Oxide Corrosion Products Research Articles

Flawless polyaniline coating for preservation and corrosion protection of ancient steel spearheads: an archaeological study from military museum, Al-Qala, Egypt

The purpose of this research was to examine the viability of applying a flawless polyaniline coating on steel spearheads to preserve them and protect them from corrosion. The spearpoints, thought to be archaeologically significant, were acquired from the Military Museum in Al-Qala, Egypt. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy were used to characterize the spearheads chemical composition and microstructure (EDX). The spearheads were determined to be constructed of steel and to have a coating of ferric oxide and other corrosion products on their exteriors. After that, a flawless polyaniline coating was electrochemically deposited onto the spearpoints in a way that was both quick and cheap. Many types of corrosion tests, such as electrochemical impedance spectroscopy and potentiodynamic polarization (PDP) readings, were used to determine the coating’s effectiveness. The steel spearheads’ findings revealed a significant improvement in their resistance to corrosion after being coated with flawless polyaniline. The coating served as a barrier, blocking out water and other corrosive substances and slowing the buildup of corrosion byproducts on the spearpoints. In conclusion, our research shows that a flawless polyaniline coating may be an effective anti-corrosion treatment for ancient steel artifacts. The approach is straightforward, cheap, and readily scalable for massive conservation efforts.

The Transition from Used Fuel Container Corrosion Under Oxic Conditions to Corrosion in an Anoxic Environment

The internationally accepted plan for nuclear fuel waste disposal is to store used nuclear fuels within a multi barrier system in a deep geological repository (DGR). Similarly, the DGR concept proposed in Canada involves isolating the used nuclear fuel in copper-coated carbon steel vessels surrounded by bentonite clay buffer boxes and gapfill material. The repository environment is anticipated to evolve from early oxic conditions to later anoxic conditions. Likewise, degradation processes of the UFCs should transition from the initial oxygen-driven processes to those governed by the availability of sulfide as oxidant. This transition may involve the conversion of accumulated (hydr)oxide-type corrosion products produced during the oxic phase to copper sulfide compounds. During this conversion, sulfide species could interact with the UFC surface in several ways depending on the type of (hydr)oxide corrosion products present, the nature and concentration of sulfide species, and other factors: Chemical conversion: in this process substitution reactions replace oxy-anions in the corrosion product layers with sulfide anions. This conversion doesn’t involve redox reactions; therefore, until the conversion is complete, the sulfide species would not drive further corrosion of the Cu surface. Over time, the (hydr)oxide corrosion product layer would be replaced by a sulfide layer.Galvanically-coupled process: in this process sulfides react with Cu metal exposed at the base of pores or cracks in the accumulated corrosion product layer, causing the Cu in those locations to oxidize and copper sulfide to form, while simultaneously expelling oxy-anions and causing cupric or cuprous ions to be reduced elsewhere in the corrosion product layer. Although the conservation of charge in these reactions would mean that no net oxidation of the surface would occur, the spatial separation of the sites where copper is oxidized (on the metal surface) and the locations where reduction occurs (in/on the corrosion product layer) would result in further damage to the Cu layer on the UFC surface, possibly localized at the sites where Cu metal is exposed. Eventually the (hydr)oxide corrosion product layer would be replaced by a sulfide layer and any localization should cease.Direct corrosion: in this process, sulfide species react with the Cu exposed at the base of pores or cracks in the accumulated corrosion product layer, causing further corrosion at these locations without the existing (hydr)oxide species being converted (i.e. process a) or galvanically reduced (i.e. process b). These sulfide species would drive further corrosion of the UFC surface, possibly localized at the sites where Cu metal is exposed. Since this process would not contribute to the removal of the (hydr)oxide corrosion product layer, it is possible that localization, if present, could continue. The research work reported here uses electrochemical and surface characterization methods to evaluate the extent to which each of these possible conversion processes take place, and the timescale/rate of conversion of (hydr)oxide layers grown in different ways. Results so far indicate a fairly rapid and quantitative conversion of oxides according to processes a) and/or b), followed by direct corrosion of the Cu surface by sulfide.

Fate and mechanistic insights into nanoscale zerovalent iron (nZVI) activation of sludge derived biochar reacted with Cr(VI)

While nanoscale zero-valent iron modified biochar (nZVI-BC) have been widely investigated for the removal of heavy metals, the corrosion products of nZVI and their interaction with heavy metals have not been revealed yet. In this paper, nZVI-BC was synthesized and applied for the removal of Cr(VI). Batch experiments indicated that the adsorption of Cr(VI) fit Langmuir isotherm, with the maximum removal capacity at 172.4 mg/g at pH 2.0. SEM-EDS, BET, XRD, FT-IR, Raman and XPS investigation suggested that reduction of Cr(VI) to Cr(III) was the major removal mechanism. pH played an important role on the corrosion of nZVI-BC, at pH 4.5 and 2.0, FeOOH and Fe3O4 were detected as the major iron oxide, respectively. Therefore, FeOOH-BC and Fe3O4-BC were further prepared and their interaction with Cr were studied. Combining with DFT calculations, it revealed that Fe3O4 has higher adsorption capacity and was responsible for the effective removal of Cr(VI) through electrostatic attraction and reduction under acidic conditions. However, Fe3O4 will continue to convert to the more stable FeOOH, which is the key to for the subsequent stabilization of the reduced Cr(III). The results showed that the oxide corrosion products of nZVI-BC were subjected to the environment, which will eventually affect the fate and transport of the adsorbed heavy metal.

Morphology and Phase Composition of Oxide Films and Corrosion Product Deposits in the Steam Generator of a BN-800 Reactor Unit

Morphology and Phase Composition of Oxide Films and Corrosion Product Deposits in the Steam Generator of a BN-800 Reactor Unit

Corrosion Behaviour of Titanium Alloy and Carbon Steel in a High-Temperature, Single and Mixed-Phase, Simulated Geothermal Environment Containing H2S, CO2 and HCl

The corrosion behaviour of a new titanium-based alloy, with nickel, molybdenum and zirconium as the main alloying elements, was studied in a simulated geothermal environment at various phase conditions of a corrosive fluid. Corrosion testing of carbon steel was also conducted for comparison. Both materials were tested at an elevated temperature between 180 and 350 °C and at a 10 bar gauge pressure in H2O containing HCl, H2S, and CO2 gases with an acidic condensate of pH = 3. The study found that the titanium alloy demonstrated good corrosion resistance in a single- and multiphase geothermal environment. In the testing volume, where the boiling of testing fluid occurred, the carbon steel was prone to localized damage of oxide, sulphide and chloride corrosion products. In the superheated testing volume, a homogeneous oxide corrosion layer was observed on the carbon steel. In the testing volume where condensation of the testing fluid occurred, a sulphide layer with an oxide sublayer was formed on the carbon steel.

Lab Case Study of Microbiologically Influenced Corrosion and Rietveld Quantitative Phase Analysis of X-ray Powder Diffraction Data of Deposits from a Refinery.

This paper reports a laboratory-based case study for the characterization of deposits from a crude cooler and reboilers in a Saudi Aramco refinery by microbiologically influenced corrosion (MIC) using microbial, metallurgic, and special analyses and correlates the Rietveld quantitative phase analysis of high-resolution X-ray powder diffraction (XRD) data of scale deposits with microbe compositions. Therefore, rapid in-field microbiological assays could be carried out to assess the potential of MIC. Based on the results, it can be highlighted that the MIC investigation showed that total bacteria and sulfate-reducing bacteria (SRB) were detected in all sampling locations. Methanogens, acid-producing bacteria, and sulfate-reducing archaea were not detected in all samples. Iron-oxidizing bacteria (IOB) were detected in the solid samples from reboilers C and D. Low loads of general bacteria and low levels of microbes with MIC potential were detected in both C and D samples. The trace amount of corrosion products in one sample and the low level of MIC microbes cannot justify the contribution of MIC microbes in the formation of accumulated solids in the system. The findings recommend conducting frequent sampling and analysis including water, oil, and solid from upstream locations to have more decisive evidence of the likelihood of the scale formation and possible contribution of MIC in the formation of deposits in the plant. Subsequently, quantitative phase analysis of XRD data of scale deposits by the Rietveld method revealed that the major phase is calcium sulfate in the form of anhydrate and the minor phases are calcium carbonate in the form of calcite and aragonite, silicon oxide in the form of quartz, and iron oxide corrosion product in the form of magnetite. The results are supported by high-resolution wavelength-dispersive X-ray fluorescence (WDXRF) results. These accurate and reproducible X-ray crystallography findings obtained from Rietveld quantitative phase analysis can guide the field engineers at the refineries and gas plants to overcome the problems of the affected equipment by drawing up the right procedures and taking preventive actions to stop the generation of these particular deposits.

Technology of laser cleaning of steel structures

Cleaning metal surfaces from oxide films, corrosion products, scale and other contaminants to obtain the required quality is one of the basic technologies in many industries. There are traditional methods of cleaning, which in the process of their evolutionary development and improvement have approached the potential technological frontier and do not fully meet the ever-growing demands of the industry. An alternative method is laser cleaning. Goal. The aim is development of modes for laser cleaning of steel structures from rust and technological contamination, which can be used both in stationary and field conditions. Method. To clean the steel structures, we used an installation based on a pulse-periodic CO2 laser with a power of 130 W. To assess the quality of the surface layer, the microhardness method and roughness measurement were used. Results. Under the action of laser radiation on the oxidized surface, reactions of reduction of oxides and evaporation of paint and varnish contaminants occur. The influence of the radiation power and the number of passes during cleaning on the surface quality, which was evaluated by the degree of metallization and roughness, was investigated. Scientific novelty. It was determined that the position of the focal plane above the surface and on the surface is the optimal position of the laser radiation for cleaning. It was found that the microhardness of the surface layer of steel after normalization and after metallization of the surface in the process of laser cleaning is practically the same. It is shown that preliminary wetting of the surface during cleaning from rust using a CO2 laser is not advisable. Water vapor is especially strong in absorbing waves in the infrared range, and carbon dioxide is an intense absorber in the mid-infrared range. Practical significance. Recommendations have been developed for the use of a repetitively pulsed CO2 laser for cleaning the steel structures of bridges, power lines, towers of television and mobile transmitters from rust and technological contamination in field conditions.

γ-radiation effects on metal oxide particles and their wetted surfaces

ABSTRACTTypical metal oxide corrosion products of structural materials have been irradiated with γ-rays in ultra-pure water to investigate the effect of radiation on the surface oxide and the nature of adsorbed water. Analysis techniques including thermal gravimetric analysis, differential thermal analysis, diffuse reflectance infrared Fourier transform spectroscopy, and X-ray photoelectron spectroscopy before and after γ-irradiation were employed to investigate surface structural effects and adsorbed water behaviour. The production of H2 in the oxide nanoparticle mixtures was investigated by gas chromatography to probe the mechanism of radiolysis in the water/oxide mixtures and the relationship with surface water. The nature of water at the surface of the oxides was affected by γ-radiation and the relationship was dependent on the particle composition. The rate of H2 production was shown to be oxide dependent, and higher rates of H2 formation were attributed to the decomposition of surface adsorbed water. Changes to the surface chemistry and H2 production rates were found to be highly dependent on the surface chemistry of the metal oxide nanoparticle and no bulk structural changes were observed.

Electrochemical behavior of CoCrMo alloy for dental applications in acidic artificial saliva containing albumin

The electrochemical properties of CoCrMo alloy immersed in different artificial saliva with or without Ca2+ and albumin were studied by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PP), and meanwhile the microstructures features, phase identification and chemical composition of the alloy were analyzed by SEM, EDS, XRD and RA-IR to further understand the electrochemical behavior of the alloy. The results indicated that the self-passivation behavior of the alloy occurred universally and was obviously distinct with each other in different acidic artificial saliva. No phase transformation was observed and the oxide layer and corrosion products exhibited amorphous nature. There was an obvious complexation of the adsorbed albumin with the alloy, and the adsorption capacity of albumin increased with the prolongation of immersion time. The adsorbed albumin presenting black stripped spots had a certain inhibition to the formation of passivation film, and Ca2+ in saliva promoted the further adsorption of albumin as an intermediate bridge, going against the improvement of the corrosion resistance of passivation film/alloy system. In addition, the passivation state of the alloy surface was changed by different methods to investigate the adsorption behavior of albumin and its influence on the further passivation behavior in depth. Hitherto, we tried to propose a model to explain the dynamic adsorption process of albumin and its influencing mechanism on the growth behavior of passivation film.

Effect of Fe, Ni and Cr on the corrosion behaviour of hypereutectic Al-Si automotive alloy in 3.5% NaCl solution at different temperature and velocity

The main focus of this study is to examine and evaluate the effect of Fe, Ni and Cr on the corrosion behaviour of hypereutectic Al-Si automotive alloy in 3.5% NaCl solution. Experimental information are attained using conventional gravimetric measurements under different solution temperature and velocity and complemented by resistivity, optical micrograph, scanning electron microscopy (SEM) and X-ray analyzer (EDX) investigations. All the alloys show the negative corrosion rate due to formation of a very thin film of aluminum oxide and hydroxid on the surface. Intermetallic particles in the alloys play a major role in passivity.
 Temperature accelerates the corrosion and the higher velocity breakdown of the passive film. It also affects the resistivity of the alloys. The results from this study also indicate that the temperature and velocity increase the negative corrosion rate of the alloys due to higher rate of chemical reactions. SEM and EDX analysis indicate that the surfaces compose with a thin layer of aluminum oxide and hydroxide corrosion products.

Revealing the evolution of microstructure, mechanical property and corrosion behavior of 7A46 aluminum alloy with different ageing treatment

Mechanical property and corrosion behavior of 7A46 aluminum alloy under peak-ageing (T6), double-ageing (DA) and pre-ageing (PA) followed by double-ageing (PA-DA) are systematically investigated. The hardness and strength of aged 7A46 alloy follows an order of T6>PA-DA > DA and the rank of corrosion resistance is PA-DA > DA > T6. The small size and dispersed distribution of matrix precipitates (MPs) lead to the high hardness and strength of T6 alloy, and low hetero-phase potential differences and less formation probability of corrosion galvanic couples enhance the corrosion resistance of PA-DA alloy. Intergranular corrosion (IGC) test results reveal that considerable potential differences promote the formation rate and the quantity of corrosive-batteries between the matrix and grain boundary precipitations (GBPs) or precipitate-free zones (PFZs). The exfoliation corrosion (EXCO) rate and the weight gain trend of corrosion products are related to the oxide film formation rate and corrosion products accumulation. Localized anodic dissolution and hydrogen embrittlement can induce the stress cracking corrosion (SCC).

Characterization of Sludge Deposits from Refineries and Gas Plants: Prerequisite Results Requirements to Facilitate Chemical Cleaning of the Particular Equipment

In this paper, the method developed by the authors to separate the inorganic materials from the hydrocarbon of the sludge deposits, which is fast and can accurately identify very small quantities of inorganic materials, has been extended to characterize the 12 types of sludge samples collected from (a) a regeneration overhead acid gas condenser, (b) water draw-off pump’s suction strainer in a gas plant, and (c) condenser, inside vessels of inlet head, and head coiler tube equipment at gas plants. The results revealed that the major phases are (a) iron sulfide corrosion products with the hydrocarbon type of mixture of diesel and lube oil for a condenser and (b) carbonate scale in the form of calcium carbonate with the hydrocarbon type of lubricant oil for sludge deposits from a suction strainer for pumps, and drilling mud in the form of barium sulfate with no organic hydrocarbon or polymer for sludge samples from a water recycling pump. Moreover, the major phases for inorganic materials built up in a condenser, inside the vessel’s inlet head, and the head coiler tube revealed that iron oxide corrosion products are found in the steam drum, and iron sulfate corrosion products are built up in the condenser. The presence of dissolved oxygen in the boiler feed water is indicated by a high wt% of iron oxide corrosion product in the form of magnetite (Fe3O4), which appeared in the inorganic materials built up in the condenser steam drum. Knowing accurately which phases and their wt% were involved in the inorganic materials can guide the field engineers to facilitate efficient cleaning of the equipment by drawing up the right procedures and taking preventive action to stop the generation of those particular sludge deposits.

Stress Generation Accompanying Intergranular Corrosion of X70 Steel

Stress corrosion cracking (SCC) is a primary degradation mode in steel oil and gas pipelines (1). Pre-emptive detection and control of SCC of pipeline steel in high-pH soils is difficult owing to a prolonged incubation stage of precursory intergranular corrosion (IGC). In high-pH environments, pipeline steel exhibits greatest susceptibility to SCC within a narrow potential range between the active dissolution peak potential and the passivation potential, which may suggest an important role for oxide formation (2,3). In the present work, IGC is characterized by electrochemical and microscopic measurements together with in situ stress monitoring. The latter method is well-suited to detect stress generated by internal grain boundary oxidation (4). Low-carbon ferritic steel samples (X70 type) were cut from a pipeline specimen. After polishing, the specimens were mounted in electrochemical cells with the front surface exposed to solution (aqueous 1 M NaHCO3at pH 8.0-8.2) and the back surface utilized for in situ stress monitoring by curvature interferometry. Prior to each constant potential corrosion experiment, the potential was held at -0.978 V vs. Ag/AgCl to cathodically reduce the native oxide layer. Corrosion morphology was examined with scanning electron microscopy (SEM) and profilometry. The internal IGC morphology was exposed at various depths by shallow-angle polishing. Fig. 1 shows the current transients at three potentials between the active current peak potential and the passivation potential. At the two highest potentials, the current density increased to maxima after initial transients associated to passive film formation, and thereafter slowly decayed. Stress measurements at the two highest potentials showed steady compressive stress generation. At -0.575 V, compressive stress increased for about 10 min and then diminished. SEM revealed that localized grain boundary attack initiated during the current rise. During the current decays at -0.521 V and -0.478 V, a micron-thick granular corrosion product accumulated on the steel surface (A in Fig. 2). This external corrosion product is likely an iron oxy-carbonate precipitate (5), with the current decay due to its mass transport resistance. Examination of the internal IGC morphology at these potentials revealed interconnected voids along grain boundaries and a primarily oxide corrosion product coating the attacked steel grains (B-D). At the greatest depths within the IGC layer (D), the oxide completely filled the corroded grain boundaries. In contrast, no oxidation on grain boundaries was detected at -0.575 V. Figs. 1 and 2 demonstrate that the potential dependence of compressive stress follows that of grain boundary oxide formation. This correspondence is evidence that the measured stress is due to steel oxidation along grain boundaries. Compressive stress generation would be localized close to the depth of maximum IGC penetration, where oxide completely fills the grain boundary (D in Fig. 2). The constant rate of stress generation associated with grain boundary oxide formation is permits IGC penetration into steel. We suggest that grain boundary oxide advance may be assisted by tensile stress in the grain boundary ahead of the oxide front, which accompanies compressive oxide stress. Such tensile stress could enhance the driving force for oxygen ion migration to the uncorroded grain boundary, so as to promote self-sustaining oxide penetration (6). ACKNOWLEDGMENT Financial support was provided by U.S. Department of Transportation, Pipeline and Hazardous Materials Safety Administration under Competitive Academic Agreement Program No. DTPH5614HCAP03 and DTPH5614HCAP01. REFERENCES 1. Y. F. Cheng, Stress Corrosion Cracking of Pipelines, Wiley, 2013. 2. R. D. Armstrong and A. C. Coates, Corros. Sci., 16, 423 (1976). 3. R. N. Parkins and S. Zhou, Corros. Sci., 39, 175 (1997). 4. Ö. Ö. Çapraz, P. Shrotriya, K. R. Hebert, J. Electrochem. Soc., 160, D501 (2013). 5. D. H. Davies and G. H. Burstein, Corrosion, 36, 416 (1980). 6. H. Gao, L. Zhang, W. D. Nix, C. V. Thompson, and E. Arzt, Acta Mater., 47, 2865 (1999). Figure 1

Modeling of Oxide Layer Formation and Corrosion Products Coagulation and Transport in Lead Coolant Using the OXID Module of the HYDRA-IBRAE/LM Code

The results of testing the models implemented in the OXID module of the thermohydraulic code HYDRA-IBRAE/LM, used to calculate an oxide film on a steel surface, transport of dissolved and disperse products, and coagulation of polydisperse solid particles in lead coolant flow, are presented. The module was developed for modeling the formation of an oxide film on the surface of the channels in a reactor facility with heavy liquid-metal coolant and transport of dissolved and disperse products of corrosion and oxidation in the loop.

Influence of oxide scales on the corrosion behaviors of B510L hot-rolled steel strips

The influence of oxide scales on the corrosion behaviors of B510L hot-rolled steel strips was investigated in this study. Focused ion beams and scanning electron microscopy were used to observe the morphologies of oxide scales on the surface and cross sections of the hot-rolled steel. Raman spectroscopy and X-ray diffraction were used for the phase analysis of the oxide scales and corrosion products. The corrosion potential and impedance were measured by anodic polarization and electrochemical impedance spectroscopy. According to the results, oxide scales on the hot-rolled strips mainly comprise iron and iron oxides. The correlation between mass gain and test time follows a power exponential rule in the damp-heat test. The corrosion products are found to be mainly composed of γ-FeOOH, Fe3O4, α-FeOOH, and γ-Fe2O3. The contents of the corrosion products are different on the surfaces of the steels with and without oxide scales. The steel with oxide scales is found to show a higher corrosion resistance and lower corrosion rate.

Shielding heat-resisting coatings for an intermetallic titanium alloy

It is shown to be possible to produce a heat-resisting coating for an intermetallic titanium alloy with the hardening ortho phase preventing changes in the surface layer of the alloy as a result of the build-up of oxide corrosion products, which can soften the component at working temperatures of 700–800°C. The results of investigation of the heat resistance properties, evolution of the composition and structure of the alloyed ion-plasma coatings on the titanium alloy with the hardening ortho-phase at working temperatures show that the deposition of the coating greatly reduces the depth in which the surface layer of the alloy changes and the thickness of this layer does not exceed 5 μm.

Free energies of (Co, Fe, Ni, Zn)Fe2O4 spinels and oxides in water at high temperatures and pressure from density functional theory: results for stoichiometric NiO and NiFe2O4 surfaces

A set of effective chemical potentials (ECPs) are derived that connect energies of (Co, Fe, Ni, Zn)Fe2O4 spinels and oxides calculated at 0 K from density functional theory (DFT) to free energies in high temperature and pressure water. The ECPs are derived and validated by solving a system of linear equations that combine DFT and experimental free energies for NiO, ZnO, Fe2O3, Fe3O4, FeO(OH), CoFe2O4, ZnFe2O4, NiFe2O4 and H2O. To connect to solution phase chemistry, a set of ECPs are also derived for solvated Ni2+, Zn2+, Fe2+ and Fe3+ ions using an analogous set of linear equations and the solid ECPs. The ECPs are used to calculate free energies of low index stoichiometric surfaces of nickel oxide (NiO) and nickel ferrite (NiFe2O4) in water as a function of temperature from 300 to 600 K at a pressure of 155 bar. Surface denuding at high temperatures is predicted, the implications of which for the formation of oxide corrosion products on heat transfer surfaces in light-water nuclear reactors are discussed.

Corrosion Behavior of Sn-3.0Ag-0.5Cu Lead-Free Solder in Potassium Hydroxide Electrolyte

The corrosion behavior of Sn-3.0Ag-0.5Cu (SAC305) solder alloy in 6 M potassium hydroxide electrolyte was investigated using polarization analysis. The results revealed that SAC305 is susceptible to corrosion because of the dissolution of the Sn phase. The corrosion potential (E corr) and corrosion current density (i corr) obtained from the sample was –1.108 V vs Hg/HgO and 1.795 × 10−4 A cm−2, respectively. In addition, microstructural and elemental characterization revealed the presence of tin oxide, Cu, and/or Ag-containing corrosion product on the surface of the corroded sample. The morphology of the samples was also observed to contain several pits, cracks, and pore-like structures.

Failure of a Waterborne Primer Applied to Zinc Coated Steel

The durability of a high‐performance coating system based on a polyester top coat and a water‐based primer system has been studied by exposure of coated panels to salt spray and 100% RH. The humidity exposure is less deleterious than the salt spray with a higher concentration of an adhesion promoter further enhancing durability in each case. XPS analysis establishes that in both cases failure occurs as a result of a weak boundary layer of zinc oxide corrosion product. Further testing of these pre‐exposed panels using a wet adhesion test and a mechanical pull‐off test confirms the superior performance of the formulation with a higher level of adhesion promoter in both tests. The locus of failure is generally in the organic phase, although those specimens at the lower end of the performance spectrum show a significant amount of failure at or close to the coating/substrate interface. For samples pre‐exposed to the salt spray, the presence of chloride ions at the failed interface indicates that such a treatment may have a deleterious effect on subsequent performance. Copyright © 2012 John Wiley & Sons, Ltd.

Influence of freezing rate on microstructure and electrochemical properties of Mg-2%Ga alloys

Equilibrium freezing curve of Mg-2%Ga (mass fraction) alloy was calculated by CALPHALD method. Microstructures of the melted Mg-2%Ga alloys solidified by iron and copper moulds, respectively, were investigated using OM and SEM. Electrochemical properties of the Mg-2%Ga alloys with different freezing rates were measured by galvanostatic, potentiodynamic and electrochemical impedance spectroscopy tests. The results show that solidification by copper mould leads to intergranular Mg 5Ga 2 compounds with small size and large number density. Less adsorbent of Mg + and oxide corrosion products occur on the surface of the Mg-2%Ga alloys solidified by copper mould, producing lower corrosion current density of 1.8×10 −5 mA/cm 2. In the galvanostatic tests with 100 mA/cm 2 current density, more negative stable potential of −1.604 V exists in the Mg-2%Ga alloys solidified by iron mould due to the lower freezing rate, which leads to smaller inductive and capacity time constants as well as shorter activity time and better electrochemical activity.