Corrosion resistance of nanostructured films of titanium diboride in mineral acid solutions

The reactions of nanostructured TiB₂ films (mean grain size is 3.3 ± 1.1 nm) with mineral acids (HCl, H₂SO₄, H₃PO₄, and HNO₃) of different concentrations have been studied. It is found that the dissolution of the films is a congruent process showing linear kinetics for the time periods in the range from 300 to 1500 min. The corrosion depth parameters of nanostructured TiB₂ films in mineral acids are determined, and their corrosion resistance is graded on a ten-point scale. The studied films show the lowest corrosion resistance in their reaction with nitric acid solutions, and the highest resistance is observed in phosphoric acid solutions.     

Corrosion inhibition of copper by potassium ethyl xanthate in acidic chloride solutions

The influence of pH and concentration of potassium ethyl xanthate (KEtX) on the spontaneous dissolution of copper in acidic chloride solutions containing oxygen has been studied. The degree of copper protection increases with an increase in pH and concentration of KEtX. An adherent layer of inhibitor is postulated to account for the protective effect. The adsorption of KEtX has been found to occur on the surface of copper according to the Langmuir isotherm. KEtX is chemically adsorbed on the copper surface.     

Corrosion characteristics of Al‐Si‐Mg/SiCp composites with varying Si/Mg molar ratio in neutral chloride solutions

The corrosion resistance of Al‐Mg‐Si/SiC_p composites produced by the pressureless infiltration method [using SiC_p preforms with 50% porosity containing rice hull ash (RHA) and four custom‐made alloys with varying Si/Mg molar ratio] was evaluated in neutral 0.1 M NaCl solutions. The deleterious phase Al₄C₃ was successfully suppressed in composites with Si/Mg molar ratios of 0.89 and 1.05, but not in those with lower Si/Mg molar ratios (0.12 and 0.49). Results of cyclic polarizations in deareated 0.1 M NaCl solutions showed that with increasing Si/Mg molar ratio, passive current density increased but pitting susceptibility decreased both for reinforced and unreinforced alloys. Immersion tests in aerated 0.1 M NaCl showed that for composites with Si/Mg molar ratios of 0.12 and 0.49 chemical degradation by hydrolysis of Al₄C₃ was followed by intense anodic dissolution at the matrix–reinforcement interface, while composites corresponding to Si/Mg molar ratios of 0.89 and 1.05 did not exhibit intense localized attack. Possible reasons for the improvement in resistance to localized corrosion are discussed.     

Corrosion of iron in highly acidic hydro-organic solutions

In acidic water-organic solvents of ethylene glycol (EGOH), propylene glycol (PGOH), methanol (MeOH) and ethanol (EtOH), iron corrosion was studied by monitoring the corrosion potential, the potentiodynamic polarization curves and electrochemical impedance diagrams. In these aqueous glycols and alcoholic solutions containing HCl having concentrations of 0.5 up to 9 M, it has been shown by electrochemical analysis (I-E curves) that dissolution mechanism of iron is similar to that one in pure acidic aqueous solutions if only we take into account the relative amount of water in the medium. Based in our experimental data, water has an important role in the transfer kinetics of protons to the metallic electrode and limits electro dissolution rate of iron. When water quantity is sufficient at the metal surface, the acidity is a governing factor in the evolution of corrosion process.     

Anodic dissolution of nickel in acidic chloride solutions

The anodic dissolution of nickel was studied galvanostatically in hydrochloric acid solutions of various concentrations. The reaction orders of chloride ion and hydrogen ion concentrations were found to be 0.5 and 1.0, respectively. An anodic Tafel slope equal to 120 ± 10 mV · decade⁻¹ was obtained. The dissolution rate of nickel at constant acid concentration was increased with stirring of the solution and increasing temperature. The activation energy, ΔH, for the anodic dissolution process was found to be 12 kcal · mol⁻¹. The presence of oxygen in solutions assisted the passivation process. The effect of addition of aniline and some of its derivatives (o-, m-, and p-anisidine) as inhibitors on the dissolution kinetics of Ni in 1 M HCl was also investigated. These compounds inhibited the anodic dissolution of nickel without affecting the Tafel slope, indicating that the adsorption of such inhibitors could not interfere with the mechanism of metal dissolution.     

Corrosion of maraging steels in chloride solutions

Abstract

The effect of a maraging heat treatment on the mechanical properties and corrosion behaviour of a special grade of heat treatable stainless steel has been studied. Although the mechanical properties such as strength and hardness of this steel are enhanced after the maraging heat treatment, its corrosion resistance is reduced as a result of the formation of intermetallic precipitates at the grain boundaries. In addition, oxide scales are formed on the surface of the specimens after the heat treatment owing to high temperature oxidation and these scales play an important role in the corrosion behaviour of the aged specimens. Corrosion studies have been conducted on underaged, peak aged, and overaged specimens with and without oxide scales and on specimens in the solution annealed condition. Impedance and potentiostatic anodic polarisation techniques in 3·5%NaCl of pH 1, 4, 7, and 10 at room temperature have been used. The corrosion performance order of this maraging steel with oxide scales in acidic, neutral and alkaline solutions is solution annealed> overaged> underaged> peak aged. The removal of oxide scales from the surface of the aged specimens by electropolishing considerably enhances the corrosion resistance without altering the performance order. The surface treated specimens behave in a similar manner to the solution annealed specimens in chloride solutions of pH 7 and 10. This tremendous increase in the corrosion resistance value is a result of the formation of a passive. film on the surface of the specimens during the surface treatment. In immersion tests the surface treated specimens show no signs of corrosion after immersion in 3·5%NaCl at room temperature for a prolonged period of time.     

Anodic dissolution of iron in acidic chloride solutions

The dissolution of iron in acidic chloride solutions was studied by both rotating disc and impedance techniques. Two regions of different slope were observed in the Tafel graphs after the effects of diffusion were eliminated. The Tafel slope was seen to change from ca. 44 to ca. 85 mV/decade at more positive potentials. The lower slope was confirmed by impedance measurements. An electrode reaction sequence is proposed for the dissolution reaction in the region where the Tafel slope is ca. 44 mV/decade.The addition of decylamine reduced the rate of the dissolution reaction, but the form of the polarization curves and the mechanism of dissolution remained unchanged. It was also established that decylamine is not involved in the electrode reaction of the dissolving polycrystalline iron.     

Corrosion properties of Co-based cemented carbides in acidic solutions

The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H₂SO₄ solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H₂SO₄ solution. The specimen in H₂SO₄ solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution.     

Corrosion behaviour and galvanic coupling of titanium and welded titanium in LiBr solutions

Corrosion resistance and galvanic coupling of Grade 2 commercially pure titanium in its welded and non-welded condition were systematically analyzed in LiBr solutions. Galvanic corrosion was evaluated through two different methods: anodic polarization (according to the Mixed Potential Theory) and electrochemical noise (using a zero-resistance ammeter). Samples have been etched to study the microstructure. The action of lithium chromate as corrosion inhibitor has been evaluated. Titanium and welded titanium showed extremely low corrosion current densities and elevated pitting potential values (higher than 1 V). The results of both methods, anodic polarization and electrochemical noise, showed that the welded titanium was always the anodic element of the pair titanium-welded titanium, so that its corrosion resistance decreases due to the galvanic effect.     

Electrochemical behavior of copper-nickel alloys in acidic chloride solutions

The electrochemical behavior of Cu-Ni alloys in acidic chloride medium was investigated. Commercial Cu-Ni alloys were investigated using potentiodynamic techniques, complemented by electrochemical impedance spectroscopy. The influence of alloy composition, chloride ion concentration and immersion time on the electrochemical response of the alloys was analyzed. Results of present investigations with pure metals (Cu and Ni) are also considered in this paper for the sake of comparison. Potentiodynamic measurements reveal that the increase in nickel content decreases the corrosion rate of the alloy and when the nickel content exceeds 30%, an increase in the corrosion rate was recorded. Also, the corrosion current density increases with increasing the concentration of chloride ions up to 0.6 M.The experimental impedance data were fitted to an equivalent circuit model representing the electrode/electrolyte interface. The relevance of the proposed model to the corrosion/passivation phenomena occurring at the electrode/solution interface was discussed.     

Corrosion inhibition of steel in alkaline chloride solutions

In this paper, the possibility of using di-sodium β-glycerophosphate (GPH) as a new non-toxic inhibitor of the corrosion of steel for concrete reinforcement was investigated. The preliminary tests reported in the paper were carried out in alkaline chloride solution which simulates the characteristics of the electrolyte in the concrete pores.     

Corrosion resistance of thermal sprayed titanium coatings in chloride solution

The corrosion behavior of a resin-sealed flame-sprayed titanium coating in 3.5% NaCl solution was investigated by electrochemical polarization measurements. The composition and structure of the sprayed film was also analyzed by scanning electron microscopy (SEM) and electron probe x-ray microanalysis (EPMA). Although an as-sprayed titanium coating exhibited no resistance to corrosion because of its porosity, the sprayed titanium sealed with epoxy or silicon resin showed an excellent resistivity with respect to chloride corrosion. Although almost half of the titanium changed to oxides, nitrides, and carbides through the wire flame spraying, the conversion of the metal to those compounds had little effect on decreasing the corrosion resistivity. The sprayed and sealed titanium coating obtained by conventional on-site thermal spraying is expected to be an economical material for chloride containing environments. Presented at United Thermal Spray Conference & Exposition ’97, 15–18 September 1997, Indianapolis, IN.     

Corrosion resistance of amorphous aluminium-molybdenum alloys in an acidic chloride environment

Pure aluminium corrodes readily in 1 M HCl solution while its supersaturated alloys with Mo, explored in this work, exhibit excellent corrosion properties due to the oxide barrier film formation on the alloy surface. The influence of Mo content in the alloy on the potential and charge distribution across the system alloy |oxide film| solution was studied in situ using dc and ac electrochemical methods. Passivity, a sudden change in electrode impedance, and the onset of secondary passivity and pitting were discussed in terms of the solute segregation, according to the solute-rich mechanism, change in the alumina structure and solid state processes connected with the transport of point defects and spatial distribution of surface charge.     

Stress corrosion cracking of aluminium brass in acidic chloride solutions

Stress corrosion cracking of aluminium brass in non-ammoniacal acidic chloride solutions has been studied. The most intense transgranular penetration was found in the range of active potentials in synthetic sea water containing 1.5 g/l. of CuCl at pH 1.5, in 1 M sodium chloride solution containing 1.5 g/l. of CuCl at pH 2.0 and in 1 M sodium chloride + 0.25M tri-sodium citrate solution containing 1.5 g/l. of CuCl at pH 4.0. Experimental evidence that high As and high P contents in the alloy can play an important role on the morphology and on the intensity of the corrosion process is presented.     

Corrosion rates of zinc,zinc coatings and steel in aerated slightly acidic chloride solutions calculated from low polarization data

A method is presented whereby electrochemical corrosion rate parameters can be calculated from low polarization data. Charge-transfer polarization portions are manually separated from the potential responses to current steps and computer fitted to an equation representing the polarization curve by an iterative least squares technique. Although accurate computer curve fitting is achieved, electrochemically calculated mass losses are typically 50–60% higher than solution analysis derived mass loss. By including the metal ion deposition reaction, electrochemically calculated mass losses are lower by between 0 and 20%, but this makes the electrochemical method redundant because metal ion concentrations are required to calculate metal equilibrium potentials. Variations in corrosion rate parameters and possible sources of measurement error are discussed.     

Leaching of complex sulphide concentrate in acidic cupric chloride solutions

The chemical analysis of a complex sulphide concentrate by emission spectrometry and X-ray diffraction shows that it contains essentially copper, lead, zinc and iron in the form of chalcopyrite, sphalerite and galena. A small amount of pyrite is also present in the ore but does not be detected with X-ray diffraction. The cupric chloride leaching of the sulphide concentrate at various durations and solid/liquid ratios at 100 ℃ shows that the rate of dissolution of the ore is the fastest in the first several hours, and after 12 h it does not evolve significantly. If oxygen is excluded from the aqueous cupric chloride solution during the leaching experiment at 100 ℃, the pyrite in the ore will not be leached. The determination of principal dissolved metals in the leaching liquor by flame atomic absorption spectrometry, and the chemical analysis of solid residues by emission spectrometry and X-ray diffraction allow to conclude that the rate of dissolution of the minerals contained in the complex sulphide concentrate are in the order of galena 〉 sphalerite 〉 chalcopyrite.     

Anodic oxidation and kinetics of titanium in 1 M chloride solutions

The anodic behaviour of titanium was investigated in 1 M chloride solutions of pH ranging from 0.0 to 12.0 by using electrochemical techniques combined with chemical analysis. No active region appears in solutions of pH above 2.0. In the passive region, the amount of titanium dissolved into the solution during 1 h oxidation decreases with increasing pH, and no dissolution occurs at pH 10.5. The thickness of passive films estimated from the electric charge accumulated on the surface increases linearly with the potential. The kinetic analysis between the anodic current and the film charge indicates that the growth of the passive film obeys the inverse logarithmic rate law, and the kinetic parameters are calculated.     

The cracking of titanium in hot aqueous sodium chloride solutions

Slow tensile straining of ASTM Grade 2 and Grade 12 titanium in 3.5% aqueous sodium chloride has revealed hydride penetration and associated cracking only in Grade 12 when the testing temperature is above about 150°C, a significant cathodic polarization is applied, and the nominal strain rate is < 10⁻⁶ s⁻¹. Similar embrittling effects are also observed as a result of long-term soaking after straining Grade 12 titanium beyond the yield point. The observations are explained on a basis of the penetration of hydrogen into the metal when passivity is locally broken down during slow straining. Considerable cathodic polarization is necessary to promote the appropriate hydrogen fugacity at exposed metal surfaces and the higher temperatures promote more rapid diffusion of hydrogen into the metal. The hydride channels that form at breaks in the passive film provide ready paths for crack propagation, and loss in ductility is accompanied by secondary cracking, both of which increase with increase in either the temperature or the time available for hydride penetration.     

Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions

The corrosion behavior of cold isostatically pressed (CIP) high purity alumina ceramics in aqueous HCl and H₂SO₄ solutions with various concentrations has been studied simultaneously at room temperature (25 °C). Corrosion tests were also performed with 0.65 mol/l HCl and 0.37 mol/l H₂SO₄ solutions at 40, 55 and 70 °C for 48 h. Chemical stability was monitored by determining the amount of Al³⁺, Mg²⁺, Ca²⁺, Na⁺ Si⁴⁺ and Fe³⁺ ions eluted in different concentrations of HCl and H₂SO₄ solutions by means of atomic absorption spectrometry (AAS). By increasing the concentration from 0.37 to 6.5 mol/l, it was notified that the corrosion susceptibility in HCl and H₂SO₄ solutions for the CIP alumina specimens at room temperature decreases.     

Corrosion rate of magnesium and its alloys in buffered chloride solutions

The effect of the buffer capacity of test solution on the corrosion rate of cast pure (>99.95 mass%) and high-purity (>99.9999 mass%) magnesium, and AZ31 and AZ91E has been studied. Their corrosion rates were measured gravimetrically in a pH 6.5 and a pH 9 borate buffer including chloride ions, and in a conventional chloride solution. Except for the AZ91E in the pH 6.5, the corrosion rates of all the examined materials depended solely on the pH of the test solution, although the materials had a variety of purity and alloying elements. Higher buffer capacity probably masked the detrimental effect of the “cathodic impurities”. The corrosion rates measured in the buffers were considered as giving the resistivity of the passive film to anodic reactions. The high-purity magnesium had a fair corrosion resistance even in conventional chloride solution.