Enhanced corrosion resistance of magnesium alloy AM60 by cerium(III) in chloride solution

Corrosion Science 56 (2012) 86–95

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Enhanced corrosion resistance of magnesium alloy AM60 by cerium(III) in chloride solution F. El-Taib Heakal a,⇑, O.S. Shehata b, N.S. Tantawy c a

Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt Physical Chemistry Department, National Research Centre, Dokki, Giza, Egypt c Girl’s College of Arts, Science and Education, Ain Shams University, Asma Fahmi Street, Cairo, Egypt b

a r t i c l e

i n f o

Article history: Received 16 August 2011 Accepted 27 November 2011 Available online 3 December 2011 Keywords: A. Magnesium B. EIS B. Polarisation B. SEM C. Neutral inhibition

a b s t r a c t Cerium(III) was utilised to enhance the corrosion resistance of AM60 in NaCl solution. Ce3+ can suppress corrosion deterioration up to 1.0 mM. Beyond that level corrosion rate increases till a steady value. Surface film resistance increases with time evolution until 24 h, then decreases and stabilizes. The converted film after 240 h immersion exhibits self-healing and thickening when re-exposed to plain chloride solution. SEM and EDX confirmed that when Ce is present as additive in solution, more compact coating is formed better than its presence as a post coating on the alloy surface before being immersed in the corrosive environment. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, magnesium alloys have received extensive recognition because of their excellent physical properties, including low density, high strength to weight ratio, excellent electrical conductivity, high thermal conductivity and good electromagnetic shielding characteristics among others [1]. Thus, in a world becoming more and more conscious of environment and safety, magnesium alloys is a topic of great relevance as they are being manufactured increasingly for light weight structural and functional parts in the automotive application and electronic industries [2,3]. Most of magnesium alloys are mainly processed by die-casting or thixo-forming processes [4]. However, the corrosion resistance of magnesium alloys is generally inadequate in many environments, especially in Cl-containing environment, primarily due to the high thermodynamic activity of magnesium metal (E0Mg2þ =Mg ¼ 2:37 V vs. SHE [5]), and the non protective nature of the passive film on the surface of these alloys [6–8], which has limited their wide spread utilization in practice. In addition, the presence of impurities and second phases acting as local cathodes cause local galvanic acceleration of corrosion [7]. A great deal of effort has been carried out using conventional ingot metallurgy to improve the corrosion performance of magnesium alloys, with considerable success. Furthermore, rapid solidification (RS) has been used [9] to explore the production of more corrosion-resistant alloys. ⇑ Corresponding author. Tel.: +20 102449048; fax: +20 2 35728099. E-mail address: [email protected] (F.E.-T. Heakal). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.11.019

Due to the strong tendency of Al to form a stable passive film [10], aluminum is the most important alloying element for magnesium. The maximum solubility for Al is 12.7 wt.% [11], and Al can improve magnesium strength. Aluminum is partly present in solid solution, and partly precipitated as a continuous b-phase (Mg17Al12) along grain boundaries, as well as in the eutectic phase constituted by a lamellar structure. Although adding Al to Mg matrix will be conducive to corrosion resistance improvement, it is reported that corrosion rate decreases rapidly when magnesium is alloyed with aluminum up to 4 wt.%, but further addition up to 9 wt.% results only in a modest improvement in corrosion resistance [10]. A large number of studies have been carried out in order to assess the effect of aluminum content, casting shape, thermal treatment and alloy microstructure on the corrosion behaviour of Mg–Al alloys immersed in chloride-containing solution [12]. Among various commercial magnesium alloys, AM (Mg–Al–Mn) series are the widely used ones, as well as AZ (Mg–Al–Zn) series. Their specific properties such as excellent energy absorption and ductility together with reasonable yield and tensile properties make them suitable to be employed for die casting process to produce parts which required good ductility and toughness such as automotive wheels [13,14]. Manganese (Mn) is usually added to suppress the harmful effect of iron on corrosion resistance through the formation of Al–(Fe, Mn) particles in the melt [15,16]. According to the isothermal section of Mg–Al–Mn ternary phase diagram, state organizations at room temperature are usually a-Mg, b-Mg17Al12 and MnAl intermetallics for Mg–Al–Mn alloy containing Mn < 1 wt.% [17]. Compared with AZ-series, there are fewer research reported on AM-series

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Test specimens were prepared from die cast Mg alloy AM60 sample containing in wt.%: 6.0 Al, 0.27–0.3 Mn and balance Mg. The electrode was embedded in Araldite epoxy resin inside a Pyrex tube leaving cross-sectional area of the specimen 0.22 cm2 to face the test solution and serving as the working electrode. Prior to each experiment the exposed surface was ground successively using a finer grades of emery papers to 1500 grit, rubbed against a smooth polishing cloth, degreased in acetone, and dried in the atmosphere before being immersed quickly in the chloride medium. Analytical grade sodium chloride (NaCl) and cerium nitrate (Ce(NO3)36H2O) reagents and triply distilled water were used to prepare stock solutions of 2.0 M NaCl and 0.25 M Ce(NO3)3. The required Ce(III)containing 0.1 M NaCl solutions over the concentration range 0–10 mM Ce(NO3)3, were then prepared from the stocks by appropriate dilutions. The electrochemical cell consisted of a typical three electrode Pyrex glass cell. A saturated calomel electrode (SCE) was used as the reference electrode to which all potentials were referred and a large platinum sheet of size 15 mm  20 mm  2 mm as the auxiliary electrode. The electrochemical workstation IM6e Zahner-elektrik, GmbH (Kronach, Germany) provided with Thales software was used as a measuring instrument. Cathodic and anodic polarisation curves were recorded at 1 mV s1 scan rate after leaving the electrode immersed freely in the test solution for 90 min to acquire a quasi-steady potential value. Corrosion current density (icorr) which is equivalent to the corrosion rate was estimated with an accuracy of more than 95% by extrapolating the most linear part of the cathodic Tafel curve , after 50 mV negative to Ecorr, back to the mixed potential of zero net current (Ecorr) [30,31]. Electrochemical

3. Results and discussion 3.1. Open circuit potential transients Fig. 1 shows the time dependence of the open circuit potential (OCP) for AM60 electrode monitored over a period of 90 min in quiescent 0.1 M NaCl solution containing various Ce3+ ion concentrations (0.0–10 mM). The transients have all similar trend, where the potential rapidly increases from the incipient of immersion, and then gradually tends towards a quasi-steady value less negative than the initial one (at t = 0). The positive shift of potential can be attributed to deposition of corrosion products on the alloy as a result of the interaction between the electrolyte and the electrode that can effectively seals its surface against further reaction [8,32]. This continues as a result of the predominance of the cathodic processes over the anodic ones until the spontaneously growing passive film acquires a stable thickness [33]. As can be clearly seen, Ce3+ addition shifts positively the steady state potential in the noble direction, suggesting that Ce3+ ion can serve as an anodic-type inhibitor. The results in general demonstrate that the inhibiting effect of Ce3+ ion could be resulted from the formation and deposition of Ce oxide and hydroxide [34] on the active anodic sites of the electrode surface via hydrolysis of cerium ions. The build-up of a Ce-containing film partially covers the a-Mg matrix in the alloy microstructure. This induces a decrease in the micro-galvanic effect, and hence reduces alloy degradation. 3.2. Polarisation behaviour Fig. 2 presents the potentiodynamic polarization Tafel plots (E-log i) of AM60 alloy scanned after an OCP stabilization period -1.48 1.0 5.0 10

-1.50

-1.52

-1.54

oc

2. Experimental

impedance spectra (EIS) were recorded at Ecorr (i.e. at the stabilised OCP) using a single sinusoidal excitation signal of 10 mV amplitude while the frequency varied over the range 10 kHz–100 mHz with five points per decade. All impedance data were fitted to an appropriate equivalent circuit, using both the real and imaginary components of the data. Scanning electron microscope (SEM) images were recorded using the JEOL JXA-840A electron probe micro-analyzer equipped with energy-dispersive X-ray (EDX) facilities. Experiments were always carried out without stirring or deaeration inside an air thermostat which kept at 25 ± 0.2 °C. To achieve reproducibility each run was repeated at least twice with freshly prepared solution.

E / V(SCE)

magnesium alloys [18]. One of the most attractive AM-series which is belong to Mg–Al system is AM60 magnesium alloy. It has been claimed [12] that this alloy when immersed in 3 wt.% NaCl solution has much lower corrosion rate than ZE41 and higher corrosion rate than AM30, AZ91, AZ31 or pure Mg. Over the past several decades, the corrosion resistance of magnesium alloy was improved by surface treatments, alloying, and other methods [19–23]. Addition of rare earth (RE) as alloying elements to the solid phase is an effective way to improve the corrosion behaviour of AM60-RE (RE = Ce or La) magnesium alloy [24]. The scavenger effect, optimized microstructure and formation of more protective corrosion product films were considered as the main key factors to enhance the corrosion resistance of RE-containing AM60 alloy surface and to the inhibition of further corrosion [18,25,26]. Recently, a chemical conversion coating based on cerium, zirconium and niobium compounds was designed and patented by Marcus and Ardelean [27]. The effect of this new coating on the corrosion resistance of AZ91 and AM50 alloys have been evaluated by electrochemical measurements and XPS data [28]. Rudd et al. [29] reported on the corrosion protection afforded by cerium, lanthanum and praseodymium conversion coatings formed on pure magnesium and on WE43 Magnesium alloy in borate buffer solutions. A significant drop in the corrosion rate was observed for the substrate coated in rare earth salt containing solutions. The study reported here was carried out on die cast AM60 magnesium alloy in order to investigate the anticorrosion properties of various Ce(NO3)3 salt additions to 0.1 M NaCl solution using electrochemical techniques (open circuit potential (OCP), polarisation curves and electrochemical impedance spectra (EIS)). The aim is to provide some insight into the fundamental mechanism for the performance of Ce3+ ion as a green inhibitor to enhance the corrosion resistance of AM60 alloy in the aggressive chloride solution. EIS data, SEM images and EDX microanalyses were used to characterise the alloy surface after immersing the alloy sample for different duration periods at 25 °C.

0.1 mM 0.0

-1.56

-1.58

-1.60 0

20

40

60

80

100

time / min Fig. 1. Variation with time of the OCP for AM60 alloy in 0.1 M NaCl solution as a function of Ce3+ ion concentration at 25 °C.

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It is generally accepted that hydrogen evolution reaction is the dominant local cathodic process during the corrosion of magnesium [37] and its alloys [8,28,38] in this type of environment via water or oxygen reduction with hydrogen release:

0

-2

log (i / A cm )

-2

0.0 0.1 1.0

-4

5.0

-8 -2.0

-1.6

-1.2

-0.8

ð1aÞ

2Hþ þ 2e ! H2 "

ð1bÞ

Moreover, it is known that [39] contrary to most conventional metals and alloys hydrogen evolution is always associated with the anodic dissolution of Mg and its alloys, where the cathodic process normally slows down while the anodic oxidation of the metal is accelerated by anodic polarisation. This is a very common phenomenon, which is known as the negative difference effect (NDE) [40]. In the broken areas or on a film-free surface, the local corrosion mechanism is consistent with the involvement of the intermediate species monovalent Mg+ which is formed via an electrochemical oxidation of magnesium, followed by its chemical reaction with water molecules to produce hydrogen and divalent Mg2+ cation [7,39]:

10 mM

Ebd

-6

2H2 O þ 2e ! 2OH þ H2 ";

-0.4

E / V(SCE) Fig. 2. Potentiodynamic polarisation plots of AM60 alloy in 0.1 M NaCl solution as a function of Ce3+ ion concentration, scanned after 90 min exposure at 25 °C. 3+

of 90 min in 0.1 M NaCl solutions containing various Ce ion concentrations. From these curves the corrosion current density (icorr), corrosion potential (Ecorr), and anodic breakdown potential (Ebd) are all evaluated and summarized in Table 1. As can be seen, icorr first decreases passing by a threshold value at 1.0 mM Ce3+ ion addition, and then increases for higher Ce3+ ion concentration. In all cases, the shape of the cathodic branches of the polarisation curves are almost the same and the curves follow Tafel behaviour. On the other hand, during the course of anodic polarization, solution composition affects the detailed features of the anodic branches. Two types of behaviour occur: (i) a sharp increase after Ecorr with a very small break in the current, as for the case in the plain chloride solution, and (ii) a well-defined current plateau after Ecorr as for the case in Ce3+-containing chloride solutions, suggesting the existence of a partially protective film on the alloy surface [8,30,35]. For magnesium alloys the current plateau extends to a certain anodic potential domain up to the Ebd. This domain corresponds to an apparent passivation zone, where the duration of its current plateau is very dependent on the solution composition and its aggressive nature. Over the potential region of the passive current zone a steady state is established between the rates of metal dissolution and passive film formation. Generally, the breakdown potential (Ebd) of a given metal or alloy in corrosive environments is considered as an indication of the capability of its anodic passive film to resist localized corrosion damage [8,30,36]. The breakdown of the passive film through its defective sites would expose the underneath substrate to the corrosive environment. Therefore, a more positive Ebd for AM60 alloy in a given solution implies that its formed anodic film is more stable in terms of its localized corrosion performance. From the results of Fig. 2 and Table 1, it can be seen that Ebd moves positively with increasing Ce3+ ion concentration, indicating that under anodic polarisation the presence of Ce3+ ion in the chloride electrolyte can effectively delay localized attack and subsequent pit propagation.

Mg ! Mgþ þ e þ

Mg þ H2 O ! Mg

ð2aÞ 2þ



þ OH þ 1=2H2 "

ð2bÞ

As this reaction proceeds local areas of alkalinity are established at which precipitation of mixed rare earth/magnesium oxides/hydroxides occur providing a barrier between the aggressive solution and the metallic surface. This mixed salt has superior protection than magnesium oxide/hydroxide layer, and thereby its breakdown resistance is much better improved, rendering Ebd to occur at more positive potential. Such anodic behaviour at high potentials is partially controlled by mass transfer [28,41] and consequently depends on the amount of Ce3+ ion added to the test chloride solution. The results indicate that the rare earth Ce improves the passivation tendency of AM60 to passivation under anodic polarisation, and thus reduces both the extent of hydrogen gas evolution on the alloy surface and the severity of the attack. On the other hand, the two corrosion parameters Ecorr and icorr exhibit an optimum Ce3+ ion concentration for which Ecorr becomes nobler and icorr acquires its minimum value. This behaviour indicates that Ce3+ ion addition makes the corrosion products of AM60 in chloride medium more compact, and promotes the formation of Al and Ce oxide and hydroxide, which improve the protectiveness of the surface film. However, for concentrations higher than 1.0 mM, Ce3+ addition possesses low activity and the stability of the surface film decreases, probably due to the big cathode and small anode effect causing dissolution to increase [18,32]. These results will be further testified by the following EIS, SEM and EDX analyses. 3.3. Electrochemical impedance behaviour 3.3.1. Influence of concentration Electrochemical impedance spectroscopy (EIS) is a non-destructive technique which provides minimal perturbative signal and thus can be proposed as a tool to follow up the stationary behaviour of metallic materials with nearly no damage for their surfaces. The EIS characteristics of AM60 alloy in 0.1 M NaCl solution containing

Table 1 Impedance and corrosion parameters of AM60 alloy after 90 min immersion in 0.1 M NaCl solutions containing various Ce3+ ion concentrations measured at 25 °C. [Ce3+] mM

Rb (kX cm2)

Cb (lF cm2)

a

Rp (X cm2)

Cp (lF cm2)

Rs)X cm2)

Ecorr (VSCE)

icorr (lA cm2)

Ebd (VSCE)

Blank 0.1 1.0 5.0 10

0.50 0.76 1.05 0.46 0.40

1.68 0.64 0.14 0.23 0.37

0.70 0.73 0.74 0.72 0.71

5.37 5.54 10.61 5.06 4.98

5.53 5.52 6.06 6.80 7.86

16.3 12.5 16.5 19.8 29.7

1.559 1.555 1.492 1.498 1.501

2.01 1.89 1.14 1.31 1.21

1.511 1.410 1.398 1.244 1.042

F.E.-T. Heakal et al. / Corrosion Science 56 (2012) 86–95

(a) 3.5

2

0.0 5.0

2.5

-40 10

2.0 -20

phase / degree

0.1

log (lZl / Ω cm )

-60

1.0 mM

3.0

1.5 0 1.0 -1

0

1

2

3

4

log (f / Hz)

(b) -600

Z''/ Ω cm

2

-400 1.0 mM

0.1

-200 0.0 10

5.0

0 0

200

400

600

800

Z'/ Ω cm2

(c)

Fig. 3. Electrochemical impedance spectra as (a) Bode and (b) Nyquist plots for AM60 alloy in 0.1 M NaCl solution as a function of Ce3+ ion concentration, traced after 90 min exposure at 25 °C. (c) Electronic equivalent circuit used in the fitting procedure of the EIS data.

various Ce3+ ion additions in the domain 0.1–10 mM were recorded at the OCP after immersing the sample for 90 min in each solution to reach a quasi stationary condition. Fig. 3a and b presents the impedance results as Bode and Nyquist plots, respectively. The general profile of the spectra and the order of magnitude of the impedance (|z|) are similar for all solutions without and with Ce3+ ion. The corrosion mechanism can be estimated by analyzing the measured impedance spectra using a proposed electronic equivalent circuit (EC) which has an acceptable fitness with the experimental EIS data. The symbols in the figure represent the measured data and the lines represent the theoretical simulated data generated using the equivalent circuit shown in Fig. 3c. As can be seen, the impedance spectra on the Bode format show resistive regions at high and low frequency (HF and LF) ranges, where log |z| tends to become constant (horizontal line) with phase angle value falling towards zero degree (h  0°). At HF the solution resistance (Rs) dominates, while at LF both Rs and Rp are the dominant parameters [42], where Rp is the polarisation resistance. But in the middle frequency (MF) range there is a marked capacitive response. The changing of solution

89

conductivity with changing its composition results in different impedance responses at HF and LF ranges [3]. On the other hand, the Nyquist format of the impedance spectra are characterised by two scarcely depressed capacitive loops, followed by an indication for an incipient inductive loop at frequencies lower than 0.1 Hz. The semicircle at the high and intermediate frequency regions is always larger than the second one in the low range of frequencies. The diameter of the large capacitive loop, as well as, the absolute impedance (|z|) at the LF limit (0.1 Hz), firstly increase with Ce3+ ion concentration up to 1.0 mM, and then decreases when Ce3+ ion addition in the chloride solution exceeds that level. This trend corroborates well with the behaviour of the corrosion rate in terms of icorr, where the corrosion rate decreases at first with increasing Ce3+ concentration and then increases for overdoses more than 1.0 mM in the chloride solution (cf. Table 1), i.e., the corrosion resistance of AM60 alloy is firstly improved, and then reduced. On the whole, the results indicate that oxide film formed spontaneously on AM60 alloy in 1.0 mM Ce3+-containing chloride solution displays optimum corrosion protection characteristics than those formed by adding lower or higher Ce3+ ion concentration, where a decrease in the corrosion resistance is detected. Generally, the impedance response of an actively corroding metal in an aqueous solution is well simulated by the classic Randles circuit of parallel resistor capacitor (RC) combination in series with the solution resistance (Rs) between the specimen and the reference electrode [42]. For an electrode surface covered with a passive film and its impedance diagrams have two merging time constants, this implies that the film consists of two layers. For this case the impedance data should be numerically fitted to an appropriate equivalent circuit with two time constants. The best fitting parameters and lowest error were obtained with the parallel model depicted in Fig. 3c. No diffusion phenomena were recognized in the present results; therefore the model neglects any diffusion process. In this circuit model, Rs is the solution resistance, Rp is the charge transfer resistance of the outer porous layer, Rb is the resistance of the inner barrier oxide layer, while Cp and Cb are the two capacitances associated with the porous and barrier layers, respectively [18,43,44]. By this way, the time constant at LF range can be related to the barrier oxide layer due to the presence of the additive, whilst the other one in the HF range is attributed to the charge transfer resistance and the double layer capacitance at the outermost porous layer of the surface film [18,43,45]. In the present work, the former time constant (RbCb) dominates the corrosion characteristics of AM60 alloy in chloride solution at all Ce3+ ion additions. However, instead of an ideal capacitance element for the barrier film (Cb), a constant phase element (CPE) was introduced in the fitting procedure that gave consistency overall the frequency range and reduced the error to an average of <3%. The impedance associated with the capacitance of the CFE is a combination of properties related to both surface and electroactive species and is described by the relation [46]:

Z ðCPEÞ ¼ Q 1 ðj2pf Þn

ð3Þ

where the complex operator j is the square root of negative one, f is the frequency in Hz = s1 of the applied ac signal in the impedance scan, and Q and n are empirically determined parameters. The admittance Q in sn O1 will be identical to the idealized capacitance (Cb) at the angular frequency x = 1 (x = 2pf rad s1). n value varies between 1.0 for purely capacitive behaviour associated with ideally flat surface and 0.5 for a porous electrode [47]. Using Thales software provided with the workstation, the fitted characteristics circuit parameters for AM60 in 0.1 M NaCl solutions as a function of Ce3+ ion concentration were all estimated and presented in Table 1. Inspection of the results indicates clearly that the resistance (Rb) of the barrier layer has an inverse trend with its capacitance (Cb). Since

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F.E.-T. Heakal et al. / Corrosion Science 56 (2012) 86–95

the passive film on the metal surface can be considered as a dielectric plate capacitor, Cb is inversely proportional to the film thickness (d) in cm following the formula [8,30,43,44,46,48]: d ¼ e0 er A=cb , where e0 is the vacuum permittivity (8.85  1012 F cm1), er is the relative dielectric constant of the film and A is the electrode area in cm2. Fig. 4 shows the influential role of small Ce3+ ion additions (61.0 mM) on the surface film stability of AM60 alloy in 0.1 M NaCl solution. Both Rb and 1/Cb sharply increase with increasing Ce3+ concentration reaching a maximum value at 1.0 mM Ce3+ addition, while icorr decreases more steeply to a minimum value. Thereafter, there is a gradual decrease in both Rb and 1/Cb commensurate with a little increase in the corrosion rate (icorr) until a relatively stabilized state at 10 mM Ce3+ ion. The results clearly demonstrate the reactivity of Ce3+ ion towards enhancing spontaneous growth of a thicker and more protective surface film on AM60 alloy via a dissolution– formation mechanism. It has been reported that AM60 magnesium alloy exhibits two phase structure as documented by SEM micrograph [17,18], Mg-rich a- and Al-rich b-phases. The standard electrochemical potential of a-phase is much lower than that of b-phase [49], therefore, the potential of the a-phase should be also lower than that of b-phase, as a result a-phase preferentially corrode than b-phase due to the micro-galvanic couple effect [12]. In aqueous solution the released Mg2+ ions from the active sites on the anodic a Mg-based phase deposit as a porous Mg(OH)2 with lower protection ability. On the other hand, in presence of Ce3+ ion, Ce(OH)3 preferentially formed owing to its lower solubility product (Ksp = 1.6  1020) than that of Mg(OH)2 (Ksp = 1.8  1011). This can be dehydrated to Ce2O3:

2CeðOHÞ3 ! Ce2 O3 þ 3H2 O

ð4Þ

In aerated chloride solution oxygen can also react with Ce(OH)3 forming Ce(OH)4, which further dehydrates to CeO2:

4CeðOHÞ3 þ O2 þ 2H2 O ! 4CeðOHÞ4 CeðOHÞ4 ! CeO2 þ 2H2 O

ð5Þ ð6Þ

the corrosion rate (icorr), commensurate with a subsequent increase in the barrier film resistance (Rb) and its relative thickness (1/Cb). Rare earth Ce makes the corrosion products compact and promotes the formation of Al and Ce oxides, which improves the protective effectiveness of the corrosion product layer [18]. This reduces the corrosion tendency of AM60 alloy and increase the stabilization of its surface. Nevertheless, the corrosion resistance deteriorates with overdose rare earth Ce3+ ion in solution (>1.0 mM). The increasing corrosion rate for large Ce3+ ion additions is attributed to a greater degree of micro-galvanic corrosion because of the greater volume fraction of the second phase, i.e. due to the increment of the cathodic phase [12,18]. This means that the second b phase has the tendency to cause micro-acceleration of the corrosion of the a-Mg matrix [3,10,12]. For AM60 alloy the volume fraction of the second phase would be the limiting factor that determines the corrosion mechanism of the alloy in chloride solution with various Ce3+ ion concentrations. Thereby, corrosion is inhibited at lower concentrations and the corrosion resistance increases to a maximum value at optimum concentration of 1.0 mM Ce3+ then deteriorates for higher additions (>1.0 mM). This is likely due to a weaker protective ability of the passive film on the sample because of increasing its corrosion rate. 3.3.2. Influence of immersion time In this part, EIS scans were monitored as a function of the exposure period for AM60 specimen suspended continuously in 0.1 M NaCl + 1.0 mM Ce3+ solution up to 240 h. The Nyquist plots at different immersion times are presented in Fig. 5a and b for short- and long-periods, respectively. As can be seen, in all cases the diagrams reveal two scarcely defined capacitive loops at high and medium frequencies as they merge into an extended depressed semicircles. The

(a)

CeO2 can also be formed via dehydration when Ce(OH)3 is further react with OH ion:

CeðOHÞ3 þ OH ! CeO2 þ 2H2 O þ e

ð7Þ

Meanwhile, possible dehydration of Mg(OH)2 can not be excluded. Once the alloy surface is covered by generated insoluble oxides of Ce and Mg deposits, they segregate the alloy from the corrosive medium, leading to suppress both of the active surface area and 8

1/Cb

1.0

2.0 6

4 0.6

3

(b)

1.8

-1

0.8

cm

5

1/C b / μF

R b / kΩ cm

2

2

icorr

7

−2

Rb

2.2

1.6

1.4

icorr / μΑ cm

1.2

2 0.4

1.2 1

0.2

0 0

2

4

6

8

1.0

10

3+

[Ce ] / mM Fig. 4. Dependence of surface film resistance (Rb) and its relative thickness (1/Cb), and corrosion rate (icorr) for AM60 alloy on Ce3+ ion concentration in 0.1 M NaCl solution measured after 90 min exposure at 25 °C.

Fig. 5. Nyquist plots for AM60 alloy in 0.1 M NaCl + 1.0 mM Ce3+ solution as a function of the immersion time for (a) short-term, and (b) long-term exposure periods at 25 °C.

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F.E.-T. Heakal et al. / Corrosion Science 56 (2012) 86–95 Table 2 Impedance parameters of AM60 alloy in 0.1 M NaCl + 1.0 mM Ce3+ solution as a function of immersion time at 25 °C. Time (h)

Rb (kX cm2)

Cb (lF cm2)

a

Rp (X cm2)

Cp (lF cm2)

Rs (X cm2)

0.5 1.0 2.0 3.0 4.0 6.0 22 46 93 136 168 240

0.94 0.99 1.62 1.81 1.98 2.27 4.36 2.25 1.37 1.23 1.00 0.93

2.22 2.21 2.01 3.28 3.22 3.22 0.25 0.22 3.01 3.45 4.46 2.574

0.77 0.80 0.75 0.81 0.82 0.82 0.46 0.41 0.80 0.79 0.82 0.85

39.09 45.63 20.17 35.18 29.13 35.53 39.36 24.82 34.43 34.80 38.94 73.86

5.43 5.31 4.77 4.77 2.68 2.50 5.41 6.73 5.20 4.32 4.50 5.11

15.18 16.28 16.72 17.82 20.65 21.56 23.96 16.87 13.43 13.67 12.40 16.59

figure shows progressive increase in the size of the semicircle with increasing the duration of exposure. However, after 24 h the size of the semicircle starts to shrink substantially with further immersion up to 168 h till almost stationary state. Based on the circuit model shown in Fig. 3c, the experimental data were analyzed and the calculated theoretical impedance parameters are listed in Table 2. The results indicate that the capacitance Cp is relatively higher than Cb. This is because Cp represents the double layer capacitance at corroding sites and Cb is that for the protected non-degrade zone on the electrode surface. The value of Rb increases rapidly from 0.94 kO cm2 at 30 min to 4.36 kO cm2 after 22 h immersion, and then decreases less sharply with time indicating reduction in surface film stability until a steady state is reached. Fig. 6 shows that the trends of Rb, which is inversely proportional to the corrosion rate, are mirrored completely by those of the OCP or Ecorr. In 1.0 mM Ce(III)-containing NaCl solution Ecorr shifts positively and then negatively with time until reach a quasi-steady value. However, the behaviour of Rb and Ecorr with immersion time can be separated into two different stages, the first being in short-term exposure up to 24 h, and the other in long-term exposure from 24 to 240 h. During the short immersion time, H+ ion can be consumed due to the initial stage of corrosion and local pH increases, indicating OH ion formation on the alloy surface. Then passivation and self-healing of anodic zone occurs with a subsequent increase in both values of Rb and Ecorr due to continuous accumulation of

-1.47

0.6

5 Rb

-1.48

1/ Cb

0.5 -1.49 0.4

3

2

0.3

1

0.2

1/C b / μF-1 cm 2

R b / kΩ cm 2

Ecorr

-1.50 -1.51 -1.52

Ecorr / V (SCE)

4

-1.53 -1.54 0

0.1 0

50

100

150

200

-1.55

250

Time / h Fig. 6. Evolution of surface film resistance (Rb) and corrosion potential (Ecorr) for AM60 alloy in 0.1 M NaCl + 1.0 mM Ce3+ solution over 240 h exposure period at 25 °C .

cerium-containing protective film on the sample surface, so further corrosion is inhibited. On the other hand, during the later stage of immersion, the destruction due to the corrosion attack of Cl ion surpasses the protection owing to corrosion product deposited film, where these two processes occur simultaneously on the electrode surface. Thus the corrosion resistance starts to decrease and Ecorr drifts towards more negative values. At last the two effects on the corrosion resistance are relatively balanced and Rb as well as Ecorr keeps at relatively stable values, which indicate that the corrosion rate is controlled by the cathodic process via a formationdissolution mechanism [18]. In favour of this behaviour is the corrosion morphologies of AM60 surfaces exposed for 2, 24 and 240 h in 0.1 M NaCl solution containing 1.0 mM Ce3+ ion at open circuit potential depicted in Fig. 7a–c, respectively. As can be seen, the sample after 24 h immersion shows no significant corrosion, confirming the good corrosion resistance of its surface film as compared to the films after 2 h or 240 h. However, the image of the film formed on the sample after 240 h immersion in a plain 0.1 NaCl solution (Fig. 7d) indicates areas with severe pitting corrosion attack observed at grain boundaries, at defects and within the grains. At this point, it is of interest to obtain interfacial information concerning the stability of the surface film formed on AM60 alloy in 0.1 M NaCl + 1.0 mM Ce3+ solution after 240 h exposure, and its propensity to protect the metallic substrate from degradation in chloride medium. The frequency response diagrams for the sample bearing this film were monitored with time in a plain 0.1 M NaCl solution. Fig. 8a presents the Nyquist plots of the obtained EIS data, and the equivalent circuit parameters which gave the best fit with the impedance results were estimated using a model analogous to the one shown in Fig. 3c. As can be seen, the diameter of the capacitance loop firstly decays after 1 h compared to the one at commencement of immersion (at 0 h), and then increases with time. The decrease during the incipient immersion may be ascribed to the weaker protective ability of the film on the sample due to pitting corrosion in some areas caused by ingress of Cl ions. Nevertheless, after a short time (>1 h) spontaneous healing occurs via accumulation of the corrosion debris in the pitting corroded zones followed by formation and thickening of the product film on the alloy. As the contact area between substrate and electrolyte is reduced, the diameter of the capacitance loop re-increases with time, subsequently the barrier film resistance (Rb) and its relative thickness (1/Cb) increase, indicating an increase in the film stability, as shown in Fig. 8b. This improvement in film performance can be attributed to a depression in the micro-galvanic couple and a decrease in the conductive character of the film. However, for longer exposure periods (>20 h) more corrosion products stifle the reaction in chloride medium [44], causing the resistance and relative thickness of the surface film to decrease and go to stabilized state after more than 100 h.

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Fig. 7. SEM micrographs of AM60 alloy immersed in (a–c) 0.1 M NaCl + 1.0 mM Ce3+ solution for: (a) 2 h, (b) 24 h, (c) 240 h; and (d) immersed in a plain 0.1 M NaCl solution for 240 h.

Z'' / Ω cm

2

-1500 -1000

0h 1 2 3 4 18 48 94

-600

8

-500

6

5 1

2

0 0

60 min

2

1, 2, 3, 4, 5, 6, 7, 8,

Z'' / Ω cm

(a) -2000

7

3 4

-400 120 15

-200 240 0.0

500

1000

1500

2000

2500

0

3000

Z' / Ω cm

2

400

600

800

1000

1200

Rb

3.0

1/Cb

0.35 2

2.5

0.30

2

2.0

Fig. 9. Nyquist plots traced after 90 min immersion in a plain 0.1 M NaCl solution, for pre-treated AM60 surface in a single 1.0 mM Ce(NO3)3 solution as a function of the treatment time.

-1

Rb / kΩcm

200

Z' /Ω cm2

0.40

1/Cb / μF cm

(b)

0

3.5

0.25 1.5

0.20

1.0

0.15

0.5 0.0

0.10 0

20

40

60

80

100

time / h Fig. 8. (a) Nyquist plots traced in a plain 0.1 M NaCl solution for the 240 h formed film on AM60 alloy in 0.1 M NaCl + 1.0 mM Ce3+ solution as a function of the exposure period at 25 °C. (b) Evolution of surface film resistance (Rb) and its relative thickness (1/Cb) with time, obtained from impedance data in (a) by fitting analysis.

3.4. Behaviour of pre-treated sample In these experiments the corrosion behaviour of chemically treated AM60 sample in 1.0 mM Ce(NO3)3 solution (pH 5.8) was tested in 0.1 M NaCl solution, as a function of the treatment time using ac impedance and dc polarisation techniques. The treatment step was made by dipping AM60 electrode in 1.0 mM Ce(NO3)3 solution for different treatment periods (0.0, 15, 60, 120, 240 min). This step leads to the formation of adherent conversion coatings, but could be removed by scratching the surface. Fig. 9 shows the Nyquist diagrams of the untreated and treated substrate traced in 0.1 M NaCl solution after 90 min immersion as a function of the treatment times. As can be seen, the diameters of the capacitive loops adopted by the treated samples are all larger than the other for the untreated one. However, upon extending the treatment time the Ce conversion film exhibits better protection, and it begins to deteriorate in

chloride medium for treatment periods exceeding 60 min. Analysis of these results, using the same EC shown in Fig. 3c, indicates that the conversion film resistance (Rb) sharply improved when the conversion time is extended to 60 min, and then it decreases (cf. Table 3 and Fig. 10). On the other hand, the relative film thickness (1/Cb) follows a reverse trend, indicating that thinner conversion films are more passive than thicker ones. The results generally reveal that the stability of preformed Ce films on AM60 by simple immersion is time dependent, as a thinner, more compact and dense films can be formed on the alloy surface when the sample is chemically treated in Ce(NO3)3 solution for short intervals up to 60 min. However, as the treatment time is extended a thicker porous conversion films with lower protective ability will be formed. Fig. 11 depicts the SEM images obtained after 90 min immersion in a plain 0.1 M NaCl solution for the 60 min and 240 min ceriumtreated AM60 samples. The two images reveal clear difference between the extent of corrosion damage in relation to the treatment period, which firmly confirms the impedance results. Potentiodynamic polarization curves of cerium treated AM60 alloy for different treatment periods, were also scanned after 90 min immersion in a plane 0.1 M NaCl solution compared with the curve for the untreated (bare) surface as shown in Fig. 12. The corrosion potential (Ecorr) and current density (icorr), as well as the breakdown potential (Ebd) were all calculated as described above and presented in Table 3, together with the impedance parameters, as a function of the conversion time. From Fig. 12, it can be seen that the apparent passivation region only appears for the treated samples and increases with the treatment time. This indicates that increasing Ce3+ ion concentration in the conversion layer decreases the tendency for anodic breakdown of the passive

93

F.E.-T. Heakal et al. / Corrosion Science 56 (2012) 86–95 Table 3 Impedance and corrosion parameters in the blank 0.1 M NaCl solution for pre-treated AM60 surface as a function of the treatment time in 1.0 mM Ce3+ solution at 25 °C. Rb (kX cm2)

Cb (lF cm2)

a

Rp (X cm2)

Cp (lF cm2)

Rs (X cm2)

Ecorr (VSCE)

icorr (lA cm2)

Ebd (VSCE)

0 15 60 120 240

0.50 0.83 0.94 0.84 0.81

1.68 2.08 2.33 1.83 1.46

0.70 0.77 0.89 0.82 0.79

5.37 101.60 164.87 66.95 58.04

5.53 5.25 5.17 4.50 4.37

16.3 10.7 16.2 12.4 21.3

1.559 1.564 1.547 1.545 1.542

2.01 0.27 0.02 0.03 0.03

1.511 1.510 1.460 1.346 1.080

0.60

0.7

0.55

0.6

0.50

0.0

-2

0.8

-2

log (i /A.cm )

0.65 2

0.9

cm

0.70

-1

1.0

1/Cb/ μF

Rb/ kΩ cm

2

Time (min)

-4 240 min. 15

-6

60

120

Rb 0.5

Ebd

0.45

1/Cb

-8 0.4

0.40 0

1

2

3

4

5

Time / h Fig. 10. Dependence of surface film resistance (Rb) and its relative thickness (1/Cb) on the treatment time for chemically treated AM60 alloy in a single 1.0 mM Ce(NO3)3 solution, measured after 90 min immersion in a plain 0.1 M NaCl solution at 25 °C.

Fig. 11. SEM micrographs for: (a) 60 min and (b) 240 min chemically treated AM60 surfaces in 1.0 mM Ce(NO3)3 solution, obtained after 90 min immersion in a plain 0.1 M NaCl solution.

film in corrosive chloride medium, where Ebd value drifts positively from 1.511 V(SCE) for the untreated surface to 1.080 V(SCE) for the 240 min cerium treated one. Moreover, Ecorr value shifts positively from 1.559 V(SCE) for the bare surface to 1.542 V(SCE) for the 240 min treated one, while icorr can be reduced by two order of magnitude as compared to its value for the untreated sample. According to Pourbaix diagram for Mg–H2O system [5], it is proposed that in neutral and low pH solutions oxidation of magnesium (Mg ? Mg2+ + 2e) takes place. Cerium oxides and hydroxides generally contain two species, namely, Ce3+and Ce4+. In slightly acidic 1.0 mM Ce(NO3)3 solution (pH 5.8) formation of Ce oxide and/or hydroxide is not thermodynamically feasible. Thus Ce3+ ions deposit on AM60 surface in the cracks and on the grains, this reduces the

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

E / V (SCE) Fig. 12. Potentiodynamic polarization plots scanned after 90 min immersion in a plain 0.1 M NaCl solution, for chemically treated AM60 surfaces in a single 1.0 mM Ce (NO3)3 solution as a function of the treatment time at 25 °C.

corrosion tendency of the alloy and increases its stabilization when the treatment time is short (not exceeding 60 min), i.e. the anodic dissolution of magnesium (mainly a-phase) is inhibited. However, on prolonging the treatment duration, more Ce3+ ions congregate on the outermost surface layer as thick porous deposits, which increase the volume fraction of the second b-phase [12]. This phase can act as cathodes in micro-galvanic cells to accelerate corrosion, thereby activates the alloy surface and enhances its corrosion rate in chloride solution with fast decrease in Rb value. Fig. 13 shows the EDX spectra (a–d) corresponding to Figs. 7a–c and 11b, respectively. It can be seen that the main elements in the surface are Mg, Al, Mn (the constituents of the substrate) and O. Ce element is also detected. The spectra indicate that the content of Ce element in Fig. 13D is much more than its extent for those samples exposed directly to Ce (III)-containing chloride solution, suggesting that Ce on their surfaces is mostly located in the inner barrier layer of the corrosion product film. Nevertheless, Ce is congregated in the outermost surface layer for the chemically treated sample in good conformity with the impedance results. It can be then concluded that, when Ce is present as additive in the corrosive chloride medium, due to the Cl-attack on the Mg substrate, acceleration in the dissolution of Mg will lead to formation of a passivating coating containing Ce3+. Moreover, the coating is compact and with good adhesion, so it can improve the corrosion resistance of magnesium alloy AM60 in 0.1 M NaCl solution. On the other hand, when Ce is present as a post conversion coating after dipping the substrate in the 1 mM Ce3+ treatment solution of pH around 6, there will be no other ingredients to promote the dissolution of Mg from the alloy. Likewise, a non-dense coating with inferior adhesion is formed on the sample, which can be easily removed by scratching the surface. It follows that, if the sample with this conversion coating is tested in 0.1 M NaCl solution, it will be easier to corrode.

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Fig. 13. (a–d) EDX spectra recorded in Figs. 7a–c, and 11b. The content (in wt.%) of Mg, Al, O and Ce element for different samples detected from EDX analysis are listed in the corresponding spectra.

4. Conclusions Addition of rare earth Ce3+ ion can effectively improve the corrosion resistance of AM60 magnesium based alloy in 0.1 M NaCl solution. Increasing Ce3+ concentration up to an optimum level of 1.0 mM, sharply decreases the corrosion rate (icorr) and shifts positively both the corrosion (Ecorr) and breakdown (Ebd) potentials. This is obviously due to formation of a thicker and more compact resistive film on the alloy sample. The stability of the surface film decreases for over dose addition of Ce3+ ion (>1.0 mM) due to the increment of the cathodic phase (b-phase). At the optimal 1.0 mM Ce3+ concentration the spontaneously formed film on the alloy characterises by increasing resistance (Rb) and relative thickness (1/Cb) with time evolution until 24 h, thereafter both Rb and 1/Cb decrease and eventually achieve a steady value at longer time (>150 h). The film formed at stabilized conditions (after 240 h immersion) exhibits spontaneous self-healing and thickening with time in a plain chloride solution. Chemically treated AM60 alloy in a single 1.0 mM Ce(NO3)3 solution showed an improved surface protection against chloride attack when the treatment period not exceeding 60 min. SEM and EDX analysis confirmed that when Ce is present as additive in the chloride solution a more compact coating can be formed to slow down effectively the corrosion rate of the alloy, better than its presence as a post conversion coating on the alloy surface before being immersed in the corrosive environment. References [1] S. Schumann, H. Friedrich, Engineering requirements, strategies and examples, in: H.E. Friedrich, B.L. Mordike (Eds.), Magnesium Technology, Metallurgy,

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