Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments

High-entropy alloys (HEAs) are a newly developed family of multi-component alloys. The potentiodynamic polarization and electrochemical impedance spectroscopy of the Al_xCrFe_1.5MnNi_0.5 alloys, obtained in H₂SO₄ and NaCl solutions, clearly revealed that the corrosion resistance increases as the concentration of aluminium decreases. The Al_xCrFe_1.5MnNi_0.5 alloys exhibited a wide passive region, which extended >1000 mV in acidic environments. The Nyquist plots of the Al-containing alloys had two capacitive loops, which represented the electrical double layer and the adsorptive layer. SEM micrographs revealed that the general and pitting corrosion susceptibility of the HEAs increased as the amount of aluminium in the alloy increased.     

Comparative evaluation of corrosion behaviour of type K thin film thermocouple and its bulk counterpart

This paper investigates the corrosion behaviour of type K thermoelements and their thin films, and compares the performance of chromel–alumel thin film thermocouple with its wire counterpart before and after exposure to 5% NaCl medium. Potentiodynamic polarisation tests reveal that chromel and alumel films are more “noble” than their respective wires. Alumel corrodes faster when coupled with chromel in films than as wires. Secondary electron micrographs and electrochemical impedance spectroscopy measurements suggest that chromel shows localised corrosion while alumel undergoes uniform corrosion. Corrosion adversely affects the thermocouple output and introduces an uncertainty in the measurement.     

The effect of molybdenum on the corrosion behaviour of the high-entropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments

The purpose of this study is to investigate the electrochemical properties of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x high-entropy alloys in three aqueous environments which simulate acidic, marine, and basic environments at ambient temperature (∼25 °C). The potentiodynamic polarisation curves of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x alloys, obtained in aqueous solutions of H₂SO₄ and NaOH, clearly revealed that the corrosion resistance of the Mo-free alloy was superior to that of the Mo-containing alloys. On the other hand, the lack of hysteresis in cyclic polarisation tests and SEM micrographs confirmed that the Mo-containing alloys are not susceptible to pitting corrosion in NaCl solution.     

Electrochemical behaviour of Cu-40Zn in 3% NaCl solution polluted by sulphides: Effect of aminotriazole

The electrochemical behaviour of Cu-40Zn alloy, in 3% NaCl medium pure and polluted by 2 ppm of S²⁻ ions, has been studied in the absence and presence of the 3-amino-1,2,4 triazole (ATA) as corrosion inhibitor. Electrochemical measurements (polarisation curves and electrochemical impedance spectroscopy) showed that sulphides accelerate the alloy corrosion. The studies revealed that ATA inhibits both cathodic and anodic reactions, indicating a mixed type of inhibition. The inhibiting effect was higher in presence of S²⁻ ions than in its absence. Scanning electron microscopy analysis showed that the inhibitor acts by preventing the adsorption of S²⁻ ions, and formation of Cu₂S at the alloy surface. The inhibition efficiency reaches 98% at a concentration of 5 × 10⁻³ M.     

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.     

Oxide properties and stress corrosion cracking behaviour for Alloy 600 in leaded caustic solutions at high temperature

The stress corrosion cracking behaviour of Alloy 600 in caustic solutions with and without PbO at 315 °C was investigated by means of slow strain rate tension tests. The characterisation of the oxide that formed on Alloy 600 was derived from transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. Lead was incorporated into the oxide in a metallic lead state and a lead oxide state, which degraded the passivity and induced PbSCC susceptibility. NiB was used as an inhibitor. It reduced the lead incorporation level in the oxide layer and decreased PbSCC susceptibility.     

Electrochemical corrosion behaviour of Bi-11Ag alloy for electronic packaging applications

This study investigated the electrochemical corrosion properties of the solders for die-attach applications in 3.5% NaCl solution. Compared with Pb-5Sn and Zn-40Sn, Bi-11Ag exhibited higher corrosion potential and relatively low corrosion current density. The ductile Ag-rich phase which dispersed in the Bi matrix was able to accommodate the stress arising from the formation of a passive layer and contributed to the two-stage passivation. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results confirm that the corrosion products comprised BiOCl, Bi₂O₃ and AgCl.     

Metal dusting behaviour of Cr-Ni steels and Ni-base alloys in a simulated syngas mixture

Long-term laboratory exposure tests for various Cr and Ni content steels and Ni-base alloys were conducted at 650 °C in a 60vol.%CO-26%H₂-11.5%CO₂-2.5%H₂O gas mixture simulating syngas environments. Upon isothermal heating, alloys with 15% and 20% Cr had many pits on the surface after a brief exposure, while no pit was found on alloys containing of 60% Ni and more than 23% Cr exposed for up to 5000 h. The thermal cycling accelerated pit initiation drastically, resulting that all test specimens except 30%Cr-60%Ni alloy suffered from metal dusting. From a measurement of pit depths, Ni proved to be an effective alloying element to retard the pit growth: growth rate for 75% Ni alloy has achieved double-digit decrease compared to that for 20% Ni. Microscopic observations has revealed that platelet graphite aligned perpendicular at the boundary of gas/metal of pits. The length of the platelet graphite for high Ni alloys was appreciably longer than that for low Ni steels. This can be interpreted from the difference of super saturation of carbon.     

Oxidation behaviour of selected wrought Ni-base high temperature alloys when used as flame tube material in modern blue flame oil burners

Modern blue flame burners are equipped with a flame tube that serves for recirculation of exhaust gas and for flame stabilization. The in operation material temperature, temperature changes and atmospheres at the flame tube have been measured. Exposure tests of four Ni-Cr-alloys with different concentrations of Al have been carried out under these conditions. The breakdown of the material was connected with the failure of the oxide layer and finally with the formation of internal aluminium nitrides. An approach has been developed for predicting the failure of the oxide layer and hence of the life time of the material.     

Effect of excimer laser surface melting on the microstructure and corrosion performance of the die cast AZ91D magnesium alloy

Excimer laser surface melting (LSM) of the die cast AZ91D alloy has been investigated in terms of microstructure and corrosion behaviour. Excimer LSM of the alloy resulted in a highly homogeneous and refined melted microstructure, which improved the corrosion resistance of the alloy. The latter was associated with the large dissolution of intermetallic phases and the enrichment of aluminium within the melted layer. An increased number of laser pulses resulted in thicker melted layers, but also in enhanced porosity and the formation of micro-cracks at the overlapping area. Both factors diminished the corrosion resistance of the laser-treated alloy.     

Effect of dissolved hydrogen on the electrochemical behaviour of Alloy 600 in simulated PWR primary water at 290 °C

With an increase in dissolved hydrogen (DH) content from 0 to 5 cm³ STP H₂/kg H₂O the electrochemical behaviour of Alloy 600 in deaerated PWR primary water at 290 °C was investigated, using corrosion potential (E_corr) monitoring, potentiodynamic polarization, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). DH content controls the E_corr of Alloy 600. Raising DH content directly promotes the cathodic process and reduces the passivity of Alloy 600 significantly. EIS results show that increasing DH content results in a thinner inner-layer oxide film and ions diffusion becomes easier. The mechanism of these DH effects is discussed.     

Heat treatment effect on the corrosion behaviour of black passivated ZnNi alloys

Electrochemical corrosion behaviour of ZnNi and black passivated ZnNi alloys in aerated and partially deaerated 5% NaCl solution was studied using dc and ac techniques. The heat treatment (24 h, 120 °C) effect on black CCC properties was analyzed. No changes in the R_p values were observed, but differences in the potentiodynamic curves and in EIS diagrams suggested a change in the corrosion mechanism. Comparative studies by AFM and electrochemical techniques showed that these changes in the corrosion behaviour could be explained by taking into account the different effect of heat treatment on the inner and outer layers of the CCC.     

Effects of solution pH and Cl on electrochemical behaviour of an Aermet100 ultra-high strength steel in acidic environments

In this work, the effects of solution pH and Cl⁻ on the electrochemical behaviour of an Aermet100 ultra-high strength steel in 0.5 M (Na₂SO₄ + H₂SO₄) solution were studied by polarization curve and electrochemical impedance spectroscopy (EIS) measurements, combined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterization. The results show that, when solution pH is below 4, the steel is in the active dissolution state, and corrosion current decreases with the increase of pH. There exists a critical pH value, above which the steel is passivated. Moreover, the oxides and hydroxides of Fe, Co, Ni and Cr are the primary components of the passive film. With addition of Cl⁻, pits are initiated on the steel electrode.     

Electrochemical behaviour of Alloy 600 tubing in thiosulphate solution

Electrochemical and surface properties of passive films formed on Alloy 600 in a thiosulphate solution were studied. Oxide films formed at various passive potentials contained a bilayer oxide, whose composition changed as a function of the applied potential resulting in a change in the impedance behaviour. Destabilisation of the oxide film at potentials within the passive region was observed, which was due to the breakdown of the oxide film and coincided with the loss of Cr within the passive film. In contrast, a higher degree of corrosion protection was obtained when the Cr content within the oxide film was elevated.     

Electrochemical investigation of the influence of laser surface melting on the microstructure and corrosion behaviour of ZE41 magnesium alloy – An EIS based study

Surface melting of a magnesium alloy, ZE41 (4%-Zn, 1%-RE) was performed to achieve electrochemical homogeneity at the surface by microstructure refinement. Large secondary precipitates are particularly known to cause severe pitting in magnesium alloys. The corrosion resistance of the laser treated and untreated alloy was investigated by potentiodynamic polarisation and electrochemical impedance spectroscopy. Contrary to the reported behaviour of other magnesium alloys (such as AZ series alloys), laser surface melting did not significantly improve the corrosion resistance of ZE41. This observation is attributed to the absence of beneficial alloying elements such as Al in ZE41 alloy.     

Electrochemical passive properties of AlxCoCrFeNi (x = 0, 0.25, 0.50, 1.00) alloys in sulfuric acids

This study investigates the electrochemical passive properties of Al_xCoCrFeNi alloys in H₂SO₄ by potentiodynamic polarization, EIS, and weight loss tests from 20 to 65 °C. Experimental results indicate that Al harms the corrosion resistance in H₂SO₄ at temperatures exceeding 27 °C owing to the porous and inferior nature of the protection oxide film of Al in these alloys. Closely examining the Arrhenius plots of corrosion current density reveals that both pre-exponential factor A and activation energy E_a increase with Al content. However, A affects corrosion current density more significantly than E_a at higher temperatures and, conversely, at lower temperatures.     

Electrochemical response of AlNiLa amorphous and devitrified alloys

The electrochemical response of Al_94−xNi₆La_x alloys (x = 4, 5, 6, 7) after different stages of devitrification was studied in 0.05 M Na₂SO₄ as well as in different concentrations [0.001 M, 0.01 M and 0.1 M] NaCl solutions. Complementary crystallization studies were carried out to elucidate the composition dependent phase evolution in these alloys. It was observed that the primary crystallization did not cause any deterioration in the corrosion resistance of the alloys as compared to the amorphous alloys. In the case of Al₈₇Ni₆La₇, there was actually an improvement in the passivating ability in benign media. The various primary crystalline phases in the different alloys investigated did not cause different electrochemical responses. However, the onset of secondary crystallization caused a reduction in the corrosion resistance in the NaCl media through a loss in passivating ability of all the alloys. This is due to increased galvanic activity as well as the loss of the amorphous phase.     

In situ impedance spectroscopy study of the electrochemical corrosion of Ti and Ti–6Al–4V in simulated body fluid at 25 °C and 37 °C

The corrosion resistance of Ti and Ti–6Al–4V was investigated through electrochemical impedance spectroscopy, EIS, potentiodynamic polarisation curves and UV–Vis spectrophotometry. The tests were done in Hank solution at 25 °C and 37 °C. The EIS measurements were done at the open circuit potential at specific immersion times. An increase of the resistance as a function of the immersion time was observed, for Ti (at 25 °C and 37 °C), and for Ti–6Al–4V (at 25 °C), which was interpreted as the formation and growth of a passive film on the metallic surfaces.     

Electrochemical behaviour of duplex stainless steels in caustic environment

Recent studies have shown that duplex stainless steels can be susceptible to general corrosion and stress corrosion cracking in high pH caustic environments. This difference in the corrosion resistance can be attributed to changes in the electrochemical behaviour of steels. The present study has shown that the corrosion rates of duplex stainless steels increase with an increase in temperature and sulphide addition to caustic environments. Moreover, alloying Fe with Cr and Ni helps to raise the corrosion potential and lower critical current density of DSS in an alkaline environment whereas Mo can be detrimental to the corrosion resistance of the steel.     

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.