Effects of combined organic and inorganic corrosion inhibitors on the nanostructure cerium based conversion coating performance on AZ31 magnesium alloy: Morphological and corrosion studies
Introduction
In recent years, magnesium and its alloys have been extensively used in large number of applications such as automobile, computer parts, and aerospace industry, because of their low density, proper castability, weldability, great thermal conductivity and good machinability [1], [2], [3], [4], [5], [6]. However, magnesium and its alloys are electrochemically reactive even in pure water. So, the low corrosion resistance of Mg alloys in corrosive environments has resulted in the restriction of the use of this invaluable metal in many applications [1], [3], [6], [7], [8], [9]. Very negative potential of magnesium alloys is responsible for their great potential to oxidation in air and corrosive electrolytes. On the other hand, the oxide film existed on these alloys can be easily dissolved in corrosive environments [10], [11], resulting in the coating protection performance decline. Many attempts have been performed in order to improve the corrosion resistance of Mg alloys. The most important of them are utilizing surface modification techniques [9], [12] such as anodizing [13], [14], electroless plating [9], [15], electroplating [9], [16], chemical conversion coating [4], [6], [9], [11], [17], [18], ion implantation [19], and organic coatings [9], [20]. Among these methods, the chemical conversion coatings are widely considered in recent years due to many advantages, such as uniform deposition [1], adhesion improvement of the subsequent coatings [11], [21], [22], cost effectiveness and simplicity in operation [11], [22]. Chromate or chromate-phosphate based conversion coatings have been widely used during last decades. However, the solutions containing hexavalent chromium compounds are toxic. Therefore, the researchers' attentions have been directed toward replacing chromates with environmentally friendly chemical treatments [10], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. Recently, the rare earth based conversion coatings are developed as environmentally friendly coatings for corrosion protection of Mg alloys [12], [28], [41], [42], [43], [44], [45]. Among various types of conversion coatings, it has been shown in many reports that the cerium based coatings can provide proper degree of corrosion protection and paint adhesion improvement [1], [6], [10], [44], [46], [47], [48], [49], [50], [51]. However, this method requires long immersion time and high temperature to achieve efficient corrosion inhibition properties [10], [30]. In addition, one major weakness of this type of coating is the creation of many micro-cracks at prolonged immersion times, resulting in the decrease of corrosion resistance of coating. Consequently, the parameters affecting the film formation process should be carefully investigated to provide efficient corrosion protection for the Mg alloys. Montemor et al. [30] showed that the chemical treatment of AZ31 Mg alloy by cerium conversion coatings enhanced its corrosion resistance. They also investigated the effectiveness of treatment time on the protection performance of the deposited film. Ramezanzadeh et al. [21] discovered that addition of Ni(II) and Co(II) additives to a Cr(III) containing solution could remarkably increase the corrosion protection performance of the deposited film. Rezaee et al. [52] investigated the effects of Mn2+ cations on the microstructure and corrosion protection performance of a room temperature phosphate conversion coating. They found that addition of Mn(II) cations significantly increased the corrosion resistance of the phosphate coating. Amini et al. [53] studied the effects of sodium dodecyl sulfate (SDS) and sodium nitrite (SN) on the phosphate coating formation on AZ31 magnesium alloy. They found that addition of SDS resulted in the increment of the corrosion protection and adhesion properties of the phosphate coating. Ramezanzadeh et al. [54] reported that addition of PVA to a room temperature zinc phosphate conversion coating resulted in the increase of corrosion protection and adhesion properties of steel substrate. Also, synergistic effects between polyvinyl alcohol, surfactants and halide ions have been reported [55], [56], [57], [58] in literature. Chen et al. [50] found that sodium dodecyl benzene sulfonate could improve the corrosion protection performance and morphological properties of Ce coating on AZ91D magnesium alloy by increasing the Ce content in the final coating. To the best of our knowledge there are a few studies dealing with the use of combination of organic and inorganic corrosion inhibitors in the cerium bath and their effects on the microstructure and corrosion protection performance of the film formed on AZ31 Mg alloy. Most of the organic corrosion inhibitors include heteroatoms like P, N, O and S which can share the lone pair of electrons with empty orbitals of metal cations, resulting in the creation of organic-inorganic chelates [59], [60]. So the use of combination of organic and inorganic corrosion inhibitors results in the creation of insoluble complexes on the metal surface, increasing the surface coverage and decreasing the coating porosity and crack [61]. In this work, a series of cerium solutions containing Mn(NO3)2·4H2O, Co(NO3)2·6H2O and polyvinyl alcohol (PVA) are prepared for chemical treatment of AZ31 Mg alloy samples at room temperature. PVA includes many OH side groups which can interact with Ce3+, Mn2+ and Co2+ cations and make PVA-Ce3+, PVA-Mn2+ and PVA-Co2+ complexes. The surface characteristics of the Ce, Ce-Mn, Ce-PVA and Ce-Mn-PVA treated Mg AZ31 substrates were investigated by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and contact angle test. Also, the corrosion resistance of the samples was examined by polarization test and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution.
Section snippets
Materials
AZ31 Mg alloy with the composition given in Table 1 was procured from Iran’s Ministry of Defense. Cerium nitrate, Ce(NO3)·5H2O, hydrogen peroxide, H2O2, sodium fluoride, NaF manganese nitrate, Mn(NO3)2·4H2O, cobalt nitrate, Co(NO3)2·6H2O, polyvinyl alcohol, PVA, sulfuric acid, H2SO4, sodium hydroxide and NaOH, were purchased from Merck Co. All of the materials were used without further purification.
AZ31 Mg alloy surface treatment procedure
AZ31 Mg alloy samples were carefully abraded by abrasive papers of 180, 600, and 1000 grits,
OCP measurement during cerium film precipitation on the Mg ZA31 alloy
The OCP values were measured in different cerium solutions on Mg ZA31 alloy during different immersion times. The bath temperature and pH were 25 °C and 3.5, respectively. Fig. 1 shows variation of OCP versus immersion time in the cerium bath with and without Mn2+ and PVA compounds. According to Fig. 1, the increase of OCP to more positive (less negative) values can be seen at initial 30 s of immersion in the cerium bath for all samples, indicating the precipitation and growth of a protective
Discussion
Electrochemical reactions occur on the surface of AZ31 Mg alloy at active anode (Mg-rich α phase) and active cathode (Al-rich β phase) [10], [47]. The anodic dissolution reaction of the Mg sample immersed in the cerium solution with pH = 3.5 takes place according to Eq. (3). This results in rapid increase of potential toward positive values due to the accumulation of the dissolved Mg2+ cations on the substrate [50]. Simultaneously, the cathodic reactions occur at Al-rich β phase through reduction
Conclusion
- 1
Addition of Mn and PVA to the Ce solution led to more uniform, denser and crack free coating deposition on the Mg AZ31 alloy surface. The conversion layer was composed of the mixture of both Ce4+ and Ce3+ compounds. The PVA-Mn2+, PVA-Mg2+ and Mn2+-PVA-Mg2+ complexes covered the Mg surface, mostly the cathodic sites and increased the corrosion resistance of Mg alloy noticeably.
- 2
Deposition of the Ce film on the Mg AZ31 alloy substrate resulted in the increase of the surface free energy and
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