Corrosion Science 49 (2007) 1540–1558

www.elsevier.com/locate/corsci

Corrosion product formation during NaClinduced atmospheric corrosion

of magnesium alloy AZ91D

Martin Jonsson a,*, Dan Persson a, Dominique Thierry b

a Corrosion and Metals Research Institute (KIMAB), Drottning Kristinas vag 48,

SE-114 28 Stockholm, Swedenb Institut de la Corrosion, 220 Rue Rivoalon, 29200 Brest, France

Received 10 July 2006; accepted 2 August 2006Available online 27 September 2006

Abstract

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with adeposition of 70 lg/cm�2 NaCl. The corrosion products formed and the surface electrolyte wereanalysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffrac-tion and Ion Chromatography. The results show that magnesium carbonates are the main solidcorrosion products formed under these conditions. The corrosion products identified were the mag-nesium carbonates hydromagnesite (Mg5 (CO3)4 (OH)24H2O) and nesquehonite (MgCO3 3H2O).The corrosion attack starts with the formation of magnesite at locations with higher NaClcontents. At 95% RH, a sequence of reactions was observed with the initial formation of magne-site, which transformed into nesquehonite after 2–3 days. Long exposures result in the formationof pits containing brucite (Mg(OH2)) covered with hydromagnesite crusts. The hydromagnesitecrusts restrict the transport of CO2 and O2 to the magnesium surface and thereby favour theformation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on thesurface at the beginning was essentially preserved during the initial corrosion process. Sincethe applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolytewas present on the surface during the whole exposure. Thus, Na+ and Cl� ions can participate inthe corrosion process during the whole time and the availability of these species will not restrict

0010-938X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2006.08.004

* Corresponding author. Tel.: +46 8 674 17 86; fax: +46 8 674 17 80.E-mail address: martin.jonsson@kimab.com (M. Jonsson).

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1541

the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosionbehaviour of AZ91D is rather controlled by factors related to the microstructure of the alloyand formation of solid carbonate containing corrosion products blocking active corrosion siteson the surface.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Magnesium; B. IR spectroscopy; C. Atmospheric corrosion

1. Introduction

Magnesium has gained considerable interest as a structural material for automotive andaerospace applications due to its low density, high specific strength and good castability.However, the use of magnesium alloys in engineering applications is mainly limited bytheir unsatisfactory corrosion properties. Thus, in order to make magnesium alloys a suit-able alternative for light weight automotive components, the understanding of the corro-sion properties of these alloys in atmospheric weathering conditions is of crucialimportance.

Despite the fact that magnesium alloys as structural materials are mostly used in atmo-spheric environments, the literature on atmospheric corrosion of magnesium alloys isscarce. Most of the studies refer to an electrochemical approach in solution [1–6]. In solu-tion magnesium suffers severe corrosion, but during atmospheric conditions in rural andindustrial environments the corrosion rate of magnesium alloys is comparable to that ofmild steel [7]. It is well known that corrosion processes in aqueous solutions are differentfrom those of atmospheric environments [8,9]. When Lindstrom et al. [9] made investiga-tions of the influence of carbon dioxide on the atmospheric corrosion of magnesium, theyconcluded that in the presence of 350 ppm CO2 the corrosion rate was only 25% of thatregistered in CO2-free air. This was explained by the formation of a protective carbonatelayer: giorgiosite, Mg5(CO3)4(OH)25H2O. Further, in the absence of CO2 heavy pittingoccurred and brucite, Mg(OH)2, was the dominant corrosion product. A number of cor-rosion products have been reported to form on magnesium in atmosphere. Godard [10]refers to field studies where hydromagnesite (Mg5 (CO3)4 (OH)24H2O), nesquehonite(MgCO3 3H2O) and lansfordite (MgCO3 Æ 5H2O) have been found after exposure for 18months in rural and industrial areas. Lansfordite (MgCO3 Æ 5H2O) has also been reportedafter exposure in urban regions in Vietnam [11].

These results are, however, not in agreement with the properties of lansfordite. Lansfor-dite is a metastable phase that occurs in nature only at temperatures below 10 �C. At tem-peratures above 10 �C lansfordite is dehydrated into nesquehonite [12,13].

In the present work, the formation of corrosion products during chloride inducedatmospheric corrosion of magnesium alloy AZ91D was studied using FTIR spectroscopy,X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). In situ FTIR micro-spectroscopy, ion chromatography and Inductively Coupled Plasma Mass Spectrometry(ICP-MS) were used to study the surface electrolyte. The aim of this work has been to gaina better understanding of the initial steps of the NaCl induced atmospheric corrosion ofmagnesium alloys.

1542 M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558

2. Experimental

2.1. Materials and exposure conditions

Corrosion test panels (130 · 100 · 3 mm) of die cast AZ91D were supplied by Dead SeaMagnesium, Ltd, Beer Sheva, Israel. The composition of the magnesium alloy is given inTable 1. The samples were machined to obtain small coupons (e.g. 45 · 15 · 3 mm). and ahole (diameter 0.4 mm) was drilled. The samples had a geometrical area of approximately6.6 cm2. The samples were sequentially polished and finally finished with 3 lm diamondpaste. The polished samples were then degreased in acetone, cleaned with ethanol in anultrasonic bath, dried in air, and finally stored in dry conditions. The AZ91D surfaceswere contaminated by 70 lg cm�2 NaCl. This level of NaCl corresponds approximatelyto marine environments that are not in the immediate vicinity of the coastline [9]. TheNaCl was saturated in 90% ethanol and was applied using a pipette. Special care was takento distribute the NaCl evenly over the magnesium surface. Samples were placed in desic-cators where the humidity was controlled using saturated salt solutions. To achieve 75%and 95% relative humidity, a saturated solution of NaCl and K2SO4, respectively, wasused [14]. The experiments were carried out at 25 �C and the atmosphere was that ofthe laboratory, with a CO2 concentration of about 350 ppm. The samples were suspendedeither vertically, using a polytetrafluoroethylene (PTFE) holder, or horizontally, for thedroplet test. The test duration was between 6 h and 4 weeks. After the investigation ofthe corrosion products was completed the samples were pickled [15].

For investigation of the corrosion rate over time and the maximum pit depth, magne-sium samples were prepared as mentioned above and contaminated with 70 lg cm�2

NaCl. The samples were exposed for 3, 6, 14, 21, 28 and 56 days. For each exposure time3 replicas were used. The samples were then pickled and the topography of one replica foreach exposure time was scanned using a Confocal microscope. For the surface electrolyteexamination one set of magnesium samples was prepared and contaminated by 70 lg cm�2

NaCl, as mentioned above. Three replicas of the samples were exposed for 1, 2, 3, 5 and 10days in 95% RH. Samples contaminated with NaCl but not exposed to humid air were alsoincluded in the study. After exposure the magnesium samples were ultrasonically cleanedfor 5 min in ultra pure water. The water containing the surface electrolyte was examinedusing Ion Chromatography (IC) and atom absorption spectrometer. In addition in situFTIR measurement was performed on a polished magnesium surface. Using an in situ cellthe surface was contaminated with 1 lg cm�2 NaCl.

2.2. Methods of analysis

The corrosion products formed on the magnesium surface were analysed using FTIRspectroscopy. Samples were analysed using both the KBr pellet method and FTIR micro-spectroscopy A small mass, typically 1 mg, of reference minerals or corrosion products

Table 1Nominal composition (weight %) of AZ91D magnesium alloy

Al Zn Mn Ni Cu Si Fe Mg

8.9 0.75 0.26 0.0008 0.0019 0.009 0.0027 remainder

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1543

was mixed with a spectroscopic grade potassium bromide (KBr) powder. The mixture wasthen grinded in a mortar and pressed to pellets in a die. The thickness and the weight of thepellets were recorded. From the KBr pellets qualitative as well as quantitative analysis ofthe corrosion products could be made. The quantitative analysis was made using Beer’slaw [16]. The FTIR spectrometer used in this study was a model Bio-Rad 175C, equippedwith a PTGS detector. For analysing corrosion products at different points on the corrod-ing magnesium surface FTIR microspectroscopy was performed using a Bio-Rad UMA500 microscope. The aperture was set at 100 · 100 lm, and the cut-off wavenumber was450 cm�1. Each spectrum consisted of 500 individual scans. In situ FTIR measurementwas performed using an in situ cell which made it possible to investigate the intensitychanges of the CO2�

3 and water bands during the corrosion process [17]. Reference miner-als, magnesite, nesquehonite and hydromagnesite were acquired from the SwedishMuseum of Natural History.

Crystalline corrosion products were identified by XRD, using an X-ray Powder Diffrac-tometer (Philips PW1050) with a graphite monochromator. SEM microscopy wasperformed using a SEM model JSM6400, equipped with a Wavelength-Dispersive Spec-trometer (WDS). The acceleration voltage was set at 20 KeV. The Confocal microscopymodel used in this study was lSurf from NanoFocus. The scanned area was 4.3 cm2 whichincluded a total of 16 * 106 measurement points. An average of the 1000 deepest measure-ment values was calculated, and the maximum pit depth was checked using a metallo-graphic microscope. The amount of water soluble chlorides was determined by IonChromatography (IC), model MetroOhm. The flow rate was 0.75 ml min�1 and 3.2 mmolL�1 Na2CO3 /1.0 mmol L�1 NaHCO3 was used as eluent. Detection was performed withan electric conductivity detector, model MetroOhm 732 IC Detector. The water solublecations, i.e. Na+- and Mg2+-ions, were analysed using an atom absorption spectrometer,model Spectra AA220 from Varian.

3. Results

3.1. Corrosion products formed at 95% and 75% relative humidity

Infrared spectra were recorded for reference minerals and synthesised compounds, hyd-romagnesite, nesquehonite, magnesite and brucite, see Figs. 1–4. The spectra obtainedwere in good agreement with those reported in the literature [18,19]. The AZ91D magne-sium samples were exposed to 75% and 95% relative humidity for 28 days in the presenceof 70 lg/cm�2 NaCl. The XRD analysis showed that brucite (Mg(OH)2) was the onlycrystalline corrosion product formed on the samples exposed for 28 days. However, ananalysis of the corrosion products with FTIR spectroscopy using KBr pellets clearlyshowed the presence of carbonate peaks; see Fig. 5. Comparing the FTIR spectra after28 days with the reference spectra of brucite (Mg(OH)2) (Fig. 4) and hydromagnesite(Fig. 1), all of the main peaks can be assigned to either brucite or hydromagnesite. Thevery distinct –OH peak at 3699 cm�1 and the shoulder at 373 cm�1 can be identified asMg(OH)2 peaks. The peaks at 3649 and 3522 cm�1 can be identified as stretch vibrationbands of the (OH)� and the H2O, respectively, of hydromagnesite. The sharp CO�2

3 asym-metric bending peaks at 1488 and 1435 cm�1 can also be attributed to hydromagnesite.The peak at 1435 cm�1 is slightly shifted from that of the reference peak at 1426 cm�1.This is probably due to the presence of a small amount of magnesite. The peak at

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596

3451

3515

854

884

433

793

3650

Abso

rban

ce

Wavenumber (cm-1)

1426

1484

Fig. 1. Infrared adsorption spectra for hydromagnesite (Mg5(CO3)4(OH)2 Æ 4(H2O)). Top left corner shows ablowup of the CO�2

3 asymmetric bending peaks.

4000 3000 2000 10000.0

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0.8

1.0

1600 1550 1500 1450 1400 1350

700

268

387

854

3563Ab

sorb

ance

Wavenumber (cm-1)

1423

1522

1475

Fig. 2. Infrared adsorption spectra for nesquehonite (MgCO3 3H2O). Top left corner shows a blowup of theCO�2

3 asymmetric bending peaks.

1544 M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558

854 cm�1 is due to the CO�23 bending vibration band. Peaks at 877, 796 and 592 cm�1 can

also be found in the reference spectra of hydromagnesite. By comparing the absorbance ofthe spectra from the corrosion products and that of the reference compounds, a roughquantitative analysis of the corrosion products could be made, using Beer’s law [16]. A cal-culation shows that after 28 days of exposure at 95% RH hydromagnesite is the main cor-rosion product and brucite amounts to about 15-20% of the corrosion product. Afterexposure for 28 days at a lower relative humidity, 75% (Fig. 6), the corrosion products

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1600 1550 1500 1450 1400 1350

729747

375

883

Abso

rban

ce

Wavenumber (cm-1)

1444

Fig. 3. Infrared adsorption spectra for magnesite (MgCO3). Top left corner shows a blowup of the CO�23

asymmetric bending peaks.

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0.0

0.2

0.4

0.6

0.8

371

468

565

3698

Abso

rban

ce

Wavenumber (cm-1)

Fig. 4. Infrared adsorption spectra for brucite (Mg(OH)2).

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1545

differ from those found at 95% RH. The main corrosion product is in this case magnesite.Magnesite is identified by the asymmetric CO�3 peak at 1447 cm�1 and the peak at878 cm�1. In addition, contributions from nesquehonite can be seen. The nesquehonitecan be identified by the asymmetric CO�3 peaks at 1476 and 1528 cm�1 and the shoulderat 1434. The peaks at 848 and 271 cm�1 also originate from nesquehonite. A peak can alsobe seen at 1461 cm�1. This peak is most likely due to Na2CO3, which has its asymmetricCO�3 peak at 1457 cm�1. It is notable that no peak for brucite can be found, which meansthat the corrosion products formed at 75% RH are exclusively due to carbonatecompounds.

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373

876 85

5

3522

592

3699

430

3649Ab

sorb

ance

Wavenumber (cm-1)

1435

1488

Fig. 5. Infrared adsorption spectra of corrosion products developed on AZ91D after 4 weeks of exposure to 95%RH in the presence of 70 lg/cm2 NaCl. Top left corner shows a blowup of the CO�2

3 asymmetric bending peaks.

4000 3500 3000 2500 2000 1500 1000 5000.2

0.3

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1600 1550 1500 1450 1400 1350

271

598

84887

83647

393

Abso

rban

ce

Wavenumber (cm-1)

1528 15

1514

76 1447

1461

Fig. 6. Infrared adsorption spectra of corrosion products developed on AZ91D after 4 weeks of exposure to 75%RH in the presence of 70 lg/cm2 NaCl. Top left corner shows a blowup of the CO�2

3 asymmetric bending peaks.

1546 M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558

3.2. Evolution of the corrosion products with the time of exposure

The amount and composition of corrosion products formed after 1, 3 and 6 days at 95%RH with a deposition of 70 lg/cm�2 NaCl were analysed with FTIR spectroscopy. Thecorrosion products were scraped off the surface of the sample and mixed with KBr andwere then analysed. The spectra obtained after different times of exposure are shown inFigs. 7–9. After 1 day (Fig. 7) a clear peak can be seen at 1446 cm�1 and small shoulderscan also be observed at 1475, 1528 cm�1. In addition, peaks can be found at 875 and

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416

575

875 85

6

1528

1475

1446

Abso

rban

ce

Wavenumber (cm-1)

Fig. 7. Infrared adsorption spectra of corrosion products developed on AZ91D after 1 day of exposure to 95%RH in the presence of 70 lg/cm2 NaCl. Top left corner shows a blowup of the CO�2

3 asymmetric bending peaks.

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0.5

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1600 1550 1500 1450 1400 1350 1300

411

590

876

1527

1436

855

1479

Abso

rban

ce

Wavenumber (cm-1)

Fig. 8. Infrared adsorption spectra of corrosion products developed on AZ91D after 3 days of exposure to 95%RH, in the presence of 70 lg/cm2 NaCl. Top left corner shows a blowup of the CO�2

3 asymmetric bending peaks.

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1547

856 cm�1. The peak at 1446 cm�1 as well as the peak at 875 cm�1 are due to magnesite.The weaker peaks at 1475 and 1528 cm�1 originate from nesquehonite. The intensity ofthe peaks suggests that after 1 day’s exposure magnesite is the main corrosion product.The broad band that can be seen around 3400 cm�1 is due to water. The water bandcan have contributions from water in the corrosion products and salt scraped off the sur-face. After 3 days of exposure (Fig. 8) the peaks at 1527 and 1479 cm�1 have increased inintensity. The peak formerly at 1446 cm�1 has now shifted to 1436 cm�1. Further the peak

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373

798

416

590

856

877

1432

1484

3651

3699

Abso

rban

ce

Wavenumber (cm-1)

Fig. 9. Infrared adsorption spectra of corrosion products developed on AZ91D after 6 days of exposure to 95%RH in the presence of 70 lg/cm2 NaCl. Top left corner shows a blowup of the CO�2

3 asymmetric bending peaks.

1548 M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558

at 856 cm�1 has increased in intensity compared to the peak at 876 cm�1. It is evident thatthe amount of nesquehonite has increased after 3 days of exposure and that the proportionof magnesite has consequently decreased. After 6 days the asymmetric CO2�

3 peak hasshifted and can be found at 1484 and 1432 cm�1 (Fig. 9). A new peak can be observedat 3651 cm�1. These peaks are consistent with the ones found for hydromagnesite.Another interesting observation is that after 6 days a sharp peak can be observed at3699 cm�1. This peak originates from brucite. Hence the main corrosion products after6 days are hydromagnesite and brucite. It was estimated from the spectra that bruciteaccounts for only about 5% of the composition of the corrosion product.

3.3. FTIR microspectroscopy

Using the microspectroscopy capability of the FTIR the corrosion products on the cor-roding surface could be examined after different exposure times at 95% RH in the presenceof 70 lg/cm�2 NaCl. The results are given in Figs. 10–12. Fig. 10 shows a typical part ofthe magnesium surface after 6 h of exposure at 95% RH. The circular dark area that canbe seen in Fig. 10 originates from a dried-out droplet of electrolyte, formed on the surface.The FTIR spectra at three different locations are given. From location A no significantinformation could be obtained. The AZ91D surface at this location is unaffected by cor-rosion. In area B, within the droplet, a diffuse carbonate peak with its maximum at1416 cm�1 can be observed. This peak is most likely due to a mixture between magnesiteand sodium carbonate at an early stage of the formation of the corrosion products. Inlocation C more apparent peaks can be found at 1434, 1471 and 1518 cm�1, together witha peak at 846 cm�1. These peaks originate from nesquehonite. A closer examination withthe light microscope reveals NaCl crystals in the black spots (SEM-EDS measurementsshowed that the Cl� concentration in the dried-out droplet is 40 times higher comparedto that of the area outside the droplet). Brucite could not be found on any part of the

A

CB

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Abso

rban

ce

Wavenumber (cm-1)

B

C

A

6H6H

Fig. 10. Right: Light Microscopy image of magnesium surface AZ91D after 6 hours of exposure to 95% RH inthe presence of 70 lg/cm2 NaCl. Black bar represents 0.5 mm. Left: FTIR spectra of points A, B and C.

Fig. 11. Right: Light Microscopy image of magnesium surface AZ91D after 48 hours of exposure to 95% RH inthe presence of 70 lg/cm2 NaCl. Black bar represents 0.5 mm. Left: FTIR spectra of points A, B and C.

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1549

surface. After 48 h of exposure (Fig. 11) the location outside the dried droplet (A) stillshowed no significant information. Inside the droplet, location B, there was a regionwhich, in higher magnification, showed less corrosion attacks. In the spectra for locationB, peaks could be found at 427, 1469 and 1535 cm�1. A small peak could also be observedat 849 cm�1. These peaks originate from nesquehonite. The thickness and the shape of thepeak at 1427 cm�1 suggest that the corrosion product at this stage also contains a smallamount of magnesite. At location C, peaks could be found at 1433, 1473 and1527 cm�1, together with a peak at 843 cm�1. The intensities and the sharpness of thepeaks indicate that in the darker spots in Fig. 11 the nesquehonite content is higher thanin location B. Brucite could not be found on any part of the surface exposed for 48 h. After10 days of exposure at 95% RH the corrosion was more severe (Fig. 12). Due to the thick-ness of the corrosion products the FTIR microspectroscopy yielded poor spectra.However, large carbonate peaks could be found. A closer examination of the spectrarevealed peaks at 1534, 1482, 1474, 1430, 876 and 851 cm�1. These peaks can be assigned

Fig. 12. Right: Light microscopy image of magnesium surface AZ91D after 10 days of exposure to 95% RH inthe presence of 70 lg/cm2 NaCl. Black bar represents 0.5 mm. Bottom: SEM image of the cross-section of thesame area. Top: FTIR spectra of points A (cross-section) and B (top-view).

1550 M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558

to either hydromagnesite or nesquehonite. In the black area in Fig. 12 the corrosion prod-uct was too thick to allow FTIR microspectroscopy investigations.

Brucite could not be observed on any part of the surface, using FTIR microspectros-copy. But evidently brucite was present after 6 days, as seen from the spectra of corrosionproducts scraped off the surface (Fig. 9). Therefore a cross-section of a pit was made bycarefully polishing the side of the sample. A SEM image of the cross-section can be viewedin Fig. 12. The cross-section was examined with the FTIR microscope. The spectra show aclear peak at 3704 cm�1. This peak originates from brucite. An investigation was alsomade on surfaces where the corrosion products had been removed with a scalpel. Insidethe unveiled pit a sharp peak was found at 3700 cm�1. SEM-EDS analysis was performedon the cross-section. The analysis showed that the chloride content was twice as high at thebottom of the pits compared to that of the top layer.

3.4. Corrosion rate and surface topography development over time

The magnesium samples exposed for 3, 6, 14, 21, 28 and 56 days were pickled afterexposure and the corrosion rate determined gravimetrically. The topography of the

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1551

surface was then scanned using a confocal microscope. From the measurement an averageof the values of the 1000 deepest corrosion pits was calculated. The corrosion rate and themaximum pit depth are given in Fig. 13. The mass loss of the samples increased rapidlyduring the first 3 days of exposure but was then essentially proportional to the exposuretime, as seen in Fig. 13. The maximum pit depth during the same time can be seen inthe same figure. The pit depth increases very rapidly during the first days of exposure.Already after 3 days the deepest pit is in the order of 70 lm. After the initial very rapidgrowth of the pits, the pit depth grows at a slower rate and the deepest pits are in the orderof 130 lm after 2 months of exposure. The maximum pit depth was also confirmed using ametallographic microscope, with similar results.

3.5. Investigation of the surface electrolyte

The composition of the surface electrolyte from samples exposed for 1, 2 3, 5 and 10days was analysed using both IC and atom absorption spectrometer. In addition, in situFTIR was conducted on a polished magnesium surface contaminated with 1 lg/cm�2

NaCl. Visual inspection of the exposed samples during exposure revealed that a water filmwas present on the samples after 1, 2 and 3 days of exposure. After 5 days the surface wascovered partially with water and partially with corrosion products, and after 10 days thewhole surface was covered with corrosion products and no water film could be seen.Chemical analysis was conducted by IC and atom absorption spectrometer. (Fig. 14) on

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Cor

rosi

on (m

g)

-140

-120

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-40

-20

0

pit depth ( μm)

Corrosionpit depth

Fig. 13. The corrosion of the AZ91D magnesium alloy versus time. The secondary y-axis (dotted line): Theaverage of the maximum pit depths formed on the AZ91D surface after different exposure times.

1552 M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558

soluble species from the exposed samples. The results showed that the amount of watersoluble Cl�-ions and Na+-ions was constant during the exposure. The amount of Cl�-ionsand Na+-ions measured was very close to the amount deposited on the surfaces beforeexposure, approximately 0.5 mg per sample. The amount of soluble Mg2+-ions presentin the surface electrolyte increases rapidly at the beginning of exposure. After 2 days of

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cent

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ions

(mg)

-7-6-5-4-3-2-1012345678

Weight loss and dry m

ass gain (mg)

Na+

Mg2+

Cl-

Dry mass gainWeight loss

Fig. 14. Investigation of the surface electrolyte. Amount of Na+ Cl� and Mg2+ ions in the surface electrolyteversus time of exposure. Secondary y-axis: Corrosion and dry mass gain versus time of exposure.

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nsity

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Intensity of CO3

- band

Fig. 15. In situ FTIR investigation of surface electrolyte. Intensity of water and carbonate bands at 1630 and at1510 cm�1, respectively, versus time.

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1553

exposure the increase in the amount of soluble Mg2+-ions levels out. In the same figure thedry mass gain and the mass loss of the samples can be seen on the secondary y-axis. Asinferred from the figure, the amount of water soluble Mg2+-ions corresponds to approxi-mately 10% of the mass loss.

The results from the in situ FTIR analysis can be seen in Fig. 15. The spectra havebands due to water at 1630 and 3400 cm�1 and carbonate at 1510 cm�1. The water bandsappear immediately after exposure to humid air, whereas the carbonate band grows grad-ually with exposure time. In Fig. 15 the intensities of the water band at 1630 cm�1 and thecarbonate band at 1510 cm�1 are plotted vs exposure time. The intensities of these bandsare proportional to the amounts of, respectively, water and carbonate that contain corro-sion products on the surface. The carbonate peak shows a rapid increase during the initial24–40 h, but then levels out. Similarly, the water peak decreases considerably during thefirst 20 h of exposure. After, that the decrease of the water band is quite slow.

4. Discussion

4.1. Corrosion products formed at 95% and 75% relative humidity

It is evident from the laboratory exposures that the corrosion products mainly containmagnesium carbonates. At 95% RH the main corrosion product formed on AZ91D is hyd-romagnesite. The cathodic reaction on magnesium in atmosphere is thought to be dueboth to water reduction and to oxygen reduction [8]:

Cathodic reaction:

H2Oþ 2e� ! H2ðgÞ þ 2OH�ðaqÞ ð1ÞO2 þ 2H2Oþ 4e� ! 4OH�ðaqÞ ð2Þ

Anodic reaction:

Mg(s)!Mg2þ(aq) + 2e� ð3Þ

The overall reaction then becomes:

Mg2þ(aq) + 2OH�(aq)!Mg(OH)2(s) ð4Þ

However, as seen in Fig. 16, which shows the thermodynamic stability diagram of the sys-tem Mg–H2O–CO2 [12], brucite is stable only at low CO2 partial pressures. At the levels ofCO2 found in ordinary atmosphere, brucite will react directly with CO2 to form magnesite.The reaction then becomes:

Mg(OH)2 + CO2!MgCO3(s) + H2O ð5Þ

Indeed, as can be seen in Fig. 7, the main corrosion product formed after 1 day of expo-sure was magnesite. As no peak for brucite can be seen, either by FTIR microscopy or theKBr pellet method, it can thus be concluded that the transition from brucite to magnesiteis very rapid. After 3 days of exposure the corrosion product has been transformed andconsists mainly of nesquehonite (Fig. 8). According to Fig. 16 this is only possible at highrelative humidity and high CO2 partial pressure. Thermodynamically, nesquehonite is sta-ble at high CO2 and H2O partial pressures. Nesquehonite may be formed in two ways, asindicated in Fig. 16, either from hydromagnesite when the CO2 pressure becomes

Fig. 16. Phase diagram for the system MgO–CO2–H2O in the solid vapour region. The main diagram isisothermal at 25 �C; inserts show a portion of the diagram calculated at 10 �C. Diagram and reactions taken from[12].

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sufficiently high or from direct hydration of magnesite. It is most probable from our exper-imental results (Figs. 7 and 8) that the latter reaction, direct hydration of magnesite, is themost probable reaction during the corrosion of AZ91D. This reaction does not involveCO2:

MgCO3(s) + 3H2O!MgCO3 � 3H2O(s) ð6Þ

After 6 days hydromagnesite was formed (Fig. 9). Hydromagnesite is the most commonhydrated magnesium carbonate and is most frequently found in nature [20]. Hydromagne-site can be formed in a number of ways according to Fig. 16. At high water vapour, partialpressures, increasing the CO2, can convert brucite into hydromagnesite directly. At higherCO2 pressures, magnesite can be hydrated to form hydromagnesite. Hydromagnesite canalso be formed by dehydration of nesquehonite:

4MgCO3 � 3H2O! 3MgCO3 �Mg(OH)2 � 3H2O + CO2 + 8H2O ð7Þ

The results of the corrosion product analyses shown in Figs. 8 and 9 indicate that nesqueh-onite is transformed into hydromagnesite according to reaction (7). At the relative humid-ity of 75% the water partial pressure and the water content in the surface electrolyte aremuch lower than that at 95% RH. It has earlier been shown that at the lower relativehumidity of 75% the rate of corrosion of AZ91D is half of that measured at 95% RH[15]. The main corrosion product at 75% RH is magnesite (Fig. 6) and the lower watercontent in the surface electrolyte in this case may inhibit the hydration into nesquehonite.

4.2. FTIR microscopy investigations

In Fig. 17 the corrosion process of AZ91D is schematically described. Initially, the ano-dic dissolution occurs at the location where electrolyte droplets are present on the surface

Fig. 17. Schematic description of the corrosion process of AZ91D in the presence of NaCl.

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and where the Cl� content is high. The SEM-EDS analysis confirmed that the Cl� concen-tration is up to 40 times higher in the droplet than in the area outside. CO2 can easilydiffuse through the water droplet, which causes a rapid formation of magnesium carbonateaccording to reactions (1)–(5) (Fig. 17A). Already after 6 h a carbonate layer can be foundon the surface of the dried-out droplet (Fig. 10). As long as sufficient oxygen is present atthe reaction sites on the surface, the cathodic reaction is probably to a large extent due tooxygen reduction (reaction (2)) [8]. In Figs. 10 and 11 it can be seen that the area just out-side the droplet (area A) is more or less unaffected, even after 2 days of exposure. Instead,at many locations on the samples exposed for 6 and 48 h an unaffected ring can be foundjust outside the droplet area. This may be due to cathodic reactions that take place in thisarea. After 2–3 days magnesite hydrated into nesquehonite. The formation of brucite wasobserved after 6 days of exposure (Fig. 9). FTIR microspectroscopy performed on thecross section of an area with corrosion attack shows that brucite is formed beneath thehydromagnesite crust, in the pits. As shown by the SEM-EDS, the Cl� content is twiceas high in the bottom of the pit compared to the top layer. The formation of brucite underthe hydromagnesite crust is probably due to hindered transport of species such as CO2 andO2 through the crust. The low CO2 pressure in the pit explains why brucite is not trans-formed into magnesite, which would be the case in ambient atmospheres, according toFig. 16. Water is present beneath the crust, and as the oxygen is depleted the cathodic reac-tion is dominated by water reduction (Reaction (1)). Closer to the bottom in the pit allcomponents for the corrosion process are present, and hence the corrosion process canproceed without exchange with the surrounding atmosphere.

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4.3. Corrosion rate and surface topography development over time

The results show that the corrosion rate during the initial first days of exposure washigh and led to localised corrosion attack on the surface. In a forthcoming work, the initialstages of the atmospheric corrosion of AZ91 will be discussed in detail. However, asshown in this work, the rapid initial corrosion attack is related to the microstructure ofthe material and microgalvanic corrosion cells formed during the corrosion process.The corrosion then proceeds rapidly due to the galvanic coupling of the nobler b-phasein the grain boundaries and the homogenised magnesium phase in the grain. After the con-sumption of the material inside the grains the corrosion process could slow down, due tothe nobler material in the grain boundaries. This is similar to the corrosion behaviour ofAZ91 in solution, studied by Song et al. [21].

4.4. Investigation of the surface electrolyte

The results of the chemical analysis show that the salt applied on the surface at thebeginning is essentially preserved during the exposure. The Cl� and Na+ ions are notbonded in sparingly soluble corrosion products, as in the case of Zn and Cu [22,23]. Thisis confirmed by the in situ IRAS measurements which show that a considerable amount ofwater is present on the surface during the whole exposure. Soluble ions play several rolesin the corrosion process of metals. First, they form a surface electrolyte layer on the sur-face during the whole exposure. This facilitates the metal dissolution process and providesa reservoir for reactants and reaction products. Secondly, Cl� ions could also play a role inthe anodic dissolution reaction, forming various surface and aqua complexes which influ-ence the anodic reaction. A third effect is that the electrolyte provides a high conductivitymedium on the surface which couples the spatially separated anodic and cathodic pro-cesses in the microgalvanic elements on the surface. However, the in situ IRAS measure-ments also indicate that the amount of water decreases slightly after prolonged exposure.This could be a result of the non-stochiometric incorporation of small amounts of Cl� andNa+ ions into the corrosion product film on the surface. Due to the presence of solubleCl� and Na+ ions a corrosive electrolyte was present on the surface during the whole expo-sure. The corrosion will therefore not be restricted by a gradual disappearance of the sur-face electrolyte, which could be the case if the Cl� and Na+ ions were imbedded ininsoluble products. The Cl� ions are also available to participate in the anodic dissolutionprocess. Furthermore, conductive paths are present on the surface for galvanic cellsbetween different phases in the alloy. Thus the availability of Cl� is probably not arate-determining factor for the initial atmospheric corrosion of AZ91D under theseconditions. Instead it is probable that the corrosion behaviour of AZ91D is controlledby factors related to the microstructure of the alloy and formation of solid carbonate con-taining corrosion products which could block active corrosion sites on the surface.

5. Conclusions

The initial NaCl induced atmospheric corrosion process resulted in the formation ofcorrosion products that mainly consisted of magnesium carbonate. After 28 days of expo-sure in humid air at 95% RH, the main corrosion products were hydromagnesite and bru-cite. A reaction sequence was observed involving different magnesium carbonates and

M. Jonsson et al. / Corrosion Science 49 (2007) 1540–1558 1557

brucite. The NaCl induced atmospheric corrosion attack at 95% RH resulted initially inthe formation of magnesite, which was subsequently transformed into nesquehonite after2–3 days of exposure. Longer exposures resulted in localised corrosion attack on the mag-nesium surface and the formation of shallow pits containing brucite covered with crusts ofcorrosion products with hydromagnesite. At 75% RH humidity the corrosion productsconsisted mainly of magnesite and the corrosion attack was less localised. Anodic dissolu-tion of the magnesium alloy resulted initially in the formation of brucite, which wasrapidly transformed into magnesite in the presence of CO2. In the initial stages of the cor-rosion attack the cathodic reactions could involve both oxygen reduction and reduction ofwater. The oxygen reduction occurred preferentially in the periphery of the electrolytedroplets on the surface, which led to alkaline conditions and an increase in CO2 depositionin these areas. After a longer exposure time a thicker layer of corrosion products wasformed on the surface. The formation of brucite in the areas with localised corrosionattack is connected with the formation of the crust of corrosion products that inhibitsthe transport of CO2 and O2 to the active magnesium surface. At this stage the cathodicprocess was dominated by the reduction of water present in the pit, which resulted in theformation of brucite. Analysis of the surface electrolyte showed that the salt applied on thesurface at the beginning was essentially preserved during the exposure. The Cl� and Na+

ions are not bonded in sparingly soluble corrosion products. Thus the surface electrolytewas present at the surface during the whole exposure and the soluble ions were present inconsiderable amounts in the surface electrolyte and could support the corrosion process. Itis suggested that initial NaCl atmospheric corrosion of AZ91D is controlled by factorsrelated to the microstructure of the alloy and formation of solid carbonate containing cor-rosion products which could block active corrosion sites on the surface.

Acknowledgements

The authors would like to thank Henrik Skogby at the Swedish Museum of NaturalHistory for helpful discussions in the mineral field and for assistance with XRD measure-ments. The authors would also like to thank Jan Y. Jonsson and Fredrik FalkenbergOutokumpu, Avesta, for all the help with the Confocal microscope.

VINNOVA (The Swedish Governmental Agency for Innovation Systems) is gratefullyacknowledge for financial support, through the VAMP32 project.

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