G. GENG et al.: ENHANCED CORROSION RESISTANCE OF A CHROMIUM-FREE CONVERSION/ORGANIC ... 257–262 ENHANCED CORROSION RESISTANCE OF A CHROMIUM-FREE CONVERSION/ORGANIC COMPOSITE COATING ON AN AZ91D MAGNESIUM ALLOY IZBOLJ[ANJE KOROZIJSKE OBSTOJNOSTI Mg ZLITINE AZ91D S KONVERZIJSKO/ORGANSKO KOMPOZITNO PREVLEKO BREZ Cr Guihong Geng 1 , Dongxin Wang 2 , Lei Zhang 1 , Weiye Chen 1 , Zhijie Yan 1,3* , Guiqun Liu 1 1 North Minzu University, School of Materials Science and Engineering, no. 204 Wenchang North Street, Xixia District, Yinchuan 750021, P. R. China 2 Northwest Rare Metal Materials Research Institute Ningxia Co., Ltd., State Key Laboratory of Special Rare Metal Materials, no. 119 Yejin Road, Dawukou District, Shizuishan 753000, P. R. China 3 North University of China, School of Materials Science and Engineering, no. 3 Xueyuan Road, Jiancaoping District, Taiyuan 030051, P. R. China Prejem rokopisa – received: 2018-08-10; sprejem za objavo – accepted for publication: 2018-11-22 doi:10.17222/mit.2018.176 A composite coating was prepared on an AZ91D magnesium alloy; first, a chromium-free potassium permanganate conversion substrate was deposited and then the substrate was further coated with epoxy resins. The surface morphology, chemical com- position and deposited products of the conversion coating were investigated with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results show that the conversion coating mainly consists of MnO2 and Al2O3 oxides, whose Pilling-Bedworth ratios (PBR) are larger than 1, indicating that the conversion coating is dense. Electrochemical-impedance-spectroscopy (EIS) plots reveal that the conversion coating shows a strong resistance to the flow of ions and electrons, demonstrating that the corrosion resistance of the AZ91D magnesium alloy is considerably enhanced. Neutral salt spray tests show that the corrosion resistance of the AZ91D magnesium alloy is substantially improved due to a composite coating consisting of a conversion deposit and an organic coating. Keywords: corrosion resistance, chromium-free coating, magnesium alloys Avtorji so pripravili kompozitno prevleko na povr{ini Mg zlitine AZ91D, tako da so najprej nanesli na povr{ino zlitine kon- verzijski substrat (nanos) iz kalijevega permanganata brez Cr, nato pa ga prevlekli {e z epoksi smolo. Povr{insko morfologijo, kemijsko sestavo in nane{ene produkte konverzijske prevleke so nato preiskovali z vrsti~no elektronsko mikroskopijo (SEM), energijsko disperzijsko spektroskopijo (EDS) in rentgensko difrakcijo (XRD). Rezultati raziskave so pokazali, da prevleka v glavnem vsebuje MnO2 in Al2O3. Vrednosti za Pilling-Bedworthovo razmerje (PBR) so ve~je kot 1, kar ka`e na to, da je izde- lana prevleka dobro zgo{~ena (brez por). Elektrokemi~na impedan~na spektroskopija (EIS) je pokazala, da ima izdelana prevleka mo~no odpornost proti toku ionov in elektronov. To potrjuje, da je korozijska obstojnost Mg zlitine AZ91D mo~no izbolj{ana. Preizkus s sprejem naravne soli je pokazal, da je korozijska odpornost Mg zlitine AZ91D znatno izbolj{ana zaradi nanosa kompozitne prevleke, sestavljene iz konverzijskega nanosa in organske prevleke. Klju~ne besede: odpornost proti koroziji, prevleke brez kroma, kemi~na pretvorba, zlitina na osnovi magnezija 1 INTRODUCTION Magnesium alloys exhibit several advantageous pro- perties, including low densities, high strength-to-weight ratio, high stiffness and good recyclability; consequently, they are promising potential applications in the auto- motive and aerospace industries. 1,2 However, the poor corrosion resistance of magnesium alloys strongly limits their engineering applications. An effective way to prevent the corrosion of magnesium alloys is coating. Various coating techniques such as chemical conversion coatings, 3–8 ceramic coatings, 9 electro- or electroless metal plating, 10 anodizing, 11 superhydrophobic coatings 12 and sputtering deposition processes, 13,14 have been deve- loped to improve the corrosion resistance of magnesium alloys. Among them, conversion coatings are common surface-protective methods, improving the corrosion resistance of magnesium alloys. They are produced with a chemical or electrochemical treatment of metal sur- faces, forming superficial layers of the substrate that are chemically bonded to the surface. In the traditional conversion technique, the coatings based on compounds with hexavalent chromium (Cr 6+ ) are widely used to protect magnesium alloys. 15 However, the increasing safety concerns regarding highly toxic chromate compound push researchers to develop chro- mate-free conversion-coating methods. In recent years, it has been reported that phosphate-permanganate conver- sion coatings effectively improve the corrosion resist- ance of magnesium alloys, which are environmentally friendly and have been shown to have similar effects on the corrosion resistance as the chromate conversion Materiali in tehnologije / Materials and technology 53 (2019) 2, 257–262 257 UDK 620.19:669.018.8:669.721.5 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(2)257(2019) *Corresponding author e-mail: zjyan@nuc.edu.cn coatings. 16–20 However, it was found that there are non-penetrating micro-cracks on the phosphate and per- manganate conversion coatings, 19,20 which considerably deteriorate the corrosion resistance of the coatings. To further improve the corrosion resistance of conversion coatings, direct exposure of the non-penetrating micro- cracks in corrosive media should be avoided. Moreover, polymeric coatings are found to considerably improve the corrosion resistance of magnesium alloys. 21,22 Based on these considerations, in the present work, a potassium permanganate conversion substrate was first deposited on the AZ91D magnesium alloy and the conversion coat- ing was subsequently covered with environmentally- friendly epoxy resins. The corrosion resistance of the prepared composite coating, composed of a conversion substrate and an organic coating, was investigated. 2 EXPERIMENTAL PART Samples of a commercial AZ91D magnesium alloy (9.27 Al, 0.25 Zn, 0.35 Mn, and balanced Mg, all in mass fractions, w/%) with dimensions of (10 × 10 × 10) mm were prepared for electrochemical impedance spectroscopies (EIS) and immersion tests. Samples with dimensions of (30 × 30 × 5) mm were prepared for adhesion tests and salt-spray tests. It is very important for a magnesium alloy to conduct a surface pretreatment prior to the chemical conversion since its surface plays a critical role in a chemical-conversion process. Here, the surfaces of the samples were metallographically ground using 600–1200 grit silicon carbide papers, rinsed with deionized water, ultrasonically degreased in acetone for 30 min and then dried with warm air. After the surface pretreatments, a chemical con- version was carried out by immersing the samples into a mixture solution of KMnO 4 (20 g/L) and Mn(H 2 PO 4 ) 2 (60 g/L) for 8–9 min at 323–333 K. After the conversion deposition, the samples were cleaned in distilled water and then dried. Later, epoxy resins were applied onto the conversion substrates. Finally, the samples coated with epoxy resins were dried by keeping them in an incubator at 333 K for 72 h. The morphologies and compositions of the conver- sion coatings were investigated with scanning electron microscopy (SEM, Hitachi S4800) and energy dispersive spectroscopy (EDS, Thermo Scientific Noran System 7). The products deposited during the conversion treatments were identified with X-ray diffraction (XRD, X’ TRA) using monochromatic Cu-K radiation. The thickness of the conversion coating was measured with 3D digital microscopy (DM, VHX-600), with which the thickness profile of the conversion coating was obtained. The corrosion resistance of the samples was evaluated after their immersion in a neutral solution of 3.5 w/% NaCl for 132 h at room temperature. The EIS tests of the con- version coatings were conducted under a three-electrode system using the Autolab PGSTAT 302 electrochemical measurement system. The samples served as the working electrodes and the saturated calomel electrode acted as the reference electrode together with a platinum counter electrode. Before the electrochemical tests, the samples covered with conversion coatings were exposed to a solution of 5 w/% NaCl in an electrolytic cell at their open circuit potentials (OCP) for 10 min. The cross-cut method was employed to evaluate the adhesion strength of the organic coating onto the conversion coating according to the ISO 2409 standard. 23 The cut flaws of the organic coating were observed after the cut actions. An adhesive cellophane tape was pasted onto the cut flaws and then the tape was peeled off them to evaluate the adhesion strength of the organic coating. 24 According to the ASTM B117.97 standard, 25 the samples covered with organic coatings were subjected to a salt-spray test to investigate their anti-corrosion properties. A salt-spray cabinet (model ZY8200) was used for the salt-spray tests. 3 RESULTS AND DISCUSSION 3.1 Morphology and composition of a conversion coat- ing The macromorphology of a conversion coating on the AZ91D magnesium alloy is presented in Figure 1a.Itis seen that a yellow brown conversion coating was depo- sited after the potassium permanganate conversion treat- ment. A SEM image and an EDS spectrum of the conversion coating are shown in Figures 1b and 1c, res- pectively. It is obvious that there are reticular micro- cracks in the conversion coating (Figure 1b). The inhomogeneous electrochemical property of the AZ91D magnesium alloy may lead to a formation of abundant micro-batteries on the surface of the alloy in the conversion solution. As a result, electrochemical corro- sion occurs during the conversion treatment, in which the -Mg matrix phase acts as the anode and the inter- metallic -Mg 17 Al 12 phase as the cathode since the corro- sion potential of Mg is lower than that of Mg 17 Al 12 . 26 The G. GENG et al.: ENHANCED CORROSION RESISTANCE OF A CHROMIUM-FREE CONVERSION/ORGANIC ... 258 Materiali in tehnologije / Materials and technology 53 (2019) 2, 257–262 Figure 1: a) Macromorphology, b) SEM image and c) EDS spectrum of a conversion coating deposited on AZ91D magnesium alloy conversion reaction first takes place in the micro-cathode area, leading to hydrogen corrosion. Subsequently, the deposited products nucleate and grow onto the phase. With the accumulation of deposits, a uniform conversion coating gradually forms on the surface of the AZ91D magnesium alloy. The hydrogen precipitation in the process of the conversion reaction results in an explosion force during the micro-bubbles floating, which is be- lieved to contribute to the microcracks on the conversion coating. Compared to the previously prepared conversion coatings, 27 the emergence of microcracks roughens the coating surface, providing a good substrate for the sub- sequent organic coating. This morphology characteristic of the conversion coating is beneficial for the improve- ment of the adhesion strength of the organic coating on the substrate and it will be discussed later. An EDS spectrum of the conversion coating indicates that the conversion coating is mainly composed of F, Mg, Al, O, P and Mn (Figure 1c). The element of F origi- nates from the hydrofluoric acid pickling in the process of pretreatment, and the elements of Mg and Al come from the chemical and electrochemical reactions of the magnesium alloy in the permanganate solution, during which these elements dissolved into the conversion coating. Figure 2 presents a DM image of a conversion coating. The line scanning profile from point C to point DinFigure 2a is shown in Figure 2b. It shows that the thickness of the conversion coating is approximately 9–10 μm and its thickness is uniform, which indicates that the chemical-conversion reaction occurred homo- geneously on the alloy surface. 3.2 Products of a conversion coating The XRD pattern of a conversion coating is shown in Figure 3. Since X-rays can penetrate a thickness of about 100 μm, 27 the sharp peaks of the XRD pattern in Figure 3 originate from both the AZ91D magnesium alloy and the conversion coating (with a thickness of about 9–10 μm). The AZ91 magnesium alloy mainly consists of an -Mg matrix phase and intermetallic -Mg 17 Al 12 phase. As a result, the products of the con- version coating are composed of MgO, MnO 2 ,Al 2 O 3 and some other unidentified phases. From the XRD pattern, it is concluded that the conversion coating mainly con- sists of oxides, which determine the corrosion resistance of the magnesium alloy. 3.3 Corrosion resistance of a conversion coating The morphologies of the samples after full-immer- sion tests are shown in Figure 4. The corrosion mor- phology of a sample with a conversion coating is shown in Figure 4a. For comparison, the corrosion morphology of the AZ91D magnesium alloy without the conversion treatment is also shown in Figure 4b. From Figure 4,it is clear that the alloy without a conversion treatment corrodes seriously and abundant corrosion holes appear on the surface (Figure 4b). On the other hand, a mild corrosion occurs on the surface of the alloy with a con- version coating (Figure 4a). From the results of the full-immersion tests, it is clear that the annual corrosion rates of the AZ91D magnesium alloy before and after the conversion treatment are 11.36 mm/a and 1.35 mm/a, respectively. This suggests that the corrosion resistance of the AZ91D magnesium alloy is considerably im- proved due to the formation of a conversion coating. The corrosion resistance of a conversion coating depends on its value of the Pilling-Bedworth ratio (PBR, the ratio of the volume of the elementary cell of a metal G. GENG et al.: ENHANCED CORROSION RESISTANCE OF A CHROMIUM-FREE CONVERSION/ORGANIC ... Materiali in tehnologije / Materials and technology 53 (2019) 2, 257–262 259 Figure 3: XRD pattern of a conversion coating deposited on AZ91D magnesium alloy Figure 2: a) DM image of a conversion coating and b) the correspond- ing line scanning profile from point C to point D oxide to the volume of the elementary cell of the corresponding metal). When the value of PBR < 1, the coating is porous and easily subjected to corrosion. When the value of PBR > 1, the coating is dense and shows favorable corrosion resistance. Based on the XRD pattern from Figure 3, the conversion coating mainly consists of dense MnO 2 (PBR=1.77) and Al 2 O 3 (PBR=1.28), which mainly substitute the porous magne- sium oxide MgO (PBR=0.81). 28 As a result, a dense oxide coating covers the magnesium matrix, which substantially improves the corrosion resistance. It can be concluded that the considerable improvement of the corrosion resistance of the AZ91D magnesium alloy is due to the formation of a passivated and dense oxide coating, whose PBR value is considerably larger than 1. This is verified with the EIS data, which is discussed in section 3.4. 3.4 EIS plots of the magnesium AZ91D alloy with a conversion coating The EIS plots of a sample with a conversion coating are shown in Figures 5a and 5b. The low frequency impedance in the Bode plot monotonously increases as the frequency drops (Figure 5a), indicating a strong resistance of the conversion coatings to the flow of ions and electrons. As a result, the corrosion resistance is enhanced. The Nyquist plot is shown in Figure 5b. The Nyquist plot exhibits two loops, which reveal that the corrosion first occurs on the conversion coating and then on the matrix. This indicates that the existence of the conversion coating prevents the corrosion from occurring directly on the matrix, which enhances the corrosion resistance. The equivalent circuit proposed for fitting the EIS data is shown in Figure 5c. In the equivalent circuit, R s , R p , R t and C d represent the solution resistance bet- ween the reference electrode and the working electrode, polarization resistance, the charge-transfer resistance and the electric double-layer capacitor, respectively. The constant phase angle element (CPE) is used to replace the pure capacitance element to reflect the dispersion effect due to the roughness of the electrode interface. The EIS results agree with the equivalent circuit. 3.5 Adhesion strength of an organic coating The adhesion-strength tests according to the ISO 2049 standard were introduced to evaluate the adhesion strength of the organic coatings. For comparison, the adhesion strength of an organic coating on the AZ91D magnesium alloy without a conversion coating is also investigated as a reference. The surface of the AZ91D- magnesium-alloy sample was treated with shot blasting before the application of the organic coating and then epoxy resins were directly deposited on its surface. According to standard ISO 2049 23 , the adhesion strength of the coating is evaluated in terms of its falling off the substrate. The morphologies of scratch patterns are shown in Figure 6, indicating that the cohesion force of the organic coating on the conversion substrate is much stronger than that on the sample without a conver- sion coating. The adhesion-strength grades of the organic coating on the samples with and without a conversion coating were evaluated to be 1 (Figure 6a)and3( Fig- ure 6b), respectively. The conversion coating is an alkaline material, and the epoxy coating is neutral or alkaline. This is favorable for the cohesion between the organic coating and the conversion coating, resulting in a much better adhesion strength. Furthermore, the epoxy resin invades into the pre-existing microcracks of the conversion coating. During the subsequent constant G. GENG et al.: ENHANCED CORROSION RESISTANCE OF A CHROMIUM-FREE CONVERSION/ORGANIC ... 260 Materiali in tehnologije / Materials and technology 53 (2019) 2, 257–262 Figure 5: a) EIS Bode plot and b) Nyquist plot of the converted AZ91D magnesium alloy in a 5 w/% NaCl solution, and c) the equiva- lent circuit for fitting the EIS data Figure 4: Corrosion morphologies of the samples: a) with a conver- sion coating and b) without conversion treatment Figure 6: Surface morphologies of the scratched patterns on the organic coatings on the surfaces of AZ91D magnesium alloy samples a) with and b) without a conversion coating high-temperature condition, the mutual penetration and diffusion between the epoxy resin and conversion coat- ing result in a mutual tolerance and synergistic effects, which enhance the adhesion strength. On the other hand, on the sample without a conversion coating, there is just a mechanical combination between the organic coating and the metal substrate. As a result, the organic coating is not well attached onto the surface of the sample with- out a conversion coating and the organic coating flakes off easily under an external force. 3.6 Corrosion resistance of the organic coating on a conversion substrate Figure 7a shows the corrosion morphology of a con- verted sample after its immersion in a solution of 3.5 w/% NaCl for two and five days at room temperature. It is found that there is a small quantity of corrosion spots on the surface of the converted sample after two days of the immersion. After five days of immersion in the NaCl so- lution, the sample without an organic coating is con- siderably damaged (bottom side in Figure 7a). Fig- ure 7b shows the corrosion morphology of a sample with a composite coating consisting of the organic coat- ing on the precursor conversion substrate under a neutral salt spray of 3.5 w/% NaCl for five days. It is clear that there is little damage on the surface of the organic coat- ing on the conversion coating. It is worth noting that the salt spray is much more corrosive than a solution with the same concentration of NaCl. The organic coating on the conversion substrate shows a better corrosion resistance under a spray than that without an organic coating, fully immersed in the solution. Based on the facts above, it can be concluded that the corrosion resistance is greatly improved due to the application of an organic coating on the conversion coating. The conversion coating provides an adhesive and rough substrate for the subsequent organic coating, and the organic substances penetrate into the micro- cracks of the conversion coating, leading to a formation of a dense composite coating. As a result, the organic coating prevents the conversion coating from direct exposure to the etching media, which further enhances the corrosion resistance. 4 CONCLUSIONS (1) A uniform and dense conversion coating is depo- sited on AZ91D magnesium using a chemical-conversion treatment in a potassium permanganate solution, and abundant reticular microcracks are found on the surface of the conversion coating. The main elements of the conversion coating are F, Mg, Al, O, P and Mn. The phase products of the conversion coating mainly include oxides MgO, MnO 2 and Al 2 O 3 . (2) EIS plots reveal a strong resistance of the con- version coating to the flow of ions and electrons, considerably preventing the corrosion of the AZ91D magnesium alloy, which is further verified with immersion tests. The improvement of the corrosion resistance of the alloy due to the conversion treatment is attributed to the formation of a dense conversion coating, mainly consisting of oxides MnO 2 and Al 2 O 3 , whose PBR values are considerably larger than 1. (3) The application of an organic coating onto the conversion substrate prevents the conversion coating from direct exposure to erosive media, which further substantially enhances the corrosion resistance of the AZ91D magnesium alloy. Acknowledgements This project was financially supported by the National Natural Science Foundation of China (Grant no. 51561001) and State Key Laboratory of Special Rare Metal Materials. 5 REFERENCES 1 J. E. Gray, B. Luan, Protective coatings on magnesium and its alloys – a critical review, J. Alloys Compd., 336 (2002), 88 2 Y. Ali, D. Qiu, B. Jiang, F. Pan, M. X. Zhang, Current research progress in grain refinement of cast magnesium alloys: A review article, J. Alloys Compd., 619 (2015), 639 3 T. Ishizaki, N. Kamiyama, K. Watanabe, A. Serizawa, Corrosion resistance of Mg(OH)(2)/Mg-Al layered double hydroxide composite film formed directly on combustion-resistant magnesium alloy AMCa602 by steam coating, Corros. Sci., 92 (2015), 76 4 C. D. Gu, W. Yan, J. L. Zhang, J. P. Tu, Corrosion resistance of AZ31B magnesium alloy with a conversion coating produced from a choline chloride-urea based deep eutectic solvent, Corros. Sci., 106 (2016), 108 5 X. Li, Z. Y. Weng, W. Yuan, X. Z. Luo, H. M. Wong, X. M. Liu, S. L. Wu, K. W. K. Yeung, Y. F. Zheng, P. K. Chu, Corrosion resistance of dicalcium phosphate dihydrate/poly(lactic-co-glycolic acid) hybrid coating on AZ31 magnesium alloy, Corros. Sci., 102 (2016), 209 6 L. J. Zhang, E. A. A. Mohammed, A. Adriaens, Synthesis and elec- trochemical behavior of a magnesium fluoride-polydopamine-stearic acid composite coating on AZ31 magnesium alloy, Surf. Coat. Tech., 307 (2016), 56 G. GENG et al.: ENHANCED CORROSION RESISTANCE OF A CHROMIUM-FREE CONVERSION/ORGANIC ... Materiali in tehnologije / Materials and technology 53 (2019) 2, 257–262 261 Figure 7: Corrosion morphologies of the sample with a conversion coating after immersion in a solution of 3.5 w/% NaCl for two days (upper side) and five days (lower side) a) and the sample with an organic coating applied onto the conversion substrate under a neutral salt spray of 3.5 w/% NaCl for five days b) 7 Z. J. Li, Y. Yuan, X. Y. Jing, Composite coatings prepared by com- bined plasma electrolytic oxidation and chemical conversion routes on magnesium-lithium alloy, J. Alloys Compd., 706 (2017), 419 8 S. Nezamdoust, D. Seifzadeh, Application of Ce-V/sol-gel composite coating for corrosion protection of AM60B magnesium alloy, Non- ferrous Met. Soc. China, 27 (2017), 352 9 S. B. Hassen, L. Bousselmi, P. Bercot, M. E. Rezrazi, E. Triki, XPS characterization and corrosion resistance of cerium-treated magne- sium coatings, Rare Metals, 30 (2011), 368 10 C. D. Gu, J. S. Lian, G. Y. Li, L. Y. Niu, Z. H. Jiang, Electroless Ni-P plating on AZ91D magnesium alloy from a sulfate solution, J. Alloys Compd., 391 (2005), 104 11 M. C. L. De Oliveria, V. S. M. Pereira, O. V. Correa, A. Antunes, Corrosion performance of anodized AZ91D magnesium alloy: effect of the anodizing potential on the film structure and corrosion behavior, J. Mater. Eng. Perform., 23 (2013), 593 12 D. Zang, R. Zhu, W. Zhang, J. Wu, X. Yu, Y. Zhang, Stearic acid modified aluminum surfaces with controlled wetting properties and corrosion resistance, Corros. Sci., 83 (2014), 86 13 H. W. Huo, Y. Li, F. H. Wang, Improvement on the corrosion resist- ance of AZ91D magnesium alloy by aluminum diffusion coating, J. Mater. Sci. Technol., 23 (2007), 379 14 S. K. Wu, S. C. Yen, T. S. Chou, A study of r.f.-sputtered Al and Ni thin films on AZ91D magnesium alloy, Surf. Coat. Tech., 200 (2006), 2769 15 M. Aveddsian, H. Baker, Magnesium and magnesium alloys, ASM Specialty Handbook, ASM, Metals Park, Ohio, 1999 16 S. Y. Jian, Y. R. Chu, C. S. Lin, Permanganate conversion coating on AZ31 magnesium alloys with enhanced corrosion resistance, Corros. Sci., 93 (2015), 301 17 C. Y. Wu, J. Zhang, State-of-art on corrosion and protection of mag- nesium alloys based on patent literatures, Trans. Nonferrous Met. Soc. China, 21 (2011), 892 18 Y. L. Cheng, H. L. Wu, Z. H. Chen, H. M. Wang, L. L. Li, Phos- phating process of AZ31 magnesium alloy and corrosion resistance of coatings, Trans. Nonferrous Met. Soc. China, 16 (2006), 1086 19 M. Zhao, S. S. Wu, J. R. Luo, Y. Fukuda, H. Nakae, A chromium- free conversion coating of magnesium alloy by a phosphate-perman- ganate solution, Surf. Coat. Tech., 200 (2006), 5407 20 K. Z. Chong, T. S. Shih, Conversion-coating treatment for mag- nesium alloys by a permanganate-phosphate solution, Mater. Chem. Phys., 80 (2003), 191 21 A. Zomorodian, M. P. Garcia, E. Moura, T. Silva, J. C. S. Fernandes, M. H. Fernades, M. F. Montemor, Corrosion resistance of a com- posite polymeric coating applied on biodegradable AZ31 magnesium alloy, Acta Biomater., 9 (2013), 8660 22 X. L. He, Y. H. Wei, L. F. Hou, Z. F. Yan, C. L. Guo, P. J. Han, Corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in NaCl solution, Rare Metals, 33 (2014), 276 23 ISO 2409:2007 (E) – Paints and Varnishes, Cross-Cut Test. CEN, Brussels, Belgium 24 JIS K5400:1990 (E) – Japanese Industrial Standard Testing Methods for Paints, Japanese Standards Association, Tokyo, Japan 25 ASTM Designation B117-97:1997 (E) – ASTM, Philadelphia 26 S. Candan, M. Unal, E. Koc, Y. Turen, E. Candan, Effects of titanium addition on mechanical and corrosion behaviours of AZ91 magne- sium alloy, J. Alloys Compd., 509 (2011), 1958 27 H. H. Elsentriecy, K. Azumi, H. Konno, Improvement in stannate chemical conversion coatings on AZ91D magnesium alloy using the potentiostatic technique, Electrochim. Acta, 53 (2007), 1006 28 M. Syvertsen, K. Aarstad, G. Trancell, T. A. Engh, in: K. U. Kainer (Ed.), Magnesium: Proceedings of 7th International Conference on Magnesium Alloys and their Applications, 2006 G. GENG et al.: ENHANCED CORROSION RESISTANCE OF A CHROMIUM-FREE CONVERSION/ORGANIC ... 262 Materiali in tehnologije / Materials and technology 53 (2019) 2, 257–262