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.     

The effects of heat treatment and zirconium on the corrosion behaviour of Mg-3Nd-0.2Zn-0.4Zr (wt.%) alloy

The corrosion behaviours of Mg-3Nd-0.2Zn (wt.%) (NZ) and Mg-3Nd-0.2Zn-0.4Zr (wt.%) (NZK) alloys were investigated in as-cast (F), solution-treated (T4) and artificially-aged (T6) conditions in 5% NaCl solution using immersion test and electrochemical measurements. The immersion test indicates that both NZ and NZK alloys exhibit better corrosion resistances in T4 and T6 states than in the F condition due to the galvanic corrosion between the cathodic Mg₁₂Nd compound and the anodic α matrix in the F condition. The NZK alloy demonstrates lower corrosion rates than the NZ alloy in three conditions, which indicates that the addition of zirconium has a beneficial effect on the corrosion resistance. It was discovered by field emission scanning electron microscope (FE-SEM) that the corrosion products of NZK-T6 formed in salt solution are composed of sandwich shape compounds, while that of NZ-T6 is composed of fine needle-like compounds and small particles. The former are more uniform and compact than the latter and can play a more protective role for the alloy. Electrochemical measurements also confirmed that the more protective film formed on the NZK than on the NZ alloy.     

Pitting corrosion of rheocast A356 aluminium alloy in 3.5 wt.% NaCl solution

In this study, the microstructure and corrosion behaviour of rheocast and gravity-cast A356 aluminium alloys were examined and compared. Scanning Kelvin probe force microscopy (SKPFM) results proved that large potential differences between iron-containing intermetallics and the α-Al matrix were responsible for the initiation of the attack at the intermetallics/α-Al interfaces. For longer immersion times, corrosion attack proceeded through the eutectic areas. Semisolid processing refined the eutectic silicon and iron-intermetallics and reduced the potential difference between secondary phases and the matrix. This resulted in improved pitting corrosion resistance of the rheocast A356 aluminium alloy.     

Localized corrosion of a 2219 aluminium alloy exposed to a 3.5% NaCl solution

This work is focused on the role of intermetallics in pitting corrosion of Al2219 alloy. Second phase particles were characterized by AES, SAM and EDX. Their behaviour in a solution of NaCl was investigated as a function of exposure time. The results confirmed the cathodic nature of the intermetallics with respect to the aluminium matrix. Corrosion products rich in aluminium and oxygen were found to progressively accumulate around the particles and iron was dissolved from the intermetallic, followed by back-deposition. Copper and manganese did not show any major activity. After 32 h of exposure the larger intermetallics were completely covered.     

Grain size effects on the oxidation of two ternary Cu-Ni-20wt.% Cr alloys at 700-800 °C in 1 atm O2

Two nanocrystalline two-phase Cu-Ni-Cr alloys, both prepared by mechanical alloying and containing about 20 at.% Cr but with different Ni contents (40 and 20 wt.%, respectively), have been oxidized in 1 atm O₂ at 700-800 °C. Their oxidation behavior has been compared with that of two cast alloys of the same composition, already studied previously, to examine the effects of a large reduction of the size of the individual phase grains and particles. The nanophase alloy with 40 wt.% Ni formed a flat external layer of chromia of regular thickness, while the corresponding cast alloy produced a very irregular chromia layer, often protruding deeply into the alloy, only after an initial stage of rather fast corrosion involving also copper and nickel, associated with some degree of internal oxidation. By oxidation at 700 °C the nanophase alloy with 20 wt.% Ni formed an irregular chromia layer associated with low corrosion rates. The corresponding cast alloy formed complex scales containing Cu, Ni and Cr oxides, extending into the alloy in the form of large pegs, even though a very irregular and discontinuous innermost chromia layer was still able to produce low corrosion rates. On the contrary, at 800 °C both alloys formed complex scales containing mixtures of the oxides of the three metal components. However, the scales grown on the cast alloy were much more irregular in thickness and formed large protrusions into the alloy. In spite of this, the corrosion kinetics of the nanophase 20 wt.% Ni alloy at 800 °C were more irregular and, except for an initial stage, less protective than that of the cast alloy with the same composition.     

The effect of T4 heat treatment on the microstructure and corrosion behaviour of Zn27Al1.5Cu0.02Mg alloy

The effect of heat treatment on the microstructure and corrosion behaviour of Zn27Al1.5Cu0.02Mg alloy was examined. The alloy was prepared by melting and casting route and then thermally processed (T4 regime). Corrosion behaviour of the as-cast and heat treated alloy was studied in 3.5 wt.% NaCl solution using immersion method and electrochemical polarization measurements. The applied heat treatment affected the alloy microstructure and resulted in increased ductility and higher corrosion resistance of the heat treated alloy. Electrochemical measurements of the corrosion rate at the free corrosion potential are in agreement with the results obtained using the weight loss method.     

Enhancement of corrosion resistance of Mg-9 wt.% Al-1 wt.% Zn alloy by a calcite (CaCO3) conversion hard coating

A calcite (CaCO₃) coating on Mg alloy, formed by chemical conversion treatment, was investigated. Aqueous with Ca²⁺ concentration of ∼220 ppm was employed in the chemical conversion treatment. Cross-sectional microstructures of the coated sample after 2 h of treatment revealed a two-layer coating structure. The corrosion current density (I_corr) of the coated sample was approximately two orders of magnitude lower than that of the untreated sample. Electrochemical impedance spectroscopy (EIS) and an appropriate equivalent circuit suggested that each of the layers of the two-layer coating effectively protects Mg alloy against corrosion.     

Electrochemical behaviour and corrosion of Mg–Y alloys

Corrosion of cast magnesium–yttrium (Y) alloys with systematic Y additions up to a nominal 18 wt.% were studied. Corrosion performance was related to the quantitative alloy microstructure and found to increase significantly with the level of alloying and volume fraction of the Mg–Y intermetallic present. In the alloy microstructures, Mg₂₄Y₅ was principally formed; the electrochemistry of which was characterised using the electrochemical microcell method. Electrochemical testing revealed the fundamental corrosion behaviour of Mg–Y alloys and elucidated the corrosion mechanisms at play.     

Corrosion behaviour of Mg-Cu and Mg-Mo composites in 3.5% NaCl

The corrosion behaviour of pure magnesium, Mg-Cu (0.3, 0.6, and 1 vol.%) and Mg-Mo (0.1, 0.3, and 0.6 vol.%) composites has been studied in 3.5% NaCl solution by weight loss and polarisation methods. Corrosion rates determined by weight loss method were considerably higher than that determined by polarisation method. The corrosion rate increased with increasing volume fraction of reinforcement in Mg-Cu and Mg-Mo composites. At the same volume fraction of reinforcement, molybdenum reinforced composite corroded faster than copper reinforced composite. The galvanic current density between Mg-Cu and Mg-Mo couples has been experimentally measured using zero resistance ammeter technique. The experimentally observed galvanic current densities were in close agreement with those obtained using mixed potential theory analysis. SEM observation of corroded samples confirmed microgalvanic activity at the matrix/reinforcement interfaces. The poor corrosion resistance of composites has been attributed to microgalvanic effects between the matrix and reinforcements and inferior quality of surface films.     

Pitting corrosion of artificially aged T6 AA2024/SiCp composites in 3.5 wt.% NaCl aqueous solution

The pitting corrosion behavior of the underaged (UA), peakaged (PA) and overaged (OA) T6 AA2024/0, 8, 14, 19, 24 vol.% 40 μm SiC_p(particles) composites was studied. The processing route used for the materials was the compocasting technique. Corrosion potentials (E_cor), pitting potentials (E_pit) as well as protection potentials (E_prot) were extracted from Double Cycle Polarization (DCP) curves contacted in aerated 3.5 wt.% NaCl aqueous solution. In addition 40 days immersion tests carried out and weight loss curves as well as total pit depth measurements were acquired. Pitting initiation and propagation as the main corrosion mechanism was driven by the aging kinetics which is ruled by the reduction in the retained vacancy concentration and at the same time by the increase in dislocation density as SiC_p volume fraction increases. Thus, alteration in pitting behavior among composites of different SiC_p content took place, although their ageing status was exactly the same.     

Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg–RE alloys: RE = Ce,La, Nd,Y, Gd

Corrosion behaviour was characterised in salt spray and in 3.5% NaCl solution saturated with Mg(OH)₂ of as-cast and solution heat-treated binary Mg–RE alloys. The corrosion rate in the immersion test for the solution heat-treated Mg–RE alloys was substantial, and was greater than that of high-purity Mg. These corrosion rates were probably caused by the particles in the microstructure and/or by Fe rich particles precipitated during the solution heat-treatment. The corrosion rate in the immersion tests for each as-cast Mg–RE alloy was greater than that of high-purity Mg, attributed to micro-galvanic acceleration caused by the second phase. Corrosion rates in salt spray had a general correlation with corrosion rates in the immersion tests, but there were differences. The values of apparent valence were always less than 2 consistent with Mg corrosion being only partly under electrochemical control.     

Degradation behaviour of AZ80E magnesium alloy exposed to phosphate buffer saline medium

Corrosion behaviour of AZ80E alloy in comparison with pure Mg was investigated in phosphate buffer saline (PBS) solution in order to assess its bioactivity. Open circuit potential and EIS results reveal that both samples exhibit self-passivation with time. The higher corrosion resistance of the alloy is discussed from the perspective of its microstructure. Anodic oxidation for the alloy surface in borate buffer solution was also attempted potentiostatically to modify its corrosion behaviour. Anodised specimen at controlled potential of 1.0 V(SCE) can improve the durability of the alloy in PBS medium. The results were further confirmed by SEM and EDX analyses.     

Effects of cerium on the compositional variations in and around inclusions and the initiation and propagation of pitting corrosion in hyperduplex stainless steels

Ce addition to a hyperduplex stainless steel increased its resistance to pitting corrosion because of the formation of stable Ce oxides and a decrease in the area of microcrevices between the matrix and inclusions that act as pit initiation sites. In addition, Cr-enriched zones were formed around Ce oxides with low Cr content in the Ce added alloy. Pitting corrosion in the base alloy initiated at the microcrevice and propagated to Cr oxides, which deteriorated the pitting corrosion resistance. However, pitting corrosion in the Ce added alloy propagated not to the stable Ce oxides but to the matrix.     

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.     

SiCp/2024Al基复合材料的点蚀行为

采用动电位阳极极化法对17%SiCp/2024Al基复合材料及其基体合金在3.5%NaCl水溶液中的耐蚀性进行了研究.结果表明:SiC颗粒的加入并不影响SiCp/2024Al基复合材料的点蚀敏感性,但与基体相比,其耐蚀性有所下降.对极化后和长期浸泡试样的腐蚀形貌观察发现:与基体相比,SiCp/2024Al基复合材料表面上的蚀孔数量相对较多,蚀孔尺寸稍小,大小分布不均匀;最大蚀孔较深,并有严重的裂缝腐蚀;裂缝腐蚀的存在会使SiCp/2024Al基复合材料的点蚀抗力明显降低.能谱分析表明:SiCp/2024Al基复合材料的腐蚀机制为富Cu阴极相与贫Cu阳极相间的电偶腐蚀,另外,SiC与Al间也存在电偶腐蚀倾向.     

Effect of heat treatment on corrosion behaviour of magnesium alloy AZ91D in simulated body fluid

The research explored ways of improving corrosion behaviour of AZ91D magnesium alloy through heat treatment for degradable biocompatible implant application. Corrosion resistance of heat-treated samples is studied in simulated body fluid at 37 °C using immersion and electrochemical testing. Heat treatment significantly affected microgalvanic corrosion behaviour between cathodic β-Mg₁₇Al₁₂ phase and anodic α-Mg matrix. In T4 microstructure, dissolution of the β-Mg₁₇Al₁₂ phase decreased the cathode-to-anode area ratio, leading to accelerated corrosion of α-Mg matrix. Fine β-Mg₁₇Al₁₂ precipitates in T6 microstructure facilitated intergranular corrosion and pitting, but the rate of corrosion was less than those of as-cast and T4 microstructures.     

Influence of Ce surface treatments on corrosion behaviour of A3xx.x/SiCp composites in 3.5 wt.% NaCl

The influence of silicon carbide particles (SiCp) proportion and matrix composition on aluminium metal matrix composites A3xx.x/SiCp modified by cerium-based conversion or electrolysis coating was evaluated in 3.5 wt.% NaCl at 22 °C using potentiodynamic polarization and gravimetric measurements. Ce-treated surfaces presented better corrosion behaviour in chloride media than original composite surfaces without treatment. Both treatments preferentially covered the intermetallic compounds and SiCp. The electrolysis afforded a higher degree of protection than conversion treatment because the coating was more extensive. Coating microstructure and nature of corrosion products were analysed by scanning electron and atomic force microscopy (SEM, AFM) and low angle X-ray diffraction (XRD).     

Wear and corrosion behaviour of Ti–13Nb–13Zr and Ti–6Al–4V alloys in simulated physiological solution

Wear and corrosion behaviour of cold-rolled Ti–13Nb–13Zr alloy, with martensitic microstructure, and Ti–6Al–4V ELI alloy, in martensitic and two-phase (α + β) microstructural conditions, was studied in a Ringer’s solution. The wear experiments were performed at room temperature with a normal load of 40 N and sliding speeds 0.26, 0.5 and 1.0 m/s. The corrosion behaviour was studied at 37 °C using open circuit potential-time measurements and potentiodynamic polarization. It was found that Ti–13Nb–13Zr alloy has a substantially lower wear resistance than Ti–6Al–4V ELI alloy in both microstructural conditions. Surface damage extent increases with sliding speed increase and is always smallest for martensitic Ti–6Al–4V ELI alloy with highest hardness. Both alloys exhibit spontaneous passivity in Ringer’s solution. Corrosion potential values are similar for all three materials. However, Ti–13Nb–13Zr and martensitic Ti–6Al–4V ELI alloys show improved corrosion resistance comparatively to Ti–6Al–4V ELI alloy with (α + β) microstructure. Martensitic Ti–6Al–4V ELI alloy possesses the best combination of both corrosion and wear resistance, although its corrosion resistance is found to be slightly higher than that of the Ti–13Nb–13Zr alloy.     

The effect of epoxy coating containing emeraldine base and hydrofluoric acid doped polyaniline on the corrosion protection of AZ91D magnesium alloy

Corrosion protection of epoxy coatings containing emeraldine base polyaniline (PANI) or hydrofluoric acid doped PANI on AZ91D magnesium alloy were studied by EIS and Pull-Off Adhesion Test. The results indicated that the addition of emeraldine base PANI or hydrofluoric acid doped PANI could improve the corrosion resistance of epoxy coating. The epoxy coating containing hydrofluoric acid doped PANI had the best performance of the corrosion protection among three systems under investigation. The corrosion product film was analyzed by XPS indicating that PANI changed the chemical structure of the corrosion film. The protective mechanism imparted by PANI was discussed.     

Pitting behavior of a bulk Ni-18 wt.% Fe nanocrystalline alloy

Three electrodeposited Ni-18 wt.% Fe samples were annealed at 400 °C for 3 h (hrs), 8 h, and 24 h to study the effects of grain size on the electrochemical properties of bulk Ni-18 wt.% Fe in 3.5 wt.% NaCl. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). Of the four materials studied, the as-received nanocrystalline alloy less sensitive to localized corrosion.