Permanganate Conversion Research Articles

Mn oxide conversion coating on AZ31B Mg alloy with hexafluorozirconate as an inhibitor in the permanganate conversion bath

Surface modification is essential for Mg alloys, including conversion coating, plasma electrolytic oxidation, organic coating, and plasma spraying. Permanganate conversion coating is a potential alternative to chromate coating for Mg alloys. A continuous MnO2 coating on commercial Mg alloys, which generally contain micrometric secondary phase particles, is still challenging. A thin, continuous MnO2 conversion coating layer on AZ31B Mg alloy was successfully prepared by adding hexafluorozirconate anions (ZrF62−) to an acidic permanganate conversion bath. AZ31B Mg alloy comprised α-Mg matrix and micrometric Al8Mn5 particles. In addition to MnO2 and MgO/Mg(OH)2, MgF2 was present in the coating formed in the presence of ZrF62−, but the amount of ZrO2 was scarce. The average thickness of the coating was reduced from 665 to 66 nm when 0.02 M ZrF62− was added. As a result, the contact resistance was reduced from 367 to 8.59 mΩ. Meanwhile, the corrosion resistance was improved. The uniformity of MnO2 coating was discussed, focusing on the Al8Mn5/Mg galvanic coupling effect.

Effect of bath temperature on the growth kinetics and characteristics of permanganate conversion coating on LZ91 magnesium alloy

Permanganate conversion coating is one of the possible alternatives to chromate conversion coating for magnesium (Mg) alloys. Permanganate conversion coating enhances the corrosion resistance of Mg alloys, which is closely related to the thickness and defects of the coating. In this study, the growth kinetics of the permanganate conversion on the LZ91, a dual-phase magnesium alloy, as affected by the bath temperature ranging from 15 to 60 °C, was detailed. The permanganate conversion coating was mainly composed of MnO2/MnOOH and MgO/Mg(OH)2. Increasing the bath temperature reduced the growth rate and thickness of the coating although the bath temperature hardly influenced the composition and crystallinity of the coating. The permanganate conversion coating formed at 15 °C for 10 s was nonuniform with a thickness ranging from 100 to 300 nm, which was prone to cracking after post-coating drying at room temperature. The presence of cracks on the permanganate conversion coating was detrimental to the corrosion resistance. On the other hand, the permanganate conversion coating formed at 40 and 60 °C for 10 s was more uniform with a thickness of less than 100 nm, yet exhibited better corrosion resistance than the counterpart formed at low temperatures (15 and 25 °C).

Stability of Octadecyltrimethoxysilane-Based Coatings on Aluminum Alloy Surface.

Long-term stability in contact with water of organosilane layers formed by octadecyltrimethoxysilane (ODTMS) on polished aluminum alloy (AA2024) through dip-coating was studied by combining SEM, water contact angle measurements, and X-ray photoelectron spectroscopy. Similar organosilane layers were formed on AA2024 coated with permanganate conversion coating, 1,2-bis(triethoxysilyl)ethane (BTSE) and hydrated SiOx as under-layers, after which their long-term durability was also tested. During immersion in water for about one month, all the samples exhibited a decrease in hydrophobicity, implying the prepared organosilane layer was not stable over time, gradually hydrolyzing and letting water interact with the underlying layer. In parallel, SEM images of one-layer samples taken after immersion showed clear signs of local electrochemical corrosion, while XPS analysis confirmed a loss of silicon from the surface layer. The highest stability over time was demonstrated by a one-layer sample prepared in an ethanol/water bath for 5 min and by a similar ODTMS layer prepared on hydrated MnOx as an under-layer.

Corrosion resistance and electrical contact resistance of a thin permanganate conversion coating on dual-phase LZ91 Mg–Li alloy

A strongly acidic permanganate conversion coating solution containing 0.1 M KMnO4 at pH 1.5 adjusted with H2SO4 was employed for LZ91 magnesium-lithium alloy, realizing the formation of a thin protective coating with a low electrical contact resistance. The corrosion resistance of LZ91 alloy was markedly improved by the permanganate conversion coating, which consisted of MgO, MnO2, and Mg(OH)2. Coating formation proceeded on both the α-Mg phase and β-Li phase in this strongly acidic permanganate solution, achieving a full coverage of permanganate conversion coating within a short treatment time. When the immersion was prolonged to 30 s, cracks were present on the permanganate conversion coating, which deteriorated the corrosion resistance substantially. Moreover, the increase in the thickness of the coating elevated the electrical contact resistance. However, the relatively thin areas on the thin permanganate conversion coating became active sites where corrosion commenced. As a result, a thin uniform permanganate conversion coating is crucial for both enhanced corrosion resistance and reduced electrical contact resistance.

Corrosion Resistance of LZ91 Magnesium Alloy with a Permanganate Conversion Coating and Electroless Ni-P Plating

With excellent specific strength and low density, magnesium alloys are extensively used in general products, such as automobiles, sport goods and even electrical appliance. Affected by increasing growth of consumer electronics, Mg-Li alloys with better formability have attracted great attention recently. Nevertheless, with noble natives of Mg and Li, corrosion is unavoidable; therefore, the passivation of magnesium-lithium alloys becomes an influential issue in industrial application, the passivation of magnesium-lithium alloys thus becomes an important issue in industrial application. Hexavalent chromium conversion treatment was widely adopted in conventional conversion coating treatment, but was restricted by RoHS due to its high toxicity. Thus, the development of alternatives to hexavalent chromium conversion coating treatment is now an urgent necessity [1-5]. In this study, LZ91 magnesium alloys were used as experimental materials. This research is aimed on the development of the chromate-free conversion coating systems applied on LZ91 alloy, basing on permanganate conversion systems. Via the formation and protection mechanism of each system, the parameter optimization can thus be achieved. The changes of process will have evident effects on both growth behaviors and corrosion properties of the passive film on LZ91 alloys. The designs of the treating solution and post-treatments take into consideration to fit the dual-phase influence the microstructure of the coating. Additionally, to verify the subsequent applications, the preparation of the permanganate conversion coating with electroless nickel-phosphorus plating is employed to evaluate the corrosion resistance and feasibility. The conversion treatment was conducted in the solution composed potassium permanganate (KMnO4) and cerium nitrate (Ce(NO3)3) with the pH approximately 2 at room temperature. After treatment, the LZ91 plate was activated in a solution at room temperature and pH = 2 with St-co-NIPAAm/Pd nanoparticles. Finally, the electroless plating layer was deposited on the samples, step sketches as shown in Fig. (a) [6-8]. Surface morphology, crystal structure and the coating composition were determined by SEM, TEM and XPS. The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and salt spray tests (SST). The results of the electrochemical measurement and SST demonstrated that the corrosion resistance of LZ91 alloy was markedly improved by the permanganate conversion treatment. However, results showed that the conversion/Ni-P composite-layer exhibited much worse corrosion resistance than that with only permanganate conversion coating. After a systematical investigation in corrosion behaviors, a possible mechanism for the deterioration of corrosion resistance is proposed in this study. Fig. (b) shows the surface microphology of the permanganate conversion coating treated for 30 s. It indicates that the surfaces are completely covered by the permanganate conversion coating with few micro cracks resulting from internal stresses. Fig. (c) shows the surface morphology of the electroless Ni-P coating deposited for 40 min on the permanganate conversion coated LZ91 alloy, showing a typical nodular-like structure. No micro-crack and pore are observed in the Ni-P film. References S. Y. Jian, Y. R. Chu, C. S. Lin, Corrosion Science 93 (2015) 301-309.C. Wang, F. Jiang, F. Wang, Corrosion Science 46 (2004) 75–89G. Bikulčius, A. Ručinskienė, A. Sudavičius, V. Burokas, A. Grigucevičienė, Surface & Coatings Technology 203 (2008) 115–120.H. Zhang, G. Yao, S. Wang, Y. Liu, H. Luo, Surface & Coatings Technology 202 (2008) 1825–1830.L. Yang, J. Li, X. Yu, M. Zhang, X. Huang, Applied Surface Science 255 (2008) 2338–2341.C. S. Lin, S. K. Fang, Journal of The Electrochemical Society 152 (2) B54-B59 (2005).H. Huo, Y. Li, F. Wang, Corrosion Science 46 (2004) 1467-1477.M. A. Chen, N. Cheng, Y. C. Ou, J.M. Li, Surface and Coatings Technology 232 (2013) 726-733.S. Y. Jian, J. L. Lee, H. B. Lee, H. H. Sheu, C. Y. Ou, M. D. Ger, Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 496–505. Figure 1

Electroless Co–P plating on magnesium alloy and its anti-corrosion properties

Electroless Co–P coating was successfully applied on AZ31 magnesium alloy through environmentally friendly cerium–lanthanum–permanganate conversion coating treatment. It is found that application of the conversion coating activates the electroless Co–P plating. Electroless nucleation centres were first formed around the conversion coating pores due to replacement reaction between the cobalt ions and the alloying elements, and then were grown to form cobalt clusters. Low phosphorous, crystalline and compact electroless coating with no obvious pores or cracks in the coating/substrate interface was obtained. The results of open circuit potential and electrochemical impedance spectroscopy measurements showed that the corrosion resistance of the AZ31 alloy was significantly increased by electroless Co–P plating.

Microstructure and Electrochemical Characterizations of Permanganate Conversion Coating on AZ31 Magnesium Alloy

Permanganate conversion coating treatment is one of the promising alternatives to hexavalent chromium conversion coating treatment for magnesium alloys. However, the permanganate conversion coating on magnesium generally contains defects such as cracks, which deteriorate the coating adhesion and corrosion resistance.1-6 In the present study, we reported the fabrication of a compact permanganate conversion coating on an AZ31 magnesium alloy in strongly acidic potassium permanganate solution. Moreover, the crack formation was largely avoided. Consequently, the corrosion resistance of the coating was improved. It has been well known that Mn(IV) oxides precipitate in the solution containing permanganate (MnO4 –) and manganous ions (Mn2+), also known as the Guyard reaction.7 This reaction has been shown to be autocatalytic in acid solution as driven by the presence of colloidal Mn(VI) oxides. This study also detailed the effects of the addition of Mn(NO3)2 or MnSO4 in the permanganate solution on the microstructure and growth of the conversion coating on the AZ31 magnesium alloy. Prior to conversion coating treatment, the AZ31 magnesium alloys were mechanically abraded up to 1200 grade emery paper. Subsequently, the specimen was rinsed in deionized water and dried with an air stream. The nitrate-containing bath was performed in the solution composed of 0.1 M potassium permanganate (KMnO4), 0.025 M manganese nitrate (Mn(NO3)2), and 0.02M potassium dihydrogen phosphate (KH2PO4) at room temperature for 60 s. The sulfate-containing bath was performed in the solution composed of 0.1 M KMnO4, 0.025 M MnSO4, and 0.02M KH2PO4 at room temperature for 60 s. After conversion coating treatment, the AZ31 plate was rinsed in deionized water and left drying overnight at room temperature. SEM, TEM, and EDX were employed to analyze the surface morphology and characterize the microstructure of the coating. The corrosion properties of the different conversion coatings were evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Finally, the EIS was conducted after immersion in 3.5 wt.% NaCl solution for different time periods up to 24 h. Figure 1 shows that the permanganate coated AZ31 displays a light brown color. As a result, the conversion coating has completely covered the AZ31 magnesium alloy after 60 s of immersion in the permanganate solution containing nitrate or sulfate anions. Figure 2(a) and (b) show the cross-sectional TEM characterization of the AZ31 after 60 s immersion in the permanganate bath containing nitrate and sulfate anions, respectively. Both coatings have comparable thickness. Moreover, the SAED of the compact layer comprised diffused halos regardless of the bath composition, suggesting that all the coatings had poor crystallinity. The P content was the main difference between the two conversion coatings. The nitrate-containing conversion coating effectively enhances the corrosion resistance of AZ31 magnesium alloys, as evaluated by EIS and potentiodynamic polarization measurements. The corrosion behaviors of the two coatings are also detailed using the EIS characterizations after different time periods of immersion in 3.5 wt.% NaCl solution. References C. S. Lin, C. Y. Lee, W. C. Li, Y. S. Chen, G. N. Fang, J. Electrochem. Soc., 153, B90 (2006).M. Zhao, S. Wu, J. Luo, Y. Fukuda, H. Nakae, Surf. Coat. Technol., 200, 5407 (2006).H. Zhang, G. C. Yao, S. L. Wang, Y. H. Liu, H. J. Luo, Surf. Coat. Technol., 202, 1825 (2008).W. Zhou, D. Shan, E. H. Han, W. Ke, Corros. Sci., 50, 329 (2008).K. Z. Chong, T. S. Shih, Mater. Chem. Phys., 80, 191 (2003).Y.L. Lee, Y.R. Chu, W.C. Li, C.S. Lin, Corros. Sci., 70, 74 (2013).M. J. Polissar, J. Phys. Chem., 39, 1057-1066 (1935). Figure 1

Permanganate conversion coating on AZ31 magnesium alloys with enhanced corrosion resistance

Necessity for developing alternatives to chromate conversion coatings has increased in the last decade. A chromate-free conversion coating treatment, mainly in permanganate solution, was developed in this study for AZ31 magnesium alloys. The coating formed after 90s of immersion consisted of a continuous nearly crack-free layer with an average thickness of 230nm. The results of the electrochemical measurements and the salt spray tests demonstrated that the corrosion resistance of the AZ31 alloy was markedly improved by the permanganate conversion treatment. Moreover, this thin permanganate conversion coating had sufficient electrical conductivity, ensuring the magnetic shielding properties of the AZ31.

Biodegradability of AZ31B Magnesium Alloy with a Permanganate Conversion Coating

Magnesium alloy has been considered as a revolutionary biodegradable material, but the fast degradation limits its development. A permanganate conversion coating for AZ31B magnesium alloy prepared, in order to improve its anti-corrosion property. The morphology and composition of the coating were characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersion spectroscopy (EDS), which showed that the coating was 40-170μm in thickness, with net-like micro-cracks, and was tightly combined with AZ31B magnesium alloy matrix. The main compositions of the coating were Mg and a series of oxides of Mn. Observed by the tests of electrochemical and immersion,the corrosion resistance of AZ31B magnesium alloy can be improved significantly through the permanganate conversion treatment

Effect of Alkaline Phosphate-Permanganate Conversion Coating on the Corrosion Resistance of AZ91D Magnesium Alloy

Alkaline phosphate-permanganate conversion coating, chrome-free conversion coating was studied for corrosion resistance of AZ91D magnesium alloy. Also, conventional acid phosphate -permanganate conversion coating was studied for comparison. Analysis and morphology observation for conversion coating layers was investigated in details by using SEM-EDS, XRD. SEM observation showed that a lot of cracks in surface and interface between conversion coating layer and AZ91D magnesium alloy substrate was observed in acid conversion coating, whereas cracks was not almost observed in alkaline conversion coating layer. SEM-EDS and XRD analysis showed that the main elements of both alkaline and acid conversion coating were Mg, O, K, P and Mn. It was found that both conversion coating layers was consisted of MgO, Mg (OH)2 and MnO2. Salt spray test showed that the alkaline conversion coating have a good corrosion resistance compared with acid conversion coating.

Preparation and Characterization of Permanganate Conversion Coating for Biodegradable AZ31B Magnesium Alloy

A permanganate conversion coating was prepared on biodegradable AZ31B magnesium alloy to improve its anti-corrosion property. Morphology, composition and corrosion resistance of the coating were studied in this paper. The SEM research showed that the coating was approximately 10-12μm in thickness with net-like microcracks. The XRD analysis indicated that the coating was mainly composed of MgO, MnO and Mn2O3. Electrochemical test showed manganese conversion treatment could significant improve the corrosion resistance of AZ31B.

Environmentally-friendly method for electroless Ni–P plating on magnesium alloy

In this paper, an environmentally-friendly procedure has been introduced for electroless nickel plating on AZ61 magnesium alloy. Magnesium alloy was firstly etched in nitric acid and then treated in a Cerium–lanthanum–permanganate (CLP) conversion coating bath. Finally, the electroless Ni–P deposition was carried out on the conversion coating from a sulfate solution. The microstructure of the CLP conversion coating was observed using Scanning Electron Microscopy (SEM), while the composition was analyzed by Energy Dispersive X-Ray Spectroscopy (EDX). Subsequently, deposited Ni–P coating was also characterized by its chemical composition, crystalline structure, morphology, and corrosion resistance. A uniform and amorphous nickel coating with high phosphorus content was successfully deposited on the pre-applied chemical conversion coating on the AZ61 alloy. An obvious passivation occurred for the nickel coating during the anodic polarization in 3.5wt.% NaCl solution. Compared with the bare and CLP treated substrates, the Ni–P deposits showed more nobler corrosion potential and also lower corrosion current density. Open circuit potential (OCP) measurements and electrochemical impedance spectroscopy (EIS) experiments were carried out in 3.5wt.% NaCl to determine the real anticorrosion performance of the Ni–P deposits.

Phosphate–permanganate conversion coatings on the AZ81 magnesium alloy: SEM, EIS and XPS studies

Abstract Conversion coatings on the magnesium alloy AZ81 were prepared using the phosphate–permanganate baths differing in composition. The corrosion behavior of the coated and uncoated alloys has been investigated by electrochemical impedance spectroscopy (EIS) and linear polarization methods. The choice of proper electric equivalent circuit (EEC) is discussed. The effect of temperature, bath composition and time of conversion as well as etching in acids before application on the corrosion resistance of the coated alloy has been evaluated. The best corrosion resistance was obtained for the samples coated in the bath containing 25 g KMnO 4 , 150 g Na 2 HPO 4 and 50 ml H 3 PO 4 in 1 dcm 3 , applied at 80 °C. Differences in the morphology and composition of coated surfaces were investigated by the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques and correlated with the corrosion resistance of the samples.

Phosphate-silicate composite coating formed on AM60 magnesium alloy

A phosphate–silicate composite coating was investigated for AM60 magnesium alloy. The coating was achieved in a phosphate–permanganate solution which involved sealing in sodium silicate solution. The morphology, composition and corrosion resistance on the coatings were analysed. The phosphate–permanganate conversion coating was shown to exhibit network feature and plenty of cracks. The coating containing increased Mn element was produced using acetic acid to adjust the pH of the phosphating bath. The corrosion resistance of the coating was improved. The phosphate–silicate composite coating became more compact. The cracks on the phosphate–permanganate conversion coating were filled in by silica gel through sealing. The main elemental compositions of the phosphate–silicate composite coating were Mg, O, K, P, Mn and Si. The electrochemical polarisation curves demonstrated that the phosphate–silicate composite coating provided more effective protection against corrosion than that of phosphate–permanganate conversion coating and traditional chromate conversion coating.

Corrosion behaviour of chemical conversion treatments on as-cast Mg–Al alloys: Electrochemical and non-electrochemical methods

Magnesium alloys are often used in as-cast conditions. So, the aim of this work is to characterize the corrosion protection of as-cast AZ91D alloys coated with simple chemical conversion (phosphate–permanganate, and cerium-based coatings). With the two coatings, the electrochemical measurements show that the corrosion protection is due to both the inhibition of cathodic and anodic reactions, because of the presence of stable CeO 2 or manganese oxides in basic pH. Nevertheless, the non-electrochemical tests of corrosion are required to bring to light the healing effect of phosphate–permanganate coating compared to Ce-coating and to describe the corrosion behaviour completely. Finally phosphoric and soda pickling associated to phosphate–permanganate conversion treatment or cerium coating are ecologically efficient alternatives to fluoride-based pickling and the chromating treatment.

Cerium–permanganate conversion coatings for a Zn–Co alloy

The present work aims at investigation of the corrosion resistance of cerium–permanganate conversion coatings (CPCC) electrolessly deposited on a Zn–Co (0.6%) alloy. For comparison, Zn–Co (0.6%) alloy with a commercial chromate treatment was also examined. CPCCs on a Zn–Co (0.6%) alloy have been studied by potentiodynamic polarization and the immersion test. The experimental results of potentiodynamic polarization and the immersion test showed that the corrosion resistance of CPCC on a Zn–Co (0.6%) alloy is close to the corrosion resistance of chromate conversion coating on Zn–Co (0.6%). The microstructure of conversion coatings was analysed by scanning electron microscopy (SEM) and the composition of conversion coatings was analysed by X-ray photoelectron spectroscopy (XPS). It has been found that the cerium–permanganate conversion coating is homogeneous as compared with the chromate coating. XPS has determined that the CPCC contains the following compounds: CeO 2(IV), Ce 2O 3(III) and Mn oxides and hydroxides.

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

A chrome-free conversion coating on magnesium–lithium alloy was obtained from a phosphate–permanganate solution. The morphology, the composition and the corrosion resistance of this coating were examined. The thin and non-penetrating cracked morphology with some deposits existed on the phosphate–permanganate conversion coating. The main elements of the conversion coating were Mg, O, K, P and Mn. The results of the electrochemical measurements and the immersion tests demonstrated that the corrosion resistance of the magnesium–lithium alloy has been improved by the phosphate–permanganate conversion treatment.

Protective coating for magnesium alloy

A phosphate–permanganate conversion coating was applied as the pretreatment process for AZ91D magnesium alloy substrate. Zn–Ni alloys were electrodeposited onto the treated AZ91D magnesium alloy from sulfate bath. The morphology and phase composition of the coatings were determined with X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). The results reveal that the conversion rate depends on pH of solution and treatment time. Salt spray and the electrochemical polarization testing were applied to evaluate the corrosion performance of phosphate–permanganate and Zn–Ni coated alloys. It was found that Ni content in deposit is a function of current density and bath composition. Zn–13 wt.% Ni coating provides very good corrosion protective function to inner AZ91D magnesium alloy. Phosphate–permanganate treatment enhances the corrosion resistance of Zn–Ni coatings.

Growth of permanganate conversion coating on 2024-Al alloy

The growth of permanganate conversion coating on aluminum 2024-T3 alloy has been studied by characterizing, with scanning Auger microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy, the coatings formed by immersion of the alloy in the coating bath (containing KMnO 4 and Na 2B 4O 7, pH 9.1) for different periods of time and at different temperatures. At room temperature, during the first 1–5 min of immersion, MnO 2 deposits are formed only on the second-phase intermetallic particles (of Al–Cu–Mg and Al–Cu–Fe–Mn types), but the coating starts to develop on the Al matrix surface after 5–10 min. The coating slows down and stops after about 150 min, with a thinner deposit over the alloy matrix. The process is accelerated at higher temperatures, for example at 68 °C it self-limits after about 3 min. The electrochemical growth process appears to follow that established for the chromate conversion coatings, although XPS does not detect significant MnO 4 − incorporation into the permanganate coatings.

Development of Permanganate-Based Coatings on Aluminum Alloy 2024-T3

Abstract Permanganate coatings show promise as replacements for chromate conversion coatings on aluminum alloys used in the aerospace industry. The development of permanganate conversion coatings on AA2024-T3 (UNS A92024) from acid solution was followed for up to 240 s using scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Rutherford backscat-tering spectroscopy (RBS). The permanganate coatings have a manganese dioxide (MnO2)-rich exterior surface covering a mixed Al-Mn oxide with a composition close to MnO2·Al2O3. RBS and AES depth profiles indicate that Mn is distributed throughout the oxide over the metal matrix, while XPS confirmed that Mn(IV) and Al(III) were both present in the coating. The onset of coating formation from permanganate solutions is rapid (<30 s) with the maximum thickness (50 nm to 70 nm) being reached after approximately 60 s. The limit on coating thickness may be due to the passivation of boehmite (AlOOH) by MnO2. In...