Electrochemical behavior of CrN coating for polymer electrolyte membrane fuel cell

CrN films on a bipolar plate in polymer electrolyte membrane fuel cells have several advantages owing to their excellent corrosion resistance and mechanical properties. Three CrN samples deposited at various radio frequency (RF) powers by RF magnetron sputtering were evaluated under potentiodynamic, potentiostatic and electrochemical impedance spectroscopy conditions. The electrochemical impedance spectroscopy data were monitored for 168 h in a corrosive environment at 70 °C to determine the coating performance at +600 mV_SCE under simulated cathodic conditions in a polymer electrolyte membrane fuel cell. The electrochemical behavior of the coatings increased with decreasing RF power. CrN films on the AISI 316 stainless steel substrate showed high protective efficiency and charge transfer resistance, i.e. increasing corrosion resistance with decreasing RF power. X-ray diffraction confirmed the formation of a CrN(200) preferred orientation at low RF power.

Influence of N2O flow rate on reliability of SiOx films deposited by SiH4-N2O gas mixture plasma

SiO_x films have several advantages as an interlayer dielectric in electronic devices owing to the strong adhesion between SiO_x and the substrate. In this study, the coating performance as a function of the N₂O flow rate was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization tests in an undisturbed environment. In addition, the coatings were examined by atomic force microscopy and Fourier transform infrared reflection spectroscopy. The SiO_x films on a stainless-steel substrate showed the highest coating performance at a N₂O flow rate of 120 sccm. This was attributed to the films having the lowest porosity value among those examined as a result of the fragmentation of SiO and SiO₂ bonds and the improved surface roughness.

Effects of crystallographic orientation on corrosion behavior of magnesium single crystals

The corrosion behavior of magnesium single crystals with various crystallographic orientations was examined in this study. To identify the effects of surface orientation on the corrosion behavior in a systematic manner, single-crystal specimens with ten different rotation angles of the plane normal from the [0001] direction to the [1010] direction at intervals of 10° were prepared and subjected to potentiodynamic polarization and potentiostatic tests as well as electrochemical impedance spectroscopy (EIS) measurements in 3.5 wt.% NaCl solution. Potentiodynamic polarization results showed that the pitting potential (E _pit) first decreased from −1.57 V _SCE to −1.64 V _SCE with an increase in the rotation angle from 0° to 40°, and then increased to −1.60 V _SCE with a further increase in the rotation angle to 90°. The results obtained from potentiostatic tests are also in agreement with the trend in potentiodynamic polarization tests as a function of rotation angle. A similar trend was also observed for the depressed semicircle and the total resistances in the EIS measurements due to the facile formation of MgO and Mg(OH)₂ passive films on the magnesium surface. In addition, the amount of chloride in the passive film was found first to increase with an increase in rotation angle from 0° to 40°, then decrease with a further increase in rotation angle, indicating that the tendency to form a more protective passive film increased for rotation angle near 0° [the (0001) plane] or 90° [the (1010) plane].

Effect of gold on the corrosion behavior of an electroless nickel/immersion gold surface finish

The performance of surface finishes as a function of the pH of the utilized plating solution was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests in 3.5 wt.% NaCl solution. In addition, the surface finishes were examined by x-ray diffraction (XRD), and the contact angle of the liquid/solid interface was recorded. NiP films on copper substrates with gold coatings exhibited their highest coating performance at pH 5. This was attributed to the films having the highest protective efficiency and charge transfer resistance, lowest porosity value, and highest contact angle among those examined as a result of the strongly preferred Au(111) orientation and the improved surface wettability.

Comparative study of ni effect on the corrosion behavior of low alloy steels in FGD and acid rain environments

The alloying effect of a small amount of nickel on low alloy steel for application to flue gas desulfurization(FGD) systems was studied. The structural characteristics of the rust layer were investigated by scanning electron microscopy(SEM). The electrochemical properties were examined by means of potentiostatic polarization test, potentiodynamic polarization test, and electrochemical impedance spectroscopy(EIS) in a modified green death solution of 16.9 vol.% H2SO4+0.35 vol.% HC1 at 60°C and an acid rain solution of 6.25 X 10-5 M H2S04+5.5 X 10-3 M NaCl at room temperature. It was found that as the amount of nickel increased, the corrosion rate increased in the modified green death solution, which seemed to result from micro-galvanic corrosion between NiS and alloy matrix. In acid rain solution, the corrosion rate decreased as the amount of nickel increased due to the repulsive force of NiFe204 rust against Cl-ions by electronegativity.

Rare earth cinnamates and their corrosion inhibition mechanisms for AS1020 steel

Speciation of the inhibitors lanthanum 2-hydroxy cinnamate and lanthanum 3-hydroxy cinnamate in solution has been evaluated and compared to the speciation of lanthanum 4-hydroxy cinnamate. The results have been correlated with corrosion inhibition efficiency for AS1020 steel in an aqueous chloride solution using a combination of analytical tools such as nuclear magnetic resonance (NMR) spectroscopy, electrospray mass spectrometry (ESMS), potentiodynamic polarisation.

In vitro studying corrosion behaviour of biocorrodible Mg alloys

The idea of bioabsorbable/biocorrodible stents has gained increasing attention in the last decade. Permanent coronary stents, traditionally made from 316L grade stainless steel, are routinely used for the treatment of blocked arteries. However, these stents can cause complications such as restenosis, thrombosis and the need for the patient to undergo prolonged antiplatelet therapy. Biodegradable metal stents provide an opportunity for the stent to remain in place for a period to ensure restoration of function and then degrade through a carefully controlled bio-corrosion process. Among the number of potentially suitable materials, Magnesium alloys have shown great promise as a stent material due to their non-toxicity [1] and the corrosion rates attainable in biological environments. However, a carefully controlled corrosion process is essential in order to avoid hyper hydrogen generation and the fatal consequences that follow. In addition uniform corrosion is a basic requirement to maintain the mechanical integrity and load bearing characteristics. Work being undertaken in our laboratories focuses on controlling the corrosion behaviour of magnesium in a simulated biological environment in the presence of protein. In the investigation reported here the Mg alloy has been examined using Scanning Electrochemical Microscope (SECM) to visualize the corrosion process and identify the corrosion pattern. Complementary bulk electrochemical techniques (EIS and potentiodynamic polarization) have been used to acquire kinetic and mechanistic information. Early results obtained by SECM have revealed the tendency towards pitting corrosion in the early stages which subsequently develops in to filiform corrosion.

Effect of nanocrystalline structure on the corrosion of a Fe20Cr alloy

The corrosion behaviour of nanocrystalline and microcrystalline Fe20Cr alloys, prepared by high energy ball milling followed by compaction and sintering, was studied in 0.05M H2SO4 and 0.05M H2SO4 + 0.5M NaCl by potentiodynamic polarization. The nanocrystalline alloy exhibited improved passivating ability and pitting resistance as described by passivation potential, critical current density, passive current density and breakdown potential. XPS and SIMS analysis revealed greater Cr content in the passive film formed on the nanocrystalline form of the alloy. The enhanced passivating ability of the nanocrystalline alloy was attributed to the formation of the passive film with higher Cr content.

Effect of the degree of crystallinity on the electrochemical behavior of Mg65Cu25Y10 and Mg70Zn25Ca5 bulk metallic glasses

The effect of varying the percent crystallinity on the electrochemical behavior of Mg65Cu25Y10 and Mg70Zn25Ca5 bulk metallic glasses was studied. The alloys were heat-treated to achieve desired microstructures ranging from fully amorphous to fully crystalline, providing a systematic basis for subsequent testing. Potentiodynamic experiments in 0.01 M sodium chloride (NaCl) were used, whereby both the amorphous and partially crystallized samples were observed to have more noble corrosion potentials and lower anodic kinetics. However, this was accompanied by more rapid cathodic kinetics relative to their fully crystalline counterparts, meaning that corrosion rates were not significantly lower in the amorphous state. To describe the electrochemical response as a function of the degree of crystallinity, differential scanning calorimetry (DSC), scanning electron microscopy, x-ray diffraction (XRD), and electrical conductivity measurements were undertaken, where it was found that crystallinity alone is not necessarily the controlling factor and microchemistry that evolves upon devitrification, plays a key role in the electrochemical response of these materials.

Selective dissolution of retained austenite in nanostructured bainitic steels

Nanostructured bainitic steels, containing bainitic ferrite laths and retained austenite films, formed at two different isothermal temperatures were compared  for corrosion behavior in chloride-containing solution using electrochemical techniques. The potentiodynamic polarization results suggest that nanostructured bainite formed at 200 °C exhibits marginally higher corrosion resistance compared with that at 350 °C. Post-corrosion analysis of the galvanostatically polarized samples revealed localized corrosion for both the steels, but the degree of attack was higher in the 350 °C steel than in the 200 °C steel. The localized corrosion attack was due to selective dissolution of the retained austenite phase. The higher volume fraction and larger size of retained austenite in the 350 °C steel as compared to that of the 200 °C steel contributed to the pronounced corrosion attack in the 350 °C steel.

Electrosprayed PLGA smart containers for active anti-corrosion coating on magnesium alloy Amlite

A novel self-healing system, consisting of poly(lactic-co-glycolic) acid (PLGA) porous particles loaded with a corrosion inhibitor, i.e. benzotriazole (BTA), has been successfully achieved via direct electro-spray deposition and subsequent epoxy spraying upon magnesium (Mg) alloy AMlite. The two-step process greatly simplified the multi-step fabrication of smart coatings reported previously. The PLGA particles demonstrate rapid response to both water and pH increase incurred by corrosion of Mg, ensuring instant and ongoing release of BTA to self-heal the protective functionality and retard further corrosion. Furthermore, nanopores in the PLGA–BTA microparticles, formed by the fast evaporation of dichloromethane during the electrospray process, also contribute to the fast release of BTA. Using Mg alloy AMlite as a model substrate which requires corrosion protection, potentiodynamic polarisation characterisation and scratch testing were adopted to reveal the anti-corrosion capability of the active coating.

Corrosion of mg alloy ZE41 and its mitigation using ionic liquids

Magnesium alloy ZE41 (Mg-Zn-RE-Zr), which is used extensively in the aerospace industry, possesses excellent mechanical properties albeit poor corrosion resistance. This work investigates the mechanism of corrosion, and the interaction between the grain boundary intermetallic phases, the zirconium (Zr)-rich regions within the grains and the bulk Mg rich matrix in both the as-cast and heat-treated conditions. The results of optical and scanning electron microscopy (SEM) show the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The Zr-rich regions play a distinct role in the early stages of corrosion with this alloy. The second part of this work investigates the interaction of two different ionic liquids (ILs) with the surface of the ZE41 alloy. ILs based on trihexyltetradecylphosphonium (P 6,6,6,14) coupled with either diphenylphosphate (DPP) or bis(trifluoromethanesulfonyl) amide (Tf 2N) have been shown to react with Mg alloy surfaces, leading to the formation of a surface film that can improve the corrosion resistance of the alloy. The interaction of the ILs with the ZE41 surface has been investigated by optical microscopy and SEM. Surface characterization has been performed using Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). The surface characterization and microscopy revealed the preferential interaction with the grain boundaries and grain boundary phases. Thus the morphology and microstructure of the Mg surface seems critical in determining the nature of the interaction with the IL. The corrosion protection of the IL films formed on the ZE41 surface was investigated by SEM and potentiodynamic polarisation.

Electrochemical and quantum chemical assessment of two organic compounds from pyridine derivatives as corrosion inhibitors for mild steel in HCl solution under stagnant condition and hydrodynamic flow

 In this study, the inhibitive performance of two pyridine derivatives as corrosion inhibitors for mild steel was examined under stagnant condition and hydrodynamic flow in HCl solution at 25. °C. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were employed. To explore the inhibitors adsorption mechanism, Langmuir isotherm and quantum chemical studies were used. The results of electrochemical measurements show that the inhibitor concentration has a positive effect on its efficiency while for hydrodynamic condition, it is vice versa. Corrosion attack morphologies were observed at stagnant and hydrodynamic conditions to verify qualitatively the results obtained by electrochemical methods. © 2013 Elsevier Ltd.

The restoration of the passivity of stainless steel Weldments in pickling solutions observed using electrochemical and surface analytical methods

In this study, a solution containing ammonium fluoride (NH4F) and nitric acid (HNO3) was used as an alternative to the conventional highly toxic pickling solution HF/HNO3 for pickling weldments of selected stainless steels including Type 316 stainless steel (UNS S31600), duplex stainless steel 2205 (UNS S32205), and super duplex stainless steel 2507 (UNS S32750). Electrochemical and surface analytical methods were used to understand the effects of pickling on the stainless steel weldments. Cyclic potentiodynamic polarization (CPP) test results indicated that the restoration of passivity of stainless steel weldments could be achieved by pickling the weldments in both HF/HNO3 solution and NH4F/HNO3 solutions. Scanning electron microscopy observation of the UNS S32750 weldment surface revealed that both the HF/HNO3 solution and the NH4F/HNO3 solution could remove the heat tint on the weldment. X-ray photoelectron spectroscopy analysis indicated that treatment in these two pickling solutions produced passive films with similar characteristics. Thus, this work suggests that the NH4F/HNO3 solution is a promising alternative to HF/HNO3 solution for the pickling of stainless steel weldments, and that the CPP test approach can be used in conjunction with surface analytical methods for further development of safer and environmentally friendly picklingsolutions.

Evaluation of corrosion behaviour of biodegradable base oils using electrochemical techniques

© 2015 Institute of Materials, Minerals and Mining. Published by Maney on behalf of the Institute. This paper describes an interesting attempt to quantitatively evaluate the corrosion behaviour of base oils using a novel approach based on electrochemical techniques. The present study evaluates the corrosion behaviour of biodegradable base oils with and without additives in an aqueous chloride solution using electrochemical measurements. Potentiodynamic polarisation and electrochemical impedance spectroscopy techniques were used to quantitatively determine the corrosion behaviour of these oils, and the results were compared to the conventional immersion tests. Both these electrochemical measurements were carried out in a three-electrode system where AS1020 mild steel alloy was used as a working electrode in a purpose made pipette cell. The results obtained from the electrochemical measurements help to evaluate the best biodegradable base oil for formulating eco-friendly industrial lubricants.