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Double-layered manganese phosphate conversion coating on magnesium alloy AZ91D: Insights into coating formation, growth and corrosion resistance
Surface & Coatings Technology 217 (2013) 147–155
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Double-layered manganese phosphate conversion coating on magnesium alloy AZ91D: Insights into coating formation, growth and corrosion resistance Xiao-Bo Chen a, b,⁎, Xian Zhou b, Trevor B. Abbott a, c, Mark A. Easton a, Nick Birbilis a, b a b c
CAST Co-operative Research Centre, Monash University, VIC 3800, Australia ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering, Monash University, VIC 3800, Australia Magontec Limited, Sydney, NSW 2000, Australia
a r t i c l e
i n f o
Article history: Received 29 September 2012 Accepted in revised form 5 December 2012 Available online 14 December 2012 Keywords: Magnesium alloy AZ91D Corrosion resistance Conversion coating Manganese phosphate Polarisation EIS
a b s t r a c t A double-layered conversion coating system, consisting of magnesium hydroxide–magnesium/manganese phosphate, was applied to magnesium alloy AZ91D using an acidic manganese nitrate and ammonium dihydrogen phosphate solution. The coating structure, composition and morphology were characterised by SEM, EDX, XRD and XPS. A coating formation mechanism is proposed, and the effect of operating parameters, i.e. pH and temperature, on coating formation was systematically investigated, with optimised conditions able to produce coatings of high corrosion resistance. Corrosion resistance of the coating was evaluated by electrochemical and salt spray testing. The double-layered coating system develops in three stages: initial substrate dissolution, formation of a dense magnesium hydroxide layer, and then co-deposition of magnesium and manganese phosphate film. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Magnesium (Mg) and its alloys are promising structural and electronic materials for diverse applications, including transportation and 3C components, due to their light weight, specific strength and conductivity [1,2]. The development and use of Mg alloys in industry remain in infancy due to poor corrosion resistance upon exposure to air and humid conditions. Two primary methodologies are nowadays being extensively explored to improve the resistance to corrosion of Mg-alloys; these include bulk alloying and the use of barrier coatings on the Mg surface [3,4]. The latter method can decrease corrosion rate of Mg alloys by a few orders of magnitude, which is more efficient in the short term compared to alloy development [3,4]. Increasingly extensive research is being devoted to protective coatings for Mg [3,4]. Chemical conversion coatings stand out from other coating types that include anodising, electroplating, electroless plating, ion implantation, etc., owing to low cost and efficiency [4,5]. In general, no power or specific facilities are required to carry out conversion coating process, significantly reducing production cost. In addition to imparting corrosion resistance, conversion coatings could also be an adhesive base for subsequent plating or painting [6,7]. Since being implemented by DOW (DOW Chemical Co., USA), chromate conversion coating, has been the benchmark corrosion protection scheme, which also offers remarkable self-healing capability. ⁎ Corresponding author at: CAST Co-operative Research Centre, Monash University, VIC 3800, Australia. Tel.: +61 3 9905 9297. E-mail address: [email protected] (X.-B. Chen). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.12.005
Modern concerns and legislation regarding toxicity present the need for alternatives to the use of chromate, and research has been carried out upon: fluoride [8], phosphate [5,9,10], phosphate–permanganate [11,12], stannate [13–15], rare earth (RE) [16–18], vanadate [19,20], titanate [7,21] and ionic liquid (IL) conversion coatings [22,23]. Among these techniques, vanadate also presents an environmental hazard; the long processing time (up to 60 min) of stannate and RE coating procedures cause significant cost increase; ILs are costly, and concentrated (and toxic) HF is indispensable for fluoride coating. Phosphate conversion coatings on the other hand, are more environmentally friendly and have been successfully exploited to protect steel and zinc against corrosion in dilute aqueous solutions. Metal-phosphate layers provide not only corrosion protection but also some specific functions to substrates, i.e., a coating bath with the necessary phosphate anions and metallic cations, including Ca2+, Mn2+/3+/4+/7+, Zn2+ etc., may improve corrosion/wear resistance and sliding properties [6,24–26]. Further, biocompatibility and bioactivity of calcium phosphate (Ca–PO4), high wear resistance and adhesion strength of zinc phosphate (Zn–PO4), and lubricity of manganese phosphate (Mn–PO4), have also been demonstrated. In the past decades, the applications of Ca–PO4 [5,10,27,28] and Zn–PO4 [29–31] conversion coatings have been successfully expanded to light-weight Mg alloys. In contrast, very few studies have focused on protecting Mg alloys with Mn 2+–PO4 conversion coatings, in spite of being expected to be more stable and corrosion resistant than their Zn and Ca peers. Han et al. [32] and Zhou et al. [9,33] reported a manganese hydrogen phosphate (MnHPO4) conversion coating for AZ31D and AZ91D alloys and proposed a mechanism of the coating formation. Cui et al. also
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investigated the growing process of an MnHPO4 conversion coating on AZ31, but presented an alternative coating growth mechanism [34]. Though these existing reports indicate that the MnHPO4 conversion coatings had desirable corrosion resistance, the immersion/coating time, up to 30 min, is much longer than the basic requirement for industrial applications. The performance of the phosphate conversion coatings is dependent on the crystal structure as well as the morphology. For example, a microcrystalline structure is usually optimal for corrosion resistance or subsequent painting. A coarse grain structure impregnated with oil, however, may be the most desirable for wear resistance. These properties can be tailored by selecting the appropriate phosphate solution, using various additives, and controlling bath temperature, pH, ion concentration, and phosphating time [3,4]. As a result, this prompts further questions regarding the experimental conditions, such as bath temperature and pH, amenable for the synthesis of stable low-valence Mn–PO4 coatings on Mg alloys and a study of their corrosion resistance. In this study, we apply an Mn 2+–PO4 conversion coating onto die-cast Mg alloy AZ91D with the view of achieving significant improvement in corrosion resistance and exploiting a simple and low cost strategy. The systematic study is aimed at developments towards a practical replacement for DOW chromate coatings. Following preliminary trials, the coating process was simplified to involve one dipping-step in an acidic Mn–PO4 solution, eliminating conventional pretreatment steps [5,33,35], which is important for upscaling. The effect of the essential coating parameters over a certain range, i.e., pH (2.0 to 6.0) and temperature (ambient to 80 °C), on the corrosion resistance of the resultant coatings was investigated, with a view to understanding the nature of the Mn–PO4 conversion coating on AZ91D, and in producing more uniform protective films. 2. Experimental section 2.1. Mn–PO4 conversion coating preparation ASTM Mg alloy AZ91D, from the Mg alloy supplier HNKWE (China) was used in this study. The AZ91D contained 9.1 wt.% Al, 0.8 wt.% Zn, 0.24 wt.% Mn, 0.031 wt.% Si, 0.0023 wt.% Fe, 0.015 wt.% Cu, and 0.0005 wt.% Ni, balance Mg, as measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Spectrometer Services, Coburg, VIC, Australia). A Toshiba 250 ton clamping force cold chamber high pressure die casting (HPDC) machine was used to cast test plates of 70 × 60 × 2 mm in size. Smaller test samples with dimensions of 20 mm × 20 mm × 2 mm were cut from these plates and were ground to 1200 grit finish and used as substrate material. Prior to the one-step coating treatment, AZ91D samples were ultrasonicated in acetone at room temperature (RT) for 15 min and then in absolute ethanol for 10 min and rinsed with deionised water thereafter. The coating solution contained 0.01 M manganese nitrate (Mn(NO3)2), 0.01 M ammonium dihydrogen phosphate (NH4H2PO4). Nitric acid (HNO3) or ammonia (NH3·H2O) was utilised to adjust pH down or up (2.0– 6.0). All chemicals used in this work were analytical grade from Sigma-Aldrich. The coating process was thus conducted by immersing pre-cleaned AZ91D specimens into the coating solution varying from RT to 80 °C for 5 min. Following this, treated samples were dried in air and kept in a desiccator for further characterisation. 2.2. Thermodynamic calculations The software MEDUSA (which is an acronym for Make Equilibrium Diagrams Using Sophisticated Algorithms) was used to make a first order calculation of the complexes and phases in a given chemical system via equilibrium formation constants. MEDUSA nominally calculates at room temperature, however manual alteration of all parameters is possible at the users' discretion using the programme edit function.
Fig. 3b illustrates how the predominance of Mg2+ in the presence of phosphate (10 mM) and Mn2+ (10 mM) varies with pH and concentration of Mg 2+ ions, while Fig. 3c presents how the predominance of phosphate in the presence of Mg2+ (500 mM) and Mn2+ (10 mM) varies with pH and concentration of phosphate ions. 2.3. Microstructural analysis of Mn–PO4 conversion coatings Surface morphology of coated AZ91D alloys was observed using scanning electron microscopy (SEM, FEI Nova Nano) fitted with energy dispersive X-ray spectroscopy (EDXS). Structure and phase composition of the AZ91D surfaces before and after coating were identified by X-ray diffraction (XRD, Philips PW1140) using Cu-Kα radiation (λ = 1.5418 Å, 40 kV, 25 mA) and a scanning speed of 0.01°/min at a 2θ range of 20–50°. Surface chemistry was analysed by X-ray photoelectron spectroscopy (XPS, Thermo K-alpha, UK) with a hemispherical analyser and the core level XPS spectra for Mn2p, Mg2p, Al2p, Zn2p, P2p, O1s and C1s were recorded. The measured binding energy values were calibrated by the C1s (hydrocarbon C–C, C–H) of 285 eV. The photoelectrons were generated by Al-Kα (1486.6 eV) primary radiation (20 kV, 15 mA). 2.4. Corrosion evaluation For electrochemical tests, a flat-cell (PAR, K0235) containing 300 mL of 0.1 M NaCl electrolyte was used with an exposed working electrode area of 1 cm 2, a saturated calomel (SCE) reference electrode and Pt-mesh counter electrode. Electrochemical experiments were performed on a BioLogic® VMP-3Z potentiostat/galvanostat using EC-Lab 10.2 software. Potentiodynamic polarisation experiments were carried out at a sweep rate of 1 mV/s after 10 min of open circuit (OCP) conditioning. The polarisation curves were used to determine corrosion current density, icorr, via a Tafel-type fit using EC-Lab software. As a general rule, fits were executed by selecting a portion of the curve that commenced > 50 mV from corrosion potential; Ecorr and icorr were subsequently estimated from the value where the fit intercepted the potential value of the true Ecorr. More generally, polarisation testing was also able to visually reveal comparative information related to the kinetics of anodic and cathodic reactions between alloys and was in contrast to salt spray testing. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 100 kHz to 10 mHz, with a sinusoidal amplitude of 10 mV and five points per decade after 10 min of open circuit (OCP) conditioning. The EIS data was analysed using EC-Lab 10.2 software. At least three separate scans were performed for each data point to ensure reproducibility. The long-term corrosion resistance of various treated AZ91D was examined by salt spray testing up to 48 h according to the ASTM B117 standard. The samples were placed at a tilting angle of 30° in a chamber containing 5 wt.% NaCl fog at 35 °C. 3. Results and discussion 3.1. OCP evolution during formation of Mn–PO4 based conversion coatings Observation of OCP evolution for AZ91D in the Mn–PO4 bath as a function of immersion time, pH and temperature, is presented in Fig. 1, illustrating the development of the conversion coatings [7,10,36]. The OCP curves were evidently affected by bath temperature (Fig. 1a). When a constant pH value of 4.0 was applied, all the OCP curves obtained at elevated temperatures (40–80 °C) initially demonstrate an abrupt potential shift in the more noble direction (first 30 s), revealing the instant protective coating formed on the substrate upon exposure to the bath [7,10]. A rapid drop in OCP at the initial stage can be observed exclusively on the case treated at RT, demonstrating removal of the spontaneously air-formed loose MgO/MgOH/MgCO3 film and subsequent
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Fig. 1. Evolution of OCP for AZ91D immersed in an Mn–PO4 bath at various temperatures (a) and pH (b). The effect of the growth temperature and pH on the coating evolution is evident.
exposure of the AZ91D metallic substrate [7,10]. No such sudden falls in OCP being observed on other samples can be attributed to the elevated temperatures (40–80 °C), which stimulated the coating formation and moderated the dissolution time duration of the oxide layer. When the dynamic equilibrium of film dissolution–precipitation was established, OCP approached a plateau. The value of the plateau potential is a function of bath temperature (Fig. 1a). Higher bath temperature produced a coating with more positive OCP and correspondingly, higher corrosion resistance [7,10]. It is evident that the coating evolution, i.e., dissolution–precipitation, was determined by bath pH value (Fig. 1b). When temperature was kept at 80 °C, the high bath-acidity (pH 2.0) led to a continuous potential shift to the more noble region, which indicates ongoing preciptation on substrate [10,36]. On the contrary, the potential reached a plateau after immersed for 300 s in the mildly acidic bath (pH 4.0), indicating the establishment of an equilibrium between dissolution and precipitation [10,36]. A peak potential can be seen in the approximately neutral bath (pH 6.0) and followed by a drop, which is likely due to the local dissolution of the existing coating [7]. In this study, the optimal immersion time of 5 min was chosen for all cases according to the OCP graph. The coating formation process and their corrosion resistance will be discussed in following sections.
3.2. Microstructural characterisation of the Mn–PO4 conversion coatings Phosphatation is an endothermic reaction [37], thus, coating procedures conducted at low temperature do not provide sufficient energy
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to satisfactorily generate phosphates. Under such circumstances, the phosphating rate is either slow or does not take place at all. Consequently, long immersion times, up to a few days, may be required to give rise to a complete phosphate film — with extremely thin coatings being undesirable for corrosion resistance. In contrast, high processing temperature can offer enough activation energy and eventually accelerate the phosphating rate. A thick phosphate film can be achieved in a relatively short time and render corrosion resistance to Mg substrates. Observation of surface morphology of bare and treated AZ91D can reveal the effect of operating parameters, i.e., pH and temperature, on the formation of the Mn–PO4 coating, as presented in Fig. 2. After removal of the surface layer containing die-spray and oxide by mechanical grinding, bare AZ91D presents a silvery metallic finish and a microstructural feature of abraded grooves. The inset EDX data demonstrates the existence of Mg (~ 89.3 wt.%), Al (~ 9.9 wt.%) and Zn (~ 0.8 wt.%), which is in agreement with the ICP-AES results. Deposition of Mn–PO4 particles and subsequent coatings was significantly influenced by growth temperature when pH was maintained at 4.0. A few newly deposited particles appeared on the substrate treated at RT for 5 min, while the density and size of the white round particles scattering over the surface was progressively increased when bath temperature was elevated from 40 to 60 °C, which is attributed to the high reaction rate incurred by high temperature. The AZ91D treated at 70 °C displays a different surface morphology, where thin flakes and round clusters were distributed unevenly over surface. With higher treating temperature, the densely packed small particles had much more energy to merge into large grains and produce a high coverage. A compact but thin coating was produced at 80 °C, consisting of Mn (2.9 at.%), P (4.8 at.%) and O (39.0 at.%), in addition to Mg (46.7 at.%) and Al (6.1 at.%), as depicted by the inset EDX spectrum (Please refer to Table S1 in the Supporting information for all the EDX elemental analysis results). The high fraction of Mg and Al detected by EDX indicates that the conversion coating was thin (~1.5 μm, Figs. 3 and S1) and the strong Mg and Al signals were mainly from the underneath AZ91D. Some cracks were obvious on the sample treated at 80 °C whose surface exhibits a uniformly light-yellow colour, which is beneficial for many applications and has a less effect on the colour of the subsequent paints [7]. Bath pH was another critical factor that significantly influenced coating morphology and thickness, particularly when coating growth was conducted at 80 °C. Descending pH from 4.0 to 2.0 resulted in a coating with higher Mn (~4.4 at.%) and P (~ 7.3 at.%) content but evident network structure, similar to the one reported by Zhou et al. by immersing AZ31 in an MnHPO4 bath at 60 °C for 30 min. The thickness was also enhanced by the strong acidity to ~2.8 μm (Fig. S1). Correlating this with the OCP curve of the case of pH 2–80 °C, it can be concluded that high acidity accelerated phosphating rate and thickened the coatings. The characteristic network structure of Mn–PO4 based conversion coatings can be correlated to hydrogen evolution process rather than dehydration effects [14]. Corrosive medium will readily penetrate this thick barrier coating and approach to substrate through the defect sites in the network structure and eventually deteriorate corrosion resistance [3,4]. Thus, defects should be avoided for the sake of corrosion resistance. Increasing pH from 4.0 to 6.0 at 80 °C, on the contrary, induced a rough though thin (~0.8 μm, Fig. S1) coating with a lower Mn (1.1 at.%) and P (1.9 at.%) fraction. This can be attributed to the low acidity that inhibited the dissolution of Mg substrate and consequently moderated the nuclei formation rate of Mn–PO4 coating. Cross-sectional micrograph and corresponding XRD spectrum (Fig. 3a) of the conversion coated AZ91D at pH 4–80 °C illustrate that the coating system (~1.5 μm thick) consisted of an interlayer and a top film. Peaks detected by XRD were ascribed to substrate AZ91D, Mg(OH)2, and (Mg/Mn)3(PO4)2, respectively. The noticeable detachment of the top layer from the intermediate film may be attributed to the sample preparation procedure, including grinding and polishing. Though not included in this study, the coating adhesion
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Fig. 2. SEM micrographs of bare and various coated AZ91D samples revealing the effect of bath pH and temperature on the formation of Mn–PO4, and EDX (inset) of the bare and coated AZ91D at pH 4–80 °C presenting the deposition of Mn and PO4 ions (scale bar 20 μm).
strength and hardness will be further characterised in the future work to clarify this issue. To further investigate the chemical state of the coating elements, XPS survey scans, high-resolution elemental analysis, and depth profiling were conducted on AZ91D treated for 5 min at pH 4–80 °C as depicted in Fig. 4. The survey scan displays the existence of Mg, O, P, and Mn at the surface. O atomic concentration reduces (62.8 to 7.8 at.%) and the Mg content increases (26.9 to 61.9 at.%) along the depth of the coating. Meanwhile, Mn and P signals were detected mainly from the outer region of the coating. Mn and P concentrations descend gradually from 3.4 and 6.6 to 0.9 and 2.4 at.%, respectively. The high-resolution scans further identify the presence of inner Mg(OH)2, and outer (Mg/Mn)3(PO4)2 films. 3.3. Formation mechanism of the double-layered coating system on AZ91D Based on the abovementioned results, a formation mechanism of the double-layered coating system is proposed and schematically depicted in Fig. 5. In this study, the evolution of the double-layered coating system on AZ91D includes three primary phases, i.e., substrate dissolution, intermediate Mg(OH)2 film deposition, and final (Mg/Mn)3(PO4)2 co-precipitation. Once Mg components were placed in the prepared acidic Mn–PO4 bath, two classic dissolution reactions (Eqs. (1) and (2)) took place, which locally generated massive OH −
and Mg 2+ ions, raising the pH in the vicinity of the metal surface (Fig. 6) [5,10,27,28]. The excess of the depleted Mg 2+ and OH − ions, compared to the small fraction of Mn 2+ and PO43− in the bath, would precipitate immediately in the form of Mg(OH)2 on metal surface (Eq. (3)) as an intermediate film, as demonstrated in Figs. 3b and 4, and schematically depicted in Fig. 5. Such Mg(OH)2 intermediate layer, especially the one produced at high temperature, say 80 °C, is a corrosion inhibitor [11,38] and led OCP suddenly ascending to more passive direction as displayed in Fig. 1. þ
2þ
þ H2 O
2þ
þ 2OH þ 2H2 ↑
Mg O þ 2H →Mg Mg þ 2H2 O→Mg 2þ
Mg
−
ð2Þ
−
þ 2OH →MgðOHÞ2 ↓
2þ
4Mn
ð1Þ
−
ð3Þ þ
þ 3H2 PO4 →MnHPO4 þ Mn3 ðPO4 Þ2 þ 5H
ð4Þ
As Zhou et al. proposed [9], in a Mn(H2PO4)2 solution, there is a balance between all ingredients at a certain temperature and pH, as described in Eq. (4). The continuous consumption of H + ions during H2 evolution decomposed the hydrolyzing equilibrium for the Mn(H2PO4)2 (Eq. (5)) that was forced to move towards right direction, resulting in insoluble Mn3PO4 conversion coating formation
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Fig. 3. Cross-sectional micrograph and XRD spectrum of the AZ91D treated at pH 4–80 °C in 0.01 M Mn(NO3)2–NH4H2PO4 solution, which suggests the existence of a Mg(OH)2– (Mg/Mn)3(PO4)2 double layer system (a) and thermo-equilibrium predominance area diagram for the Mn2+, Mg2+ and PO43− ions calculated using the MEDUSA software package (b and c). It can be seen that Mg(OH)2 (exists as intermediate layer) is the dominate deposition when bath pH is high, and then Mg3(PO4)2 and Mn3(PO4)2 precipitate orderly along with pH decrease (outer layer).
on AZ91D surface. Thus, after immediate formation of an Mg(OH)2 intermediate layer, insoluble Mn3(PO4)2 nuclei were produced on top of the Mg(OH)2 film and expanded to cover the entire surface after 5 min. Due to the existence of Mg2+ ions, co-precipitation of (Mg/Mn)3(PO4)2 was available, as revealed in Fig. 3c. Similar co-precipitation of (Fe/Mn)3(PO4)2 has been noted on steel treated in Mn–PO4 bath [6]. As the phosphating process proceeded, AZ91D substrate dissolution was moderated by limited protection and the amount of Mg2+ ions in the solution gradually decreased, resulting in a declined concentration profile of Mg2+ and ascending concentration profile of Mn2+ in the phosphate layer. The XPS depth profile in Fig. 4 demonstrates that the outer layer of the Mn–PO4 phosphate deposition includes ions from the coating bath while inner layer contained higher fraction of the ions from the AZ91D substrate [24]. It should be pointed out that Zhou et al. reported that Mn–PO4 nuclei directly grew on substrate rather than on an Mg(OH)2 intermediate layer, which may be
ascribed to the etching treatment prior to immersion and high Mn concentration (0.10–0.25 M) applied in the bath [9]. 3.4. Corrosion resistance of conversion coated AZ91D alloy Immersing Mg coupons into acidic phosphate baths resulted in a crystalline product that drastically reduces the electrical conductivity of the surface and plays as a barrier to isolate corrosive electrolyte from the substrate. Since corrosion relies on the flow of electrons between anodes and cathodes that exist on a heterogeneous surface like AZ91D, decrease in the electrical conductivity, and separation between metal and electrolyte will significantly retard the corrosion process [39]. In this study, corrosion resistance of various AZ91D specimens was evaluated via potentiodynamic polarisation curves and EIS in 0.1 M NaCl, and salt spray according to the ASTM B117 standard.
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Fig. 4. XPS analysis of the Mg–PO4 conversion coating, survey, high resolution of O1s, Mg2p, Mn2p, and P2p, and depth profile of the coating formed after 5 min of immersion at pH 4–80 °C, revealing the existence of elements of O, Mg, P, and Mn, and their distribution over the coating system.
Because analysis of potentiodynamic curves is not always in agreement with the ‘long-term’ corrosion rate measured by salt spray [7], the analysis of various AZ91D samples was used exclusively herein to demonstrate the instant anodic and cathodic reaction mechanisms, as presented in Fig. 7a (variable temperature) and b (variable pH). With respect to the effect of temperature, at a constant pH (4.0), it can be
seen in Fig. 7a that the cathodic reactions were tremendously inhibited by the conversion coating treatment, which has also been reported by other researchers [30,40], though the reason is still unclear. It may be attributed to the formation of intermediate insoluble Mg(OH)2 which acts as an inhibitor of oxygen and consequent cathodic kinetics, similar to that of chromate coating on Al alloys as Kendig and Buchheit
Fig. 5. Schematic presentation of the formation of the Mn–PO4 coating on AZ91D.
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Fig. 6. The pH change of the coated AZ91 alloy during the immersion process at various conditions.
presented [41]. It is evident that in the anodic branch even above Ecorr, the corrosion current density still remained almost the same indicating the passive nature of these layers [1,42]. A breakdown potential point and passivation region can be seen in all the anodic branches, revealing that the anodic dissolution reaction was also retarded [28]. In contrast, corrosion current density (icorr), an indicator of corrosion resistance, was drastically regulated by bath temperature, same as the OCP results (Fig. 1a). Specifically, icorr of AZ91D was reduced by an order of magnitude by the coating formed at 80 °C, as illustrated in Fig. 7a and Table 1. In terms of the effect of pH, both higher (pH 2.0) and lower acidity (pH 6.0) were a detrimental factor on the corrosion resistance when treated at 80 °C. It is obvious that the corrosion current density in the anodic branch rose steeply immediately above the Ecorr revealing the pit formation on the AZ91D treated at pH 2.0. The least corrosion resistance of the samples treated at pH 2.0 may be attributed to the network structure. The existence of incomplete double-layered coating system incurred in the bath with pH 6.0 could not fulfil a satisfactory inhibiting role in corrosion either. The corrosion resistance of MnPO4 coated AZ91D was further examined using EIS (Fig. 7c and d). An example of a calculated curve is presented to illustrate the goodness of fit (Fig. S2). EIS can provide instantaneous data on the impedance of a surface subject to polarisation, which is directly proportional to the corrosion resistance (i.e., inversely proportional to corrosion rate) [43,44]. The Nyquist plots of bare AZ91D coupon exhibit a capacitive loop at high frequencies, a capacitive loop at intermediate frequencies, and an inductive arc at low frequencies. The capacitive loop at high and medium frequency can be ascribed to the characteristic of an electric double layer and loose naturally formed oxide film on the substrate, respectively [27]. Meanwhile, the presence of the inductive arc suggests the occurrence of relaxation of absorbed species, which is generally related to the exposure of the AZ91D and the subsequent release of Mg 2+ ions and Mg(OH)2 [45]. Only one capacitive loop can be observed in the EIS plots of all the coated AZ91D, indicating the passive nature of the coatings. The increase in diameter of the capacitive loop correlates to the increase in the surface film corrosion resistance [46]. The EIS data were simulated using the equivalent circuit presented in Fig. 7f (EC-Lab package, version 10.2). The parameters Rs (the solution resistance), Q (constant phase element representing a non-ideal capacitance related to the coating), Rc (the coating resistance), Qdl (constant phase element representing a non-ideal capacitance related to the double layer), Rct (the charge transfer resistance) and W (representing the Warburg element which represents the diffusion of species) were calculated. This equivalent circuit was chosen due to being the most simple representation of a filmed surface that can
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accommodate local breakdown/defects [7,29,44]. Based on the fitted results in Table 1, it is observed be found that the low frequency impedance (determined at 10 mHz) and the total resistance (Rt) of the coatings were increasing as a function of bath temperature, and pH 4.0 was favourable to give rise to the most protective film. Accordingly, the AZ91D coated at pH 4.0 and 80 °C had the largest polarisation resistance, indicating that the coating system was effective as a barrier to protect AZ91D alloy against corrosion in 0.1 M NaCl. After 48 h salt spray, the area fraction corroded, which is simply the ratio of corroded area versus the total area exposed to NaCl fog, was measured and listed in Table 1. Compared to bare AZ91D, the corrosion area fraction was reduced by the proposed Mn–PO4 conversion coating treatment. The coatings obtained at pH 4-(RT ~ 50 °C) offered a mild protection, suffering 5–10% corrosion after salt spray test for 48 h. Improved protection is observed on coatings at pH 4 (60–70 °C), where only 1–5% area corroded. The minimum corrosion area fraction (less than 1%) was noted on the case of pH 4–80 °C. The optimal Mn–PO4 conversion coating thus can offer a corrosion protection similar to that of the DOW conversion coating and other conversion coatings with high protection efficiency [7,47]. Moreover, high temperature, though has less remarkable influence on the improvement of corrosion rate than that of pH, always favours the anticorrosion performance of the phosphate coatings [30].
4. Conclusions A series of Mn–PO4 containing conversion coatings were produced by a simple immersion method capable of improving the corrosion resistance of Mg alloy AZ91D. A coating formation mechanism was proposed with the aid of thermodynamic equilibrium calculations. Upon exposure to the coating bath, the matrix Mg dissolved and released, Mg 2+, H2 gas and OH − ions, increasing the pH in the vicinity of solid–liquid interface. The pH increase facilitated the formation of thin Mg(OH)2 intermediate layer on the substrate, which was then verified by SEM and XPS analysis. Finally, a top coating in form of (Mg/Mn)3(PO4)2 was produced due to their decreasing solubility along with decreasing pH, as predicted from the thermodynamic equilibrium diagram and confirmed by XPS results. Calculations could provide a theoretical rationalisation to engineering protective coating formation for AZ91D. The effect of coating growth parameters, pH and temperature, on coating morphology and subsequent corrosion resistance was systematically studied. It was found that pH was a key factor to determining coating thickness and characteristics. A thicker (~2.8 μm) surface film obtained at high acidity (pH 2.0) was associated with a network of severe structural defects due to hydrogen evolution. Because cracks are detrimental to corrosion resistance, a mild acidity (pH 4.0) resulted in a more compact Mn–PO4 conversion coating with optimal corrosion resistance. Higher alkalinity (pH 6.0), however, retarded the release of Mg2+ ions from the substrate, which led to a coating with low corrosion resistance. The operating temperature also had an influence on the coating formation, albeit only minor to pH. Due to its exothermic characteristic, the phosphating reaction rate heavily depends on the energy offered by the heating process. In general, the higher temperature, the quicker the phosphating process. Overall, the instant growth of a dense and thin Mg(OH)2 intermediate layer and a complete coverage by an (Mg/Mn)3(PO4)2 top layer at pH 4–80 °C exhibited the best corrosion resistance in corrosive environments, displaying a corrosion area fraction less than 1% after 48 h of salt spray testing. With respect to the salt spray evaluation, the proposed Mn–PO4 surface film outperformed chromate conversion coatings. Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.surfcoat.2012.12.005.
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Fig. 7. Potentiodynamic polarisation curves of various AZ91D obtained in 0.1 M NaCl at a sweep rate of 1 mV/s (a and b), Nyquist plots of the various AZ91D obtained in 0.1 M NaCl with a frequency range from 100 kHz to 10 mHz (c and d), e) 3D plot of the corrosion current density icorr (z axis) against treating temperature (x axis) and pH (y axis), and f) the equivalent circuit used for analysis of the EIS data of various AZ91D. (Rs is solution resistance, Rc is the resistance and constant phase element of a film on the sample surface, and Rct, Q and Qdl are the charge transfer resistance of a coating, constant phase element of the charge transfer, charge transfer capacitance of a double-layer, respectively. W is Warburg element accounting the diffusion of species).
Table 1 The corrosion area fraction after 48 h of the salt spray test, corrosion current density (icorr) and film resistance (Rc, generated from the Nyquist plots in Fig. 7c and d using the equivalent circuit presented in Fig. 7f) of various AZ91D. Coating parameters
pH 4-RT
pH 4–40 °C
pH 4–50 °C
pH 4–60 °C
pH 4–70 °C
pH 4–80 °C
pH 2–80 °C
pH 6–80 °C
Bare AZ91D
Corrosion area fraction, % icorr, 10−6 A cm−2 Rc, ohm
5–10 4.5 ± 0.5 2950 ± 360
5–10 4.1 ± 0.6 3470 ± 170
5–10 3.6 ± 1.1 4110 ± 320
1–5 2.7 ± 0.6 4410 ± 170
1–5 2.3 ± 0.2 4870 ± 110
b1 1.1 ± 0.2 6110 ± 80
15–30 11.5 ± 0.8 7580 ± 140
15–25 11.9 ± 0.2 1790 ± 180
70–80 18.0 ± 0.8 3640 ± 210
X.-B. Chen et al. / Surface & Coatings Technology 217 (2013) 147–155
Acknowledgement The CAST CRC was established under, and is funded in part by the Australian Commonwealth Government Cooperative Research Centre Scheme. The authors acknowledge the facilities (SEM-EDX and XPS), and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at the RMIT Microscopy & Microanalysis Facility. The Australian Research Council (Centre of Excellence for Design in Light Metals) and Victorian State Government for the establishment of the Victorian Facility for Light Metals Surface are gratefully acknowledged. References [1] W.-J. Xu, J.-L. Song, J. Sun, Y. Lu, Z.-Y. Yu, ACS Appl. Mater. Interfaces 3 (2011) 4404. [2] B.L. Mordike, T. Ebert, Mater. Sci. Eng., A 302 (2001) 37. [3] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [4] X.-B. Chen, N. Birbilis, T.B. Abbott, Corrosion 67 (2011) 035005. [5] X.-B. Chen, T. Abbott, N. Birbilis, Corros. Sci. 55 (2012) 226. [6] C.-M. Wang, H.-C. Liau, W.-T. Tsai, Surf. Coat. Technol. 201 (2006) 2994. [7] Y.C. Yang, C.Y. Tsai, Y.H. Huang, C.S. Lin, J. Electrochem. Soc. 159 (2012) C226. [8] K.Y. Chiu, M.H. Wong, F.T. Cheng, H.C. Man, Surf. Coat. Technol. 202 (2007) 590. [9] W.Q. Zhou, D.Y. Shan, E.H. Han, W. Ke, Corros. Sci. 50 (2008) 329. [10] Y.W. Song, D.Y. Shan, E.H. Han, Corros. Sci. 51 (2009) 62. [11] C.S. Lin, C.Y. Lee, W.C. Li, Y.S. Chen, G.N. Fang, J. Electrochem. Soc. 153 (2006) B90. [12] M. Mosiałek, G. Mordarski, P. Nowak, W. Simka, G. Nawrat, M. Hanke, R.P. Socha, J. Michalska, Surf. Coat. Technol. 206 (2011) 51. [13] H. Huo, Y. Li, F. Wang, Corros. Sci. 46 (2004) 1467. [14] F. Zucchi, A. Frignani, V. Grassi, G. Trabanelli, C. Monticelli, Corros. Sci. 49 (2007) 4542. [15] H.H. Elsentriecy, K. Azumi, H. Konno, Electrochim. Acta 53 (2007) 1006. [16] C. Wang, S.L. Zhu, F. Jiang, F.H. Wang, Corros. Sci. 51 (2009) 2916. [17] C.S. Lin, S.K. Fang, J. Electrochem. Soc. 152 (2005) B54. [18] K. Brunelli, M. Dabalà, I. Calliari, M. Magrini, Corros. Sci. 47 (2005) 989. [19] K.H. Yang, M.D. Ger, W.H. Hwu, Y. Sung, Y.C. Liu, Mater. Chem. Phys. 101 (2007) 480.
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Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Double-layered manganese phosphate conversion coating on magnesium alloy AZ91D: Insights into coating formation, growth and corrosion resistance Xiao-Bo Chen a, b,⁎, Xian Zhou b, Trevor B. Abbott a, c, Mark A. Easton a, Nick Birbilis a, b a b c
CAST Co-operative Research Centre, Monash University, VIC 3800, Australia ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering, Monash University, VIC 3800, Australia Magontec Limited, Sydney, NSW 2000, Australia
a r t i c l e
i n f o
Article history: Received 29 September 2012 Accepted in revised form 5 December 2012 Available online 14 December 2012 Keywords: Magnesium alloy AZ91D Corrosion resistance Conversion coating Manganese phosphate Polarisation EIS
a b s t r a c t A double-layered conversion coating system, consisting of magnesium hydroxide–magnesium/manganese phosphate, was applied to magnesium alloy AZ91D using an acidic manganese nitrate and ammonium dihydrogen phosphate solution. The coating structure, composition and morphology were characterised by SEM, EDX, XRD and XPS. A coating formation mechanism is proposed, and the effect of operating parameters, i.e. pH and temperature, on coating formation was systematically investigated, with optimised conditions able to produce coatings of high corrosion resistance. Corrosion resistance of the coating was evaluated by electrochemical and salt spray testing. The double-layered coating system develops in three stages: initial substrate dissolution, formation of a dense magnesium hydroxide layer, and then co-deposition of magnesium and manganese phosphate film. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Magnesium (Mg) and its alloys are promising structural and electronic materials for diverse applications, including transportation and 3C components, due to their light weight, specific strength and conductivity [1,2]. The development and use of Mg alloys in industry remain in infancy due to poor corrosion resistance upon exposure to air and humid conditions. Two primary methodologies are nowadays being extensively explored to improve the resistance to corrosion of Mg-alloys; these include bulk alloying and the use of barrier coatings on the Mg surface [3,4]. The latter method can decrease corrosion rate of Mg alloys by a few orders of magnitude, which is more efficient in the short term compared to alloy development [3,4]. Increasingly extensive research is being devoted to protective coatings for Mg [3,4]. Chemical conversion coatings stand out from other coating types that include anodising, electroplating, electroless plating, ion implantation, etc., owing to low cost and efficiency [4,5]. In general, no power or specific facilities are required to carry out conversion coating process, significantly reducing production cost. In addition to imparting corrosion resistance, conversion coatings could also be an adhesive base for subsequent plating or painting [6,7]. Since being implemented by DOW (DOW Chemical Co., USA), chromate conversion coating, has been the benchmark corrosion protection scheme, which also offers remarkable self-healing capability. ⁎ Corresponding author at: CAST Co-operative Research Centre, Monash University, VIC 3800, Australia. Tel.: +61 3 9905 9297. E-mail address: [email protected] (X.-B. Chen). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.12.005
Modern concerns and legislation regarding toxicity present the need for alternatives to the use of chromate, and research has been carried out upon: fluoride [8], phosphate [5,9,10], phosphate–permanganate [11,12], stannate [13–15], rare earth (RE) [16–18], vanadate [19,20], titanate [7,21] and ionic liquid (IL) conversion coatings [22,23]. Among these techniques, vanadate also presents an environmental hazard; the long processing time (up to 60 min) of stannate and RE coating procedures cause significant cost increase; ILs are costly, and concentrated (and toxic) HF is indispensable for fluoride coating. Phosphate conversion coatings on the other hand, are more environmentally friendly and have been successfully exploited to protect steel and zinc against corrosion in dilute aqueous solutions. Metal-phosphate layers provide not only corrosion protection but also some specific functions to substrates, i.e., a coating bath with the necessary phosphate anions and metallic cations, including Ca2+, Mn2+/3+/4+/7+, Zn2+ etc., may improve corrosion/wear resistance and sliding properties [6,24–26]. Further, biocompatibility and bioactivity of calcium phosphate (Ca–PO4), high wear resistance and adhesion strength of zinc phosphate (Zn–PO4), and lubricity of manganese phosphate (Mn–PO4), have also been demonstrated. In the past decades, the applications of Ca–PO4 [5,10,27,28] and Zn–PO4 [29–31] conversion coatings have been successfully expanded to light-weight Mg alloys. In contrast, very few studies have focused on protecting Mg alloys with Mn 2+–PO4 conversion coatings, in spite of being expected to be more stable and corrosion resistant than their Zn and Ca peers. Han et al. [32] and Zhou et al. [9,33] reported a manganese hydrogen phosphate (MnHPO4) conversion coating for AZ31D and AZ91D alloys and proposed a mechanism of the coating formation. Cui et al. also
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investigated the growing process of an MnHPO4 conversion coating on AZ31, but presented an alternative coating growth mechanism [34]. Though these existing reports indicate that the MnHPO4 conversion coatings had desirable corrosion resistance, the immersion/coating time, up to 30 min, is much longer than the basic requirement for industrial applications. The performance of the phosphate conversion coatings is dependent on the crystal structure as well as the morphology. For example, a microcrystalline structure is usually optimal for corrosion resistance or subsequent painting. A coarse grain structure impregnated with oil, however, may be the most desirable for wear resistance. These properties can be tailored by selecting the appropriate phosphate solution, using various additives, and controlling bath temperature, pH, ion concentration, and phosphating time [3,4]. As a result, this prompts further questions regarding the experimental conditions, such as bath temperature and pH, amenable for the synthesis of stable low-valence Mn–PO4 coatings on Mg alloys and a study of their corrosion resistance. In this study, we apply an Mn 2+–PO4 conversion coating onto die-cast Mg alloy AZ91D with the view of achieving significant improvement in corrosion resistance and exploiting a simple and low cost strategy. The systematic study is aimed at developments towards a practical replacement for DOW chromate coatings. Following preliminary trials, the coating process was simplified to involve one dipping-step in an acidic Mn–PO4 solution, eliminating conventional pretreatment steps [5,33,35], which is important for upscaling. The effect of the essential coating parameters over a certain range, i.e., pH (2.0 to 6.0) and temperature (ambient to 80 °C), on the corrosion resistance of the resultant coatings was investigated, with a view to understanding the nature of the Mn–PO4 conversion coating on AZ91D, and in producing more uniform protective films. 2. Experimental section 2.1. Mn–PO4 conversion coating preparation ASTM Mg alloy AZ91D, from the Mg alloy supplier HNKWE (China) was used in this study. The AZ91D contained 9.1 wt.% Al, 0.8 wt.% Zn, 0.24 wt.% Mn, 0.031 wt.% Si, 0.0023 wt.% Fe, 0.015 wt.% Cu, and 0.0005 wt.% Ni, balance Mg, as measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Spectrometer Services, Coburg, VIC, Australia). A Toshiba 250 ton clamping force cold chamber high pressure die casting (HPDC) machine was used to cast test plates of 70 × 60 × 2 mm in size. Smaller test samples with dimensions of 20 mm × 20 mm × 2 mm were cut from these plates and were ground to 1200 grit finish and used as substrate material. Prior to the one-step coating treatment, AZ91D samples were ultrasonicated in acetone at room temperature (RT) for 15 min and then in absolute ethanol for 10 min and rinsed with deionised water thereafter. The coating solution contained 0.01 M manganese nitrate (Mn(NO3)2), 0.01 M ammonium dihydrogen phosphate (NH4H2PO4). Nitric acid (HNO3) or ammonia (NH3·H2O) was utilised to adjust pH down or up (2.0– 6.0). All chemicals used in this work were analytical grade from Sigma-Aldrich. The coating process was thus conducted by immersing pre-cleaned AZ91D specimens into the coating solution varying from RT to 80 °C for 5 min. Following this, treated samples were dried in air and kept in a desiccator for further characterisation. 2.2. Thermodynamic calculations The software MEDUSA (which is an acronym for Make Equilibrium Diagrams Using Sophisticated Algorithms) was used to make a first order calculation of the complexes and phases in a given chemical system via equilibrium formation constants. MEDUSA nominally calculates at room temperature, however manual alteration of all parameters is possible at the users' discretion using the programme edit function.
Fig. 3b illustrates how the predominance of Mg2+ in the presence of phosphate (10 mM) and Mn2+ (10 mM) varies with pH and concentration of Mg 2+ ions, while Fig. 3c presents how the predominance of phosphate in the presence of Mg2+ (500 mM) and Mn2+ (10 mM) varies with pH and concentration of phosphate ions. 2.3. Microstructural analysis of Mn–PO4 conversion coatings Surface morphology of coated AZ91D alloys was observed using scanning electron microscopy (SEM, FEI Nova Nano) fitted with energy dispersive X-ray spectroscopy (EDXS). Structure and phase composition of the AZ91D surfaces before and after coating were identified by X-ray diffraction (XRD, Philips PW1140) using Cu-Kα radiation (λ = 1.5418 Å, 40 kV, 25 mA) and a scanning speed of 0.01°/min at a 2θ range of 20–50°. Surface chemistry was analysed by X-ray photoelectron spectroscopy (XPS, Thermo K-alpha, UK) with a hemispherical analyser and the core level XPS spectra for Mn2p, Mg2p, Al2p, Zn2p, P2p, O1s and C1s were recorded. The measured binding energy values were calibrated by the C1s (hydrocarbon C–C, C–H) of 285 eV. The photoelectrons were generated by Al-Kα (1486.6 eV) primary radiation (20 kV, 15 mA). 2.4. Corrosion evaluation For electrochemical tests, a flat-cell (PAR, K0235) containing 300 mL of 0.1 M NaCl electrolyte was used with an exposed working electrode area of 1 cm 2, a saturated calomel (SCE) reference electrode and Pt-mesh counter electrode. Electrochemical experiments were performed on a BioLogic® VMP-3Z potentiostat/galvanostat using EC-Lab 10.2 software. Potentiodynamic polarisation experiments were carried out at a sweep rate of 1 mV/s after 10 min of open circuit (OCP) conditioning. The polarisation curves were used to determine corrosion current density, icorr, via a Tafel-type fit using EC-Lab software. As a general rule, fits were executed by selecting a portion of the curve that commenced > 50 mV from corrosion potential; Ecorr and icorr were subsequently estimated from the value where the fit intercepted the potential value of the true Ecorr. More generally, polarisation testing was also able to visually reveal comparative information related to the kinetics of anodic and cathodic reactions between alloys and was in contrast to salt spray testing. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 100 kHz to 10 mHz, with a sinusoidal amplitude of 10 mV and five points per decade after 10 min of open circuit (OCP) conditioning. The EIS data was analysed using EC-Lab 10.2 software. At least three separate scans were performed for each data point to ensure reproducibility. The long-term corrosion resistance of various treated AZ91D was examined by salt spray testing up to 48 h according to the ASTM B117 standard. The samples were placed at a tilting angle of 30° in a chamber containing 5 wt.% NaCl fog at 35 °C. 3. Results and discussion 3.1. OCP evolution during formation of Mn–PO4 based conversion coatings Observation of OCP evolution for AZ91D in the Mn–PO4 bath as a function of immersion time, pH and temperature, is presented in Fig. 1, illustrating the development of the conversion coatings [7,10,36]. The OCP curves were evidently affected by bath temperature (Fig. 1a). When a constant pH value of 4.0 was applied, all the OCP curves obtained at elevated temperatures (40–80 °C) initially demonstrate an abrupt potential shift in the more noble direction (first 30 s), revealing the instant protective coating formed on the substrate upon exposure to the bath [7,10]. A rapid drop in OCP at the initial stage can be observed exclusively on the case treated at RT, demonstrating removal of the spontaneously air-formed loose MgO/MgOH/MgCO3 film and subsequent
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Fig. 1. Evolution of OCP for AZ91D immersed in an Mn–PO4 bath at various temperatures (a) and pH (b). The effect of the growth temperature and pH on the coating evolution is evident.
exposure of the AZ91D metallic substrate [7,10]. No such sudden falls in OCP being observed on other samples can be attributed to the elevated temperatures (40–80 °C), which stimulated the coating formation and moderated the dissolution time duration of the oxide layer. When the dynamic equilibrium of film dissolution–precipitation was established, OCP approached a plateau. The value of the plateau potential is a function of bath temperature (Fig. 1a). Higher bath temperature produced a coating with more positive OCP and correspondingly, higher corrosion resistance [7,10]. It is evident that the coating evolution, i.e., dissolution–precipitation, was determined by bath pH value (Fig. 1b). When temperature was kept at 80 °C, the high bath-acidity (pH 2.0) led to a continuous potential shift to the more noble region, which indicates ongoing preciptation on substrate [10,36]. On the contrary, the potential reached a plateau after immersed for 300 s in the mildly acidic bath (pH 4.0), indicating the establishment of an equilibrium between dissolution and precipitation [10,36]. A peak potential can be seen in the approximately neutral bath (pH 6.0) and followed by a drop, which is likely due to the local dissolution of the existing coating [7]. In this study, the optimal immersion time of 5 min was chosen for all cases according to the OCP graph. The coating formation process and their corrosion resistance will be discussed in following sections.
3.2. Microstructural characterisation of the Mn–PO4 conversion coatings Phosphatation is an endothermic reaction [37], thus, coating procedures conducted at low temperature do not provide sufficient energy
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to satisfactorily generate phosphates. Under such circumstances, the phosphating rate is either slow or does not take place at all. Consequently, long immersion times, up to a few days, may be required to give rise to a complete phosphate film — with extremely thin coatings being undesirable for corrosion resistance. In contrast, high processing temperature can offer enough activation energy and eventually accelerate the phosphating rate. A thick phosphate film can be achieved in a relatively short time and render corrosion resistance to Mg substrates. Observation of surface morphology of bare and treated AZ91D can reveal the effect of operating parameters, i.e., pH and temperature, on the formation of the Mn–PO4 coating, as presented in Fig. 2. After removal of the surface layer containing die-spray and oxide by mechanical grinding, bare AZ91D presents a silvery metallic finish and a microstructural feature of abraded grooves. The inset EDX data demonstrates the existence of Mg (~ 89.3 wt.%), Al (~ 9.9 wt.%) and Zn (~ 0.8 wt.%), which is in agreement with the ICP-AES results. Deposition of Mn–PO4 particles and subsequent coatings was significantly influenced by growth temperature when pH was maintained at 4.0. A few newly deposited particles appeared on the substrate treated at RT for 5 min, while the density and size of the white round particles scattering over the surface was progressively increased when bath temperature was elevated from 40 to 60 °C, which is attributed to the high reaction rate incurred by high temperature. The AZ91D treated at 70 °C displays a different surface morphology, where thin flakes and round clusters were distributed unevenly over surface. With higher treating temperature, the densely packed small particles had much more energy to merge into large grains and produce a high coverage. A compact but thin coating was produced at 80 °C, consisting of Mn (2.9 at.%), P (4.8 at.%) and O (39.0 at.%), in addition to Mg (46.7 at.%) and Al (6.1 at.%), as depicted by the inset EDX spectrum (Please refer to Table S1 in the Supporting information for all the EDX elemental analysis results). The high fraction of Mg and Al detected by EDX indicates that the conversion coating was thin (~1.5 μm, Figs. 3 and S1) and the strong Mg and Al signals were mainly from the underneath AZ91D. Some cracks were obvious on the sample treated at 80 °C whose surface exhibits a uniformly light-yellow colour, which is beneficial for many applications and has a less effect on the colour of the subsequent paints [7]. Bath pH was another critical factor that significantly influenced coating morphology and thickness, particularly when coating growth was conducted at 80 °C. Descending pH from 4.0 to 2.0 resulted in a coating with higher Mn (~4.4 at.%) and P (~ 7.3 at.%) content but evident network structure, similar to the one reported by Zhou et al. by immersing AZ31 in an MnHPO4 bath at 60 °C for 30 min. The thickness was also enhanced by the strong acidity to ~2.8 μm (Fig. S1). Correlating this with the OCP curve of the case of pH 2–80 °C, it can be concluded that high acidity accelerated phosphating rate and thickened the coatings. The characteristic network structure of Mn–PO4 based conversion coatings can be correlated to hydrogen evolution process rather than dehydration effects [14]. Corrosive medium will readily penetrate this thick barrier coating and approach to substrate through the defect sites in the network structure and eventually deteriorate corrosion resistance [3,4]. Thus, defects should be avoided for the sake of corrosion resistance. Increasing pH from 4.0 to 6.0 at 80 °C, on the contrary, induced a rough though thin (~0.8 μm, Fig. S1) coating with a lower Mn (1.1 at.%) and P (1.9 at.%) fraction. This can be attributed to the low acidity that inhibited the dissolution of Mg substrate and consequently moderated the nuclei formation rate of Mn–PO4 coating. Cross-sectional micrograph and corresponding XRD spectrum (Fig. 3a) of the conversion coated AZ91D at pH 4–80 °C illustrate that the coating system (~1.5 μm thick) consisted of an interlayer and a top film. Peaks detected by XRD were ascribed to substrate AZ91D, Mg(OH)2, and (Mg/Mn)3(PO4)2, respectively. The noticeable detachment of the top layer from the intermediate film may be attributed to the sample preparation procedure, including grinding and polishing. Though not included in this study, the coating adhesion
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Fig. 2. SEM micrographs of bare and various coated AZ91D samples revealing the effect of bath pH and temperature on the formation of Mn–PO4, and EDX (inset) of the bare and coated AZ91D at pH 4–80 °C presenting the deposition of Mn and PO4 ions (scale bar 20 μm).
strength and hardness will be further characterised in the future work to clarify this issue. To further investigate the chemical state of the coating elements, XPS survey scans, high-resolution elemental analysis, and depth profiling were conducted on AZ91D treated for 5 min at pH 4–80 °C as depicted in Fig. 4. The survey scan displays the existence of Mg, O, P, and Mn at the surface. O atomic concentration reduces (62.8 to 7.8 at.%) and the Mg content increases (26.9 to 61.9 at.%) along the depth of the coating. Meanwhile, Mn and P signals were detected mainly from the outer region of the coating. Mn and P concentrations descend gradually from 3.4 and 6.6 to 0.9 and 2.4 at.%, respectively. The high-resolution scans further identify the presence of inner Mg(OH)2, and outer (Mg/Mn)3(PO4)2 films. 3.3. Formation mechanism of the double-layered coating system on AZ91D Based on the abovementioned results, a formation mechanism of the double-layered coating system is proposed and schematically depicted in Fig. 5. In this study, the evolution of the double-layered coating system on AZ91D includes three primary phases, i.e., substrate dissolution, intermediate Mg(OH)2 film deposition, and final (Mg/Mn)3(PO4)2 co-precipitation. Once Mg components were placed in the prepared acidic Mn–PO4 bath, two classic dissolution reactions (Eqs. (1) and (2)) took place, which locally generated massive OH −
and Mg 2+ ions, raising the pH in the vicinity of the metal surface (Fig. 6) [5,10,27,28]. The excess of the depleted Mg 2+ and OH − ions, compared to the small fraction of Mn 2+ and PO43− in the bath, would precipitate immediately in the form of Mg(OH)2 on metal surface (Eq. (3)) as an intermediate film, as demonstrated in Figs. 3b and 4, and schematically depicted in Fig. 5. Such Mg(OH)2 intermediate layer, especially the one produced at high temperature, say 80 °C, is a corrosion inhibitor [11,38] and led OCP suddenly ascending to more passive direction as displayed in Fig. 1. þ
2þ
þ H2 O
2þ
þ 2OH þ 2H2 ↑
Mg O þ 2H →Mg Mg þ 2H2 O→Mg 2þ
Mg
−
ð2Þ
−
þ 2OH →MgðOHÞ2 ↓
2þ
4Mn
ð1Þ
−
ð3Þ þ
þ 3H2 PO4 →MnHPO4 þ Mn3 ðPO4 Þ2 þ 5H
ð4Þ
As Zhou et al. proposed [9], in a Mn(H2PO4)2 solution, there is a balance between all ingredients at a certain temperature and pH, as described in Eq. (4). The continuous consumption of H + ions during H2 evolution decomposed the hydrolyzing equilibrium for the Mn(H2PO4)2 (Eq. (5)) that was forced to move towards right direction, resulting in insoluble Mn3PO4 conversion coating formation
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Fig. 3. Cross-sectional micrograph and XRD spectrum of the AZ91D treated at pH 4–80 °C in 0.01 M Mn(NO3)2–NH4H2PO4 solution, which suggests the existence of a Mg(OH)2– (Mg/Mn)3(PO4)2 double layer system (a) and thermo-equilibrium predominance area diagram for the Mn2+, Mg2+ and PO43− ions calculated using the MEDUSA software package (b and c). It can be seen that Mg(OH)2 (exists as intermediate layer) is the dominate deposition when bath pH is high, and then Mg3(PO4)2 and Mn3(PO4)2 precipitate orderly along with pH decrease (outer layer).
on AZ91D surface. Thus, after immediate formation of an Mg(OH)2 intermediate layer, insoluble Mn3(PO4)2 nuclei were produced on top of the Mg(OH)2 film and expanded to cover the entire surface after 5 min. Due to the existence of Mg2+ ions, co-precipitation of (Mg/Mn)3(PO4)2 was available, as revealed in Fig. 3c. Similar co-precipitation of (Fe/Mn)3(PO4)2 has been noted on steel treated in Mn–PO4 bath [6]. As the phosphating process proceeded, AZ91D substrate dissolution was moderated by limited protection and the amount of Mg2+ ions in the solution gradually decreased, resulting in a declined concentration profile of Mg2+ and ascending concentration profile of Mn2+ in the phosphate layer. The XPS depth profile in Fig. 4 demonstrates that the outer layer of the Mn–PO4 phosphate deposition includes ions from the coating bath while inner layer contained higher fraction of the ions from the AZ91D substrate [24]. It should be pointed out that Zhou et al. reported that Mn–PO4 nuclei directly grew on substrate rather than on an Mg(OH)2 intermediate layer, which may be
ascribed to the etching treatment prior to immersion and high Mn concentration (0.10–0.25 M) applied in the bath [9]. 3.4. Corrosion resistance of conversion coated AZ91D alloy Immersing Mg coupons into acidic phosphate baths resulted in a crystalline product that drastically reduces the electrical conductivity of the surface and plays as a barrier to isolate corrosive electrolyte from the substrate. Since corrosion relies on the flow of electrons between anodes and cathodes that exist on a heterogeneous surface like AZ91D, decrease in the electrical conductivity, and separation between metal and electrolyte will significantly retard the corrosion process [39]. In this study, corrosion resistance of various AZ91D specimens was evaluated via potentiodynamic polarisation curves and EIS in 0.1 M NaCl, and salt spray according to the ASTM B117 standard.
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Fig. 4. XPS analysis of the Mg–PO4 conversion coating, survey, high resolution of O1s, Mg2p, Mn2p, and P2p, and depth profile of the coating formed after 5 min of immersion at pH 4–80 °C, revealing the existence of elements of O, Mg, P, and Mn, and their distribution over the coating system.
Because analysis of potentiodynamic curves is not always in agreement with the ‘long-term’ corrosion rate measured by salt spray [7], the analysis of various AZ91D samples was used exclusively herein to demonstrate the instant anodic and cathodic reaction mechanisms, as presented in Fig. 7a (variable temperature) and b (variable pH). With respect to the effect of temperature, at a constant pH (4.0), it can be
seen in Fig. 7a that the cathodic reactions were tremendously inhibited by the conversion coating treatment, which has also been reported by other researchers [30,40], though the reason is still unclear. It may be attributed to the formation of intermediate insoluble Mg(OH)2 which acts as an inhibitor of oxygen and consequent cathodic kinetics, similar to that of chromate coating on Al alloys as Kendig and Buchheit
Fig. 5. Schematic presentation of the formation of the Mn–PO4 coating on AZ91D.
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Fig. 6. The pH change of the coated AZ91 alloy during the immersion process at various conditions.
presented [41]. It is evident that in the anodic branch even above Ecorr, the corrosion current density still remained almost the same indicating the passive nature of these layers [1,42]. A breakdown potential point and passivation region can be seen in all the anodic branches, revealing that the anodic dissolution reaction was also retarded [28]. In contrast, corrosion current density (icorr), an indicator of corrosion resistance, was drastically regulated by bath temperature, same as the OCP results (Fig. 1a). Specifically, icorr of AZ91D was reduced by an order of magnitude by the coating formed at 80 °C, as illustrated in Fig. 7a and Table 1. In terms of the effect of pH, both higher (pH 2.0) and lower acidity (pH 6.0) were a detrimental factor on the corrosion resistance when treated at 80 °C. It is obvious that the corrosion current density in the anodic branch rose steeply immediately above the Ecorr revealing the pit formation on the AZ91D treated at pH 2.0. The least corrosion resistance of the samples treated at pH 2.0 may be attributed to the network structure. The existence of incomplete double-layered coating system incurred in the bath with pH 6.0 could not fulfil a satisfactory inhibiting role in corrosion either. The corrosion resistance of MnPO4 coated AZ91D was further examined using EIS (Fig. 7c and d). An example of a calculated curve is presented to illustrate the goodness of fit (Fig. S2). EIS can provide instantaneous data on the impedance of a surface subject to polarisation, which is directly proportional to the corrosion resistance (i.e., inversely proportional to corrosion rate) [43,44]. The Nyquist plots of bare AZ91D coupon exhibit a capacitive loop at high frequencies, a capacitive loop at intermediate frequencies, and an inductive arc at low frequencies. The capacitive loop at high and medium frequency can be ascribed to the characteristic of an electric double layer and loose naturally formed oxide film on the substrate, respectively [27]. Meanwhile, the presence of the inductive arc suggests the occurrence of relaxation of absorbed species, which is generally related to the exposure of the AZ91D and the subsequent release of Mg 2+ ions and Mg(OH)2 [45]. Only one capacitive loop can be observed in the EIS plots of all the coated AZ91D, indicating the passive nature of the coatings. The increase in diameter of the capacitive loop correlates to the increase in the surface film corrosion resistance [46]. The EIS data were simulated using the equivalent circuit presented in Fig. 7f (EC-Lab package, version 10.2). The parameters Rs (the solution resistance), Q (constant phase element representing a non-ideal capacitance related to the coating), Rc (the coating resistance), Qdl (constant phase element representing a non-ideal capacitance related to the double layer), Rct (the charge transfer resistance) and W (representing the Warburg element which represents the diffusion of species) were calculated. This equivalent circuit was chosen due to being the most simple representation of a filmed surface that can
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accommodate local breakdown/defects [7,29,44]. Based on the fitted results in Table 1, it is observed be found that the low frequency impedance (determined at 10 mHz) and the total resistance (Rt) of the coatings were increasing as a function of bath temperature, and pH 4.0 was favourable to give rise to the most protective film. Accordingly, the AZ91D coated at pH 4.0 and 80 °C had the largest polarisation resistance, indicating that the coating system was effective as a barrier to protect AZ91D alloy against corrosion in 0.1 M NaCl. After 48 h salt spray, the area fraction corroded, which is simply the ratio of corroded area versus the total area exposed to NaCl fog, was measured and listed in Table 1. Compared to bare AZ91D, the corrosion area fraction was reduced by the proposed Mn–PO4 conversion coating treatment. The coatings obtained at pH 4-(RT ~ 50 °C) offered a mild protection, suffering 5–10% corrosion after salt spray test for 48 h. Improved protection is observed on coatings at pH 4 (60–70 °C), where only 1–5% area corroded. The minimum corrosion area fraction (less than 1%) was noted on the case of pH 4–80 °C. The optimal Mn–PO4 conversion coating thus can offer a corrosion protection similar to that of the DOW conversion coating and other conversion coatings with high protection efficiency [7,47]. Moreover, high temperature, though has less remarkable influence on the improvement of corrosion rate than that of pH, always favours the anticorrosion performance of the phosphate coatings [30].
4. Conclusions A series of Mn–PO4 containing conversion coatings were produced by a simple immersion method capable of improving the corrosion resistance of Mg alloy AZ91D. A coating formation mechanism was proposed with the aid of thermodynamic equilibrium calculations. Upon exposure to the coating bath, the matrix Mg dissolved and released, Mg 2+, H2 gas and OH − ions, increasing the pH in the vicinity of solid–liquid interface. The pH increase facilitated the formation of thin Mg(OH)2 intermediate layer on the substrate, which was then verified by SEM and XPS analysis. Finally, a top coating in form of (Mg/Mn)3(PO4)2 was produced due to their decreasing solubility along with decreasing pH, as predicted from the thermodynamic equilibrium diagram and confirmed by XPS results. Calculations could provide a theoretical rationalisation to engineering protective coating formation for AZ91D. The effect of coating growth parameters, pH and temperature, on coating morphology and subsequent corrosion resistance was systematically studied. It was found that pH was a key factor to determining coating thickness and characteristics. A thicker (~2.8 μm) surface film obtained at high acidity (pH 2.0) was associated with a network of severe structural defects due to hydrogen evolution. Because cracks are detrimental to corrosion resistance, a mild acidity (pH 4.0) resulted in a more compact Mn–PO4 conversion coating with optimal corrosion resistance. Higher alkalinity (pH 6.0), however, retarded the release of Mg2+ ions from the substrate, which led to a coating with low corrosion resistance. The operating temperature also had an influence on the coating formation, albeit only minor to pH. Due to its exothermic characteristic, the phosphating reaction rate heavily depends on the energy offered by the heating process. In general, the higher temperature, the quicker the phosphating process. Overall, the instant growth of a dense and thin Mg(OH)2 intermediate layer and a complete coverage by an (Mg/Mn)3(PO4)2 top layer at pH 4–80 °C exhibited the best corrosion resistance in corrosive environments, displaying a corrosion area fraction less than 1% after 48 h of salt spray testing. With respect to the salt spray evaluation, the proposed Mn–PO4 surface film outperformed chromate conversion coatings. Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.surfcoat.2012.12.005.
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Fig. 7. Potentiodynamic polarisation curves of various AZ91D obtained in 0.1 M NaCl at a sweep rate of 1 mV/s (a and b), Nyquist plots of the various AZ91D obtained in 0.1 M NaCl with a frequency range from 100 kHz to 10 mHz (c and d), e) 3D plot of the corrosion current density icorr (z axis) against treating temperature (x axis) and pH (y axis), and f) the equivalent circuit used for analysis of the EIS data of various AZ91D. (Rs is solution resistance, Rc is the resistance and constant phase element of a film on the sample surface, and Rct, Q and Qdl are the charge transfer resistance of a coating, constant phase element of the charge transfer, charge transfer capacitance of a double-layer, respectively. W is Warburg element accounting the diffusion of species).
Table 1 The corrosion area fraction after 48 h of the salt spray test, corrosion current density (icorr) and film resistance (Rc, generated from the Nyquist plots in Fig. 7c and d using the equivalent circuit presented in Fig. 7f) of various AZ91D. Coating parameters
pH 4-RT
pH 4–40 °C
pH 4–50 °C
pH 4–60 °C
pH 4–70 °C
pH 4–80 °C
pH 2–80 °C
pH 6–80 °C
Bare AZ91D
Corrosion area fraction, % icorr, 10−6 A cm−2 Rc, ohm
5–10 4.5 ± 0.5 2950 ± 360
5–10 4.1 ± 0.6 3470 ± 170
5–10 3.6 ± 1.1 4110 ± 320
1–5 2.7 ± 0.6 4410 ± 170
1–5 2.3 ± 0.2 4870 ± 110
b1 1.1 ± 0.2 6110 ± 80
15–30 11.5 ± 0.8 7580 ± 140
15–25 11.9 ± 0.2 1790 ± 180
70–80 18.0 ± 0.8 3640 ± 210
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