Mechanism of Corrosion Protection of Anodized Magnesium Alloys

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1 Materials Transactions, Vol. 49, No. 5 (2008) pp to 1064 Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, IV #2008 The Japan Institute of Metals Mechanism of Corrosion Protection of Anodized Magnesium Alloys Koji Murakami 1, Makoto Hino 1, Kiyomichi Nakai 2, Sengo Kobayashi 2, Atsushi Saijo 3 and Teruto Kanadani 4 1 Industrial Technology Research Institute of Okayama Prefectural Government, Okayama , Japan 2 Graduate School of Science and Engineering, Ehime University, Matsuyama , Japan 3 Hori Metal Finishing Industry, Co. Ltd., Takahashi , Japan 4 Faculty of Engineering, Okayama University of Science, Okayama , Japan Corrosion resistance of anodized surfaces on high-purity magnesium (99.95 mass%), rolled sheets of ASTM AZ31B (Mg-2.9Al-0.85Zn) magnesium alloy and die-cast plates of ASTM AZ91D (Mg-9.1Al-0.75Zn) magnesium alloy has been studied. Anodization was conducted by environment-friendly electrolysis whose electrolyte consists of phosphate and ammonium salt. The anodized surface was covered with amorphous film, and showed only discoloration during salt spray test where formation of corrosion product (magnesium hydroxide) was well suppressed within 605 ks. Even when the anodized surfaces were trenched with ceramic knife to form locally exposed substrate, corrosion was well suppressed by formation of new type of dense protective films for each substrate which consist of oxygen, magnesium, aluminum and phosphorus. Anodic polarization curves indicate that the anodized surfaces show sacrificial function due to the thermodynamically unstable state of phosphorus in the anodized layers and its resulting release of electrons. From the viewpoint of kinetics in corrosion on the anodzed surfaces, the curves show that the anodized layers dissolve quite slowly into the electrolyte compared with the case of the untreated substrates. The excellent corrosion protectivity obtained by the anodization is considered to be based on the formation of a dense protective film on the exposed area, as well as sacrificial function of the amorphous anodized layer. [doi: /matertrans.mc200718] (Received October 5, 2007; Accepted March 3, 2008; Published April 16, 2008) Keywords: magnesium alloy, surface treatment, anodization, corrosion protection, sacrificial function 1. Introduction Magnesium alloys are increasingly utilized recently to reduce fuel consumption of vehicles by reducing their weight. Suppression of oscillation, shielding of electromagnetic wave and recyclability of the alloys are also advantages in electric and electronic products as well as in automotive applications. However, magnesium is one of the materials which bear stain most easily because of its quite low potential region where metallic magnesium can exist in wet environment. 1,2) As protective coatings for magnesium alloys, conventional anodizations by HAE 3,4) and Dow17 5 7) treatments have successfully been utilized, but these methods need harmful chemical agents such as chromium oxide (VI) and fluoride which have recently been restricted by RoHS (Restriction of the use of certain Hazardous Substances in electrical and electronic equipment) and current trend for reducing environmental load. Another protection of magnesium alloys by anodization is performed using an electrolyte which consists of phosphate and ammonium salt. 8 12) Because of the simplicity of the electrolyte, its environmental load is quite lighter compared with those of Dow17 and HAE. In the previous research, 12) the anodized surface obtained by the above environment-friendly anodization was shown to have ideal sacrificial function for ASTM AZ91D magnesium alloy. The main purpose of this research is to clarify the corrosion protectivity of the anodized surfaces on other magnesium alloys with low aluminum concentration and the mechanisms of corrosion protection of the surfaces. 2. Experimental Procedure Cast high-purity magnesium (99.95 mass%), rolled sheets of ASTM AZ31B (Mg-2.9Al-0.85Zn) magnesium alloy, and die-cast plates of AZ91D (Mg-9.1Al-0.75Zn) magnesium alloy were used as substrates. Chemical composition of each substrate is shown in Table 1. Anodization was conducted by using a solution of phosphate and ammonium salt, 8 12) and the thickness of the anodized layers was 10 mm for each substrate. The structure of the anodized layers was analyzed by X-ray diffraction and electron microscopy. X-ray diffraction patterns were taken under Seemann-Bohlin geometry of! ¼ 1 with parallel beam optics. The specimens for cross-sectional observation and elemental analysis by electron probe microanalysis (EPMA) with wavelengthdispersive X-ray spectrometer were prepared by using argon ion beam of acceleration voltage 5 kv. 13) Corrosion protectivity was evaluated by salt spray test (SST, JIS Z2371) with trenches on the anodized surfaces 12) and by electrochemical measurement (linear sweep voltammetry, sweep rate 1 mv/s). The electrolyte was a solution of 5 mass% sodium chloride whose ph was 6.5, the counter electrode was a plate of titanium coated with platinum, the reference electrode was a standard calomel electrode (SCE) with a saturated solution of potassium chloride, and each working electrode had a square window of mm 2. For qualitative assessment of the formation of magnesium phosphate layer due to the changes of the anodized layer in wet environment, high-purity magnesium was pickled by phosphoric acid, then, immersed in solutions of trisodium phosphate dodecahydrate (Na 3 PO 4 12H 2 O) of ph 3, 7 or 11 for 30 s at 298 K. Here, the ph of the solutions was set by adding phosphoric acid to the solution of trisodium phosphate dodecahydrate whose concentration was originally 100 kg/m 3. Here, the concentration of phosphate is based on a conventional conversion treatment which utilizes manganese phosphate. 14)

2 1058 K. Murakami et al. Table 1 Chemical composition of substrates (mass%). substrate Al Mn Zn Si Cu Ni Fe Mg High-purity <0:001 <0: >99:95 magnesium AZ31B bal. AZ91D bal. Anodized layer Substrate Mg 17 Al 12 Mg 17 Al 12 Fig. 1 Cross-sectional backscattered electron images and compositions of the anodized layers [ High-purity magnesium, AZ31B, AZ91D, (d) Compositions of the anodized layers]. 3. Experimental Results 3.1 Structure of anodized surface Figure 1 shows the cross-sectional microstructure of the anodized surfaces and concentrations of oxygen, magnesium, aluminum and phosphorus in the anodized layers obtained by EPMA. Many pores are observed in the anodized layers (Figs. 1 ) some of which link the surface and the substrate. The atomic ratio of the elements in the anodized layers is shown in Fig. 1(d), where atomic percent of oxygen varies from 50 to 70 at.% and that of phosphorus from 10 to 20 depending on substrate. Figure 2 shows the X-ray diffraction patterns taken from the anodized surfaces. The anodized surfaces show broad scattering peaks at 20 < 2 <40 for each substrate as previously reported, 12) and the overlying peaks indicate the magnesium matrix of the substrates. 3.2 Salt spray test Figure 3 shows the anodized surfaces of AZ31B after salt spray test for 86.4 and 605 ks. The anodized surfaces were free from visible corrosion products and had only slight discoloration along the trenches. No remarkable corrosion product was found even in the magnified optical images of Fig. 2 X-ray diffraction patterns taken from the anodized surfaces [ High-purity magnesium, AZ31B, AZ91D]. the trenches, but the initially metallic surface of the trenches showed discoloration into dark red or black. Figure 4 shows the backscattered electron images of the trenched areas after salt spray test for 605 ks. Although the trenched areas had no anodized films and the substrates were

3 Mechanism of Corrosion Protection of Anodized Magnesium Alloys 1059 Discoloration 5 mm 5 mm Fig. 3 Appearance of the anodized surfaces of AZ31B with trenches after salt spray test [ 86.4 ks, 605 ks]. Anodized surface Anodized surface 100 µm 50 µm Anodized surface 50 µm Fig. 4 Backscattered electron images of the trenched areas on the anodized surfaces after salt spray test for 605 ks [ High-purity magnesium, AZ31B, AZ91D]. exposed to the wet environment containing chloride ion, the areas show no remarkable corrosion product (magnesium hydroxide) for every substrate. While a cracked surface is newly formed on the trenched surface in the case of highpurity magnesium (Fig. 4) and AZ31B (Fig. 4), such morphology can not be remarkably observed in the case of AZ91D (Fig. 4). Figure 5 shows the cross-sectional analyses of the anodized surface with trench on high-purity magnesium after salt spray test for 605 ks. A cracked film which consists of oxygen, magnesium, and phosphorus is newly formed in the trench, and remarkable corrosion or formation of magnesium hydroxide is well suppressed. The cracked film is also observed between the original anodized layer and the substrate. Figure 6 shows the cross-sectional analyses of the anodized surface with trench on AZ31B after salt spray test for 605 ks. Regeneration of a film which consists of oxygen,

4 1060 K. Murakami et al. Original anodized layer Regenerated film Substrate 50 µm (d) Fig. 5 Cross-sectional analyses of the trenched area on the anodized high-purity magnesium after salt spray test for 605 ks [ Backscattered electron image, O-K, Mg-K, (d) P-K]. magnesium, aluminum and phosphorus is identified also in this case, but the film is thinner compared with the case of high-purity magnesium (Fig. 5). Figure 7 shows the cross-sectional analyses of the anodized surface with trench on AZ91D after SST for 605 ks. The regenerated film consists of oxygen, magnesium, aluminum and phosphorus as in the case of AZ31B (Fig. 6). While the thickness of the regenerated film at the edge of the trench is 2 3 mm, that inside the trench is about 500 nm. 3.3 Electrochemical properties of anodized surfaces Figure 8 shows the anodic polarization curves of the anodized surfaces. Anodization can successfully suppress the anodic current or corrosion rate for each substrate, and each corrosion potential is shifted to less noble side by mv compared with that of the untreated substrate. The anodized surfaces showed local corrosion as the potential proceeded into the anodic side. Figure 9 shows the surface morphology after immersion of high-purity magnesium into the trisodium phosphate solutions. The treatment by the solution of ph ¼ 11 (Fig. 9(d)) showed no visible reaction on the substrate which resulted in almost the same surface as that obtained by pickling (Fig. 9), and the treatment by the solution of ph ¼ 3 (Fig. 9) resulted in intense dissolution of the substrate. On the other hand, the solution of ph ¼ 7 brought about precipitation of salt and its resulting local coating on the substrate. 4. Discussion 4.1 Structure of anodized layer As Figs. 1 and 2 show, the porous anodized layer is mainly amorphous for each substrate, and fine crystallites of magnesium oxide (MgO) and spinel (MgAl 2 O 4 ) are thought to exist in the amorphous matrix according to the previous report. 12) The mechanism of the formation of the anodized layer, which has been discussed in the case of AZ91D, 12) is basically applicable to the cases of high-purity magnesium and AZ31D, and the difference of the morphology according to the chemical composition of substrate has been observed. 15) There, diameter of the pores is reported to decrease as aluminum content of substrate increases supposedly because of the change in solidifying temperature of the melt generated by local discharge during electrolysis, and the tendency is also confirmed in Figs. 1. While magnesium and aluminum in the anodized layers are taken from the substrate, oxygen and phosphorus are taken from water molecules and phosphates in the electrolyte during the rapid solidification. By assuming that valences of oxygen, magnesium, aluminum and phosphorus are 2, 2, 3 and 5, respectively, 12) composition of the anodized layers in the case of AZ91D is roughly expressed as (MgO) x (MgAl 2 O 4 ) 17 x (Al 2 O 3 ) x 16 (P 2 O 5 ) 6:5 from Fig. 1(d). The summed amount of oxygen in this chemical formula is 52.5 which is less than the measured value 68, and the gap is thought to be due to water molecules picked up in the anodized layer. Here, elements of oxygen, magnesium, aluminum and phosphor do not necessarily form the above

5 Mechanism of Corrosion Protection of Anodized Magnesium Alloys 1061 Original anodized layer Regenerated film Substrate Mg 17 Al 12 (d) (e) Fig. 6 Cross-sectional analyses of the trenched area on the anodized AZ31B after salt spray test for 605 ks [ Backscattered electron image, O-K, Mg-K, (d) Al-K, (e) P-K]. stoichiometric compounds which have long-range order to show sharp peaks for X-ray diffraction, but the elements in the amorphous anodized layer are assumed to be close to these compounds in terms of short-range order. The same discussion shows (MgO) x (MgAl 2 O 4 ) 32 x - (Al 2 O 3 ) x 31:5 (P 2 O 5 ) 5 for AZ31B and (MgO) 28 (P 2 O 5 ) 10:5 for high-purity magnesium. In the cases of AZ31B and highpurity iron, the summed amount of oxygen is 58.5 and 80.5, respectively, while the measured values are 57 and 51. The difference, in which oxygen is calculated to be excessive according to the above assumption, is considered to be due to an inappropriate assumption for the valences of magnesium and phosphorus. That is, chemical state for each element should be MgO 1 and PO 2:5 ". 4.2 Mechanism of corrosion protection The discoloration in Fig. 3 does not correspond to formation of magnesium hydroxide, but to disappearance of the original anodized layer as observed beside the trenches in Fig. 4. Those areas are thought to be anodic sites where the anodized layer dissolves into sodium chloride solution during salt spray test. This mode of corrosion well suppresses intense corrosion of magnesium alloys or formation of magnesium hydroxide, and the anodized layers on high-

6 1062 K. Murakami et al. Regenerated film Mg 17 Al 12 (d) (e) Fig. 7 Cross-sectional analyses of the trenched area on the anodized AZ91D after salt spray test for 605 ks [ Backscattered electron image, O-K, Mg-K, (d) Al-K, (e) P-K]. purity magnesium and AZ31B are considered to have sacrificial function as reported in the case of AZ91D 12) although the element which is oxidized in the anodized layer and the change in its oxidation number are not certain and should be specified in the future work. This effect is also quantitatively confirmed in Fig. 8, where anodization is found to shift the corrosion potential for each substrate to less noble side compared with the untreated ones, and potential

7 Mechanism of Corrosion Protection of Anodized Magnesium Alloys 1063 Fig. 8 Anodic polarization curves of the raw and anodized surfaces obtained in 5 mass% sodium chloride solution [ High-purity magnesium, AZ31B, AZ91D, Sweep rate 1 mv/s]. regions are observed which correspond to dissolution of the anodized layers before corrosion of the substrates occurs. The shift of the corrosion potentials to less noble side by anodization suggests thermodynamically unstable state of phosphorus and/or magnesium in the anodized layers and its resulting release of electrons (oxidation) compared with the untreated substrates. However, oxidation number of magnesium in the anodized layers is thought to be 2, therefore, electrons which belong to phosphorus in the anodized layers show stronger tendency to be caught by hydrogen ions in the solution, then, hydrogen gas is generated (reduction). Here, the modified chemical expression for magnesium and phosphorus in the anodized layer proposed in 4.1 (MgO 1,PO 2:5 " ) also means that magnesium or phosphorus can be further oxidized in wet environment to increase their oxidation number and stabilize. Since this tendency is stronger in the cases of high-purity magnesium and AZ31B, whose corrosion resistance is lower, it is thought that the original anodized layer shows effective sacrificial protection even for less noble magnesium alloys. On the other hand, the decrease of the anodic currents means that the corrosion rate in the above sacrificial effect is quite low, that is, the anodized layers show long duration in a wet environment. This is the kinetically preferable feature of the anodized layers which can successfully suppress intense corrosion of magnesium in a long-term use. The regenerated films which consist of oxygen, magnesium, aluminum and phosphorus (Figs. 5, 6, 7) are thought to be obtained by dissolution of the anodized layers and following formation of an insoluble or poorly soluble salt as magnesium phosphate (Mg 3 (PO 4 ) 2 4H 2 O, Mg 3 (PO 4 ) 2 8H 2 O) or magnesium hydrophosphate (MgHPO 4 3H 2 O, MgHPO 4 7H 2 O). When the formation rate of the above dense inert film surpasses that of magnesium hydroxide, corrosion which spoils the appearance of a product can be well suppressed. Although the films show some cracks in them, their adhesion to the substrate is better than that of magnesium hydroxide, 12) and the front of each crack where the substrate is exposed can be covered successfully with the inert film mentioned above. Thus, the areas covered with the regenerated films are thought to avoid generation of magnesium hydroxide or visible corrosion as long as the original anodized layer exists on the surface and keeps (d) Fig. 9 Surface morphology of high-purity magnesium after conversion treatment [ After pickling, ph ¼ 3, ph ¼ 7, (d) ph ¼ 11].

8 1064 K. Murakami et al. dissolving slowly into the wet environment. Here, electrons generated by dissolution or oxidation of magnesium in the substrate are thought to be cousumed by the reduction reaction of hydrogen ion (2 H þ þ 2e!¼H 2 ) which occurs near the anode sites. Since thickness of regenerated film is the largest in the case of high-purity magnesium and the second largest in the case of AZ31D, the formational rate of the film is thought to become larger for a substrate whose corrosion resistivity is lower. That is, the regenerated film is the result of reaction of substrate (magnesium) with dissolved chemical agents as phosphate ion (PO 3 4 ), hydrogen phosphate ion (HPO2 4 ), dihydrogen phosphate ion (H 2 PO 4 ), or aqueous phosphoric acid (H 3 PO 4 ) at certain ph. From the point that a film is obtained at ph ¼ 7 in the conversion treatment (Fig. 9) which can successfully cover the highly active surface of magnesium exposed by trenching, the ph of the environment near the trench is nearly 7 where hydrogen phosphate ion (HPO 2 4 ) and dihydrogen phosphate ion (H 2 PO 4 ) predominate. This is explained by setting ph ¼ 7 (½H þ Š¼10 7 ) for K a1 ¼½H þ Š½H 2 PO 4 Š= ½H 3 PO 4 Š¼7:510 3, K a2 ¼½H þ Š½HPO 2 4 Š=½H 2PO 4 Š¼ 6:2 10 8, K a3 ¼½H þ Š½PO 3 4 Š=½HPO2 4 Š¼2: A schematic illustration of dissolution of the original anodized layer and regeneration of the protective film is shown in Fig Conclusion In this research, substrates of high-purity magnesium, ASTM AZ31B magnesium alloy, and ASTM AZ91D magnesium alloy were anodized by electrolysis in a solution which consists of phosphate and ammonium salt. The anodized surfaces were subjected to salt spray test and electrochemical measurements for evaluation of corrosion protectivity, and the structure was analyzed by X-ray diffraction and electron microscopy. The main results can be summarized as follows. (1) The anodized layers obtained on each substrate suppress visible corrosion of magnesium in wet environment even though the layers contain many pores. The excellent corrosion resistance is thought to be due to sacrificial effect of the anodized layers. (2) The protectivity is supposedly based on electrochemically less nobleness of the anodized layers, whose structure is mainly amorphous and dissolving rate is quite low. The less nobleness is due to the thermodynamically unstable state of phosphorus in anodized layers and its resulting release of electrons, and the low dissolving rate is the preferable kinetic feature which assures long-term use. That is, ideal sacrificial function is obtained for each substrate in terms of thermodynamics and kinetics. (3) In addition to the ideal sacrificial function, formation of dense protective films which show good adhesion on the areas where the original anodized layer is lost prevents the substrate from bearing remarkable corrosion product of magnesium hydroxide. Substrate REFERENCES Amorphous anodized layer (O, Mg, (Al), P) Environment for conversion treatment Protective film (O-Mg-(Al)-P) Fig. 10 Schematic illustration of the mechanism of corrosion protection during salt spray test [ before test, dissolution of anodized layer, formation of protective layer on the exposed areas of the substrate]. 1) M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solutions, (National Association of Corrosion Engineers, Houston, TX 1974), ) R. B. Mears and C. D. Brown: Corrosion Nat. Assoc. of Corr. Eng. 1 (1945) ) M. Takaya: J. Jpn. Inst. Light Metals 37 (1987) ) M. Takaya: Jitsumu Hyomen Gijutsu 35 (1988) ) Military Specifications and Standards, MIL-M-45202C (1981). 6) Japanese Industrial Standards, JIS H 8651 (1995). 7) American Society for Testing and Materials, ASTM D (1998). 8) T. F. Barton: US-Patent (1998). 9) M. Hino, M. Hiramatsu, K. Murakami and T. Kanadani: J. Surf. Fin. Soc. Jpn. 54 (2003) ) A. Saijo, M. Hino, M. Hiramatsu, K. Murakami and T. Kanadani: J. Surf. Fin. Soc. Jpn. 56 (2005) ) M. Hino, M. Hiramatsu, K. Murakami, A. Saijo and T. Kanadani: J. Jpn. Inst. Light Metals 56 (2006) (in Japanese). 12) K. Murakami, M. Hino, M. Hiramatsu, K. Nakai, S. Kobayashi, A. Saijo and T. Kanadani: Mater. Trans. 48 (2007) ) K. Murakami, M. Hiramatsu, M. Hino, K. Sueoka and R. Nakanishi: J. Surf. Fin. Soc. Jpn. 58 (2007) (in Japanese). 14) D. Hawke and D. L. Albright: Metal Fin. 93(10) (1995) ) M. Hino, K. Murakami, K. Muraoka, A. Saijo and T. Kanadani: To be published in J. Jpn. Inst. Light Metals 57 (2007) (in Japanese).

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