Effect of plasma electrolytic oxidation coating on the stress corrosion cracking behaviour of wrought AZ61 magnesium alloy

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10⁻⁶ s⁻¹ and 1 × 10⁻⁷ s⁻¹, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.     

Corrosion behavior of the composite ceramic coating containing zirconium oxides on AM30 magnesium alloy by plasma electrolytic oxidation

The deterioration process of a plasma electrolytic oxidation (PEO) coating containing zirconium oxides on AM30 magnesium alloy in 3.5 wt.% NaCl solution have been investigated. The coating consists of an outer porous layer and an inner dense layer. The content of MgF₂ is high in the pores and an MgO-rich layer is evident in the inner layer. The corrosion resistance of the outer layer gradually decreases in the initial immersion stage (96 h) due to the decomposition of MgO, and the deterioration of the inner layer is delayed by the blocking effect of the outer layer.     

Oxidation behaviour of cast aluminium matrix composites with Ce surface coatings

The oxidation behaviour of SiC-reinforced aluminium matrix composites (A3xx.x/SiCp) has been studied after Ce-based treatments. Kinetics data of oxidation process were obtained from gravimetric tests performed at different temperatures (350, 425 and 500 °C). The nature of the oxidation layer was analyzed by scanning electron and atomic force microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and X-ray diffraction. The extent of oxidation degradation in untreated composites was preferentially localized in matrix/SiCp interfaces favouring the MgO formation. Ce coatings favoured a uniform oxidation of the composite surface with MgAl₂O₄ spinel formation. This oxide increased the surface hardness of the materials.     

Corrosion behaviour of AIP NiCoCrAlYSiB coating in salt spray tests

NiCoCrAlYSiB coatings were deposited by arc ion plating (AIP) and annealed/pre-oxidised under various conditions. The corrosion behaviour of as-deposited and annealed/pre-oxidised coatings was studied by salt spray testing in a neutral mist of 5 wt% NaCl at 35 °C for 200 h. The results showed that the as-deposited NiCoCrAlYSiB coating behaved poorly while the annealed and pre-oxidised ones performed much better in salt spray tests. The dense microstructure in annealed coatings and formation of α-Al₂O₃ scales on the surface during pre-oxidation improved the corrosion resistance in salt spray test. The corrosion process was investigated from the aspects of corrosion products, and its electrochemical mechanism was proposed as well.     

Formation of micro-arc oxidation coatings on AZ91HP magnesium alloys

In a neutral solution, coating formation by micro-arc oxidation (MAO) on AZ91HP magnesium alloy was studied. The process involved the substrate dissolution and coating development. During the first 5 s, the sample mass decreased, indicating substrate dissolution dominating the process. After 5 s, the sample mass began to increase and the coating development began to predominate the process. The coating was firstly nucleated on α phase and sparks initially appeared mainly on the edges of the sample. With treating proceeded, the coating was becoming uniform; meanwhile, microscopic pores of anodic coatings increased in size and decreased in number.     

Formation of self-repairing anodized film on ACM522 magnesium alloy by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) on a die-casting ACM522 Mg alloy was conducted in aqueous silicate and phosphate solutions. The corrosion behavior of the anodized ACM522 Mg alloy was investigated in detail. During the investigation, the self-repairing behavior of the anodized films was evaluated by a salt spray test for 168 h, and the mechanism of this self-repairing was discussed in terms of thermodynamic equilibrium constants. Furthermore, the effects of additives to the phosphate solution on the self-repairing behavior were examined, and the addition of Na₂B₄O₇ was found to effectively reduce cracks on a self-repaired film.     

Film formation in the second step of micro-arc oxidation on magnesium alloys

Magnesium alloys were anodized firstly in a neutral solution and then in an alkaline solution by micro-arc oxidation (MAO). The second step of MAO was a competition between old film dissolution and new film formation. The curves of voltage and mass gain with time showed that in the initial stage, the old film was dissolved. Surface morphologies and EDX analyses indicated that a new film was formed after 1 min. The transition from the process dominated by old film dissolution to that by new film formation took place when the new film had completely covered the old film.     

Plasma electrolytic oxidation of pre-anodized aluminium

The influence of an anodizing pre-treatment in sulphuric acid is investigated on plasma electrolytic oxidation (PEO) of aluminium in silicate electrolyte under constant rms current. The presence of the anodic film is shown to promote the establishment of a micro-arc regime that is favourable for growth of the PEO coating. The incorporation of the pre-formed film into the coating appears to proceed by thermal transformation of the anodic alumina, accompanied by formation of oxide beneath the pre-formed layer. The final coatings contain α- and γ-Al₂O₃, with increased concentrations of silicon, sodium and potassium in an outer region of the coating.     

Characterization of the plasma electrolytic oxidation of aluminium in sodium tungstate

Morphology, composition and microhardness of oxide coatings formed during DC plasma electrolytic oxidation (PEO) of aluminium in sodium tungstate were investigated. Oxide coatings were characterized by optical emission spectroscopy, AFM, SEM-EDS and XRD. Oxide coatings morphology is strongly dependent on process duration, while microhardness decreases with extended PEO time. The microdischarges characteristics were studied as well and it is shown that size of microdischarges becomes larger, while the surface density of microdischarge sites becomes lower, with increasing PEO time. Optical emission spectrum of microdischarges has several intensive band peaks caused by electronic transition in Al, W, Na, O, H atoms.     

Corrosion behaviour of silicon-carbide-particle reinforced AZ92 magnesium alloy

The corrosion behaviour of silicon-carbide-particle (SiCp) reinforced AZ92 magnesium alloy manufactured by a powder metallurgy process was evaluated in 3.5 wt.% NaCl solution, neutral salt fog (ASTM B 117) and high relative humidity (98% RH, 50 °C) environments. The findings revealed severe corrosion of AZ92/SiC/0-10p materials in salt fog environment with formation of corrosion products consisting of Mg(OH)₂ and (Mg,Al)_x(OH)_y. The addition of SiCp increased the corrosion rate and promoted cracking and spalling of the corrosion layer for increasing exposure times. Composite materials revealed higher corrosion resistance in high humidity atmosphere with almost no influence of SiCp on the corrosion behaviour.     

Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.     

Growth characteristics and corrosion resistance of micro-arc oxidation coating on pure magnesium for biomedical applications

Protective micro-arc oxidation (MAO) coating was prepared on pure magnesium in aqueous solution without toxic components under a constant current density. The whole anodizing process can be divided into four stages based on the differences in voltage, sparking behaviour and oxygen evolution. SEM micrographs show that a thick and dense coating can be obtained after anodizing for adequate time. The corrosion resistance of the samples with anodic coatings was evaluated by drop experiments and potentiodynamic polarization tests. The results confirm that the thick and dense coating can efficiently protect the magnesium substrate from corrosion in aggressive environments.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

Anodizing of magnesium alloy AZ31 in alkaline solutions with silicate under continuous sparking

Anodization is a useful technique for forming protective films on magnesium alloys and improves its corrosion resistance. Based on the alkaline electrolyte solution with primary oxysalt developed previously, the optimum secondary oxysalt was selected by comparing the anti-corrosion property of anodic film. The structure, component and surface morphology of anodic film and cross-section were analyzed using energy dispersion spectrometer (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion process was detected by electrochemical impedance spectroscopy (EIS). The results showed that secondary oxysalt addition resulted in different anodizing processes, sparking or non-sparking. Sodium silicate was the most favorable additive of electrolyte, in which anodic film with the strongest corrosion resistance was obtained. The effects of process parameters, such as silicate concentration, applied current density and temperature, were also investigated. High temperature did not improve anti-property of anodic film, while applying high current density resulted in more porous surface of film.     

Detrimental effect of particle sol-gel coating on the corrosion behavior of A380-SiC composite

In this study, the corrosion susceptibility of aluminum matrix composites reinforced with artificially oxidized SiO₂ and sol-gel Fe/TiO₂ coated silicon carbide particles (SiC_p) has been investigated. Corrosion behavior of the composites, fabricated by the liquid metal infiltration technique, was established in chloride containing alkaline environments by cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS) techniques. It has been found that, sol-gel coating of SiC particles with Fe/TiO₂ has a detrimental effect on the corrosion characteristics of A380-SiC metal matrix composite.     

Corrosion behaviour of thermally sprayed Al and Al/SiCp composite coatings on ZE41 magnesium alloy in chloride medium

Thermally sprayed Al and Al/SiCp composite coatings have been deposited on ZE41 magnesium alloy and mechanical compaction at room temperature was applied to the Al and Al/SiCp coatings to reduce their porosity. Corrosion behaviour of coated samples was evaluated and compared to that of uncoated substrate in 3.5 wt.% NaCl solution using electrochemical measurements. Al and Al/SiCp composite coatings reduced the corrosion current density of Mg-Zn alloys by three and two orders of magnitude, respectively, and reductions up to four orders of magnitude were obtained after mechanical compaction.     

High temperature oxidation behaviour of a TiAl-Al2O3 intermetallic matrix composite

A TiAl-based intermetallic matrix composite has been produced through sintering of mechanically milled Al/TiO₂ composite powder. The composite contains 42-50 vol.% of α-Al₂O₃ as the particulate reinforcement phase. Oxidation experiments were carried out at 800-900 °C in air up to 500 h to evaluate its oxidation and scale spallation resistance. A cast Ti-50at.%Al alloy was also tested for comparison. The composite samples showed much lower oxidation mass gain than the cast alloy under all testing conditions. Moreover, the composite samples exhibited extremely strong scale spallation resistance. Spallation could never be recorded and observed even under long-time intensive cyclic oxidation exposure. Based on the kinetic and microstructural studies, the mechanisms for the improved oxidation and spallation resistance are discussed.     

Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiCp)

The influence of silicon carbide (SiCp) proportion and matrix composition on four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) immersed in 1-3.5 wt% NaCl at 22 °C was investigated by potentiodynamic polarization. The kinetics of the corrosion process was studied on the basis of gravimetric measurements. The nature of corrosion products was analysed by scanning electron microscopy (SEM) and low angle X-ray diffraction (XRD). The corrosion damage in Al/SiCp composites was caused by pitting attack and by nucleation and growth of Al₂O₃ · 3H₂O on the material surface. The main attack nucleation sites were the interface region between the matrix and the reinforcement particles. The corrosion process was influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement and saline concentration.     

Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate–vanadate salt

Hot corrosion studies of thermal barrier coatings (TBCs) with different YSZ/LaMgAl₁₁O₁₉ (LaMA) composite coating top coats were conducted in 50 wt.% Na₂SO₄ + 50 wt.% V₂O₅ molten salt at 950 °C for 60 h. Results indicate that TBCs with composite coating top coats exhibit superior oxidation and hot corrosion resistances to the TBC with the traditional YSZ top coat, especially for which has a LaMA overlay. The presence of LaMA can effectively restrain the destabilization of YSZ at the expense of its own partial degradation. The hot corrosion mechanism of LaMA coating and the composite coatings have been explored.     

Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte

Spark anodizing of aluminium at 5 A dm⁻² in sodium metasilicate/potassium hydroxide electrolytes is studied, with particular emphasis on the mechanism of coating growth, using transmission electron microscopy and surface analytical techniques, with coatings typically 10 μm, or more, thick. Two-layered coatings develop by deposition of an outer layer based on amorphous silica, associated with low levels of alkali-metal species, at the coating surface and growth of an inner, mainly alumina-based, layer, with an amorphous region next to the metal/coating interface. Formation of crystalline phases in the inner layer, mainly γ-Al₂O₃, with some α-Al₂O₃ and occasional δ-Al₂O, is assisted by local heating, and possibly also by ionic migration processes, arising from the rapid coating growth at sites of breakdown. Due to local access of electrolyte species in channels created by breakdown events, the silicon content in the inner coating regions varies widely, ranging from negligible levels to about 10 at.%. Silica deposition at the coating surface and formation of Al₂SiO₅ and Al₆Si₂O₁₃ phases is promoted by increased time of anodizing and concentration of metasilicate in the electrolyte. However, at sufficiently high concentration of metasilicate and pH, when more extreme voltage fluctuations accompany breakdown, the two-layered nature of coatings is replaced by a mixture of aluminium-rich and silicon-rich regions throughout the coating thickness.