The effects of anions on the behaviour of scratched iron electrodes in aqueous solutions

Two distinct types of electrochemical behaviour are observed when iron electrodes held at constant potential are rapidly scratched in the presence of reactive anions in aqueous solutions. The results presented here deal with the effects of bicarbonate, chloride and phosphate ions in alkaline solutions. Chloride and phosphate each accelerate the first oxidative step in the reaction by direct formation of adsorbed surface complexes with Fe(I). Bicarbonate does not accelerate the first oxidative step and does not complex with Fe(I) at a significant rate. It does, however, enter the second oxidative step by reacting at a significant rate with adsorbed FeOH to give an Fe(II) complex, thereby maintaining a bare reaction surface at higher potentials. The results are compared with those obtained in non-aggressive electrolytes of similar pH. In all cases the Fe(I) intermediate forms to complete monolayer coverage at potentials below the Fe/Fe(II) reversible potentials.     

Repassivation behaviour of stressed aluminium electrodes in aqueous chloride solutions

The combined influence of mechanical stress and chemical environment on the pitting susceptibility of aluminium electrodes was studied by integrating open circuit potential measurements and electrochemical repassivation modelling. The transient degradation of aluminium oxide owing to a tensile stress field and its subsequent repassivation are observed as an abrupt negative potential excursion followed by a slow decay of the corrosion potential back towards its initial value prior to the mechanical loading. Experiments were performed and an electrochemical repassivation model was established to quantitatively describe the dependence of oxide dissolution rate and the reciprocal of cathodic Tafel slope on chloride concentration.     

The influence of sulphate,chloride and nitrate anions on the cyclic strain-enhanced dissolution behaviour of mild steel

The transient electrochemical behaviour of mild steel during cyclic plastic deformation in 0.5 M aqueous solutions of sulphate, chloride and nitrate ions was studied in the pH range 3–14. In all solutions dissolution current transients were observed during both tensile and compressive strain cycles. The magnitude of the dissolution effects, however, depended upon the solution composition, pH, potential and the extent of strain cycling. Analysis of the current transient characteristics revealed that for the anion concentration employed in this study, sulphate appeared to exhibit more aggressiveness than chloride and both these exerted a greater disruptive influence on surface films than nitrate ions. Tests in 0.25 M Na₂SO₄ revealed that the effect of sulphate could not be attributed simply to the increased ionic strength of the solution.     

The influence of some aldehydes on the corrosion and anodic behaviour of aluminium in sodium hydroxide solution

A set of aliphatic and aromatic aldehydes has been examined for their effectiveness in preventing the corrosion of aluminum (2S) in 1M NaOH solution and their influence on the anodic polarization behaviour. It has been found that the aromatic compounds are generally more effective as corrosion inhibitors than the aliphatic inhibitive nature of a solution.     

Effect of small amounts of alloyed tin on the electrochemical behaviour of aluminium in sodium chloride solution

Binary model AlSn alloys containing 30–1000 ppm (by weight) Sn were investigated by electrochemical polarization in 5 wt% chloride solution and subsequent characterization of corrosion morphology. In the homogenized and rolled condition, tin concentration only slightly affected electrochemical behaviour up to 500 ppm, and the pitting potentials were all about −0.8 V_SCE. However, alloy containing 1000 ppm Sn was significantly activated by lowering of the passivity-breakdown potential to −1.38 V_SCE. Annealing at 300 °C caused significant segregation of Sn to the metal surface, and all specimens, independent of bulk Sn concentration, became nearly similarly active with breakdown potentials around −1.2 V_SCE. Corrosion on 300 °C-annealed specimens was uniformly distributed by polarization below the bulk pitting potential of −0.76 V_SCE. Moreover, the activation effect was temporary, and corrosion was significantly reduced as the segregated Sn was etched away from the surface. Thick oxide, formed during water quenching on high Sn concentration samples, containing 500 and 1000 ppm Sn, introduced partial passivation during polarization test. Annealing at 600 °C caused increasing activation with increasing Sn concentration, caused by Sn enrichment at the metal surface by dealloying of aluminium during anodic polarization in chloride solution. Corrosion was localized in the form of grain boundary corrosion for alloyed Sn concentration less than and equal to 500 ppm and pitting following the triple grain boundaries for 1000 ppm.     

The corrosion of aluminium in solutions of sodium fluoride and sodium chloride

Abstract

The corrosion of 99·997% aluminium by 0·1 N-sodium fluoride and chloride has been examined in the presence of air over the pH range 2—13. When a solution of either salt was saturated with aluminium oxide, corrosion was arrested.

Under anaerobic conditions, 0·1 N-sodium fluoride attacked the air-formed oxide film and corrosion was rapid, in contrast no corrosion was observed in 0·1 N-sodium chloride solution.     

Electrochemical synthesis of polypyrrole on aluminium in different anions and corrosion protection of aluminium

In this study, polypyrrole was deposited on the aluminium in different anions (CO₃²⁻, NO₂⁻, CrO₄²⁻, DBS⁻). The contribution of anions to formation of polypyrrole film was investigated by using cyclic voltammetry (CV) technique. The effect of polypyrrole film on the corrosion of aluminium was searched in 0.1 M HCI solution by using potentiostatic method. For this purpose, polarization curves were obtained, corrosion current density (icorr), corrosion potential (Ecorr), polarization resistance (Rp) were determined from the polarization curves. Moreover, the percent efficiency of coating was calculated. The complexes of pyrrole pentamers and different anions (CO₃²⁻, NO₂⁻, CrO₄²⁻, DBS⁻) were studied using ab initio quantum chemical at the Hartree-Fock (HF) levels with STO-3G, 3-21G, 6-31G(d,p) [13] basis sets and HOMO-LUMO energy gap is calculated by B3LYP method with 3-21G* and 6-31G(d,p) basis sets. The polypyrrole film obtained in CrO₄²⁻ anion is determined to be the most effective in prevention to pitting corrosion of aluminium as experimental and theoretical.     

The electrochemical behaviour of mild steel in phosphate-borate-sulphide solutions

The localized corrosion of mild steel in neutral buffered solutions containing sodium sulphide is studied using potentiostatic and potentiodynamic techniques complemented by scanning electron microscopy and energy dispersive X-ray analysis. The breakdown potential depends linearly with the logarithm of the sodium sulphide concentration. At potentials close to ? 0.70 V (SCE), the nucleation and growth of a ferrous sulphide layer replacing the initial ferrous hydroxide film occurs prior to pit growth. In the ? 0.70 to ? 0.60 V range the logarithm of the induction time for pit initiation decreases linearly with the reciprocal of the applied potential. At potentials more positive than ? 0.60 V the oxide formation hinders pit growth leading to a partial passivation. The results suggest that two-dimensional salt islands are formed before pitting as it has been postulated for iron in halide-containing solutions.     

The electrochemical study of pitting corrosion of aluminium in chloride solutions

The electrochemical behaviour of pure Al in different Cl^? media similar to those formed inside growing pits have been investigated. Artificial pit measurements confirm the previous assumption that saturated AlCl₃ must be formed at the pit bottom as a condition for the attack. Experimental results support the idea that a competitive process involving passivation and activation can occur depending on the potential, the pH and the Cl^? concentration. Rather than the potential for pitting initiation, the pitting potential of Al appears to be the point at which the passivation phenomena become very slow.     

The influence of sn on the electrochemical behaviour of A1

The electrochemical behaviour of an AI electrode containing small amounts of Sn has been studied in an acetate buffer of pH = 5.25. The potentiostatic step method was used. The experiments showed that two types of oxide are formed, depending on the potential at which the oxide was formed and on the Sn content of the Al. The cathodic hydrogen evolution reaction was dependent on the oxide type formed, while the anodic dissolution reaction was independent of the oxide type. The cathodic reaction showed two Tafel slopes which indicated that two cathodic reaction mechanisms occurred. An explanation of the results based on the non-stoichiometry of the oxide is given.     

The influence of zinc cations on steel oxidation in nitrate solutions

The influence of zinc nitrate additives on the protective properties of the magnetite coating (MC) obtained by the oxidation of low-carbon steel in the nitrate solution at temperatures of 96–98°C has been studied by physical, electrochemical, and corrosion methods. It has been shown that, under these conditions, iron zinc spinel ZnFeO₄, which is introduced into an MC, can be formed. Low concentrations ($ C_{Zn(NO_3 )_2 } $ C_{Zn(NO_3 )_2 } = 25–150 mg/l) of it increase the corrosion stability of steel with MC in water containing chlorides and sulfates and also at the periodic moisture condensation of the sample surface. At $ C_{Zn(NO_3 )_2 } $ C_{Zn(NO_3 )_2 } > 500 mg/l, MC formation stops.     

Spectrophotometric study of corrosion inhibition of aluminium in orthophosphoric acid aqueous solutions by using sodium molybdate

ABSTRACT

The kinetics of the corrosion of aluminium in the orthophosphoric acid aqueous solution was investigated spectrophotometrically and gravimetrically. Sodium molybdate was applied as the inhibitor of the corrosion process. The analysis of the results obtained indicated that there are at least two species that might be useful for the inhibition of the corrosion. These are the products from the reduction of molybdophosphoric acid with aluminium. The mechanism of the inhibition of the corrosion depends on the initial concentration of sodium molybdate.     

The role of chelating agents on the corrosion mechanisms of aluminium in alkaline aqueous solutions

The influence of 1,2-diaminoethane (DAE) on the mechanism of aluminium corrosion in KOH solutions at pH 13 was investigated by combining time-resolved inductively coupled plasma optical emission spectrocopy, open-circuit potential measurements and X-ray photoelectron spectroscopy. In pure KOH solutions, a very slow corrosion rate is initially observed, corresponding to the dissolution of the native oxide layer. Following this incubation stage, the corrosion rate is increasing due to the formation and oxidation of Al hydride, until a steady state is reached. DAE behaves as a strong initial corrosion accelerator, due to synergistic effects with hydroxyl ions and a dissolution mechanism in three successive steps has been proposed: (i) a rapid initial dissolution induced by the formation and detachment from the surface of bidentate (chelate) Al-DAE metal bound surface complexes; (ii) a slower step ascribed to the formation and release of monodentate Al-DAE metal bound surface complexes and (iii) a final step dominated by direct oxidation of surface aluminium hydride by hydroxyl species as in pure KOH.     

Electrochemical corrosion behaviour of microcrystalline aluminium in acidic solutions

The electrochemical corrosion behaviour of microcrystalline pure aluminium coating, fabricated by a magnetron sputtering technique, has been investigated in both 0.5 mol/l NaCl and 0.5 mol/l Na₂SO₄ acidic (pH = 2) aqueous solutions. The corrosion resistance of the microcrystalline Al coating has deteriorated more compared with that of the cast pure Al in Na₂SO₄ acidic solution. However, its oxide film has a higher pitting resistance in the NaCl acidic solution. Chloride ions play a big role in the formation of the oxide film on the microcrystalline Al coating. The higher pitting resistance was attributed to the more acidic isoelectric point which the oxide film achieved.     

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.     

Doping competition of anions during the electropolymerization of pyrrole in aqueous solutions

The doping competition of anions in the electropolymerization of pyrrole was studied in aqueous solutions containing two kinds of anions with the same concentration of 0.1 mol dm^-3. The composition analysis of the polypyrrole (PPy) films prepared shows that the doping amount of p-toluene sulfonate (TsO^-) prevails over that of perchlorate, chloride and nitrate anions. The conductivity of the PPy film is close to the higher value of the two PPy films doped with a single anion. When polymerization is performed in the solution containing TsO^- and NO₃^-, TsO^-is mainly doped at Structure I, while NO₃^- is mainly at Structure II of the PPy film.     

Effect of heat treatment on electrochemical behaviour of aluminium alloy AA3005

The effect of heat treatment on the electrochemical properties of bare metal samples of aluminium alloy 3005 in an acidified sodium chloride solution was investigated. Annealing at increasing temperatures in the range 150-500°C causes negative potential transients of increasing magnitude and longevity for samples exposed at open circuit. Electrochemical characterisation shows the potential transients to be caused by an increased anodic reaction rate of the annealed material surfaces as compared to the cold rolled, hard temper substrate. It is suggested that this increased anodic activity during short time exposure of the bare metal samples may be attributed to changes in the protective properties of the surface oxide as a result of the high temperature heat treatment. The mechanisms controlling short term electrochemical properties of bare metal samples may not be identical to those controlling the filiform corrosion susceptibility of coated products and a cautious approach is recommended when using electrochemical techniques for predicting corrosion properties.     

Effect of heat treatment on electrochemical behaviour of binary aluminium model alloys

In order to understand the role of various trace, impurity and alloying elements on the electrochemical activation of aluminium in chloride solution, binary alloys of type Al-Pb, Al-Bi, Al-In, Al-Sn, Al-Mg and Al-Zn were investigated by use of electrochemical and surface-analytical techniques. Al-Pb and Al-Bi alloys were electrochemically activated as a result of high temperature heat treatment. However, alkaline etching or mechanical polishing significantly reduced the activation. Alloys Al-In and Al-Sn were electrochemically activated regardless of surface condition. The activation of Al-Pb alloys is attributed to enrichment of lead, presumably in metallic form, at the metal-oxide interface by segregation and diffusion along the grain boundaries. In the presence of chloride, lead destabilises the surface oxide, giving rise to unexpected oxidation of the underlying aluminium at potentials well below the pitting potential. It is suggested that the resulting detachment of the Pb from the aluminium surface causes repassivation. Aluminium is not activated by Mg or Zn as a result of high temperature heat treatment.     

Influence of some anions on the stress corrosion of low alloy steels in aqueous solutions

Polarization and stress corrosion behaviour of two types of low alloy steels, AISI 4130X and AISI 4145, were studied in agitated 0.05 M sodium acetate and 0.05 M trisodium phosphate solutions saturated with natural gas at room temperature. The magnitude of currents, shown in potentiokinetic polarization curves in the range of —0.8 V(SCE) to +0.8 V(SCE) in these media, is very small compared with earlier results obtained in sulphate and chloride solutions.Stress corrosion experiments conducted using a slow strain rate technique under cathodic conditions indicate the onset of hydrogen embrittlement on both types of steel in sulphate, chloride, acetate and phosphate solutions. Under anodic conditions, the results in 0.05 M sodium acetate solutions indicate strong susceptibility to stress corrosion compared with no susceptibility in 0.05 M trisodium phosphate solutions if there is no onset of pitting. These results are in contrast with results showing strong anodic dissolution and consequent absence of stress corrosion under anodic conditions in sulphate and chloride media.Results of Auger and IR analyses of anodic surface films formed in acetate and phosphate solutions are presented and discussed in relation to their observed mechanical behaviour.