrosion and Metals Research Institute (KIMAB), Stockholm.
Corrosion and Metals Research Institute (KIMAB), Stockholm.
Corrosion and Metals Research Institute (KIMAB), Stockholm.
2007 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 154, no 11, C684-C691 p.Article in journal (Refereed) Published
Abstract [en]

Magnesium, with its low density, is a very interesting metal in applications in which weight is important. In this work the initiation of corrosion attacks on magnesium alloy AZ91D has been investigated under atmospheric conditions, using confocal laser scanning microscopy and SKPFM. The corrosion attack starts in the alpha phase in larger grains and is under atmospheric conditions initiated at the boundary between the alpha phase and the eutectic alpha-/beta phase. The SKPFM measurement shows that a high Volta potential difference exists between the larger grains and the surrounding eutectic alpha-/beta phase and the beta phase. A microgalvanic element is formed in the thin adlayer on the surface, with anodic Mg dissolution in the alpha phase and the cathodic reaction primarily in the eutectic alpha-/beta phase. The initiation in the larger grains could be explained by the smaller aluminum content in the larger grains due to the solidification process. Even though intermetallic Al-Mn particles show a high Volta potential difference with the connecting alpha phase, they are not involved in the initiation of the corrosion. This is explained by the position within the microstructure of Al-Mn particles embedded in the beta phase and located away from the alpha phase.

Place, publisher, year, edition, pages
2007. Vol. 154, no 11, C684-C691 p.
Keyword [en]
Atmospheric corrosion; Confocal microscopy; Corrosion prevention; Grain size and shape; Reaction kinetics; Solidification; Cathodic reactions; Confocal laser scanning microscopy; Microgalvanic elements
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-7685DOI: 10.1149/1.2779957ISI: 000249787900042OAI: oai:DiVA.org:kth-7685DiVA: diva2:12784
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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