| Project Team | Institution | Role | ||
| Professor Andrej Atrens | The University of Queensland | Associate Investigator | ||
| Dr Nick Birbilis | Monash University | Program Leader | ||
| Mr Ming Liu | The University of Queensland | Postgraduate Student | ||
| Dr Ross Marceau | The University of Sydney | Research Fellow | ||
| Dr Kevin Ralston | Monash University | Research Fellow | ||
| Mr Aaron Südholz | Monash University | Postgraduate Student | ||
| Dr Zhiming Zhi | Monash University | Research Fellow | ||
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Executive Summary
Project B2 has two distinct themes. The first is the development of corrosion resistant light alloys, namely Mg and Al that suffer from corrosion in most aqueous environments and atmospheres.
With regards to corrosion resistant Mg alloy development, research has been conducted into alloying Mg with a range of binary, ternary and quaternary additions in order to explore composition space and in order to test hypotheses that will make the next generation of such alloys more damage tolerant. Such work has also included collaboration with the CAST-CRC and international exchange both to and from Monash. Additionally, research has continued into the study of Mg alloys for bio-resorbable implant applications. This is one of those opportunities to harness the utility of Mg alloys - and their subsequent corrosion prone characteristics - for beneficial purposes. The formation of small coherent atomic solute clusters within the matrix does not appear to cause the model alloy under investigation (Al-1.1Cu-1.7Mg (at%)) to become susceptible to pitting. Additionally, precipitates with minimum dimension less than approximately 3nm or smaller were also observed to have similar pitting resistances as an as-quenched solid solution of Al-1.1Cu-1.7Mg (at%), both of which were observed to have excellent pitting resistance.
Work upon the durability of Mg alloys used in transport continues with an extensive experimental program in motion to provide better understanding of the corrosion mechanism under interrupted salt spray conditions. This understanding may push the environmental envelope for the effective use of unprotected use of Mg alloys in auto service.
Work in this project builds on prior XPS studies and involved collaboration with Swiss Federal Laboratories for Materials Testing and Research (EMPA) and the Ecole Nationale Supérieure de Chimie de Paris (ENSCP), with the utility of ToF-SIMS in the study of surface products upon Mg.
Project B2 has two distinct themes. The first is the development of corrosion resistant light alloys, namely Mg and Al that suffer from corrosion in most aqueous environments and atmospheres.
With regards to corrosion resistant Mg alloy development, research has been conducted into alloying Mg with a range of binary, ternary and quaternary additions in order to explore composition space and in order to test hypotheses that will make the next generation of such alloys more damage tolerant. Such work has also included collaboration with the CAST-CRC and international exchange both to and from Monash. Additionally, research has continued into the study of Mg alloys for bio-resorbable implant applications. This is one of those opportunities to harness the utility of Mg alloys - and their subsequent corrosion prone characteristics - for beneficial purposes. The formation of small coherent atomic solute clusters within the matrix does not appear to cause the model alloy under investigation (Al-1.1Cu-1.7Mg (at%)) to become susceptible to pitting. Additionally, precipitates with minimum dimension less than approximately 3nm or smaller were also observed to have similar pitting resistances as an as-quenched solid solution of Al-1.1Cu-1.7Mg (at%), both of which were observed to have excellent pitting resistance.
Work upon the durability of Mg alloys used in transport continues with an extensive experimental program in motion to provide better understanding of the corrosion mechanism under interrupted salt spray conditions. This understanding may push the environmental envelope for the effective use of unprotected use of Mg alloys in auto service.
Work in this project builds on prior XPS studies and involved collaboration with Swiss Federal Laboratories for Materials Testing and Research (EMPA) and the Ecole Nationale Supérieure de Chimie de Paris (ENSCP), with the utility of ToF-SIMS in the study of surface products upon Mg.
Project Aims/Targets
Theme A: Design and development of corrosion resistant light alloys
Explore and develop new alloy systems based on Mg and Al that exhibit superior corrosion resistance. This will include studying and optimizing the microstructure-corrosion relationship and elucidating the factors that dictate localized corrosion initiation for (Mg and Al based) light alloys; allowing for rational alloy design to follow.
Theme A: Design and development of corrosion resistant light alloys
Explore and develop new alloy systems based on Mg and Al that exhibit superior corrosion resistance. This will include studying and optimizing the microstructure-corrosion relationship and elucidating the factors that dictate localized corrosion initiation for (Mg and Al based) light alloys; allowing for rational alloy design to follow.
Theme B: Automotive Mg alloys
Address the lack of the basic scientific understanding of the factors controlling Mg corrosion in intermittent salt spray as applicable to auto applications and to develop a methodology of predicting galvanic corrosion applicable to auto service of Mg coupled to steel.
Address the lack of the basic scientific understanding of the factors controlling Mg corrosion in intermittent salt spray as applicable to auto applications and to develop a methodology of predicting galvanic corrosion applicable to auto service of Mg coupled to steel.
Design of Mg alloys which exhibit lower rates of corrosion
Testing of custom produced Mg alloys (in collaboration with CAST-CRC) was carried out by High Pressure Die Casting. Corrosion testing of such alloys built on the results of 2008, by expanding into alloying with ternary and quaternary combinations. Figure B15 reveals the summary of results that shows the impact of RE alloying additions is not straightforward in that a monotonic increase in corrosion rate is not seen with increasing RE additions (which is expected for elementary systems and additions). Instead, it is evident that relatively low corrosion rate values can be maintained in the instances where total RE loading is made by small additions of ternary or quarternary combinations of alloying elements. The scientific reasons relating to the electrochemical activity of the second phases responsible for this phenomenon have been isolated, and such technologies are being exploited in the development of future alloys.
Testing of custom produced Mg alloys (in collaboration with CAST-CRC) was carried out by High Pressure Die Casting. Corrosion testing of such alloys built on the results of 2008, by expanding into alloying with ternary and quaternary combinations. Figure B15 reveals the summary of results that shows the impact of RE alloying additions is not straightforward in that a monotonic increase in corrosion rate is not seen with increasing RE additions (which is expected for elementary systems and additions). Instead, it is evident that relatively low corrosion rate values can be maintained in the instances where total RE loading is made by small additions of ternary or quarternary combinations of alloying elements. The scientific reasons relating to the electrochemical activity of the second phases responsible for this phenomenon have been isolated, and such technologies are being exploited in the development of future alloys.
Design of Al alloys with superior corrosion resistance
This project has adopted a bottom-up approach in an attempt to quantify the critical microstructural feature size (viz. precipitate size) that is capable of triggering a cascade of pitting events and eventual degradation of corrosion resistance. This was accomplished via exploiting the well characterised hardening response in a model alloy, Al-1.1Cu-1.7Mg (at%), for which pitting resistance of the alloy was tracked with ageing time and hence microstructural evolution. Corrosion performance and microstructural characterisation were carried out using combination of electrochemical testing, coupled with high resolution scanning transmission electron microscopy (HRSTEM) and atom probe tomography (APT). Results indicate, at least for this particular alloy, that second-phase features below a critical size of approximately 3nm can be tolerated from a corrosion perspective. This study has potentially wide consequences in the understanding of aluminium alloy corrosion initiation and the development of aluminium alloys for corrosion resistance.
Figure B17 reveals the massive (3 orders of magnitude) increase in pitting rate following a critical aging time being reached.
This project has adopted a bottom-up approach in an attempt to quantify the critical microstructural feature size (viz. precipitate size) that is capable of triggering a cascade of pitting events and eventual degradation of corrosion resistance. This was accomplished via exploiting the well characterised hardening response in a model alloy, Al-1.1Cu-1.7Mg (at%), for which pitting resistance of the alloy was tracked with ageing time and hence microstructural evolution. Corrosion performance and microstructural characterisation were carried out using combination of electrochemical testing, coupled with high resolution scanning transmission electron microscopy (HRSTEM) and atom probe tomography (APT). Results indicate, at least for this particular alloy, that second-phase features below a critical size of approximately 3nm can be tolerated from a corrosion perspective. This study has potentially wide consequences in the understanding of aluminium alloy corrosion initiation and the development of aluminium alloys for corrosion resistance.
Figure B17 reveals the massive (3 orders of magnitude) increase in pitting rate following a critical aging time being reached.
Overall, the pitting resistance of Al-1.1Cu-1.7Mg (at%) alloy in dilute NaCl electrolytes was observed to be closely tied to the evolution of microstructure incurred from aging at elevated temperatures. Once S-phase precipitates have grown to have a minimum dimension of ~8nm or greater the microstructure was determined to be approximately 3 orders of magnitude more susceptible to pitting. This brings notion that there likely exists a critical maximum size for precipitates in Al-Cu-Mg alloys below which second phase precipitates cease to act as distinct electrochemical entities. It is speculated that the matrix oxide could be able to bridge “small” precipitates, whilst “large” precipitates pose too large an obstacle leading to a breakdown in passive film stability. The ability to define a critical feature sizes in the context of corrosion of Al alloys is important in emerging damage accumulation models and in the 'bottom up' design of Al alloys.
Theme B: Factors controlling Mg corrosion
An exploratory study of the corrosion of Mg alloys during interrupted salt spray testing was carried out in collaboration with the Swiss Federal Laboratories for Materials Testing and Research (EMPA). A first systematic investigation was carried out to understand the corrosion of common Mg alloys (pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt% NaCl by measuring hydrogen evolution (Figure B19) and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3wt% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)2 surface film on Mg alloys is probably formed by a precipitation reaction when the Mg2+ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120 – X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle.
An exploratory study of the corrosion of Mg alloys during interrupted salt spray testing was carried out in collaboration with the Swiss Federal Laboratories for Materials Testing and Research (EMPA). A first systematic investigation was carried out to understand the corrosion of common Mg alloys (pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt% NaCl by measuring hydrogen evolution (Figure B19) and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3wt% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)2 surface film on Mg alloys is probably formed by a precipitation reaction when the Mg2+ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120 – X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle.
The ToF-SIMS technique has been utilized to assess surface chemistry of Mg specimens. Figure B20 shows a presents a typical ToF-SIMS negative ion mass spectrum for polished pure Mg after 2 min immersion in ultra pure water.
Figure B21 (p. 84) presents a typical ToF-SIMS negative ion depth profile for polished pure Mg after 2 min immersion in ultra pure water. The distribution of the ionized fragments, all measured simultaneously, are plotted versus Cs+ ion sputtering time.
~260 s etching corresponds to a film thickness of ~25 nm based on the etch rate estimated from the sputter yield. At a sputtering time of ~5 s, the intensity of the 24MgOOH- signal reached a maximum whereas the 24MgOO- signal continued to increase. Thereafter, the 24MgOOH- signal decreased continually until the metal/oxide interface whereas the 24MgOO- signal continued to increase and showed a flat peak around a sputtering time of 200 s, just before the interface. That indicated that the surface layer had relatively more hydroxide in the outer layer and more oxide in the inner layer, although, the ToF-SIMS profile shown in Figure B17 cannot be neatly divided into two surface layers corresponding to the two indicated by our prior XPS works.
Figure B17 also indicates that the 24MgHHH- signal had a significant maximum at a sputtering time of 100 s and thereafter decreased with increasing etching time. The 25MgHHH- signal had the same shape with a relatively lower intensity. The intensity ratio 25MgHHH-/24MgHHH- corresponds closely to 0.125, the ratio of isotopic abundance 25Mg/24Mg. These signals were quite weak or nearly zero for the first 50 s sputtering and after the profile went into the substrate, so they cannot be related to the presence of MgO, Mg(OH)2 or metallic Mg. Thus, the 24MgHHH- and 25MgHHH- signals indicate that MgH2 exists inside the surface layer on pure Mg after immersion in ultra pure water. The MgH2 appears to be some distance from metallic Mg. The distribution of MgH2 in the surface film on pure Mg measured in this work was similar to that measured in the prior work on A13Mg2.
These data provide compelling evidence for the existence of MgH2 on the corroded surface of magnesium and that MgH2 can form during the exposure of Mg to pure water at the open circuit potential.
~260 s etching corresponds to a film thickness of ~25 nm based on the etch rate estimated from the sputter yield. At a sputtering time of ~5 s, the intensity of the 24MgOOH- signal reached a maximum whereas the 24MgOO- signal continued to increase. Thereafter, the 24MgOOH- signal decreased continually until the metal/oxide interface whereas the 24MgOO- signal continued to increase and showed a flat peak around a sputtering time of 200 s, just before the interface. That indicated that the surface layer had relatively more hydroxide in the outer layer and more oxide in the inner layer, although, the ToF-SIMS profile shown in Figure B17 cannot be neatly divided into two surface layers corresponding to the two indicated by our prior XPS works.
Figure B17 also indicates that the 24MgHHH- signal had a significant maximum at a sputtering time of 100 s and thereafter decreased with increasing etching time. The 25MgHHH- signal had the same shape with a relatively lower intensity. The intensity ratio 25MgHHH-/24MgHHH- corresponds closely to 0.125, the ratio of isotopic abundance 25Mg/24Mg. These signals were quite weak or nearly zero for the first 50 s sputtering and after the profile went into the substrate, so they cannot be related to the presence of MgO, Mg(OH)2 or metallic Mg. Thus, the 24MgHHH- and 25MgHHH- signals indicate that MgH2 exists inside the surface layer on pure Mg after immersion in ultra pure water. The MgH2 appears to be some distance from metallic Mg. The distribution of MgH2 in the surface film on pure Mg measured in this work was similar to that measured in the prior work on A13Mg2.
These data provide compelling evidence for the existence of MgH2 on the corroded surface of magnesium and that MgH2 can form during the exposure of Mg to pure water at the open circuit potential.
Ongoing works also indicated a need for careful examination of the use of Tafel extrapolation for Mg. For research that nevertheless does intend to use Tafel extrapolation to elucidate corrosion of Mg associated with service, it is strongly recommended that these measurements be complemented by the use of at least two of the three other simple measurement methods: (i) weight loss rate, (ii) hydrogen evolution rate and (iii) rate of Mg++ leaving the metal surface.
Work continued into the electrochemical reactivity, surface composition and corrosion mechanisms of the complex metallic alloy Al3Mg2 carried out in collaboration with Patrik Schmutz of the Swiss Federal Laboratories for Materials Testing and Research (EMPA) and Phillippe Marcus of the Ecole Nationale Supérieure de Chimie de Paris (ENSCP), France. A corrosion mechanism is proposed for Al3Mg2, based on electrochemical tests, XPS, and depth profiling using XPS and ToF-SIMS. After short (~2 min) solution exposure, the surface consists of a surface film above dealloying. The dealloying is attributed to selective Mg dissolution and the surface rearrangement of Al into islands, although the metallic Al could alternatively be formed by two reduction reactions. The surface film thickness was ~10 nm. After exposure to ultra pure water, the composition was AlMg1.3O0.2(OH)5.1 corresponding to Al(OH)3×1.1Mg(OH)2×0.2MgO. After exposure to 0.01M Na2SO4, the composition was AlMg0.2O0.4(OH)2.5 corresponding to Al(OH)3×0.1Al2O3×0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH2. Three possibilities are identified for MgH2 formation. Al(OH)3 formation is attributed to a precipitation reaction. Bulk nanoporous Al3Mg2 formation is predicted to be possible by Mg dealloying of Mg17Al12.
Work continued into the electrochemical reactivity, surface composition and corrosion mechanisms of the complex metallic alloy Al3Mg2 carried out in collaboration with Patrik Schmutz of the Swiss Federal Laboratories for Materials Testing and Research (EMPA) and Phillippe Marcus of the Ecole Nationale Supérieure de Chimie de Paris (ENSCP), France. A corrosion mechanism is proposed for Al3Mg2, based on electrochemical tests, XPS, and depth profiling using XPS and ToF-SIMS. After short (~2 min) solution exposure, the surface consists of a surface film above dealloying. The dealloying is attributed to selective Mg dissolution and the surface rearrangement of Al into islands, although the metallic Al could alternatively be formed by two reduction reactions. The surface film thickness was ~10 nm. After exposure to ultra pure water, the composition was AlMg1.3O0.2(OH)5.1 corresponding to Al(OH)3×1.1Mg(OH)2×0.2MgO. After exposure to 0.01M Na2SO4, the composition was AlMg0.2O0.4(OH)2.5 corresponding to Al(OH)3×0.1Al2O3×0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH2. Three possibilities are identified for MgH2 formation. Al(OH)3 formation is attributed to a precipitation reaction. Bulk nanoporous Al3Mg2 formation is predicted to be possible by Mg dealloying of Mg17Al12.
Future Activity Plan
In regards to durability of automotive Mg alloys, this project will need to focus on applied outcomes in order to begin to apply a methodology of predicting galvanic corrosion applicable to auto service of Mg coupled to steel. Additionally, the work will identify the limits and environmental envelope (and the techniques best needed to asses this envelope) for Mg as applicable to auto applications.
With regards to the development of corrosion resistant alloys, for Mg, alloys work will concentrate upon in-house production of new alloys (comprised of conventional alloying elements) for presentation and testing as corrosion resistant versions of existing alloys. This is based on the accumulation of information that has already been obtained. Such work, coupled with detailed microstructural analyses will be genuinely design oriented. Work will continue on the development of Al alloys with enhanced pitting resistance by exploiting the notion of being able to develop 'electrochemically heterogeneous' alloys that may be able to achieve increase in mechanical properties without the attendant loss in corrosion properties (where a typical inverse correlation is seen between the two).
In regards to durability of automotive Mg alloys, this project will need to focus on applied outcomes in order to begin to apply a methodology of predicting galvanic corrosion applicable to auto service of Mg coupled to steel. Additionally, the work will identify the limits and environmental envelope (and the techniques best needed to asses this envelope) for Mg as applicable to auto applications.
With regards to the development of corrosion resistant alloys, for Mg, alloys work will concentrate upon in-house production of new alloys (comprised of conventional alloying elements) for presentation and testing as corrosion resistant versions of existing alloys. This is based on the accumulation of information that has already been obtained. Such work, coupled with detailed microstructural analyses will be genuinely design oriented. Work will continue on the development of Al alloys with enhanced pitting resistance by exploiting the notion of being able to develop 'electrochemically heterogeneous' alloys that may be able to achieve increase in mechanical properties without the attendant loss in corrosion properties (where a typical inverse correlation is seen between the two).