An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water,... more An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al 3 Mg 2 and Mg 17 Al 12. The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH) 2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al 3 Mg 2 was AlMg 1:4 O 0:2 ðOHÞ 5:4 whilst on Mg 17 Al 12 the composition was AlMg 2:5 ðOHÞ 8 .
Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direc... more Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direct observations. There were two important effects. In 0.1 M NaCl, the corrosion rate increased with increasing Y content due to increasing amounts of the Y-containing intermetallic. In 0.1 M Na 2 SO 4 , the corrosion rate decreased with increasing Y content above 3%, attributed to a more protective surface film, despite the intermetallic. The corrosion rate evaluated by electrochemical impedance spectroscopy was somewhat smaller than that evaluated from H evolution as expected from the Mg corrosion mechanism. A mechanism is proposed for filiform corrosion. Direct in situ corrosion observations revealed that a predominant feature was hydrogen evolution from particular parts of the alloy surface.
A corrosion mechanism is proposed for Al 3 Mg 2 , based on electrochemical tests, XPS, and depth ... more A corrosion mechanism is proposed for Al 3 Mg 2 , 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 AlMg 1.3 O 0.2 (OH) 5.1 corresponding to Al(OH) 3 Á1.1 Mg(OH) 2 Á0.2MgO. After exposure to 0.01 M Na 2 SO 4 , the composition was AlMg 0.2 O 0.4 (OH) 2.5 corresponding to Al(OH) 3 Á0.1Al 2 O 3 Á0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH 2. Three possibilities are identified for MgH 2 formation. Al(OH) 3 formation is attributed to a precipitation reaction. Bulk nanoporous Al 3 Mg 2 formation is predicted to be possible by Mg dealloying of Mg 17 Al 12 .
Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydro... more Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydrogen pipelines is a prospective mode of energy transmission during the transition to renewables. The risk of hydrogen embrittlement (HE) in pipeline steels must first be quantified to ensure safe pipeline operation. This review provides an overview of HE in pipeline steels. Most pipeline steels have reduced ductility when exposed to hydrogen partial pressures of 100 bar and above. Higher-strength pipeline steels (X80 and X100) have been found to undergo HE at ∼50 bar hydrogen. Hydrogen-induced subcritical crack growth in pipeline steels has not been reported in the literature. There are few articles on HE in pipeline welds, with some indications that the weld is more susceptible to HE, and some indications that it is less. The relationship between hydrogen pressure and absorbed hydrogen concentration has not been evaluated. Gaps in knowledge are identified in the conclusions.
An XPS investigation was carried out on the surface film formed by exposure to high-purity water,... more An XPS investigation was carried out on the surface film formed by exposure to high-purity water, on mechanically polished Mg and the two Mg-Al intermetallic compounds: Al3Mg2 and Mg17Al12. The result for mechanically polished pure Mg indicates that a film of MgO covered by a Mg(OH)2 layer, formed by the reaction of MgO with water vapour in the air. On immersion in distilled water, this film was hydrated to a duplex film with an inner MgO layer next to the Mg metal and an external porous layer of hydroxide. For both intermetallics, there was preferential dissolution of magnesium from the mechanically ground surface and also during aqueous immersion. After immersion, there was a 10 nm thick, stable film on the surface; the film composition on Al3Mg2 was whilst that on Mg17Al12 was .
ABSTRACT Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solut... more ABSTRACT Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solution saturated with Mg(OH)(2). The intrinsic corrosion rate measured with weight loss, P-W = 0.25 +/- 0.07 mm y(-1), was slightly smaller than that for high-purity Mg. Some specimens had somewhat higher corrosion rates attributed to localised corrosion. The average corrosion rate measured from hydrogen evolution, P-AH, was lower than that measured with weight loss, P-W, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism is suggested. (c) 2013 Elsevier Ltd. All rights reserved.
Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on ... more Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on pure magnesium by immersion for 2 min in ultra pure water. The ToF-SIMS data indicates that there is magnesium hydride within the surface film. The presence of MgH 2 is a result of the Mg corrosion mechanism.
A first systematic investigation was carried out to understand the corrosion of common Mg alloys ... more 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 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 3 wt.% 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 Mg 2+ 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 electrochemical measurements of the corrosion rate, based on the ''corrosion current" at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.
A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium ... more A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium alloy. Polarization curves and electrochemical impedance diagrams plotted in Na 2 SO 4 electrolyte showed its high protective effect. Analysis of the chemical composition by X-ray photoelectron spectroscopy indicated that the coating mainly consisted of (i) magnesium metaborate and metaphosphate, (ii) MgF 2 and ZrF 4 , and (iii) Nb 2 O 5 , ZrO 2 and MgO. A higher concentration of fluorine at both interfaces and an enrichment in Zr compared to Nb were revealed by SEM and EDS analyses. Thus, Zr-based compounds and MgF 2 play a key role in the anti-corrosion ability of the coating.
The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in ... more The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in 3% NaCl, of (i) high-pressure die-cast (HPDC) model Mg-Al alloys, (ii) low-purity Mg, (iii) high-purity (HP) Mg and (iv) HP Mg heat treated at 550°C. HPDC is the most important route for the production of Mg components. The corrosion of the model alloys was dominated by the Fe impurity element. The present research identified the appearance of the Fe-rich particles in the microstructure. In high magnification ($1000Â to 5000Â) secondary electron images, they appear as small white features, typically less than 1 lm in diameter. In order to understand the impurity tolerance limits, (i) the appropriate corrosion literature was summarised and reviewed and (ii) Mg phase diagrams were calculated using the Pandat software package. Calculated phase diagrams can explain (i) the tolerance levels for Fe and Cu and (ii) the production of high-purity castings by means of control of melt conditions; this has high significance for the production of quality castings from recycled Mg. A full analysis requires that the Mg database be extended to include Ni, Co and some RE. The Fe tolerance limit is $5-10 ppm for cast HP Mg heat treated at 550°C. Analysis of the Mg corrosion literature indicates that several studies have been dominated by the Fe impurity content and have not dealt with the stated aims; it means that the full chemical composition should be reported in all studies of the corrosion of Mg alloys.
Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydro... more Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydrogen pipelines is a prospective mode of energy transmission during the transition to renewables. The risk of hydrogen embrittlement (HE) in pipeline steels must first be quantified to ensure safe pipeline operation. This review provides an overview of HE in pipeline steels. Most pipeline steels have reduced ductility when exposed to hydrogen partial pressures of 100 bar and above. Higher-strength pipeline steels (X80 and X100) have been found to undergo HE at ∼50 bar hydrogen. Hydrogen-induced subcritical crack growth in pipeline steels has not been reported in the literature. There are few articles on HE in pipeline welds, with some indications that the weld is more susceptible to HE, and some indications that it is less. The relationship between hydrogen pressure and absorbed hydrogen concentration has not been evaluated. Gaps in knowledge are identified in the conclusions.
A first systematic investigation was carried out to understand the corrosion of common Mg alloys ... more 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 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 3 wt.% 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 Mg 2+ 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 electrochemical measurements of the corrosion rate, based on the ''corrosion current" at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.
Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on ... more Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on pure magnesium by immersion for 2 min in ultra pure water. The ToF-SIMS data indicates that there is magnesium hydride within the surface film. The presence of MgH 2 is a result of the Mg corrosion mechanism.
An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water,... more An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al 3 Mg 2 and Mg 17 Al 12 . The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH) 2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al 3 Mg 2 was AlMg 1:4 O 0:2 ðOHÞ 5:4 whilst on Mg 17 Al 12 the composition was AlMg 2:5 ðOHÞ 8 .
An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water,... more An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al 3 Mg 2 and Mg 17 Al 12 . The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH) 2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al 3 Mg 2 was AlMg 1:4 O 0:2 ðOHÞ 5:4 whilst on Mg 17 Al 12 the composition was AlMg 2:5 ðOHÞ 8 .
A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium ... more A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium alloy. Polarization curves and electrochemical impedance diagrams plotted in Na 2 SO 4 electrolyte showed its high protective effect. Analysis of the chemical composition by X-ray photoelectron spectroscopy indicated that the coating mainly consisted of (i) magnesium metaborate and metaphosphate, (ii) MgF 2 and ZrF 4 , and (iii) Nb 2 O 5 , ZrO 2 and MgO. A higher concentration of fluorine at both interfaces and an enrichment in Zr compared to Nb were revealed by SEM and EDS analyses. Thus, Zr-based compounds and MgF 2 play a key role in the anti-corrosion ability of the coating.
A corrosion mechanism is proposed for Al 3 Mg 2 , based on electrochemical tests, XPS, and depth ... more A corrosion mechanism is proposed for Al 3 Mg 2 , 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 AlMg 1.3 O 0.2 (OH) 5.1 corresponding to Al(OH) 3 Á1.1 Mg(OH) 2 Á0.2MgO. After exposure to 0.01 M Na 2 SO 4 , the composition was AlMg 0.2 O 0.4 (OH) 2.5 corresponding to Al(OH) 3 Á0.1Al 2 O 3 Á0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH 2 . Three possibilities are identified for MgH 2 formation. Al(OH) 3 formation is attributed to a precipitation reaction. Bulk nanoporous Al 3 Mg 2 formation is predicted to be possible by Mg dealloying of Mg 17 Al 12 .
Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direc... more Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direct observations. There were two important effects. In 0.1 M NaCl, the corrosion rate increased with increasing Y content due to increasing amounts of the Y-containing intermetallic. In 0.1 M Na 2 SO 4 , the corrosion rate decreased with increasing Y content above 3%, attributed to a more protective surface film, despite the intermetallic. The corrosion rate evaluated by electrochemical impedance spectroscopy was somewhat smaller than that evaluated from H evolution as expected from the Mg corrosion mechanism. A mechanism is proposed for filiform corrosion. Direct in situ corrosion observations revealed that a predominant feature was hydrogen evolution from particular parts of the alloy surface.
The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in ... more The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in 3% NaCl, of (i) high-pressure die-cast (HPDC) model Mg-Al alloys, (ii) low-purity Mg, (iii) high-purity (HP) Mg and (iv) HP Mg heat treated at 550°C. HPDC is the most important route for the production of Mg components. The corrosion of the model alloys was dominated by the Fe impurity element. The present research identified the appearance of the Fe-rich particles in the microstructure. In high magnification ($1000Â to 5000Â) secondary electron images, they appear as small white features, typically less than 1 lm in diameter. In order to understand the impurity tolerance limits, (i) the appropriate corrosion literature was summarised and reviewed and (ii) Mg phase diagrams were calculated using the Pandat software package. Calculated phase diagrams can explain (i) the tolerance levels for Fe and Cu and (ii) the production of high-purity castings by means of control of melt conditions; this has high significance for the production of quality castings from recycled Mg. A full analysis requires that the Mg database be extended to include Ni, Co and some RE. The Fe tolerance limit is $5-10 ppm for cast HP Mg heat treated at 550°C. Analysis of the Mg corrosion literature indicates that several studies have been dominated by the Fe impurity content and have not dealt with the stated aims; it means that the full chemical composition should be reported in all studies of the corrosion of Mg alloys.
An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water,... more An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al 3 Mg 2 and Mg 17 Al 12. The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH) 2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al 3 Mg 2 was AlMg 1:4 O 0:2 ðOHÞ 5:4 whilst on Mg 17 Al 12 the composition was AlMg 2:5 ðOHÞ 8 .
Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direc... more Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direct observations. There were two important effects. In 0.1 M NaCl, the corrosion rate increased with increasing Y content due to increasing amounts of the Y-containing intermetallic. In 0.1 M Na 2 SO 4 , the corrosion rate decreased with increasing Y content above 3%, attributed to a more protective surface film, despite the intermetallic. The corrosion rate evaluated by electrochemical impedance spectroscopy was somewhat smaller than that evaluated from H evolution as expected from the Mg corrosion mechanism. A mechanism is proposed for filiform corrosion. Direct in situ corrosion observations revealed that a predominant feature was hydrogen evolution from particular parts of the alloy surface.
A corrosion mechanism is proposed for Al 3 Mg 2 , based on electrochemical tests, XPS, and depth ... more A corrosion mechanism is proposed for Al 3 Mg 2 , 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 AlMg 1.3 O 0.2 (OH) 5.1 corresponding to Al(OH) 3 Á1.1 Mg(OH) 2 Á0.2MgO. After exposure to 0.01 M Na 2 SO 4 , the composition was AlMg 0.2 O 0.4 (OH) 2.5 corresponding to Al(OH) 3 Á0.1Al 2 O 3 Á0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH 2. Three possibilities are identified for MgH 2 formation. Al(OH) 3 formation is attributed to a precipitation reaction. Bulk nanoporous Al 3 Mg 2 formation is predicted to be possible by Mg dealloying of Mg 17 Al 12 .
Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydro... more Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydrogen pipelines is a prospective mode of energy transmission during the transition to renewables. The risk of hydrogen embrittlement (HE) in pipeline steels must first be quantified to ensure safe pipeline operation. This review provides an overview of HE in pipeline steels. Most pipeline steels have reduced ductility when exposed to hydrogen partial pressures of 100 bar and above. Higher-strength pipeline steels (X80 and X100) have been found to undergo HE at ∼50 bar hydrogen. Hydrogen-induced subcritical crack growth in pipeline steels has not been reported in the literature. There are few articles on HE in pipeline welds, with some indications that the weld is more susceptible to HE, and some indications that it is less. The relationship between hydrogen pressure and absorbed hydrogen concentration has not been evaluated. Gaps in knowledge are identified in the conclusions.
An XPS investigation was carried out on the surface film formed by exposure to high-purity water,... more An XPS investigation was carried out on the surface film formed by exposure to high-purity water, on mechanically polished Mg and the two Mg-Al intermetallic compounds: Al3Mg2 and Mg17Al12. The result for mechanically polished pure Mg indicates that a film of MgO covered by a Mg(OH)2 layer, formed by the reaction of MgO with water vapour in the air. On immersion in distilled water, this film was hydrated to a duplex film with an inner MgO layer next to the Mg metal and an external porous layer of hydroxide. For both intermetallics, there was preferential dissolution of magnesium from the mechanically ground surface and also during aqueous immersion. After immersion, there was a 10 nm thick, stable film on the surface; the film composition on Al3Mg2 was whilst that on Mg17Al12 was .
ABSTRACT Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solut... more ABSTRACT Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solution saturated with Mg(OH)(2). The intrinsic corrosion rate measured with weight loss, P-W = 0.25 +/- 0.07 mm y(-1), was slightly smaller than that for high-purity Mg. Some specimens had somewhat higher corrosion rates attributed to localised corrosion. The average corrosion rate measured from hydrogen evolution, P-AH, was lower than that measured with weight loss, P-W, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism is suggested. (c) 2013 Elsevier Ltd. All rights reserved.
Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on ... more Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on pure magnesium by immersion for 2 min in ultra pure water. The ToF-SIMS data indicates that there is magnesium hydride within the surface film. The presence of MgH 2 is a result of the Mg corrosion mechanism.
A first systematic investigation was carried out to understand the corrosion of common Mg alloys ... more 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 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 3 wt.% 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 Mg 2+ 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 electrochemical measurements of the corrosion rate, based on the ''corrosion current" at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.
A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium ... more A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium alloy. Polarization curves and electrochemical impedance diagrams plotted in Na 2 SO 4 electrolyte showed its high protective effect. Analysis of the chemical composition by X-ray photoelectron spectroscopy indicated that the coating mainly consisted of (i) magnesium metaborate and metaphosphate, (ii) MgF 2 and ZrF 4 , and (iii) Nb 2 O 5 , ZrO 2 and MgO. A higher concentration of fluorine at both interfaces and an enrichment in Zr compared to Nb were revealed by SEM and EDS analyses. Thus, Zr-based compounds and MgF 2 play a key role in the anti-corrosion ability of the coating.
The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in ... more The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in 3% NaCl, of (i) high-pressure die-cast (HPDC) model Mg-Al alloys, (ii) low-purity Mg, (iii) high-purity (HP) Mg and (iv) HP Mg heat treated at 550°C. HPDC is the most important route for the production of Mg components. The corrosion of the model alloys was dominated by the Fe impurity element. The present research identified the appearance of the Fe-rich particles in the microstructure. In high magnification ($1000Â to 5000Â) secondary electron images, they appear as small white features, typically less than 1 lm in diameter. In order to understand the impurity tolerance limits, (i) the appropriate corrosion literature was summarised and reviewed and (ii) Mg phase diagrams were calculated using the Pandat software package. Calculated phase diagrams can explain (i) the tolerance levels for Fe and Cu and (ii) the production of high-purity castings by means of control of melt conditions; this has high significance for the production of quality castings from recycled Mg. A full analysis requires that the Mg database be extended to include Ni, Co and some RE. The Fe tolerance limit is $5-10 ppm for cast HP Mg heat treated at 550°C. Analysis of the Mg corrosion literature indicates that several studies have been dominated by the Fe impurity content and have not dealt with the stated aims; it means that the full chemical composition should be reported in all studies of the corrosion of Mg alloys.
Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydro... more Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydrogen pipelines is a prospective mode of energy transmission during the transition to renewables. The risk of hydrogen embrittlement (HE) in pipeline steels must first be quantified to ensure safe pipeline operation. This review provides an overview of HE in pipeline steels. Most pipeline steels have reduced ductility when exposed to hydrogen partial pressures of 100 bar and above. Higher-strength pipeline steels (X80 and X100) have been found to undergo HE at ∼50 bar hydrogen. Hydrogen-induced subcritical crack growth in pipeline steels has not been reported in the literature. There are few articles on HE in pipeline welds, with some indications that the weld is more susceptible to HE, and some indications that it is less. The relationship between hydrogen pressure and absorbed hydrogen concentration has not been evaluated. Gaps in knowledge are identified in the conclusions.
A first systematic investigation was carried out to understand the corrosion of common Mg alloys ... more 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 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 3 wt.% 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 Mg 2+ 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 electrochemical measurements of the corrosion rate, based on the ''corrosion current" at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.
Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on ... more Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on pure magnesium by immersion for 2 min in ultra pure water. The ToF-SIMS data indicates that there is magnesium hydride within the surface film. The presence of MgH 2 is a result of the Mg corrosion mechanism.
An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water,... more An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al 3 Mg 2 and Mg 17 Al 12 . The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH) 2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al 3 Mg 2 was AlMg 1:4 O 0:2 ðOHÞ 5:4 whilst on Mg 17 Al 12 the composition was AlMg 2:5 ðOHÞ 8 .
An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water,... more An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al 3 Mg 2 and Mg 17 Al 12 . The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH) 2 layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al 3 Mg 2 was AlMg 1:4 O 0:2 ðOHÞ 5:4 whilst on Mg 17 Al 12 the composition was AlMg 2:5 ðOHÞ 8 .
A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium ... more A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium alloy. Polarization curves and electrochemical impedance diagrams plotted in Na 2 SO 4 electrolyte showed its high protective effect. Analysis of the chemical composition by X-ray photoelectron spectroscopy indicated that the coating mainly consisted of (i) magnesium metaborate and metaphosphate, (ii) MgF 2 and ZrF 4 , and (iii) Nb 2 O 5 , ZrO 2 and MgO. A higher concentration of fluorine at both interfaces and an enrichment in Zr compared to Nb were revealed by SEM and EDS analyses. Thus, Zr-based compounds and MgF 2 play a key role in the anti-corrosion ability of the coating.
A corrosion mechanism is proposed for Al 3 Mg 2 , based on electrochemical tests, XPS, and depth ... more A corrosion mechanism is proposed for Al 3 Mg 2 , 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 AlMg 1.3 O 0.2 (OH) 5.1 corresponding to Al(OH) 3 Á1.1 Mg(OH) 2 Á0.2MgO. After exposure to 0.01 M Na 2 SO 4 , the composition was AlMg 0.2 O 0.4 (OH) 2.5 corresponding to Al(OH) 3 Á0.1Al 2 O 3 Á0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH 2 . Three possibilities are identified for MgH 2 formation. Al(OH) 3 formation is attributed to a precipitation reaction. Bulk nanoporous Al 3 Mg 2 formation is predicted to be possible by Mg dealloying of Mg 17 Al 12 .
Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direc... more Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direct observations. There were two important effects. In 0.1 M NaCl, the corrosion rate increased with increasing Y content due to increasing amounts of the Y-containing intermetallic. In 0.1 M Na 2 SO 4 , the corrosion rate decreased with increasing Y content above 3%, attributed to a more protective surface film, despite the intermetallic. The corrosion rate evaluated by electrochemical impedance spectroscopy was somewhat smaller than that evaluated from H evolution as expected from the Mg corrosion mechanism. A mechanism is proposed for filiform corrosion. Direct in situ corrosion observations revealed that a predominant feature was hydrogen evolution from particular parts of the alloy surface.
The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in ... more The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in 3% NaCl, of (i) high-pressure die-cast (HPDC) model Mg-Al alloys, (ii) low-purity Mg, (iii) high-purity (HP) Mg and (iv) HP Mg heat treated at 550°C. HPDC is the most important route for the production of Mg components. The corrosion of the model alloys was dominated by the Fe impurity element. The present research identified the appearance of the Fe-rich particles in the microstructure. In high magnification ($1000Â to 5000Â) secondary electron images, they appear as small white features, typically less than 1 lm in diameter. In order to understand the impurity tolerance limits, (i) the appropriate corrosion literature was summarised and reviewed and (ii) Mg phase diagrams were calculated using the Pandat software package. Calculated phase diagrams can explain (i) the tolerance levels for Fe and Cu and (ii) the production of high-purity castings by means of control of melt conditions; this has high significance for the production of quality castings from recycled Mg. A full analysis requires that the Mg database be extended to include Ni, Co and some RE. The Fe tolerance limit is $5-10 ppm for cast HP Mg heat treated at 550°C. Analysis of the Mg corrosion literature indicates that several studies have been dominated by the Fe impurity content and have not dealt with the stated aims; it means that the full chemical composition should be reported in all studies of the corrosion of Mg alloys.
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