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The influence of microstructure on the corrosion behaviour of AZ91D studied by scanning Kelvin probe force microscopy and scanning Kelvin probe
Swedish Corrosion Institute, Stockholm.
Institut de la Corrosion, Brest.
Institut de la Corrosion, Brest.
2006 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 48, no 5, 1193-1208 p.Article in journal (Refereed) Published
Abstract [en]

beta-Mg17Al12, eta-Al8Mn5 and an alpha-magnesium phase have been synthesized from pure components by controlled solidification procedures. These phases have been studied using different electrochemical techniques including the scanning Kelvin probe (SKP). From the results; it was possible to determine the nobility and the rate of the cathodic reaction of the different phases. Measurements have also been made on an AZ91D Mg-alloy using scanning Kelvin probe force microscopy (SKPFM) and field emission gun scanning electron microscopy (FEG-SEM). The results show that the Volta potentials measured with the scanning Kelvin probe (SKP) on bulk intermetallics are comparable with those recorded with the SKPFM on the AZ91D alloy. It is shown that SKPFM provides information on the local nobility of the different intermetallic particles and phases on the submicron scale. Both the eta-Al8Mn5 phase and the beta-Mg17Al12 phase in AZ91D showed a more noble potential than the alpha-magnesium phase. It is also shown that the aluminium-rich coring along the grain boundaries results in measurable changes in the Volta potential. Finally, the role of the beta-Mg17Al12 phase and the eta-Al8Mn5 phase in the corrosion behaviour of AZ91D is discussed in term of local nobility, surface coverage of the cathode and the cathodic activity of the different phases.

Place, publisher, year, edition, pages
2006. Vol. 48, no 5, 1193-1208 p.
Keyword [en]
AFM; Atmospheric corrosion; Magnesium; Scanning Kelvin probe; Atomic force microscopy; Cathodic protection; Corrosion resistant alloys; Electrochemical corrosion; Intermetallics; Magnesium printing plates; Microstructure; Scanning electron microscopy; Solidification; Coring; Corrosion behavior; Scanning Kelvin probe force microscopy; Scanning Kelvin probes
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-7684DOI: 10.1016/j.corsci.2005.05.008ISI: 000237624500011OAI: oai:DiVA.org:kth-7684DiVA: diva2:12783
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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