Anodic Behaviour of 17–7 Precipitation Hardening Stainless Steel in Acid Media

Abstract

The anodic behaviour of 17–7 precipitation hardening stainless steel in 0.5 M H₂SO₄ and 0.5 M H₂SO₄ + Cl^? solutions was investigated in detail. The coulometric areas of the two anodic peaks in the potentiodynamic curve in the chloride-free solution are dependent upon the duration of the free corrosion between the preliminary cathodic activation and the starting of the potentiodynamic scan. When the free corrosion time exceeds 6 minutes the peak at the more active potential disappears completely.

The influence of the free corrosion time was much less evident in the presence of chloride ions; in these conditions anodic currents were greater.

It was also established that the two anodic peaks correspond to the active-passive transitions relative to the two phases composing the steel: austenite and δ-ferrite. This was seen by potentiostatically etching the steel in the active dissolution ranges of both anodic peaks.

Finally it was shown, both micrographically and by comparing the anodic curves of pickled and polished specimens, that ordinary nitric–hydrofluoric pickling solutions selectively attack the austenite.     

Poly(N-methylaniline) coatings on stainless steel by electropolymerization

Poly(N-methylaniline) (PNMA) coatings have been electropolymerized on 304 stainless steel alloy by potentiodynamic, galvanostatic and potentiostatic synthesis techniques from aqueous solutions of 0.1 M N-methylaniline (NMA) and 0.3 M oxalic acid. Characterization of PNMA coatings was carried out by cyclic voltammetry, UV-Vis and FTIR spectroscopy techniques. Corrosion behavior of PNMA coated stainless steel electrodes was investigated using linear anodic potentiodynamic polarization, Tafel test, chronoamperometry and electrochemical impedance spectroscopy (EIS) techniques in 0.5 M aqueous HCl solutions. Corrosion test results showed that PNMA coatings possessed protection to uncoated stainless steel against corrosion.     

Inhibitory efficiency of oxalic N-phenylhydrazide-N′-phenylthiosemicarbazide on corrosion of carbon steel in 0·5M H2SO4

The inhibitory effect of an oxalic type organic compound oxalic N-phenylhydrazide-N′-phenylthiosemicarbazide on the corrosion of carbon steel in 0·5M H₂SO₄ aqueous solution was investigated by different experimental methods; gravimetry, voltammetry and electrochemical impedance spectroscopy. The surface state of the electrode after immersion of the steel coupons in corrosion test solutions, in absence and in presence of the inhibitor was characterised by scanning electron microscopy. The potentiodynamic polarisation measurements showed that the presence of the inhibitor in 0·5M H₂SO₄ decreased both cathodic and anodic processes. The action of this inhibitor is therefore of a mixed type. The electrochemical impedance measurements showed the adsorption of the inhibitor. The inhibitory efficiency reaches a value of 95% at an inhibitor concentration of 10^?3M.     

Corrosion behavior of hafnium in anhydrous isopropanol and acetonitrile solutions containing bromide ions

The corrosion behaviors of hafnium in Et₄NBr isopropanol and acetonitrile(ACN) solutions were investigated using electrochemical measurements, ICP-AES and SEM techniques. Results revealed that the open circuit potential gets more positive due to the increased passivity of the surface oxide film with increasing immersion time until it reaches a steady state value. The potentiodynamic anodic polarization curves did not exhibit an active dissolution region near corrosion potential due to the presence of an oxide film on the electrode surface, which was followed by pitting corrosion. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allowed the pitting potential (?_pit) and the repassivation potential (?_p) to be determined. ?_pit increased with increasing potential scanning rate but decreased with increasing temperature, Br^? concentration and ACN concentration. The impedance spectra showed that the resistances of the solution and charge transfer decreased with the increase of ACN concentration.     

Spontaneous and anodic oxidation of Tantalum and Zirconium in phosphate solutions

Spontaneous and anodic oxidation of Ta and Zr have been studied in 0.1 M solutions of H₃PO₄, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄. Below 10 A/cm² the dependence of the spontaneous-oxidation rate on potential follows a Tafel relation. Te results indicate that the partial anodic reaction of film formation is balanced by the cathodic reduction of dissolved oxygen. The kinetics of anodic oxidation are discussed on the basis of a one phase oxide file liable to contamination by phosphate anions. Oxide growth follows the high field ionic conduction even below the oxygen evolution potential. The simple exponential relation of Güntherschulze and Betz is valid. This is supported by the linearity obtained between (1) reciprocal capacity and the logarithm of current density i and (2) the potential E and log i for oxide formation at constant charge. Approximate calculations of the effective activation distance show that the degree of contamination by anions is the same in the four equimolar solutions indepent of PH.     

Role of ClO4 in breakdown of tin passivity in NaOH solutions

The anodic behaviour of a tin electrode in NaOH solutions containing different concentrations of NaClO₄ was studied by employing potentiodynamic, potential transient under constant current density methods and complemented with scanning electron microscopy (SEM). In perchlorate-free NaOH solutions, the E/i response exhibits active/passive transition. The active region involves two anodic peaks corresponding to the formation of Sn(II) and Sn(IV) species respectively. The permanent passive layer is duplex and consists of SnO and SnO₂. Additions of NaClO₄ to the alkali solution, accelerates the active dissolution of tin and tends to breakdown the duplex passive layer at a certain breakdown potential. SEM examination confirms the occurrence of film breakdown. The breakdown potential decreases with an increase in ClO₄⁻ concentration, but increases with increasing both OH⁻ concentration and scan rate. The potential-time transients display that the incubation time for pit initiation decreases with increasing both ClO₄⁻ concentration and anodic current density.     

Anodisches Verhalten und Korrosion von Titan in methanolischen Lösungen

Anodic behaviour and corrosion of titanium in methanolic solutions The anodic behaviour of titanium in methanolic solutions containing halides or water in various concentrations has been investigated and the result were compared with the result obtained by corrosion tests performed on ?U”? bend specimens in similar solutions. Polarization curves can predict the susceptibility to titanium to stress corrosion cracking as in the case the metal surface is not passivable. These conditions can be obtained in the presence of low water content (i.e. 0.1% H₂O) and of activating ions as Cl^? and Br^? even in very small concentration (10^5?, 10^4? M). Presence of higher water contents and/or of F^? or I^? ions can to some extent produce passivation of the metal surface and cracking does not occur. A lower water amount in the solution (i.e. 130 ppm) eliminates any possibility of passivation of the metal surface, so that cracking can occur even in the absence of chlorides or bromides. High Cl^? concentration in the solution. (i.e. 0.1 M NaCl + 0.1% H₂O) can produce intergranular attack even in the absence of applied stress.     

Investigation of wear behavior of titanium oxide films, produced by anodic oxidation, on commercially pure titanium in vacuum conditions

The wear performances of titanium oxide films, produced by anodic oxidation, in vacuum conditions were investigated. Anodic oxidation treatments were carried out at − 3 and 40 °C temperatures using H₂SO₄(1.5 M)-H₃PO₄(0.3 M) solution and voltage of 200 V. Wear tests were performed at ambient air, pressures of 10^− 3 mbar and 10^− 6 mbar. Anodizing process produced a porous oxide layer on the surface. Although the pore size decreased with increasing process temperature, surface roughness decreased. Hardness results showed that anodic oxidation at lower temperatures produced oxide film with higher hardness. Wear rate of CP-Ti significantly decreased with anodic oxidation treatment in all wear conditions since oxide film acted like a solid lubricant. The best wear resistance was obtained from the hard-anodized samples both in ambient air and vacuum conditions.     

Electrochemical studies on the electrodeposited Zn-Ni-Co ternary alloy in different media

The electroplating of ternary Zn-Ni-Co alloy, surface morphology and corrosion resistance were investigated and contrasted with the characteristics of Zn-Ni electrodeposits. The investigation of electrodeposition was carried out using cyclic voltammetry and galvanostatic techniques, while potentiodynamic polarization resistance and anodic linear sweeping voltammetry techniques were used for corrosion study. Under the examined conditions, the electrodeposition of the alloy was of anomalous type. It was found that the obtained Zn-Ni-Co alloy exhibited more preferred surface appearance and better corrosion resistance compared to Zn-Ni alloy that electrodeposited at similar conditions. During the cathodic scan of cyclic voltammetry, a cathodic peak at − 574 mV is appeared and correlated with the deposition of sulfur liberated from the reduction of sulphate group in the presence of H⁺. Up to four anodic peaks were obtained by cyclic voltammetry technique, two correlated with zinc oxidation from pure deposited Zn and γ-Ni₅Zn₂₁ phases and two correlated with oxidation of cobalt and nickel, were observed. The phase structure, surface morphology and chemical composition of the deposits were characterized by means of X-ray diffraction analysis, scanning electron microscopy and atomic absorption spectroscopy, respectively.     

Evaluation of 2-mercaptobenzimidazole as corrosion inhibitor for mild steel in phosphoric acid

The inhibiting action of 2-mercaptobenzimidazole on the corrosion of mild steel in phosphoric acid (H₃PO₄) solution has been investigated by weight loss and polarization. The studies reveal that the inhibitor is effective for the inhibition of mild steel over a wide concentration range of H₃PO₄ solutions and retards the anodic and cathodic corrosion reactions with emphasis on the former.     

Removal of magnesium and calcium from electric furnace titanium slag by H3PO4 oxidation roasting–leaching process

H₃PO₄ oxidation roasting followed by HCl acid leaching was proposed to remove magnesium and calcium from electric furnace titanium slag containing 3.12% MgO and 0.86% CaO. XRF, XRD and SEM techniques were used to characterize the composition, mineral phase component and microstructure of the titanium slag. The H₃PO₄ oxidation thermodynamic, mineral phase transformation, microstructure, element distribution in titanium slag during H₃PO₄ oxidation process and leaching process were investigated. The thermodynamic analysis indicated that H₃PO₄ could promote the decomposition of MgTi₂O₅ and CaSiO₃. The results indicated that H₃PO₄ could effectively promote the transformation of titanium-bearing mineral to rutile and enrich the impurities in M_xTi_3–xO₅ into phosphate which could be removed by acid leaching process. Under the studied conditions, the leaching rates of magnesium and calcium reached 94.68% and 87.19%, respectively. The acid leached slag containing 0.19% MgO and 0.13% CaO (mass fraction) was obtained.     

Corrosion behavior of iron thin film in deaerated phosphate solutions by an electrochemical quartz crystal microbalance

The corrosion rates of iron thin film in deaerated phosphate solutions were measured by an electrochemical quartz crystal microbalance as a function of solution pH or phosphorus concentration in solutions. The dependences of corrosion rate and corrosion potential on solution pH and phosphorus concentration have suggested that the corrosion mechanism changes in the vicinity of pH 5 and H₂PO₄⁻ contributes to both anodic dissolution of iron and hydrogen evolution. The corrosion mechanism which contains iron-phosphate-hydroxide complex ion as an adsorbed intermediate was proposed to explain the experimental results.     

Study of inhibition mechanism and efficiency of indole-5-carboxylic acid on corrosion of copper in aerated 0·5M H2 SO4

Abstract

Inhibition of the corrosion of copper in aerated 0·5M sulphuric acid solutions containing various concentrations of indole-5-carboxylic acid was studied in the temperature range 25–55°C using potentiodynamic curves (Tafel lines), weight loss, analytical methods, and determination of double layer capacitance. The corrosion rates reveal good corrosion inhibition, up to 95% in the concentration range of 1 × 10^-4–4 × 10^-3M. Tafel anodic slopes in inhibited acid solutions are considerably higher than those in uninhibited acid solutions. This points to a change in the corrosion mechanism of copper in the presence of indole-5-carboxylic acid. Under these conditions, copper could electro-oxidise primarily to Cu⁺ rather than to Cu²⁺, forming slightly soluble [Cu–(indole-5-carboxylic)_n ]⁺_(ads) complexes. Corrosion rates determined by the weight loss method in both the absence and presence of inhibitor are much higher (on average by a factor of about 6) than those obtained with the potentiodynamic method. This points to a limitation of the Tafel line extrapolation method in corrosion rate determination. The double layer capacitance–potential curves indicate considerable adsorption of the inhibitor over a wide potential range (-600 to +200 mV with regard to E_corr ).     

铜在磷酸溶液中的电化学抛光研究

通过阳极极化和电化学阻抗谱测试,研究了铜在磷酸溶液中进行电化学抛光的电化学行为。研究发现:随着磷酸浓度的增加,铜在0.2,0.4,0.6,0.8 V四个电位下电化学抛光后的粗糙度都呈现先减小、后增大的趋势,在磷酸质量分数为55%时达最低值;EIS图谱拟合的Rs值的变化反映了磷酸盐粘膜层电阻和铜表面氧化膜电阻的变化,此外,随着磷酸浓度的增加,EIS中第一个容抗半圆的弛豫时间延长,铜的溶解反应速度加快。     

Potentiodynamic behaviour of the nickel electrode in acid media

The electrochemical behaviour of nickel has been investigated in H₂SO₄ and HCl solutions of various concentrations by means of the potentiodynamic method. In very dilute solutions, the low anodic current is controlled by the presence of pre-existing film. At higher concentrations the film is removed and dissolution occurs. In H₂SO₄ solutions, pseudopassivity, primary passivity, transpassivity and secondary passivity are observed prior to oxygen evolution. In all these regions, the rate of dissolution increases with acid concentration. In HCl solutions, the anodic polarization curves exhibit a current peak followed by a passivation region; the latter may not involve complete passivity. In the active region, the uniform dissolution is controlled by diffusion processes. In the passive region, local attack in the form of pitting occurs. In both acid solutions, the cathodic and anodic polarization curves satisfy Tafel's relation.     

Inhibiting effects of butyl triphenyl phosphonium bromide on corrosion of mild steel in 0.5 M sulphuric acid solution and its adsorption characteristics

Butyl triphenyl phosphonium bromide (BuTPPB) has been evaluated as a corrosion inhibitor for mild steel in 0.5 M H₂SO₄ solutions using galvanostatic polarisation and potentiostatic polarisation measurements. The study was also complemented by infra red (IR) spectroscopy, scanning electron microscopy (SEM) and quantum chemical calculations. Galvanostatic polarisation measurements showed that the presence of BuTPPB in aerated 0.5 M H₂SO₄ solutions decreases corrosion currents to a great extent and the corrosion rate decreases with increasing inhibitor concentration at a constant temperature. At 298K, inhibition efficiency was found to be 94.5% for 10⁻⁷ M BuTPPB which increased to about 99% for the BuTPPB concentration of 10⁻² M. The effect of temperature on the corrosion behaviour of mild steel was studied at five different temperatures ranging from 298 to 338K. The polarisation curves clearly indicate that BuTPPB acts as a mixed type inhibitor. Adsorption of BuTPPB on the mild steel surface follows the Langmuir isotherm.Potentiostatic polarisation measurements showed that passivation was observed only for lower BuTPPB concentrations (10⁻⁵ and 10⁻⁷ mol l⁻¹) for the mild steel in 0.5 M H₂SO₄. IR and SEM investigations also confirmed the adsorption of BuTPPB on the mild steel surface in 0.5 M H₂SO₄ solutions. The molecular parameters obtained using PM3 semi-empirical method, were correlated with the experimentally measured inhibitor efficiencies.     

An XPS study of passive films on Nickel and alloy 600 in acids

Compositions of surface films formed on nickel and Alloy 600 in I M HCI, 0.5 M H₂SO₄ and I/3 M H₃PO₁ solutions were investigated as a function of polarization potential. The main constituent of surface films formed on Ni in 0.5 M H₂SO₄ or 1/3 M H₃PO₄ solution was hydrated nickel oxyhydroxide, in which the ratio of O₂ to OH⁻ increased when passivation occurred. The surface films formed on Ni and Alloy 600 at lower potentials in 0.5 M H₂SO₄ solution contain S² ions other than SOP_4²⁻ ions, whereas S²⁻ ions were not incorporated in the passive film. Passivation of Alloy 600 took place by the formation of hydrated chromium oxyhydroxide. Pitting led to no substantial change in the average composition of the film.     

Corrosion inhibition of carbon steel in tetra-n-butylammonium bromide aqueous solution by benzotriazole and Na3PO4

The corrosion inhibition behavior of benzotriazole, Na₃PO₄ and their mixture on carbon steel in 20 wt.% (0.628 mol l⁻¹) tetra-n-butylammonium bromide aerated aqueous solution was investigated by weight-loss test, potentiodynamic polarization measurement, electrochemical impedance spectroscopy and scanning electron microscope/energy dispersive X-ray techniques. The inhibition action of BTA or SP or inhibitors mixture on the corrosion of carbon steel is mainly due to the inhibition of anodic process of corrosion. The results revealed that inhibitors mixtures have shown synergistic effects at lower concentration of inhibitors. At 2 g l⁻¹ BTA and 2 g l⁻¹ SP showed optimum enhanced inhibition compared with their individual effects.     

Electrochemical corrosion behavior of electroless Ni–P coating in NaCl and H2SO4 solutions

Electrochemical corrosion behavior of electroless Ni–P coating in NaCl and H₂SO₄ solutions were studied by potentiodynamic polarization curves and electrochemical impedance spectra techniques, as well as the corrosion morphology was characterized. The results indicate that electroless Ni–P coating with about 25 µm is stable in 30 days immersion in NaCl solution. Although it was corroded with prolonged immersion days, the corrosive medium has not penetrated through the coating. During the H₂SO₄ concentration ranging from 5 to 10%, the corrosion current density of electroless Ni–P coating increased due to the intensified anodic dissolution process; in 15% H₂SO₄ solution, electroless Ni–P coating shows obvious anodic passivation effect.     

A newly synthesized glycine derivative to control uniform and pitting corrosion processes of Al induced by SCN anions - Chemical, electrochemical and morphological studies

A newly synthesized glycine derivative (termed GlyD), 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride, was used to inhibit uniform and pitting corrosion processes of Al in 0.50 M KSCN solutions (pH 6.8) at 25 °C. For uniform corrosion inhibition study, Tafel extrapolation, linear polarization resistance and impedance methods were used, complemented with SEM examinations. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectrometry) was also used to test validity of corrosion rate measured by Tafel extrapolation method. GlyD inhibited uniform corrosion, even at low concentrations, reaching a value of inhibition efficiency up to 97% at a concentration of 5 × 10⁻³ M. Results obtained from the different corrosion evaluation techniques were in good agreement. This new synthesized glycine derivative was also used to control pit nucleation and growth on the pitted Al surface based on cyclic polarization, potentiostatic and galvanostatic measurements. The pitting potential (E_pit) and the repassivation potential (E_rp) increased by the addition of GlyD. Thus GlyD suppressed pit nucleation and propagation. Nucleation of pit was found to take place after an incubation time (t_i). The rate of pit nucleation and growth decreased with increase in inhibitor concentration. Morphology of pitting was also studied as a function of the applied anodic potential and solution temperature. Cross-sectional view of pitted surface revealed the formation of large distorted hemispherical and narrow deep pits. GlyD was much better than Gly in controlling uniform and pitting corrosion processes of Al in these solutions.