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permanganate conversion coating on AZ31 magnesium alloy
Corrosion Science 70 (2013) 74–81
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Effect of permanganate concentration on the formation and properties of phosphate/permanganate conversion coating on AZ31 magnesium alloy Y.L. Lee, Y.R. Chu, W.C. Li, C.S. Lin ⇑ Department of Materials Science and Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 12 September 2012 Accepted 7 January 2013 Available online 16 January 2013 Keywords: A. Magnesium B. SEM B. TEM B. EIS C. Passive films
a b s t r a c t This study investigated the role of permanganate in phosphate solutions in the formation and corrosion resistance of phosphate/permanganate coatings on AZ31 magnesium alloys. Experimental results showed that permanganate was reduced to manganese (IV) oxide together with the dissolution of magnesium during the conversion coating treatment. Adding more permanganate to the phosphate solution resulted in a thinner coating with a compact magnesium oxide layer contacting the AZ31 plate. Moreover, the thinner coating had fewer cracks and displayed higher polarization resistance and corrosion resistance than the thicker counterpart formed in the solution with less permanganate. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Conversion coating treatments have been extensively used to enhance the corrosion resistance of magnesium alloys, one of the highly regarded materials for applications in which structural weight is of concern. Hexavalent chromium conversion coating treatments, which are known to produce highly corrosion-resistant films on magnesium alloys [1,2], are now being restricted due to extremely toxic hexavalent chromium ions [3]. Non-chromate conversion coating treatments for magnesium alloys have thus received ever-increasing attention, including stannate, rare-earth metal salt, phosphate, titanate, and phosphate/permanganate conversion coatings [1,4–30]. Hawke et al. showed that the phosphate/ permanganate conversion coating on AM60B magnesium alloy displayed a wide color spectrum, ranging from a pale yellow interference tint to a medium brown with increasing thickness [12]. Chong et al. compared the conversion coatings formed in phosphate–permanganate and chromate solutions and found that both coatings had an equivalent corrosion protection potential for AZ61A, AZ80A, AZ91D magnesium alloys, and pure magnesium [15]. Zucchi et al. studied the formation of stannate and phosphate–permanganate conversion coatings on an AZ31 magnesium alloy and investigated the protective performances of the coatings using an in situ EIS technique [17]. Zhou et al. studied the structure and formation mechanism of phosphate conversion coatings on an AZ91D magnesium alloy and proposed a possible formation mechanism for the phosphate coating [18]. Chen et al. investigated ⇑ Corresponding author. Tel.: +886 2 33665240; fax: +886 2 23634562. E-mail address: [email protected] (C.S. Lin). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.01.014
the effect of Ca2+ and PO3 concentrations on the formation of 4 calcium phosphate conversion coatings on an AZ91D magnesium alloy [19]. Ishizaki et al. prepared an anticorrosive multilayered coating on an AZ31 magnesium alloy via a phosphate conversion treatment, followed by a steam curing treatment [20]. Li et al. studied the zinc phosphate conversion coatings formed on an AZ91D magnesium alloy in the phosphating bath with varying amounts of ethanolamine and found that the conversion coatings displayed a duplex structure [21]. Compared to the conventional chromate conversion coating, the phosphate/permanganate coating must be thicker to have a corrosion resistance comparable to its chromate counterpart [12,16]. While a thicker coating can provide better corrosion resistance, the adhesion of the phosphate/ permanganate conversion coating deteriorates with increasing thickness due to the occurrence of severe cracking [15,16,23]. It has been shown that the thickness of phosphate/permanganate conversion coatings can be controlled by the pH and temperature of the electrolyte, as well as the immersion time. In general, thicker coatings are formed in the electrolyte with lower pHs and at lower temperatures [16,23]. The growth rate of the coating obeys a parabolic law, i.e. the growth rate decreases with continued immersion [12,23]. Permanganate in a phosphate solution acts as a strong oxidizing agent to accelerate the dissolution of magnesium in acidic media [12]. The subsequent proton discharge results in an increase in pH near the surface of magnesium. As a result, magnesic ion (Mg2+) precipitates as magnesium hydroxide (Mg(OH)2) on the substrate, leading to the formation of the conversion coating [12,14,15,23]. Concurrent with the oxidation of magnesium, permanganate can be reduced to form lower-valence oxides that are
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potentially incorporated in the conversion coating [12–16,23]. Consequently, the presence of permanganate in a phosphate solution influences the growth and modifies the microstructure of the conversion coating. Nevertheless, the effect of permanganate on the microstructure of phosphate/permanganate conversion coatings has yet to be elucidated. This study investigated the microstructure and properties of the conversion coating on an AZ31 magnesium alloy in a phosphate solution with the addition of 0.063–0.51 M potassium permanganate (KMnO4). The effect of permanganate in the solution on the growth kinetics of the phosphate/permanganate coating was also discussed. 2. Experimental 2.1. Conversion coating treatment The material used in this study is the commercial AZ31 plate. To avoid the effect of the air-formed surface oxide layer, the AZ31 plate was mechanically abraded using emery paper up to 2000 grit, rinsed with deionized water, cleaned in acetone ultrasonically, and finally dried in a stream of hot air. The conversion coating treatment was conducted in the solution composed of 0.87 M anhydrate ammonium phosphate (NH4H2PO4) and 0.063–0.51 M KMnO4 at 60 °C for 10 min. After treatment, the AZ31 plate was thoroughly rinsed in deionized water and then left for drying in room temperature air overnight. 2.2. Microstructural characterization The surface morphology of the phosphate/permanganate coating was investigated using scanning electron microscopy (SEM). To prepare cross-sectional specimens, two AZ31 plates with conversion coatings facing each other were glued together using an M-Bond epoxy. Thin slices were then cut using a low-speed diamond saw. After mechanical grinding and polishing using a dimpling machine, the cross section was observed under the SEM to measure the thickness of the coating. To prepare a cross-sectional TEM specimen, the other side of the specimen was further thinned by mechanical grinding and dimpling, and finally by ionbeam thinning. The TEM specimen was observed via JEOL100CXII and JEOL3010 TEM. The composition of the coating was analyzed by the energy dispersive spectrometry (EDS) using an electron probe of 10 nm in diameter and the crystal structure was identified by the selected area electron diffraction (SAED) technique. The thickness of the coating was measured from the cross-sectional SEM or TEM micrographs, and the thickness of each specimen was reported as an average of at least 10 measurements on at least three micrographs for each specific coating. Finally, the X-ray photoelectron spectrum of the conversion coating was measured using an ESCA250 (VG Science Inc., UK) spectrometer with monochromatic Mg Ka radiation. The pressure in the spectrometer was approximately 109 Torr during the measurement. Binding energy of the peak was referenced to the C 1s peak at 285 eV. 2.3. Property evaluation The salt spray test according to the ASTM-B117 standard was employed to evaluate the corrosion resistance of the phosphate/ permanganate coatings. The coated AZ31 plate was placed at a tilted angle of 30° in a chamber containing 5 wt% sodium chloride (NaCl) fog. The potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS) measurement were conducted in a solution composed of 0.05 M NaCl and 0.10 M sodium sulfate (Na2SO4) using a three-electrode cell, in which a platinum wire and a saturated calomel electrode (SCE) were used
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as the counter and reference electrodes, respectively. The potentiodynamic polarization measurements were conducted by sweeping the potential from 200 mV vs. open circuit potential (OCP) to +500 mV vs. OCP at a scan rate of 1 mV/s after a steady OCP was reached. Each EIS spectrum was recorded at the OCP in frequencies ranging from 105 to 102 Hz, with a sinusoidal signal amplitude of 10 mV. The electrochemical characterizations and salt spray tests were performed on three AZ31 plates treated in a specific permanganate/phosphate conversion solution to confirm the reproducibility of the tests.
3. Results 3.1. Microstructure of the phosphate/permanganate conversion coating A variety of colors were observed on the coatings formed in the solution with various amounts of permanganate. The coating formed in the solution containing 0.063 M KMnO4 displayed a dark brown color. The color of the coating was, in order, light brown, golden yellow, and light yellow as the KMnO4 concentration was increased from 0.25 to 0.51 M. In general, the color of the coating became lighter when more permanganate was added in the solution. Fig. 1 shows the surface morphology of the AZ31 plate after 10 min of immersion in the solution containing different amounts of permanganate. Cracks were observed on all the phosphate/ permanganate coated AZ31 plates. Cracks have been commonly seen on phosphate/permanganate conversion coatings and are likely due to dehydration during post-immersion [15,16,23]. Severe cracking was observed on the AZ31 plates treated in the solution with 0.063 M KMnO4 (Fig. 1a), whereas fewer and smaller cracks were observed on those treated in solutions containing more permanganate. Fig. 2 shows the cross-sectional TEM characterization of an AZ31 plate treated in the solution with 0.063 M KMnO4. The conversion coating comprised three layers: a porous layer in contact with the substrate, a compact layer as the intermediate layer, and a cellular overlay as marked, in order, by 1, 2 and 3 in Fig. 2a. Note that the outer region of the cellular layer shown in Fig. 2a had been sputtered away during ion beam thinning. The SAED pattern taken from the compact layer contained diffuse halos (inset in Fig. 2a), indicating the compact layer was amorphous. The cellular overlay was also an amorphous structure as characterized by its SAED pattern (not shown here). The porous layer in contact with the substrate contained magnesium, oxygen, and trace of phosphorus and aluminum species (Fig. 2b), suggesting that this layer mainly comprised magnesium hydroxide/oxide. The compact layer was also composed of magnesium, oxygen, phosphorus, aluminum species, in which the phosphorus content was larger than that of the porous layer (Fig. 2c). In addition to magnesium, oxygen, and phosphorus, manganese signals were also detected in the cellular layer (Fig. 2d). Fig. 3 shows the cross-sectional TEM characterization of an AZ31 plate treated in the solution with 0.38 M KMnO4. The coating also consisted of three layers, including a compact layer contacting the substrate, a pore-containing intermediate layer, and a thin top layer, as marked by 1, 2, and 3 in Fig. 3a. Inset in Fig. 3a is a SAED pattern taken from the compact inner layer, showing that this compact layer was crystalline magnesium oxide (MgO). This is consistent with the EDS spectrum taken from the compact layer, which mainly consisted of magnesium and oxygen signals (Fig. 3b). The pore-containing layer contained magnesium, oxygen, phosphorus, and aluminum species (Fig. 3c), and displayed an amorphous structure as illustrated by its SAED pattern (not shown
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Fig. 1. Surface morphology of the AZ31 plates after 10 min of immersion in the phosphate solution with the addition of (a) 0.063, (b) 0.25, (c) 0.38, and (d) 0.51 M KMnO4.
Fig. 2. Cross-sectional TEM characterization of an AZ31 plate after 10 min of immersion in the solution containing 0.063 M KMnO4: (a) bright-field image, (b, c and d) are the EDS spectrums taken, in order, from the porous layer, compact layer, and cellular overlay. Inset in (a) is a SAED pattern taken from the compact layer.
here). In addition to magnesium, oxygen and phosphorus species, the thin top layer contained a significant amount of manganese species (Fig. 3d). Fig. 4 shows the thickness of the phosphate/permanganate conversion coating as a function of solution KMnO4 concentration. The coatings formed at lower KMnO4 concentrations (0.063 M and 0.25 M) were relatively thick and its thickness was measured on the cross-sectional SEM micrographs. In contrast, the coatings
formed at higher KMnO4 concentrations (0.38 M and 0.51 M) were apparently thin, as measured on the cross-sectional TEM micrographs. It is evident in Fig. 4 that the coating was thinner as more permanganate was added to the solution. For example, the thickness of the coatings formed in solutions containing 0.063 M and 0.51 M KMnO4 were approximately 8 and 1 lm, respectively. Fig. 5a shows a general survey XPS spectrum of a phosphate/ permanganate coating formed in the solution containing 0.063 M
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Fig. 3. Cross-sectional TEM characterization of anAZ31 plate after 10 min of immersion in the solution containing 0.38 M KMnO4: (a) bright-field image, and (b, c and d) are the EDS spectrums taken, in order, from the inner compact layer, pore-containing layer, and thin top layer. Inset in (a) is a SAED pattern taken from the compact layer.
coating formed at lower permanganate concentrations can be associated with thicker coatings with more and larger cracks. 3.2. Properties of the phosphate/permanganate conversion coating
Fig. 4. The thickness of the phosphate/permanganate coating on AZ31 plates as a function of the solution permanganate concentration.
KMnO4. The main constituents of the coating surface included magnesium, manganese, oxygen, and phosphorus species. Highresolution XPS scans (Fig. 5b) further show the Mn 2p3/2 binding energy appearing at approximately 643.2 eV, indicating the presence of Mn4+ species in the coating [31]. Moreover, the Mn species were present mainly as Mn4+ regardless of the permanganate concentration in the conversion coating solution, as shown in Fig. 5b. Because manganese (IV) oxide generally displays a brown–black color [45], the dependence of the color of the coating on the solution permanganate concentration is apparent not due to the valence of the Mn species. Instead, the darker brown color of the
To evaluate the protective properties of phosphate/permanganate conversion coatings, both the phosphate/permanganate coated AZ31 plates and a bare AZ31 plate [30] were tested using potentiodynamic polarizations and EIS. The potentiodynamic polarization curves of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates are shown in Fig. 6. For the AZ31 plates with conversion coatings, the Ecorr shifted toward the noble direction and the icorr decreased with the increase in the solution permanganate concentrations. Notably, the icorr decreased approximately one order of magnitude as the solution permanganate concentration was increased from 0.063 M to 0.51 M. Moreover, the anodic current densities were remarkably suppressed with increasing solution permanganate concentration, suggesting the corrosion resistance of the coating increased as more permanganate was added to the phosphate conversion solution. Fig. 6 also shows that the bare AZ31 plate had a slightly higher Ecorr and lower icorr compared with the AZ31 plates treated at 0.063 M permanganate. It is likely that severe cracking of the AZ31 treated at 0.063 M permanganate markedly deteriorates the corrosion resistance of the coating. Fig. 7 shows the Nyquist plots of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates. The Nyquist plots of the bare AZ31 plate and the AZ31 plate with the coating formed in the solution with 0.063 M KMnO4 were characterized by a capacitive loop in the high frequency, accompanied by an inductive loop in the low frequency. This inductive loop can be attributed to the phenomena of adsorption and desorption of Mg+ species on the surface of the Mg substrate [32–34], suggesting
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Fig. 5. (a) General XPS survey of the coating formed in the solution containing 0.063 M KMnO4, and (b) a high-resolution XPS scan showing the presence of manganese (IV) species on the coating formed in the solution with different KMnO4 concentrations.
that some areas of the coating were broken and the substrate had suffered the attack of the corrosive media in the immersion period in the test solution [35]. Conversely, the AZ31 plates with phosphate/permanganate coatings formed at higher permanganate concentrations (0.25–0.51 M) showed a single capacitive loop in which the diameter increased with increasing permanganate concentration. As a result, the coating formed in the solution containing 0.51 M KMnO4 had the largest impedance modulus value at low frequencies, indicating that the protective property of the coating increased as more permanganate was added to the solution. In order to further understand the corrosion process of the phosphate/permanganate conversion coating, the EIS spectrums were fitted to the electrical equivalent circuit using the ZSimpWin software. All the plots in the Fig. 7 were fitted using the model shown in Fig. 8a [36–38], except those of the bare AZ31 plate and the AZ31 plate with the coating formed in 0.063 M KMnO4, which were fitted using the model shown in Fig. 8b [30] by taking into account the presence of the inductive loop in the low frequency range. The Rs in the circuit represents the solution
resistance between the specimen and the reference electrode. The constant phase element CPEc and Rc are the surface coating capacitance and the resistance experienced by the electrolyte within the pore length of the coating, respectively. Instead of a capacitor, the CPE was used to account for a non-ideal capacitive behavior, which is generally considered to be related to the surface roughness [39–41]. The CPEdl is the double layer capacitance and the Rct represents the charge transfer resistance. Table 1 lists the values of the different electric elements of the bare AZ31 plate and the distinct phosphate/permanganate coated AZ31 plates. Compared to the bare AZ31 plate, the AZ31 plate treated at 0.063 M permanganate had a lower Rc and a higher Rct, suggesting the phosphate/permanganate coating with large cracks offered limited protection on the AZ31 plate. The Rc and Rct markedly increased as the permanganate concentration was increased from 0.25 M to 0.51 M, which was parallel with the decrease in the population density and size of cracks on the coatings. Moreover, the value of CPEdl decreased over three orders of magnitude for AZ31 plates with coatings formed in solutions containing 0.25 M,
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Fig. 6. Potentiodynamic polarization curves of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates tested in a solution composed of 0.05 M NaCl and 0.10 M Na2SO4 after a steady OCP is reached. The measurements are conducted by sweeping the potential from 200 mV vs. OCP to +500 mV vs. OCP at a scan rate of 1 mV/s.
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0.38 M and 0.51 M KMnO4, indicating a comparatively lower exposure of the metal/liquid interface to the test solution [42]. Also, the AZ31 plate treated in the solution with 0.51 M KMnO4 had a highest value of Rct. This indicates that the charge transfer between the coating and the substrate became more difficult, implying the coating formed in the phosphate conversion solution containing 0.51 M KMnO4 acted as a better barrier to inhibit the penetration of aggressive electrolyte to the metal surface [43]. Moreover, the CPEdl decreased nearly four orders of magnitude when the solution permanganate concentration was increased from 0.063 M to 0.51 M. Because the double layer capacitance is proportional to the exposure area at the interface between the substrate and the solution, the relatively lower capacitance indicates that a less defective area was developed on the coating formed in the solution with 0.51 M KMnO4 [42]. Table 2 shows the effect of the solution permanganate concentration on the percentage of corroded area after different salt spray exposure times. After 1 h of the salt spray test, small pitting spots were observed on the bare AZ31 plate and all the phosphate/permanganate coated AZ31 plates. The percentage of corroded area of the bare AZ31 plate was comparable to that of the AZ31 plate treated at 0.063 M permanganate during the whole time intervals of the salt spray test. Both exhibited the worst corrosion resistance among the various AZ31 plates. After 24 h of exposure in the salt spray chamber, the corroded area fractions of the AZ31 plates with and without phosphate/permanganate coatings were equal or larger than 50% except the AZ31 plate treated in the solution with 0.51 M KMnO4, which had a corroded area fraction less than 10%. Again, this agrees well with the potentiodynamic polarization and EIS evaluations. That is, the phosphate/permanganate coating formed in the solution with 0.51 M KMnO4 effectively improves the corrosion resistance of the AZ31 magnesium alloy. 4. Discussion 4.1. Effect of permanganate on the growth kinetic of phosphate/ permanganate conversion coatings
Fig. 7. Nyquist plots of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates tested in a solution composed of 0.05 M NaCl and 0.10 M Na2SO4 after a steady OCP is reached. Each EIS spectrum is recorded at the OCP in frequencies ranging from 105 to 102 Hz, with a sinusoidal signal amplitude of 10 mV.
Fig. 8. Equivalent circuit models for the simulation of the Nyquist plots (a) without, (b) with an inductive loop in the low frequencies.
Accelerators, which are generally a kind of oxidizing agent, in phosphating baths for steel are necessary to enhance the dissolution of the substrate, and to modify the constituents and the corrosion resistance of the resulting conversion coating [44]. Permanganate is known as an ideal accelerator for the phosphate treatment of magnesium alloys [12]. This is because permanganate can be reduced to lower-valence oxides, though not to metallic manganese, due to the powerful reducing action of magnesium [12]. The incorporation of lower-valence manganese oxides into a phosphate conversion coating can improve the corrosion resistance of the coating, whereas the elimination of metallic manganese avoids the galvanic corrosion of the coating [12]. When magnesium is immersed in a phosphate/permanganate solution, the major oxidation reaction is the dissolution of magnesium (Reaction 1) or the direct oxidation of magnesium to magnesium oxide (Reaction 2) in the presence of oxygen.
Mg ! Mg2þ þ 2e
ð1Þ
1 Mg þ O2 ! MgO 2
ð2Þ
While Reactions 1 and 2 are parallel reactions, the amount of permanganate in the solution apparently determines their interplay. That is, the first layer directly contacting the AZ31 plate was found to be magnesium hydroxide and magnesium oxide in the phosphate solution with low permanganate concentrations (0.063 M) and high permanganate concentrations (0.38 M and 0.51 M), respectively.
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Table 1 EIS simulation results of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates tested in a solution composed of 0.05 M NaCl and 0.10 M Na2SO4 after a steady OCP is reached. Each EIS spectrum is recorded at the OCP in frequencies ranging from 105 to 102 Hz, with a sinusoidal signal amplitude of 10 mV.
Rs (X cm2) CPEc (sn lX1 cm2) Rc (X cm2) CPEdl (sn lX1 cm2) Rct (X cm2) L (henry) Total error (%)
Bare AZ31
0.063 M KMnO4
0.25 M KMnO4
0.38 M KMnO4
0.51 M KMnO4
69.2 ± 7.14 19.1 ± 2.55 1620 ± 203 2510 ± 191 58.1 ± 6.28 34,500 ± 4690
67.7 ± 1.10 40.8 ± 10.2 444 ± 23.6 8540 ± 713 119 ± 33.0 2460 ± 212
69.0 ± 1.65 110 ± 7.64 1410 ± 177 0.408 ± 0.115 283 ± 63.6 –
72.6 ± 5.02 85.4 ± 15.3 1540 ± 106 0.446 ± 0.102 321 ± 115 –
74.0 ± 2.43 51.0 ± 15.3 2860 ± 220 0.204 ± 0.076 783 ± 106 –
3.7 ± 0.6
5.3 ± 0.6
5.6 ± 1.5
6.0 ± 1.0
8.3 ± 1.5
Table 2 Corroded area fraction of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates as a function of the salt spray time according to the ASTM-B117 standard.
Corroded Corroded Corroded Corroded Corroded Corroded
fraction fraction fraction fraction fraction fraction
% % % % % %
(1 h) (2 h) (3 h) (6 h) (12 h) (24 h)
Bare AZ31
0.063 M KMnO4
0.25 M KMnO4
0.38 M KMnO4
0.51 M KMnO4
5 10 15 >50 >50 >50
5 <10 10 15 30 >50
3 5 <10 15 20 50
3 <5 <10 10 15 50
<1 <3 <3 <5 <5 <10
When the permanganate concentration is low, dissolution of magnesium prevails over the direct oxidation of magnesium, giving rise to abundant Mg2+ on the surface of the substrate. As the surface pH rises due to hydrogen discharge, Mg2+ ions precipitate as magnesium hydroxide and/or phosphate, in which PO3 4 results from the dissociation of H2 PO taking place at a pH higher than 12 [45]. As 4 a result, a thicker conversion coating is formed on the AZ31 plate, as shown in Fig. 4. In contrast, the formation of magnesium oxide is promoted at high permanganate concentrations. Moreover, the compact magnesium oxide layer effectively retards the oxidation and dissolution of the AZ31 plate. The resulting coating is, therefore, thinner than that formed in the solution with less permanganate (Fig. 4). It is not immediately clear as why permanganate in the phosphate solution enhances the formation of magnesium oxide on an A31 plate. Permanganate is not completely stable due to its tendency of decomposition, which causes oxygen evolution and manganous ion (Mn2+) formation in acid media [46–48], as shown in Reaction 3. However, the decomposition rate is generally slow in dilute acid solution but can be accelerated by light, heat, acid, and manganese (IV) oxide [46,48].
4MnO4 þ 12Hþ ! 4Mn2þ þ 6H2 O þ 5O2
ð3Þ
In this present study, there is few precipitation or sedimentation of manganese (IV) oxide even at the largest permanganate concentration studied, i.e., 0.51 M. Accordingly, the reaction between permanganate and manganous ions to form manganese (IV) oxide (reaction 4), which is known as the Guyard Reaction, is slow in acid solution even at high temperatures [49]. Moreover, once the manganese (IV) oxide precipitates, it may further inhibit the Guyard Reaction [50].
2MnO4 þ 3Mn2þ þ 2H2 O ! 5MnO2 þ 4Hþ
ð4Þ
The dissolved oxygen as a function of permanganate was measured using the WTW Oxi 330i oxygen measuring instrument (Hoskin Scientific Ltd., Canada). It was found that the average dissolved oxygen in 0.87 M NH4H2PO4 solution at 50 °C was around 6 mg/l. After the addition of permanganate, the dissolved oxygen was measured every minute up to 5 min and then every 5 min up to 1 h. It was found that the dissolved oxygen increased with increasing permanganate concentrations (data not shown) and the maximum dissolved oxygen measured was approximately
6.58 mg/l. Because the dissolved oxygen only slightly increased with the presence of permanganate, the formation of magnesium oxide at high permanganate concentrations seems not to be related to the increased oxygen concentration in the phosphate solution. More studies are essential to clarify the role of permanganate in the formation of magnesium oxide. In addition to being an accelerator, as commonly seen in the literature [12,44], this present study further shows that permanganate in a phosphate solution can be an inhibitor that retards the growth of the conversion coating on magnesium via the formation of magnesium oxide. Reduction of permanganate, on one hand, promotes the dissolution of magnesium and the subsequent growth of the conversion coating composed of magnesium hydroxide/phosphate and manganese (IV) oxide. On the other hand, excess permanganate enhances the direct oxidation of magnesium to oxide, which, in turn, retards the growth of the phosphate/permanganate conversion coating. Cracks are frequently seen in phosphate/permanganate and cerium conversion coatings on magnesium [7–11,15,16,23,24]. These cracks have been shown to evolve during the post-immersion treatment such as the drying process. Dehydration of the conversion coating is likely to occur because the coating is mainly composed of magnesium hydroxide/phosphate and manganese (IV) oxide. Shrinkage in volume due to dehydration induces tensile stresses, leading to the formation of cracks on the coating. Higher stresses caused by the larger volume shrinkage associated with a thicker coating account for the fact that the thicker coating generally suffers more severe cracking, as shown in Fig. 1. In contrast, the thinner coating contains a smaller percentage of magnesium hydroxide and is formed preferably in the solution containing more permanganate. 4.2. Corrosion resistance and adhesion of phosphate/permanganate conversion coatings As mentioned in the previous section, the presence of permanganate in a phosphate solution influences the various chemical reactions taking place on the surface of an AZ31 plate. Therefore, the microstructure, growth kinetics, and corrosion resistance of the coating are strongly dependent on the concentration of permanganate in the solution. The total impedance of the coating increases with the solution permanganate concentration, as shown
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in Fig. 7. This suggests a less defective phosphate/permanganate conversion coating is formed in the phosphate solution containing more permanganate. Specifically, the coating formed in the solution containing 0.063 M KMnO4 contains large, through-coating cracks and provides inferior corrosion protection on the AZ31 plate. In contrast, increasing solution permanganate concentration to 0.51 M KMnO4 results in a coating that suffers a milder corrosion attack after 24 h of the salt spray test (Table 2). This is because the corrosion of the conversion coating proceeds with its dissolution in a corrosive environment. A thinner phosphate/permanganate coating with fewer cracks apparently provides adequate corrosion protection on AZ31 plates. When the corrosion behaviors of the phosphate/permanganate coated AZ31 plates are compared with those of the bare AZ31 plate, it is not immediately clear as why the order regarding the corrosion resistance is inconsistent for the salt spray and electrochemical tests. One possible explanation is that the salt spray test is performed in the fog of 5 wt% NaCl, whereas the electrochemical tests are conducted in the solution containing 0.05 M NaCl and 0.10 M Na2SO4. Since the solution containing 0.05 M NaCl and 0.10 M Na2SO4 is milder than that of 5 wt% NaCl, the salt spray tests may not be closely related to the electrochemical results. However, insignificant difference was measured for the electrochemical tests in 5 wt% NaCl (data not shown). Accordingly, the electrochemical tests were conducted in mild test solutions, which are commonly employed as the test solution for magnesium alloys with conversion coatings [7,9,17,22,24,26,27,29]. 5. Conclusions The microstructure and properties of the phosphate/permanganate conversion coating on an AZ31 magnesium alloy were investigated in this study with emphasis on the effect of solution permanganate concentration. It was found that when the treatment time was set at 10 min, adding more permanganate to a solution containing 0.87 M NH4H2PO4 resulted in a thinner conversion coating with smaller cracks. As a result, the corrosion resistance of the coating increased as more permanganate was added to the solution. Both of the phosphate/permanganate coatings formed in the solutions containing, respectively, 0.063 M and 0.38 M KMnO4 were composed of three layers. The layers directly in contact with the substrate were a porous magnesium hydroxide/oxide layer and a compact magnesium oxide layer for the coating formed in the solutions containing 0.063 M and 0.38 M KMnO4, respectively. It is likely that the compact magnesium oxide layer was formed via the direct oxidation of magnesium. As an oxidizing agent, permanganate in a phosphate solution accelerated the dissolution of magnesium and the subsequent precipitation of the conversion coating. However, permanganate can also retard the growth of the coating by suppressing the dissolution of the magnesium substrate via the formation of the compact magnesium oxide layer. This retardation is beneficial for the formation of a compact phosphate/permanganate conversion coating with enhanced corrosion protection on an AZ31 plate. Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under
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Grant No. 952221E002080MY3. L.C. Wang, National Sun Yat-sen University, is acknowledged for her assistance with the TEM work. References [1] T. Biestek, J. Weber, Electrolytic and Chemical Conversion Coatings, first ed., Portcullis Press Ltd., Redhill, Surrey, UK, 1976. [2] H. Bhatt, Plat. Surf. Finish. 93 (10) (2006) 20–23. [3] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88–113. [4] M.A. Gonzalez-Nunez, C.A. Nunze-Lopez, P. Skeldon, G.E. Thompson, H. Karimzadeh, P. Lyon, T.E. Wilks, Corros. Sci. 37 (1995) 1763–1772. [5] M.A. Gonzalez-Nunez, P. Skeldon, G.E. Thompson, H. Karimzadeh, Corrosion 55 (1999) 1136–1143. [6] C.S. Lin, H.C. Lin, K.M. Lin, W.C. Lai, Corros. Sci. 48 (2006) 93–109. [7] L. Rudd, C.B. Breslin, F. Mansfeld, Corros. Sci. 42 (2000) 275–288. [8] M. Dabalà, K. Brunelli, E. Napolitani, M. Magrini, Surf. Coat. Technol. 172 (2003) 227–232. [9] K. Brunelli, M. Dabalà, I. Calliari, M. Magrini, Corros. Sci. 47 (2005) 989–1000. [10] C.S. Lin, S.K. Fang, J. Electrochem. Soc. 152 (2005) B54–B59. [11] C.S. Lin, W.J. Li, Mater. Trans. 47 (2006) 1020–1025. [12] D. Hawke, D.L. Albright, Met. Finish. 93 (10) (1995) 34–38. [13] H. Umehara, S. Terauchi, M. Takaya, Mater. Sci. Forum. 350 (2000) 273–282. [14] H. Umehara, M. Takaya, Y. Kojima, Mater. Trans. 42 (2001) 1691–1699. [15] K.Z. Chong, T.S. Shih, Mater. Chem. Phys. 80 (2003) 191–200. [16] M. Zhao, S. Wu, J. Luo, Y. Fukuda, H. Nakae, Surf. Coat. Technol. 200 (2006) 5407–5412. [17] F. Zucchi, A. Frignani, V. Grassi, G. Trabanelli, C. Monticelli, Corros. Sci. 49 (2007) 4542–4552. [18] W. Zhou, D. Shan, E.H. Han, W. Ke, Corros. Sci. 50 (2008) 329–337. [19] X.B. Chen, N. Birbilis, T.B. Abbott, Corros. Sci. 55 (2012) 226–232. [20] T. Ishizaki, R. Kudo, T. Omi, K. Teshima, T. Sonoda, I. Shigematsu, M. Sakamoto, Electrochim. Acta 62 (2012) 19–29. [21] Q. Li, S. Xu, J. Hu, S. Zhang, X. Zhong, X. Yang, Electrochim. Acta 55 (2010) 887– 894. [22] M.F. Montemor, A.M. Simones, M.J. Carmezim, Appl. Surf. Sci. 253 (2007) 6922–6931. [23] C.S. Lin, C.Y. Lee, W.C. Li, Y.S. Chen, G.N. Fang, J. Electrochem. Soc. 153 (2006) B90–B96. [24] M. Mosialek, G. Mordarsku, P. Nowak, W. Simka, G. Nawrat, M. Hanke, R.P. Socha, J. Michalska, Surf. Coat. Technol. 206 (2011) 51–62. [25] H. Zhang, G.C. Yao, S.L. Wang, Y.H. Liu, H.J. Luo, Surf. Coat. Technol. 202 (2008) 1825–1830. [26] H. Elsentriecy, K. Azumi, H. Konno, Electrochim. Acta 53 (2008) 4267–4275. [27] H. Ardelean, I. Frateur, P. Marcus, Corros. Sci. 50 (2008) 1907–1918. [28] C. Wang, S. Zhu, F. Jiang, F. Wang, Corros. Sci. 51 (2009) 2916–2923. [29] E. Rocca, C. Juers, J. Steinmetz, Corros. Sci. 52 (2010) 2172–2178. [30] Y.C. Yang, C.Y. Tsai, Y.H. Huang, C.S. Lin, J. Electrochem. Soc. 159 (2012) C226– C232. [31] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Physical Electronics Division, Eden Prairie, Minnesota, 1992. [32] G. Song, A. Atrens, D. St. John, X. Wu, J. Nairn, Corros. Sci. 39 (1997) 1981–2004. [33] G. Baril, G. Galicia, C. Deslouis, N. Pébére, B. Tribollet, V. Vivier, J. Electrochem. Soc. 154 (2) (2007) C108–C113. [34] G. Galicia, N. Pebere, B. Tribollet, V. Vivier, Corros. Sci. 51 (2009) 1789–1794. [35] F. Liu, D. Shan, Y. Song, E. Han, W. Ke, Corros. Sci. 53 (2011) 3845–3852. [36] A. Doner, R. Solmaz, M. Ozcan, G. Kardas, Corros. Sci. 53 (2011) 2902–2903. [37] F.E. Heakal, O.S. Shehata, N.S. Tantawy, Corros. Sci. 56 (2012) 86–95. [38] H. Gao, Q. Li, F.N. Chen, Y. Dai, F. Luo, L.Q. Li, Corros. Sci. 53 (2011) 1401–1407. [39] U. Rammelt, G. Reinhard, Corros. Sci. 27 (1987) 373–382. [40] M. Keddam, H. Takenouti, Electrochim. Acta 33 (1988) 445–448. [41] A.P. Borosy, L. Nyikos, T. Pajkossy, Electrochim. Acta 36 (1991) 163–165. [42] P.C. Banerjee, R.K.S. Raman, Electrochim. Acta 56 (2011) 3790–3798. [43] H.S. Ryu, J. Ryu, D.S. Park, S.H. Hong, J. Electrochem. Soc. 158 (2011) C23–C28. [44] G. Górecki, Met. Finish. 90 (8) (1992) 27–29. [45] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, second ed., NACE, Houston, Texas, 1974. [46] D.A. Skoog, D.M West, Fundamentals of Analytical Chemistry, third ed., Holt, Rinehart & Winston, New York, 1976. [47] J.W. Ladbury, C.F. Cullis, Chem. Rev. 58 (1958) 403–438. [48] E. Narita, T. Okabe, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 661–666. [49] M.J. Polissar, J. Phys. Chem. 39 (1935) 1057–1066. [50] A.W. Adamson, J. Phys. Colloid Chem. 55 (1951) 293–303.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Effect of permanganate concentration on the formation and properties of phosphate/permanganate conversion coating on AZ31 magnesium alloy Y.L. Lee, Y.R. Chu, W.C. Li, C.S. Lin ⇑ Department of Materials Science and Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 12 September 2012 Accepted 7 January 2013 Available online 16 January 2013 Keywords: A. Magnesium B. SEM B. TEM B. EIS C. Passive films
a b s t r a c t This study investigated the role of permanganate in phosphate solutions in the formation and corrosion resistance of phosphate/permanganate coatings on AZ31 magnesium alloys. Experimental results showed that permanganate was reduced to manganese (IV) oxide together with the dissolution of magnesium during the conversion coating treatment. Adding more permanganate to the phosphate solution resulted in a thinner coating with a compact magnesium oxide layer contacting the AZ31 plate. Moreover, the thinner coating had fewer cracks and displayed higher polarization resistance and corrosion resistance than the thicker counterpart formed in the solution with less permanganate. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Conversion coating treatments have been extensively used to enhance the corrosion resistance of magnesium alloys, one of the highly regarded materials for applications in which structural weight is of concern. Hexavalent chromium conversion coating treatments, which are known to produce highly corrosion-resistant films on magnesium alloys [1,2], are now being restricted due to extremely toxic hexavalent chromium ions [3]. Non-chromate conversion coating treatments for magnesium alloys have thus received ever-increasing attention, including stannate, rare-earth metal salt, phosphate, titanate, and phosphate/permanganate conversion coatings [1,4–30]. Hawke et al. showed that the phosphate/ permanganate conversion coating on AM60B magnesium alloy displayed a wide color spectrum, ranging from a pale yellow interference tint to a medium brown with increasing thickness [12]. Chong et al. compared the conversion coatings formed in phosphate–permanganate and chromate solutions and found that both coatings had an equivalent corrosion protection potential for AZ61A, AZ80A, AZ91D magnesium alloys, and pure magnesium [15]. Zucchi et al. studied the formation of stannate and phosphate–permanganate conversion coatings on an AZ31 magnesium alloy and investigated the protective performances of the coatings using an in situ EIS technique [17]. Zhou et al. studied the structure and formation mechanism of phosphate conversion coatings on an AZ91D magnesium alloy and proposed a possible formation mechanism for the phosphate coating [18]. Chen et al. investigated ⇑ Corresponding author. Tel.: +886 2 33665240; fax: +886 2 23634562. E-mail address: [email protected] (C.S. Lin). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.01.014
the effect of Ca2+ and PO3 concentrations on the formation of 4 calcium phosphate conversion coatings on an AZ91D magnesium alloy [19]. Ishizaki et al. prepared an anticorrosive multilayered coating on an AZ31 magnesium alloy via a phosphate conversion treatment, followed by a steam curing treatment [20]. Li et al. studied the zinc phosphate conversion coatings formed on an AZ91D magnesium alloy in the phosphating bath with varying amounts of ethanolamine and found that the conversion coatings displayed a duplex structure [21]. Compared to the conventional chromate conversion coating, the phosphate/permanganate coating must be thicker to have a corrosion resistance comparable to its chromate counterpart [12,16]. While a thicker coating can provide better corrosion resistance, the adhesion of the phosphate/ permanganate conversion coating deteriorates with increasing thickness due to the occurrence of severe cracking [15,16,23]. It has been shown that the thickness of phosphate/permanganate conversion coatings can be controlled by the pH and temperature of the electrolyte, as well as the immersion time. In general, thicker coatings are formed in the electrolyte with lower pHs and at lower temperatures [16,23]. The growth rate of the coating obeys a parabolic law, i.e. the growth rate decreases with continued immersion [12,23]. Permanganate in a phosphate solution acts as a strong oxidizing agent to accelerate the dissolution of magnesium in acidic media [12]. The subsequent proton discharge results in an increase in pH near the surface of magnesium. As a result, magnesic ion (Mg2+) precipitates as magnesium hydroxide (Mg(OH)2) on the substrate, leading to the formation of the conversion coating [12,14,15,23]. Concurrent with the oxidation of magnesium, permanganate can be reduced to form lower-valence oxides that are
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potentially incorporated in the conversion coating [12–16,23]. Consequently, the presence of permanganate in a phosphate solution influences the growth and modifies the microstructure of the conversion coating. Nevertheless, the effect of permanganate on the microstructure of phosphate/permanganate conversion coatings has yet to be elucidated. This study investigated the microstructure and properties of the conversion coating on an AZ31 magnesium alloy in a phosphate solution with the addition of 0.063–0.51 M potassium permanganate (KMnO4). The effect of permanganate in the solution on the growth kinetics of the phosphate/permanganate coating was also discussed. 2. Experimental 2.1. Conversion coating treatment The material used in this study is the commercial AZ31 plate. To avoid the effect of the air-formed surface oxide layer, the AZ31 plate was mechanically abraded using emery paper up to 2000 grit, rinsed with deionized water, cleaned in acetone ultrasonically, and finally dried in a stream of hot air. The conversion coating treatment was conducted in the solution composed of 0.87 M anhydrate ammonium phosphate (NH4H2PO4) and 0.063–0.51 M KMnO4 at 60 °C for 10 min. After treatment, the AZ31 plate was thoroughly rinsed in deionized water and then left for drying in room temperature air overnight. 2.2. Microstructural characterization The surface morphology of the phosphate/permanganate coating was investigated using scanning electron microscopy (SEM). To prepare cross-sectional specimens, two AZ31 plates with conversion coatings facing each other were glued together using an M-Bond epoxy. Thin slices were then cut using a low-speed diamond saw. After mechanical grinding and polishing using a dimpling machine, the cross section was observed under the SEM to measure the thickness of the coating. To prepare a cross-sectional TEM specimen, the other side of the specimen was further thinned by mechanical grinding and dimpling, and finally by ionbeam thinning. The TEM specimen was observed via JEOL100CXII and JEOL3010 TEM. The composition of the coating was analyzed by the energy dispersive spectrometry (EDS) using an electron probe of 10 nm in diameter and the crystal structure was identified by the selected area electron diffraction (SAED) technique. The thickness of the coating was measured from the cross-sectional SEM or TEM micrographs, and the thickness of each specimen was reported as an average of at least 10 measurements on at least three micrographs for each specific coating. Finally, the X-ray photoelectron spectrum of the conversion coating was measured using an ESCA250 (VG Science Inc., UK) spectrometer with monochromatic Mg Ka radiation. The pressure in the spectrometer was approximately 109 Torr during the measurement. Binding energy of the peak was referenced to the C 1s peak at 285 eV. 2.3. Property evaluation The salt spray test according to the ASTM-B117 standard was employed to evaluate the corrosion resistance of the phosphate/ permanganate coatings. The coated AZ31 plate was placed at a tilted angle of 30° in a chamber containing 5 wt% sodium chloride (NaCl) fog. The potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS) measurement were conducted in a solution composed of 0.05 M NaCl and 0.10 M sodium sulfate (Na2SO4) using a three-electrode cell, in which a platinum wire and a saturated calomel electrode (SCE) were used
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as the counter and reference electrodes, respectively. The potentiodynamic polarization measurements were conducted by sweeping the potential from 200 mV vs. open circuit potential (OCP) to +500 mV vs. OCP at a scan rate of 1 mV/s after a steady OCP was reached. Each EIS spectrum was recorded at the OCP in frequencies ranging from 105 to 102 Hz, with a sinusoidal signal amplitude of 10 mV. The electrochemical characterizations and salt spray tests were performed on three AZ31 plates treated in a specific permanganate/phosphate conversion solution to confirm the reproducibility of the tests.
3. Results 3.1. Microstructure of the phosphate/permanganate conversion coating A variety of colors were observed on the coatings formed in the solution with various amounts of permanganate. The coating formed in the solution containing 0.063 M KMnO4 displayed a dark brown color. The color of the coating was, in order, light brown, golden yellow, and light yellow as the KMnO4 concentration was increased from 0.25 to 0.51 M. In general, the color of the coating became lighter when more permanganate was added in the solution. Fig. 1 shows the surface morphology of the AZ31 plate after 10 min of immersion in the solution containing different amounts of permanganate. Cracks were observed on all the phosphate/ permanganate coated AZ31 plates. Cracks have been commonly seen on phosphate/permanganate conversion coatings and are likely due to dehydration during post-immersion [15,16,23]. Severe cracking was observed on the AZ31 plates treated in the solution with 0.063 M KMnO4 (Fig. 1a), whereas fewer and smaller cracks were observed on those treated in solutions containing more permanganate. Fig. 2 shows the cross-sectional TEM characterization of an AZ31 plate treated in the solution with 0.063 M KMnO4. The conversion coating comprised three layers: a porous layer in contact with the substrate, a compact layer as the intermediate layer, and a cellular overlay as marked, in order, by 1, 2 and 3 in Fig. 2a. Note that the outer region of the cellular layer shown in Fig. 2a had been sputtered away during ion beam thinning. The SAED pattern taken from the compact layer contained diffuse halos (inset in Fig. 2a), indicating the compact layer was amorphous. The cellular overlay was also an amorphous structure as characterized by its SAED pattern (not shown here). The porous layer in contact with the substrate contained magnesium, oxygen, and trace of phosphorus and aluminum species (Fig. 2b), suggesting that this layer mainly comprised magnesium hydroxide/oxide. The compact layer was also composed of magnesium, oxygen, phosphorus, aluminum species, in which the phosphorus content was larger than that of the porous layer (Fig. 2c). In addition to magnesium, oxygen, and phosphorus, manganese signals were also detected in the cellular layer (Fig. 2d). Fig. 3 shows the cross-sectional TEM characterization of an AZ31 plate treated in the solution with 0.38 M KMnO4. The coating also consisted of three layers, including a compact layer contacting the substrate, a pore-containing intermediate layer, and a thin top layer, as marked by 1, 2, and 3 in Fig. 3a. Inset in Fig. 3a is a SAED pattern taken from the compact inner layer, showing that this compact layer was crystalline magnesium oxide (MgO). This is consistent with the EDS spectrum taken from the compact layer, which mainly consisted of magnesium and oxygen signals (Fig. 3b). The pore-containing layer contained magnesium, oxygen, phosphorus, and aluminum species (Fig. 3c), and displayed an amorphous structure as illustrated by its SAED pattern (not shown
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Fig. 1. Surface morphology of the AZ31 plates after 10 min of immersion in the phosphate solution with the addition of (a) 0.063, (b) 0.25, (c) 0.38, and (d) 0.51 M KMnO4.
Fig. 2. Cross-sectional TEM characterization of an AZ31 plate after 10 min of immersion in the solution containing 0.063 M KMnO4: (a) bright-field image, (b, c and d) are the EDS spectrums taken, in order, from the porous layer, compact layer, and cellular overlay. Inset in (a) is a SAED pattern taken from the compact layer.
here). In addition to magnesium, oxygen and phosphorus species, the thin top layer contained a significant amount of manganese species (Fig. 3d). Fig. 4 shows the thickness of the phosphate/permanganate conversion coating as a function of solution KMnO4 concentration. The coatings formed at lower KMnO4 concentrations (0.063 M and 0.25 M) were relatively thick and its thickness was measured on the cross-sectional SEM micrographs. In contrast, the coatings
formed at higher KMnO4 concentrations (0.38 M and 0.51 M) were apparently thin, as measured on the cross-sectional TEM micrographs. It is evident in Fig. 4 that the coating was thinner as more permanganate was added to the solution. For example, the thickness of the coatings formed in solutions containing 0.063 M and 0.51 M KMnO4 were approximately 8 and 1 lm, respectively. Fig. 5a shows a general survey XPS spectrum of a phosphate/ permanganate coating formed in the solution containing 0.063 M
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Fig. 3. Cross-sectional TEM characterization of anAZ31 plate after 10 min of immersion in the solution containing 0.38 M KMnO4: (a) bright-field image, and (b, c and d) are the EDS spectrums taken, in order, from the inner compact layer, pore-containing layer, and thin top layer. Inset in (a) is a SAED pattern taken from the compact layer.
coating formed at lower permanganate concentrations can be associated with thicker coatings with more and larger cracks. 3.2. Properties of the phosphate/permanganate conversion coating
Fig. 4. The thickness of the phosphate/permanganate coating on AZ31 plates as a function of the solution permanganate concentration.
KMnO4. The main constituents of the coating surface included magnesium, manganese, oxygen, and phosphorus species. Highresolution XPS scans (Fig. 5b) further show the Mn 2p3/2 binding energy appearing at approximately 643.2 eV, indicating the presence of Mn4+ species in the coating [31]. Moreover, the Mn species were present mainly as Mn4+ regardless of the permanganate concentration in the conversion coating solution, as shown in Fig. 5b. Because manganese (IV) oxide generally displays a brown–black color [45], the dependence of the color of the coating on the solution permanganate concentration is apparent not due to the valence of the Mn species. Instead, the darker brown color of the
To evaluate the protective properties of phosphate/permanganate conversion coatings, both the phosphate/permanganate coated AZ31 plates and a bare AZ31 plate [30] were tested using potentiodynamic polarizations and EIS. The potentiodynamic polarization curves of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates are shown in Fig. 6. For the AZ31 plates with conversion coatings, the Ecorr shifted toward the noble direction and the icorr decreased with the increase in the solution permanganate concentrations. Notably, the icorr decreased approximately one order of magnitude as the solution permanganate concentration was increased from 0.063 M to 0.51 M. Moreover, the anodic current densities were remarkably suppressed with increasing solution permanganate concentration, suggesting the corrosion resistance of the coating increased as more permanganate was added to the phosphate conversion solution. Fig. 6 also shows that the bare AZ31 plate had a slightly higher Ecorr and lower icorr compared with the AZ31 plates treated at 0.063 M permanganate. It is likely that severe cracking of the AZ31 treated at 0.063 M permanganate markedly deteriorates the corrosion resistance of the coating. Fig. 7 shows the Nyquist plots of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates. The Nyquist plots of the bare AZ31 plate and the AZ31 plate with the coating formed in the solution with 0.063 M KMnO4 were characterized by a capacitive loop in the high frequency, accompanied by an inductive loop in the low frequency. This inductive loop can be attributed to the phenomena of adsorption and desorption of Mg+ species on the surface of the Mg substrate [32–34], suggesting
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Fig. 5. (a) General XPS survey of the coating formed in the solution containing 0.063 M KMnO4, and (b) a high-resolution XPS scan showing the presence of manganese (IV) species on the coating formed in the solution with different KMnO4 concentrations.
that some areas of the coating were broken and the substrate had suffered the attack of the corrosive media in the immersion period in the test solution [35]. Conversely, the AZ31 plates with phosphate/permanganate coatings formed at higher permanganate concentrations (0.25–0.51 M) showed a single capacitive loop in which the diameter increased with increasing permanganate concentration. As a result, the coating formed in the solution containing 0.51 M KMnO4 had the largest impedance modulus value at low frequencies, indicating that the protective property of the coating increased as more permanganate was added to the solution. In order to further understand the corrosion process of the phosphate/permanganate conversion coating, the EIS spectrums were fitted to the electrical equivalent circuit using the ZSimpWin software. All the plots in the Fig. 7 were fitted using the model shown in Fig. 8a [36–38], except those of the bare AZ31 plate and the AZ31 plate with the coating formed in 0.063 M KMnO4, which were fitted using the model shown in Fig. 8b [30] by taking into account the presence of the inductive loop in the low frequency range. The Rs in the circuit represents the solution
resistance between the specimen and the reference electrode. The constant phase element CPEc and Rc are the surface coating capacitance and the resistance experienced by the electrolyte within the pore length of the coating, respectively. Instead of a capacitor, the CPE was used to account for a non-ideal capacitive behavior, which is generally considered to be related to the surface roughness [39–41]. The CPEdl is the double layer capacitance and the Rct represents the charge transfer resistance. Table 1 lists the values of the different electric elements of the bare AZ31 plate and the distinct phosphate/permanganate coated AZ31 plates. Compared to the bare AZ31 plate, the AZ31 plate treated at 0.063 M permanganate had a lower Rc and a higher Rct, suggesting the phosphate/permanganate coating with large cracks offered limited protection on the AZ31 plate. The Rc and Rct markedly increased as the permanganate concentration was increased from 0.25 M to 0.51 M, which was parallel with the decrease in the population density and size of cracks on the coatings. Moreover, the value of CPEdl decreased over three orders of magnitude for AZ31 plates with coatings formed in solutions containing 0.25 M,
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Fig. 6. Potentiodynamic polarization curves of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates tested in a solution composed of 0.05 M NaCl and 0.10 M Na2SO4 after a steady OCP is reached. The measurements are conducted by sweeping the potential from 200 mV vs. OCP to +500 mV vs. OCP at a scan rate of 1 mV/s.
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0.38 M and 0.51 M KMnO4, indicating a comparatively lower exposure of the metal/liquid interface to the test solution [42]. Also, the AZ31 plate treated in the solution with 0.51 M KMnO4 had a highest value of Rct. This indicates that the charge transfer between the coating and the substrate became more difficult, implying the coating formed in the phosphate conversion solution containing 0.51 M KMnO4 acted as a better barrier to inhibit the penetration of aggressive electrolyte to the metal surface [43]. Moreover, the CPEdl decreased nearly four orders of magnitude when the solution permanganate concentration was increased from 0.063 M to 0.51 M. Because the double layer capacitance is proportional to the exposure area at the interface between the substrate and the solution, the relatively lower capacitance indicates that a less defective area was developed on the coating formed in the solution with 0.51 M KMnO4 [42]. Table 2 shows the effect of the solution permanganate concentration on the percentage of corroded area after different salt spray exposure times. After 1 h of the salt spray test, small pitting spots were observed on the bare AZ31 plate and all the phosphate/permanganate coated AZ31 plates. The percentage of corroded area of the bare AZ31 plate was comparable to that of the AZ31 plate treated at 0.063 M permanganate during the whole time intervals of the salt spray test. Both exhibited the worst corrosion resistance among the various AZ31 plates. After 24 h of exposure in the salt spray chamber, the corroded area fractions of the AZ31 plates with and without phosphate/permanganate coatings were equal or larger than 50% except the AZ31 plate treated in the solution with 0.51 M KMnO4, which had a corroded area fraction less than 10%. Again, this agrees well with the potentiodynamic polarization and EIS evaluations. That is, the phosphate/permanganate coating formed in the solution with 0.51 M KMnO4 effectively improves the corrosion resistance of the AZ31 magnesium alloy. 4. Discussion 4.1. Effect of permanganate on the growth kinetic of phosphate/ permanganate conversion coatings
Fig. 7. Nyquist plots of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates tested in a solution composed of 0.05 M NaCl and 0.10 M Na2SO4 after a steady OCP is reached. Each EIS spectrum is recorded at the OCP in frequencies ranging from 105 to 102 Hz, with a sinusoidal signal amplitude of 10 mV.
Fig. 8. Equivalent circuit models for the simulation of the Nyquist plots (a) without, (b) with an inductive loop in the low frequencies.
Accelerators, which are generally a kind of oxidizing agent, in phosphating baths for steel are necessary to enhance the dissolution of the substrate, and to modify the constituents and the corrosion resistance of the resulting conversion coating [44]. Permanganate is known as an ideal accelerator for the phosphate treatment of magnesium alloys [12]. This is because permanganate can be reduced to lower-valence oxides, though not to metallic manganese, due to the powerful reducing action of magnesium [12]. The incorporation of lower-valence manganese oxides into a phosphate conversion coating can improve the corrosion resistance of the coating, whereas the elimination of metallic manganese avoids the galvanic corrosion of the coating [12]. When magnesium is immersed in a phosphate/permanganate solution, the major oxidation reaction is the dissolution of magnesium (Reaction 1) or the direct oxidation of magnesium to magnesium oxide (Reaction 2) in the presence of oxygen.
Mg ! Mg2þ þ 2e
ð1Þ
1 Mg þ O2 ! MgO 2
ð2Þ
While Reactions 1 and 2 are parallel reactions, the amount of permanganate in the solution apparently determines their interplay. That is, the first layer directly contacting the AZ31 plate was found to be magnesium hydroxide and magnesium oxide in the phosphate solution with low permanganate concentrations (0.063 M) and high permanganate concentrations (0.38 M and 0.51 M), respectively.
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Table 1 EIS simulation results of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates tested in a solution composed of 0.05 M NaCl and 0.10 M Na2SO4 after a steady OCP is reached. Each EIS spectrum is recorded at the OCP in frequencies ranging from 105 to 102 Hz, with a sinusoidal signal amplitude of 10 mV.
Rs (X cm2) CPEc (sn lX1 cm2) Rc (X cm2) CPEdl (sn lX1 cm2) Rct (X cm2) L (henry) Total error (%)
Bare AZ31
0.063 M KMnO4
0.25 M KMnO4
0.38 M KMnO4
0.51 M KMnO4
69.2 ± 7.14 19.1 ± 2.55 1620 ± 203 2510 ± 191 58.1 ± 6.28 34,500 ± 4690
67.7 ± 1.10 40.8 ± 10.2 444 ± 23.6 8540 ± 713 119 ± 33.0 2460 ± 212
69.0 ± 1.65 110 ± 7.64 1410 ± 177 0.408 ± 0.115 283 ± 63.6 –
72.6 ± 5.02 85.4 ± 15.3 1540 ± 106 0.446 ± 0.102 321 ± 115 –
74.0 ± 2.43 51.0 ± 15.3 2860 ± 220 0.204 ± 0.076 783 ± 106 –
3.7 ± 0.6
5.3 ± 0.6
5.6 ± 1.5
6.0 ± 1.0
8.3 ± 1.5
Table 2 Corroded area fraction of a bare AZ31 plate and the various phosphate/permanganate coated AZ31 plates as a function of the salt spray time according to the ASTM-B117 standard.
Corroded Corroded Corroded Corroded Corroded Corroded
fraction fraction fraction fraction fraction fraction
% % % % % %
(1 h) (2 h) (3 h) (6 h) (12 h) (24 h)
Bare AZ31
0.063 M KMnO4
0.25 M KMnO4
0.38 M KMnO4
0.51 M KMnO4
5 10 15 >50 >50 >50
5 <10 10 15 30 >50
3 5 <10 15 20 50
3 <5 <10 10 15 50
<1 <3 <3 <5 <5 <10
When the permanganate concentration is low, dissolution of magnesium prevails over the direct oxidation of magnesium, giving rise to abundant Mg2+ on the surface of the substrate. As the surface pH rises due to hydrogen discharge, Mg2+ ions precipitate as magnesium hydroxide and/or phosphate, in which PO3 4 results from the dissociation of H2 PO taking place at a pH higher than 12 [45]. As 4 a result, a thicker conversion coating is formed on the AZ31 plate, as shown in Fig. 4. In contrast, the formation of magnesium oxide is promoted at high permanganate concentrations. Moreover, the compact magnesium oxide layer effectively retards the oxidation and dissolution of the AZ31 plate. The resulting coating is, therefore, thinner than that formed in the solution with less permanganate (Fig. 4). It is not immediately clear as why permanganate in the phosphate solution enhances the formation of magnesium oxide on an A31 plate. Permanganate is not completely stable due to its tendency of decomposition, which causes oxygen evolution and manganous ion (Mn2+) formation in acid media [46–48], as shown in Reaction 3. However, the decomposition rate is generally slow in dilute acid solution but can be accelerated by light, heat, acid, and manganese (IV) oxide [46,48].
4MnO4 þ 12Hþ ! 4Mn2þ þ 6H2 O þ 5O2
ð3Þ
In this present study, there is few precipitation or sedimentation of manganese (IV) oxide even at the largest permanganate concentration studied, i.e., 0.51 M. Accordingly, the reaction between permanganate and manganous ions to form manganese (IV) oxide (reaction 4), which is known as the Guyard Reaction, is slow in acid solution even at high temperatures [49]. Moreover, once the manganese (IV) oxide precipitates, it may further inhibit the Guyard Reaction [50].
2MnO4 þ 3Mn2þ þ 2H2 O ! 5MnO2 þ 4Hþ
ð4Þ
The dissolved oxygen as a function of permanganate was measured using the WTW Oxi 330i oxygen measuring instrument (Hoskin Scientific Ltd., Canada). It was found that the average dissolved oxygen in 0.87 M NH4H2PO4 solution at 50 °C was around 6 mg/l. After the addition of permanganate, the dissolved oxygen was measured every minute up to 5 min and then every 5 min up to 1 h. It was found that the dissolved oxygen increased with increasing permanganate concentrations (data not shown) and the maximum dissolved oxygen measured was approximately
6.58 mg/l. Because the dissolved oxygen only slightly increased with the presence of permanganate, the formation of magnesium oxide at high permanganate concentrations seems not to be related to the increased oxygen concentration in the phosphate solution. More studies are essential to clarify the role of permanganate in the formation of magnesium oxide. In addition to being an accelerator, as commonly seen in the literature [12,44], this present study further shows that permanganate in a phosphate solution can be an inhibitor that retards the growth of the conversion coating on magnesium via the formation of magnesium oxide. Reduction of permanganate, on one hand, promotes the dissolution of magnesium and the subsequent growth of the conversion coating composed of magnesium hydroxide/phosphate and manganese (IV) oxide. On the other hand, excess permanganate enhances the direct oxidation of magnesium to oxide, which, in turn, retards the growth of the phosphate/permanganate conversion coating. Cracks are frequently seen in phosphate/permanganate and cerium conversion coatings on magnesium [7–11,15,16,23,24]. These cracks have been shown to evolve during the post-immersion treatment such as the drying process. Dehydration of the conversion coating is likely to occur because the coating is mainly composed of magnesium hydroxide/phosphate and manganese (IV) oxide. Shrinkage in volume due to dehydration induces tensile stresses, leading to the formation of cracks on the coating. Higher stresses caused by the larger volume shrinkage associated with a thicker coating account for the fact that the thicker coating generally suffers more severe cracking, as shown in Fig. 1. In contrast, the thinner coating contains a smaller percentage of magnesium hydroxide and is formed preferably in the solution containing more permanganate. 4.2. Corrosion resistance and adhesion of phosphate/permanganate conversion coatings As mentioned in the previous section, the presence of permanganate in a phosphate solution influences the various chemical reactions taking place on the surface of an AZ31 plate. Therefore, the microstructure, growth kinetics, and corrosion resistance of the coating are strongly dependent on the concentration of permanganate in the solution. The total impedance of the coating increases with the solution permanganate concentration, as shown
Y.L. Lee et al. / Corrosion Science 70 (2013) 74–81
in Fig. 7. This suggests a less defective phosphate/permanganate conversion coating is formed in the phosphate solution containing more permanganate. Specifically, the coating formed in the solution containing 0.063 M KMnO4 contains large, through-coating cracks and provides inferior corrosion protection on the AZ31 plate. In contrast, increasing solution permanganate concentration to 0.51 M KMnO4 results in a coating that suffers a milder corrosion attack after 24 h of the salt spray test (Table 2). This is because the corrosion of the conversion coating proceeds with its dissolution in a corrosive environment. A thinner phosphate/permanganate coating with fewer cracks apparently provides adequate corrosion protection on AZ31 plates. When the corrosion behaviors of the phosphate/permanganate coated AZ31 plates are compared with those of the bare AZ31 plate, it is not immediately clear as why the order regarding the corrosion resistance is inconsistent for the salt spray and electrochemical tests. One possible explanation is that the salt spray test is performed in the fog of 5 wt% NaCl, whereas the electrochemical tests are conducted in the solution containing 0.05 M NaCl and 0.10 M Na2SO4. Since the solution containing 0.05 M NaCl and 0.10 M Na2SO4 is milder than that of 5 wt% NaCl, the salt spray tests may not be closely related to the electrochemical results. However, insignificant difference was measured for the electrochemical tests in 5 wt% NaCl (data not shown). Accordingly, the electrochemical tests were conducted in mild test solutions, which are commonly employed as the test solution for magnesium alloys with conversion coatings [7,9,17,22,24,26,27,29]. 5. Conclusions The microstructure and properties of the phosphate/permanganate conversion coating on an AZ31 magnesium alloy were investigated in this study with emphasis on the effect of solution permanganate concentration. It was found that when the treatment time was set at 10 min, adding more permanganate to a solution containing 0.87 M NH4H2PO4 resulted in a thinner conversion coating with smaller cracks. As a result, the corrosion resistance of the coating increased as more permanganate was added to the solution. Both of the phosphate/permanganate coatings formed in the solutions containing, respectively, 0.063 M and 0.38 M KMnO4 were composed of three layers. The layers directly in contact with the substrate were a porous magnesium hydroxide/oxide layer and a compact magnesium oxide layer for the coating formed in the solutions containing 0.063 M and 0.38 M KMnO4, respectively. It is likely that the compact magnesium oxide layer was formed via the direct oxidation of magnesium. As an oxidizing agent, permanganate in a phosphate solution accelerated the dissolution of magnesium and the subsequent precipitation of the conversion coating. However, permanganate can also retard the growth of the coating by suppressing the dissolution of the magnesium substrate via the formation of the compact magnesium oxide layer. This retardation is beneficial for the formation of a compact phosphate/permanganate conversion coating with enhanced corrosion protection on an AZ31 plate. Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under
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