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Atmospheric corrosion of field exposed magnesium alloy AZ91D
Corrosion and Metals Research Institute (KIMAB), Stockholm.
Corrosion and Metals Research Institute (KIMAB), Stockholm.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Corrosion Science.ORCID iD: 0000-0002-9453-1333
2008 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 50, no 5, 1406-1413 p.Article in journal (Refereed) Published
Abstract [en]

The magnesium alloy AZ91D was exposed in three different types of atmospheric environment, viz. urban, rural and marine exposure sites. Corrosion rates, corrosion products formed, and the influence of the microstructure on the corrosion behaviour of the alloy were investigated. The corrosion rate of AZ91D exposed in the marine environment was 4.2 mu m/year, and in the rural and urban environments 2.2 and 1.8 mu m/year, respectively. The main corrosion product found was magnesium carbonate hydromagnesite (Mg-5(CO3)(4)(OH)(2)center dot 4H(2)O), which was formed at all three exposure sites. The corrosion attack started in the alpha-phase in larger grains at the boundary between the alpha-phase and the eutectic alpha-/beta-phase. Microgalvanic elements were formed with the eutectic alpha-/beta-Mg phase as cathodic site and the alpha-Mg grains as anodes. The Al-Mn particles played a minor roll in the initiation process, even though these particles are the most noble in the microstructure and thus the driving force for a corrosion attack around these particles could be expected to be high. A close resemblance was observed between the corrosion mechanisms operating under the field-exposure conditions described here and the mechanisms operating under the previously reported laboratory conditions.

Place, publisher, year, edition, pages
2008. Vol. 50, no 5, 1406-1413 p.
Keyword [en]
Magnesium alloys; Atmospheric corrosion; Corrosion product; Corrosion rate; Field-exposure; Anodes; Atmospheric corrosion; Carbonates; Eutectics; Corrosion product; Field-exposure
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-7683DOI: 10.1016/j.corsci.2007.12.005ISI: 000256184800024Scopus ID: 2-s2.0-42049102024OAI: oai:DiVA.org:kth-7683DiVA: diva2:12782
Note
QC 20100802. Uppdaterad från manuskript till artikel i tidskrift 20100802.Available 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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