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The Atmospheric Corrosion of Magnesium Alloys: Influence of Microstructure and Environments
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
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 [en]
Materials science, metallurgy
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-4545ISBN: 978-91-7178-799-6 (print)OAI: oai:DiVA.org:kth-4545DiVA: diva2:12786
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
List of papers
1. Atmospheric Corrosion of Magnesium Alloys: Influence of Temperature, Relative Humidity, and Chloride Deposition
Open this publication in new window or tab >>Atmospheric Corrosion of Magnesium Alloys: Influence of Temperature, Relative Humidity, and Chloride Deposition
2004 (English)In: Corrosion, ISSN 0010-9312, E-ISSN 1938-159X, Vol. 60, no 4, 356-362 p.Article in journal (Refereed) Published
Abstract [en]

Laboratory tests have been performed to determine how climatic parameters, e.g., relative humidity, temperature, and the amount of sodium chloride (NaCl), influence the corrosion rate of magnesium alloys AZ91D (UNS M11916) and AM50 (UNS M10500). The effect of the surface state also has been investigated. The exposures were performed at 75%, 85016, and 95% relative humidity (RH) and at 25degreesC and 35degreesC. The amount of NaCl ranged from 14 mug cm(-2) to 240 mug cm(-2). The corrosion rate of both materials increased as a function of temperature, RH, and amount of NaCl. A strong influence of the surface state, i.e., as-cast or polished, was observed mainly due to the combined effect of an active surface layer and the roughness of as-cast surfaces.

Keyword
Atmospheric corrosion; Automobiles; Chlorine; Climatic parameters; Magnesium alloys; Relative humidity
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-7681 (URN)000220794900004 ()
Note
QC 20100802Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2017-12-14Bibliographically approved
2. Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D
Open this publication in new window or tab >>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
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.

Keyword
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:nbn:se:kth:diva-7682 (URN)10.1016/j.corsci.2006.08.004 (DOI)000245420400038 ()
Note
QC 20100802Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2017-12-14Bibliographically approved
3. Atmospheric corrosion of field exposed magnesium alloy AZ91D
Open this publication in new window or tab >>Atmospheric corrosion of field exposed magnesium alloy AZ91D
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.

Keyword
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:nbn:se:kth:diva-7683 (URN)10.1016/j.corsci.2007.12.005 (DOI)000256184800024 ()2-s2.0-42049102024 (Scopus ID)
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
4. The influence of microstructure on the corrosion behaviour of AZ91D studied by scanning Kelvin probe force microscopy and scanning Kelvin probe
Open this publication in new window or tab >>The influence of microstructure on the corrosion behaviour of AZ91D studied by scanning Kelvin probe force microscopy and scanning Kelvin probe
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.

Keyword
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:nbn:se:kth:diva-7684 (URN)10.1016/j.corsci.2005.05.008 (DOI)000237624500011 ()
Note
QC 20100802Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2017-12-14Bibliographically approved
5. The Initial Steps of Atmospheric Corrosion on Magnesium Alloy AZ91D
Open this publication in new window or tab >>The Initial Steps of Atmospheric Corrosion on Magnesium Alloy AZ91D
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.

Keyword
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:nbn:se:kth:diva-7685 (URN)10.1149/1.2779957 (DOI)000249787900042 ()
Note
QC 20100802Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2017-12-14Bibliographically approved
6. A comparison between the corrosion properties of magnesium alloy AZ91D and AM50 from a microstructural perspective
Open this publication in new window or tab >>A comparison between the corrosion properties of magnesium alloy AZ91D and AM50 from a microstructural perspective
(English)Manuscript (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7686 (URN)
Note
QC 20100802Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2010-08-02Bibliographically approved

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Citation style
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  • harvard1
  • ieee
  • modern-language-association-8th-edition
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