Composite cerium oxide/titanium oxide thin films for corrosion protection of AZ91D magnesium alloy via sol–gel process

Anti‐corrosive composite cerium oxide/titanium oxide (CeO₂/TiO₂) thin films were successfully prepared on an AZ91D magnesium alloy substrate by applying cerium oxide (CeO₂) thin films as the inner layer with a sol–gel process. Composition and surface morphology of the thin films were analyzed using X‐ray diffraction (XRD) and scanning electron microscope (SEM). XRD showed that the composite films consisted of cerianite and anatase phases. The wettability of the thin films was evaluated by water contact angles measurements. Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests were used to evaluate the corrosion behavior of the bare substrate and coated samples in 3.5 wt% sodium chloride solution (3.5 wt% NaCl). The results demonstrated that titanium oxide (TiO₂) thin film mainly dominated the corrosion resistance of samples and the composite films with excellent hydrophilicity could significantly improve the corrosion resistance of AZ91D magnesium alloy.     

Effect of cerium addition on microstructure and corrosion resistance of die cast AZ91 magnesium alloy

A novel AZ91 Ce containing magnesium alloy characterized by excellent corrosion resistance is fabricated by adding rare earth Ce (cerium) in the form of a Mg‐Ce master alloy. The metallographic investigation shows that Ce added to AZ91 can obviously decrease the size of β‐Mg₁₇Al₁₂ and forms Al₁₁Ce₃ intermetallic compounds in the shape of fine needles. The corrosion tests and electrochemical measurements indicate that the corrosion resistance of AZ91 Ce containing magnesium alloy is obviously higher than that of AZ91. Furthermore, increasing the content of Ce in the magnesium alloy can further enhance the corrosion resistance. X‐ray photoelectron spectroscopy (XPS) reveals that Ce can be incorporated into corrosion products in the form of CeO₂ in the course of corrosion. Based on the preliminary analysis, the addition of Ce can improve the corrosion resistance of AZ91 by decreasing the size of β‐Mg₁₇Al₁₂ and enhancing the protective effectiveness of corrosion products.     

Microstructural characteristics and corrosion property of non‐chromate surface treatments on AZ91D magnesium alloy

Chromate conversion coatings can be successfully used for corrosion protection of magnesium alloys. However, the environmental laws have imposed severe restrictions on chromate use in many countries. In this study, a novel protective environmental‐functionally gradient coating was formed on AZ91D magnesium alloy by non‐chromate surface treatments, which consisted of pre‐etching followed by cerium‐based chemical conversion before applying the sol–gel CeO₂ film. It was determined by the analysis of X‐ray diffraction that the gradient coating was mainly composed of CeO₂. The calculation, based on the Scherrer formula, further revealed the formation of nanocrystalline structure in the coating. Scanning electron microscopy (SEM) observations showed that the coating was homogeneous and compact, no obvious cracked structure occurred. According to the immersion tests, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests, the corrosion resistance of AZ91D magnesium alloy was found to be greatly improved by means of this novel environmental‐functionally gradient coating.     

2-Hydroxy-4-methoxy-acetophenone as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy

The inhibition behavior of 2-hydroxy-4-methoxy-acetophenone (paeonol) as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy was investigated in 0.05 wt.% NaCl solution by means of polarization curve, AC impedance, weight loss measurement, scanning electron microscopy, Fourier transformation infrared spectroscopy, ultraviolet analysis, and computer molecular simulation. The results show that paeonol can inhibit the corrosion of AZ91D. The maximum inhibition efficiency is achieved when paeonol concentration is 50 ppm by weight in this study. It is proposed that paeonol chelates with Mg to form a paeonol-Mg complex mixing with the original Mg(OH)₂ film on the surface to inhibit the anodic dissolution of AZ91D.     

Enhanced corrosioon resistance by sol‐gel‐based ZrO2‐CeO2 coatings on magnesium alloys

The physical, chemical and mechanical properties of magnesium alloys make them attractive materials for automotive and aerospace applications. However, these materials are susceptible to corrosion and wear. This work discusses the potential of using sol‐gel based coatings consisting of ZrO₂ and 15 wt.% of CeO₂. The CeO₂ component provides enhanced corrosion protection, while ZrO₂ impart corrosion as well as wear resistance. Coating deposition was performed by the dip coating technique on two magnesium alloy substrates with different surface finishes: AZ91D (as‐casted, sand‐blasted, and machined) and AZ31 (rolled and machined). All as‐deposited coatings (xerogel coatings) were then subjected to 10 h annealilng: a temperature of 180°C was applied to the AZ91D alloy and 140°C to the AZ31 alloy. Morphological and structural properties of the annealed coatings were investigated by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Coating composition was examined using energy dispersive X‐ray analysis. Adhesion of the annealed ZrO₂‐CeO₂ coatings on the substrates, assessed by scratch tests, showed critical loads indicative of coating perforation of up to 32 N. Hardness and elasticity, measured using depth‐sensing nanoindentation tests, gave a hardness and elastic modulus of 4.5 GPa and 98 GPa, respectively. Salt spray corrosion tests performed on these coatings showed superior corrosion resistance for AZ91D (as‐casted and machined) and AZ31 (machined), while severe corrosion was observed for the AZ31 (rolled) and AZ91D (sand‐blasted) magnesium alloy substrates.     

Characterization of plasma electrolytic oxide formed on AZ91 Mg alloy in KMnO4 electrolyte

The aim of this work is to investigate microstructure, corrosion resistance characteristics and nanohardness of the oxide layer on AZ91 Mg alloy by applying different voltage with KMnO₄ contained solution. There are lots of closed pores that are filled with another oxide compound compared with the typical surface morphology with pore coated until 350 V of coating voltage. The thickness of oxide layer increases with increasing coating voltage. The oxide layer formed on AZ91 Mg alloy in electrolyte with potassium permanganate consists of MgO and Mn₂O₃. Corrosion potential of the oxide layer on AZ91 Mg alloy obtained at different plasma electrolytic oxidation(PEO) reaction stages increases with increasing coating voltage. The corrosion resistance of AZ91 Mg alloy depends on the existence of the manganese oxide in the oxide layer. The inner barrier layer composed of the MgO and Mn₂O₃ may serve as diffusion barrier to enhance the corrosion resistance and may partially explain the excellent anti-corrosion performance in corrosion test. Nanohardness values increase with increasing coating voltage. The increase in the nanohardness may be due to the effect of manganese oxide in the oxide layer on AZ91 Mg alloy coated from electrolyte containing KMnO₄.     

Electrodeposition of zinc on AZ91D magnesium alloy pre‐treated by stannate conversion coatings

A stannate chemical conversion process followed by an activation procedure was employed as the pre‐treatment process for AZ91D magnesium alloy substrate. Zn was electroplated onto the pre‐treated AZ91D magnesium alloy surface from pyrophosphate bath to improve the corrosion resistance and the solderability. The surface morphologies of conversion coating and zinc coating were examined with scanning electron microscope (SEM). The phase composition of conversion coating was investigated by X‐ray diffraction (XRD). The electrochemical corrosion behavior of the coatings in the corrosive solution was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The experimental results showed that the activated stannate chemical conversion coating provided a suitable interface between zinc coating and the AZ91D magnesium alloy substrate. The corrosion resistance of the AZ91D substrate was improved by the zinc coating.     

Characterization and corrosion studies of ceria thin film based on fluorinated AZ91D magnesium alloy

The CeO₂ thin film was prepared via sol-gel method on fluorinated AZ91D magnesium alloy surfaces. The surface morphology, composition and the corrosion resistance of the film were investigated in details using scanning electron microscope, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy as well as potentiodynamic polarization tests. It was found that small amount of MgO and MgF₂ were encapsulated in CeO₂ thin film. The electrochemical measurement results demonstrated that the CeO₂ thin film on fluorinated AZ91D magnesium alloy could improve the corrosion resistance approximately by two orders of magnitude compared with that of the bare substrate.     

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 μg/cm⁻² NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg₅ (CO₃)₄ (OH)₂4H₂O) and nesquehonite (MgCO₃ 3H₂O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH₂)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO₂ and O₂ to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na⁺ and Cl⁻ ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.     

The influence of NaCl and CO2 on the atmospheric corrosion of magnesium alloy AZ91

The influence of NaCl and CO₂ on the atmospheric corrosion of magnesium alloy AZ91 is studied in the laboratory. Samples were exposed under carefully controlled air and flow conditions; the relative humidity was 95%, the temperature was 22.0°C and the concentration of CO₂ was < 1 ppm or 350 ppm. Different amounts of sodium chloride (0–70 μg/cm²) were added before exposure. The corrosion products were analyzed by gravimetry, ion chromatography, X‐ray diffraction and scanning electron microscopy. Mass gain and metal loss results are reported. The combination of high humidity and NaCl is very corrosive towards AZ91. However, the NaCl‐induced corrosion is inhibited by ambient concentrations of CO₂. Exposure in the absence of CO₂ gives rise to heavy pitting, with brucite, Mg(OH)₂, being the dominant corrosion product. In the presence of CO₂ a layer of hydrated magnesium hydroxy carbonate, Mg₅(CO₃)₄(OH)₂ · 5 H₂O forms. A tentative corrosion mechanism is presented that explains the behavior in the two environments.     

Corrosion behaviour of Mg/Al alloys in high humidity atmospheres

The influence of relative humidity (80–90–98% RH) and temperature (25 and 50 °C) on the corrosion behaviour of AZ31, AZ80 and AZ91D magnesium alloys was evaluated using gravimetric measurements. The results were compared with the data obtained for the same alloys immersed in Madrid tap water. The corrosion rates of AZ alloys increased with the RH and temperature and were influenced by the aluminium content and alloy microstructure for RH values above 90%. The initiation of corrosion was localised around the Al–Mn inclusions in the AZ31 alloy and at the centre of the α‐Mg phase in the AZ80 and AZ91D alloys. The β‐Mg₁₇Al₁₂ phase acted as a barrier against corrosion.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

Microstructure,corrosion behavior,and surface mechanical properties of Fe oxide coatings on biomedical ZK60 Mg alloy

The Mg‐5.5Zn‐0.6Zr (in wt%, ZK60) alloy has been surface modified by dual Fe&O ion implantation and deposition (II&D) under different O₂ fluxes from 0 to 40 sccm. The microstructure is investigated by glancing angle X‐ray diffraction, atomic force microscope, and scanning electron microscopy. The results show that the modified layers, with a gradient microstructure of outer deposition region and inner implantation region, are composed of α‐Fe + Fe&Mg mixture, FeO/Fe‐rich oxide + Fe&Mg mixture, and α‐Fe₂O₃/Fe‐rich oxide + Fe&Mg mixture for 0, 10, and 40 sccm O₂ fluxes, respectively. The electrochemical and immersion tests in 37°C Hank's solution indicate an improvement in corrosion behavior under 0 and 10 sccm O₂ fluxes, but a deterioration in corrosion resistance under 40 sccm O₂ flux. In addition, the nanoindentation tests suggest that the dual Fe&O II&D simultaneously enhances the surface hardness and elastic modulus due to the formation of Fe and its oxide coatings.     

Enhanced corrosion resistance of magnesium and its alloys through the formation of cerium (and aluminium) oxide surface films

Cerium (and aluminium) oxide layers were formed on magnesium and its alloys (AZ91) by chemical surface treatment with or without subsequent annealing. The corrosion behaviour modifications provided by the formation of these surface films were studied by means of different electrochemical and surface analysis techniques. The electrochemical behaviour, studied in sodium sulphate (Na₂SO₄) solution, showed (i) a marked shift of the corrosion potential towards more positive values, (ii) a slight inhibition of the cathodic reaction and (iii) a significant decrease of the anodic dissolution current. X‐ray photoelectron spectroscopy (XPS) was used for the characterisation of the composition of the deposited films and of the changes in the film composition during the electrochemical corrosion tests. The components of some oxide films are cerium dioxide (CeO₂), aluminium oxide (Al₂O₃) and aluminium hydroxide (Al(OH)₃). Other metallic mixed oxide films were obtained as a function of the solution composition. Very little (or no) change in the oxide film composition during the cathodic and anodic polarization experiments was observed from XPS measurements. Chemical treatment provides thick and moderately adherent protective oxide films. Annealing under oxygen further improves the beneficial effect of the chemical treatment.     

Corrosion and self‐healing behaviour of AZ91D magnesium alloy in ethylene glycol/water solutions

Corrosion behaviour of magnesium alloy‐based engine parts in cooling system is an urgent fundamental issue in automotive field where magnesium alloys are increasingly used. In the present work, the corrosion behaviour of AZ91D magnesium alloys in various ethylene glycol/water solutions was studied by electrochemical measurements and immersion tests at room temperature. The surfaces of the samples after immersion tests were examined using scanning electron microscope (SEM) and X‐ray diffraction (XRD). The results showed that the corrosion rates of AZ91D magnesium alloys decreased with the increase of ethylene glycol concentration in ethylene glycol/water solutions and the corrosion process was dominated by pitting corrosion. A continuous protective film transferred from corrosion products was formed on the corroded surface after sufficient immersion duration in ethylene glycol/water solutions, which is able to heal the corrosion pits. The self‐healing behaviour inhibited the further corrosion of AZ91D magnesium alloy.     

Preparation and characterization of inorganic and organic coatings on AZ91D magnesium alloy with electroless plating pretreatment

In this paper, a protective coating scheme was applied for the corrosion protection of AZ91D magnesium alloy. Electroless Ni coating (EN coating) as bottom layer, electrodeposited Ni coating (ENN coating), and silane‐based coating (ENS coating) as top layer, respectively, were successfully prepared on AZ91D magnesium alloy by combination techniques. Scanning electron microscopy and X‐ray diffraction were employed to investigate the surface and phase structure of coatings, respectively. The electrochemical corrosion behaviors of coatings in neutral 3.5 wt% NaCl solution were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The corrosion testing showed that the three kinds of coatings all could provide corrosion protection for AZ91D magnesium alloy to a certain extent, and the corrosion resistance of ENN and ENS was superior to EN. In order to further study the corrosion protection properties of ENN and ENS, a comparative investigation on the evolution of EIS of ENN and ENS was carried out by dint of immersion test in neutral 3.5 wt% NaCl solution. The results indicated that, compared with ENN, the ENS could provide longer corrosion protection for AZ91D magnesium alloy. It is significant to determine the barrier effect of each coating, which could provide reference for industry applications.     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

Influence of potassium pyrophosphate in electrolyte on coated layer of AZ91 Mg alloy formed by plasma electrolytic oxidation

The effect of potassium pyrophosphate in the electrolyte on plasma electrolytic oxidation (PEO) process for AZ91 Mg alloy was investigated. The morphologies and chemical compositions of the coating layer on the AZ91 Mg alloy were evaluated and corrosion resistance was also estimated by potentiodynamic polarization analysis. The coating layer on AZ91 Mg alloy coated from the Bath 2 containing 0.03 mol/L of potassium pyrophosphate for 360 s exhibited considerably dense structure and contained 11%–18% (mass fraction) of phosphorous. The higher content of phosphorous of coating layer coated from Bath 2 could be detected at the bottom of oxide layer, which strongly implied that the phosphorous ion might be concentrated at the barrier layer. Corrosion potential of coating layer of AZ91 Mg alloy increased and corrosion current density decreased with increasing the concentration of potassium pyrophosphate. The polarization resistance (R_p) of coating layer of AZ91 Mg alloy coated from Bath 2 was 4.65×10⁷ Ω/cm², which was higher than that (R_p=3.56×10⁴ Ω/cm²) of the sample coated from electrolyte without potassium pyrophosphate. The coating layer coated from Bath 2 containing 0.03 mol/L potassium pyrophosphate exhibited the best corrosion resistance.     

AZ91D镁合金微弧氧化工艺参数的优化

利用自制微弧氧化装置在硅酸盐体系中对AZ91D镁合金进行微弧氧化处理.采用4因素3水平正交试验,从考察膜层厚度、表面粗糙度和耐蚀性出发,确定了AZ91D镁合金在硅酸盐体系中的最佳工艺参数.结果表明：在最佳工艺条件下,微弧氧化膜呈多孔结构、孔径较小,裂纹较少,分布均匀,膜层较为致密;微弧氧化膜由MgO、Mg2SiO4、MgAl2O4和少量的SiO2组成;室温下,在质量分数为3.5%的NaCl中性溶液中浸泡168 h后,膜层表面未出现明显的点蚀现象,耐蚀性较镁合金基体有了很大提高.     

Corrosion behaviour of die-cast AZ91D magnesium alloy in aqueous sulphate solutions

The corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions was investigated by SEM, FTIR and polarization measurements. For immersion times less than 48 h, no pitting corrosion occurred and only generalized corrosion was apparent. According to the polarization curves, the corrosion rate order of the die-cast AZ91D Mg alloy in three aqueous solutions was: NaCl > MgSO₄ > Na₂SO₄. The main corrosion products were Mg(OH)₂ and MgAl₂(SO₄)₄·22H₂O in the sulphate solutions and the product film was compact. Precipitation of MgAl₂(SO₄)₄·22H₂O required a threshold immersion time.