Environmental factors affecting the corrosion behaviour of reinforcing steel. V. Role of chloride and sulphate ions in the corrosion of reinforcing steel in saturated Ca(OH)2 solutions

The corrosion behaviour of reinforcing steel in saturated naturally aerated Ca(OH)₂ solutions in absence and presence of different concentrations of NaCl, NH₄Cl, Na₂SO₄ and (NH₄)₂SO₄ is followed by measuring of the open circuit potential complemented with SEM and EDS investigation. These salts cause breakdown of passivity and initiation of pitting corrosion. The rates of oxide film thickening by OH⁻ ions and oxide film destruction by the aggressive ions follow a direct logarithm law and depend on the concentration and type of aggressive salts anions and cations. The values of the activation energies for oxide film thickening are calculated and discussed.     

Transport processes in passivity breakdown—II. Full hydrolysis of the metal ions

Transport process analysis shows that passivity breakdown can be associated with a depletion of OH^? ligands at the metal-solution interphase. In a previous publication (J. electrochem. Soc., 123, 464 (1976)), assuming only the first step of cation hydrolysis, important pH changes were shown to be possible at the initial steps of pitting. A calculation based on full hydrolysis is attempted in the present paper. New relations between pitting potential and pH were found, which could not be explained by the previous simplified treatment. The pitting potential of bivalent metals, such as iron, nickel, cadmium, zinc and cobalt was found to be pH-independent up to a pH value of around 9 to 10, but pH-dependent for higher pH-values. This pH dependence should be equal to the Tafel slope for those metals in the pit-like solution. In the case of trivalent metal ions, such as aluminium, on the other hand, the external pH contribution is buffered by the various hydrolysis steps of the metal ion. As a result of this, the condition for pitting is not affected by the external pH value over a wide range of pH values, an observation that though reported by previous authors, has not so far been theoretically explained.     

The electrodissolution and passivation of mild steel in alkaline sulphide solutions

The electrodissolution and passivation of mild steel in alkaline sulphide solutions at 25°C is investigated by the use of steady state and transient electrochemical techniques and by scanning electron microscopy and energy dispersive X-ray analysis. The electrodissolution and passivation processes are explained by the initial competitive adsorption of OH^?, HS^? and H₂O and the subsequent charge transfer and proton transfer processes involving the OH and HS adsorbed species. The former is responsible for the passivation of the metal by an oxide metal layer while the second species undergoes two different reactions yielding in one case a precipitated mackinawite film on the metal, which is a poorly protective layer, and in the other a soluble Fe(II) species which can be detected by chemical analysis. The contribution of the reactions starting from each adsorbed species depends on the HS^?/OH^? concentration ratio in the solution. The initiation of pitting on iron is also explained through the postulated reaction pattern.     

On the pitting corrosion of tin in aqueous solutions

Four independent techniques were employed to prove that Sn undergoes pitting corrosion. The first was based on the measurement of the variation of the open circuit potential of the Sn electrode in aerated Cl^? solutions of various concentrations. Steady-state potentials were attained slowly and erratically from negative values, and were more positive the higher the dilution. Sn electrodes prepassivated in CrO₄^2? solutions responded readily to additions of Cl^? ion. The potentials developed were more noble than those measured in presence of the passivator alone, and changed to positive values with the increase of the concentration of the pitting corrosion agent. Attack was under cathodic control.Galvanostatic polarization of the Sri electrode was carried out in 0.005 to 0.1M NaOH, in the presence of various additions of Cl^?. Above a certain Cl^? content, contingent upon the alkali concentration, the aggressive anion prevented the evolution of Oa on the electrode, and oscillations in the E?t curves were recorded. Competitive adsorption of Cl^? and OH^? is assumed to occur, which affected both the quantity of electricity, Q_p, consumed along the oxide formation steps, and the rate of potential increase, dE/dt, following oxidation. Plots of the two variables as a function of the Cl^? ion concentration exhibited a definite break at the value characteristic for the initiation of pitting attack. Potentiodynamic polarization showed that the pitting corrosion potential progressively shifts towards negative values as the concentration of the aggressive agent in solution was increased. The pitting corrosion currents resulting from the addition of Cl^? to Sn electrodes prepassivated in CrO₄^2? solutions were measured. The dependence of the maximum currents on solution composition is explained on the basis of competitive adsorption.     

Transport processes in passivity breakdown—III. Full hydrolysis plus ion migration plus buffers

Transport process analysis shows that passivity breakdown can be associated with a depletion of OH ligands at the metal solution interphase. In previous publications (J. electrochem. Soc. 123, 464, 1976 and Corros. Sci. 21, 551, 1981) pit model calculations were performed assuming that the aggressive anion solution was acting as a supporting electrolyte. In the present paper a pit model calculation was developed for pitting in diluted aggressive anion solutions. The new model was applied to the case of pitting of zinc in borate buffered diluted NaCl solutions. The model gives a good account of the experimental findings of this system.     

Breakdown of cadmium passivity in alkaline solution

The electrochemical behaviour of a Cd anode was investigated in 0.05–2 M NaOH solutions by the potentiodynamic technique. The polarization curves exhibit active to passive transition prior to oxygen evolution. The threshold potential of the active dissolution is very close to the reversible potential of the system Cd/Cd(OH)₂. X-Ray diffraction and XPS measurements reveal that the passive layer is composed of both Cd(OH)₂ and CdO. The influence of increasing amounts of Cl^?, Br^? or I^? ions on the anodic behaviour of Cd in NaOH solution has been investigated. The halides stimulate the anodic dissolution in the active region and tend to break-down the passive layer in the passive region, leading to pitting attack. The pitting potential shifts to more negative values with increasing halide ion concentration but to the reverse direction with increasing alkali concentration.     

Passivity and passivity breakdown of zinc anode in alkaline medium

The electrochemical behaviour of zinc in NaOH solutions has been investigated by using potentiodynamic technique and complemented by X-ray analysis. The E/i curves exhibit active, passive and transpassive regions prior to oxygen evolution. The active region displays two anodic peaks. The passivity is due to the Formation of a compact Zn(OH)₂ film on the anode surface. The transpassive region is assigned to the electroformation of ZnO₂. The reverse sweep shows an activation anodic peak and one catholic peak prior to hydrogen evolution. The influence of increasing additives of NaCl, NaBr and Nal on the anodic behaviour of zinc in NaOH solutions has been studied. The halides stimulate the active dissolution of zinc and tend to break down the passive film, leading to pitting corrosion. The aggressiveness of the halide anions towards the stability of the passive film decreases in the order: I^? > Br^? > Cl^?. The susceptibility of zinc anode to pitting corrosion enhances with increasing the halide ion concentration but decreases with increasing both the alkali concentration and the sweep rate.     

Localized acidification as the cause of passivity breakdown of high purity zinc

The passivity breakdown of high purity zinc was studied in borate buffered sodium chloride solutions. Based on previous ion-migration calculations (Corros. Sci. 24, 517, 1984) synthetic pit-like solutions were prepared, and the behaviour of zinc in those solutions was studied. The experimental observations allowed for a quantitative evaluation of the equation for pitting potentials:

$\text{E}{}_{\mathrm{<mtext>p</mtext>}}\text{= E}{}^1{}_{\mathrm{<mtext>c</mtext>}}\text{+ η + Δφ.}$

This equation, derived from the localized acidification mechanism for pitting, was found to give a complete account of the experimental observations of passivity breakdown of zinc. The OH- ion depletion, at the metal-solution interface, proved to be the cause of passivity breakdown of high purity zinc in borate buffered sodium chloride solutions.     

The effect of microstructure on the pitting tendency of a heat-resistant aluminium alloy (AAA 2618)

The effect of microstructural heterogeneity on the pitting tendency of aluminium alloy AAA2618 (2.5 Cu, 1.4 Fe, 1.8 Mg, 1.1 Ni was studied after solution treatment for 5 h at 530°C, followed by quenching in water at 80°C, and subsequent ageing for 20 h at 200°C. The microstructure was examined by optical microscopy, transmission electron microscopy, and electron-probe microanalysis. The pitting tendency was evaluated by measuring (a) the induction time for passivity breakdown in 0.1 N H₂SO₄ + 10^?3 N NaCl at ? 500mV(SCE), and (b) the fraction of pitted area after 6 min exposure to treatment (a). Significant differences in pitting tendency between the two treatment versions are attributed to the weak passive film on Al₂CuMg, the strengthening phase in the aged alloy.     

Effect of molybdate ions as corrosion inhibitors of iron in neutral aqueous solutions

Abstract

The effect of molybdate ions on the corrosion of Fe in neutral solutions was investigated by electrochemical measurements (dc polarisation and impedance spectroscopy) together with gravimetric determinations. Studies were conducted in solutions containing sodium hydrogen/sodium sulphate salts with molybdate concentrations ranging between 10^?4 and 10^?2M at pH 8 and 9. Mass loss measurements indicated that about 10^?3M of molybdate was necessary in order to inhibit completely the corrosion of Fe at room temperature. The potentiodynamic polarisation and electrochemical impedance studies gave indications about the mechanism of action of the MoO^2?₄ ion. They also showed that the inhibiting effect of the oxyanion is increased in the presence of dissolved oxygen. Spontaneous passivation of the corroding Fe electrode could happen only in the presence of dissolved oxygen at concentrations greater than 10^?4M. However, passivity was also obtained under potentiodynamic polarisation conditions in deaerated solutions. The effect of the oxygen was attributed to displacement of the corrosion potential into the region of selfpassivation of the steel. Finally, the results indicated that the corrosion inhibition of Fe in neutral solutions by molybdate ions was largely insensitive to pH over the range from 8 to 9.     

Passivity breakdown and stress corrosion cracking of α-brass in sodium nitrite solutions

The stress corrosion cracking (SCC) susceptibility of pure copper and four α-brasses of different zinc alloy concentration in NaNO₂ 1 M solution, at various pH values, was studied by means of potentiodynamic polarisation curves and constant potential slow strain rate experiments. The results confirmed that passivity rupture was a necessary condition for SCC of α-brass and copper in nitrite solutions. Susceptibility to SCC was observed only when the potential was equal to or higher than a certain critical value at which passivity breakdown was triggered by the slow dynamic straining of the metal. Cracks were found only in those metal areas where passivity rupture had taken place.     

Effect of halide anions and temperature on initiation of pitting in Cr–Mn–N and Cr–Ni steels

Abstract

The pitting corrosion of Cr₁₈Mn₁₂N and Cr₁₈Ni₉ steels in halide solutions (F^?, Cl^?, Br^? and I^?) has been investigated. The study involved cyclic potentiodynamic polarisation tests with subsequent examination of the specimens by both optical and scanning electron microscopy. Values of the critical concentrations of halide ions, [X^?]_cr, beyond which pitting occurs, as well as breakdown potentials for pitting in chloride solution, have been established. In addition, the effect of the temperature over the range of 5–80°C on the critical chloride ion concentration [Cl^?]_cr has been investigated and it has been found that temperature has a negligible effect beyond 40°C.     

Effect of halide ions on passivation and pitting Corrosion of copper in alkaline solutions

The passivity of copper in NaOH and borate buffer solutions containing chloride, bromide and fluoride ions was studied by using cyclic voltammetry and potentiostatic current transient measurements. At scan rates ≥ 20 mVs^?1, the addition of halide ions does not nearly affect the cyclic voltammograms in the absence of pitting. But they differ considerably in presence of pitting. The pitting potential was found to depend on the solution pH and decreased linearly with increase of logarithm of halide ion concentration. The current transients in the passivity as well as in the pitting potential regions were analyzed. Before the pitting, i-time curves were rather similar to those obtained in the plain solutions. At times > 20 seconds, the current varies linearly with the reciprocal of the square root of time indicating diffusional characteristics of the metal corrosion through the passive layer. In presence of pitting, the pitting current versus time relations fit the Engell-Stolica equation.     

The breakdown of passivity of iron and nickel by fluoride

The breakdown of passivity of iron and nickel by fluoride is investigated in acid solutions. Localized corrosion is observed for iron at pH ? 5 whereas nickel shows general corrosion. In strong acids only general corrosion for both metals is obtained. The breakdown of passivity of iron in acids occurs in two stages. Fluoride leads first to an increased passive current density ( 0.5 mA cm^?2) where Fe³⁺ ions are formed. After a non-reproducible induction period of minutes to hours a steep increase of the dissolution current density is obtained going along with Fe³⁺ ion production. Apparently, at this stage the passive layer is removed leading to a similar situation as on the surface of pits during localized corrosion caused by other halides.     

Electrochemical Investigation of the Nucleation and Propagation of Pits in Iron-Chromium Alloys

Abstract

Electrochemical measurements performed on single crystals and polycrystalline samples of Fe 13 Cr and Fe 16 Cr alloys in Na₂SO₄ + NaCl solutions differing in Ci^? ion concentration, have shown that during pitting the anodic current I increases with time t:

I～t^b Depending on experimental conditions, b may vary from 2 to 6. At b = 2 all the pits nucleate simultaneously, so that their number does not change with time of anodic polarisation. At b > 2 the number of pits increases with time.

Galvanostatic polarisation of polycrystalline samples in solutions containing much fewer Cl_? ions than SO₄^2? ions does not lead to a steady potential. Under these conditions periodic oscillations of potential occur, several hundred millivolts in amplitude. This phenomenon is probably due to the inhibiting action of SO₄^2? ions on the nucleation of pits by Cl^? ions, and it occurs only at certain concentration ratios of these two ions in the electrolyte. Inhibition of pit nucleation is also revealed during potentio-static and potentiokinetic measurements at potential values several hundred millivolts more positive than the breakdown potential measured galvanostatically.

An interpretation of above phenomena is given for both single crystals and polycrystalline alloy samples.     

Breakdown of passive film on nickel in borate solutions containing halide anions

The effect of Cl⁻, Br⁻ and I⁻ anions as aggressive agents on the anodic behaviour of nickel electrode in deaerated Na₂B₄O₇ solutions have been investigated by galvanostatic polarization technique. Lower concentrations of the halide anions have no effect on the mechanism of nickel passivation. An increase in the halide anions concentration causes oscillation of the potential in the oxygen evolution region. This could be attributed to the destruction of the passivity by halide anions and repassivation of the film by anodic current and/or OH⁻ anions. Higher aggressive anion concentrations cause breakdown of the passive film and initiated pitting corrosion. As the temperature increases, the breakdown potential is shifted towards the more negative direction. On the other hand, as the pH of the solution increases, the breakdown potential is shifted toward more positive direction, indicating increased protection of the passive film. The activation energy, , of the oxide film formation in the presence of Cl⁻ anions was calculated and was found to be 21 kJ/mol.     

Eine charakterisierung der lochfraßkorrosion des nickels—I. Die bestimmung der lochfraßpotentiale in neutralen und alkalischen lösungen

Quasistationary values for the characteristic pitting potentials for nickel were determined by means of potentiokinetic polarization measurements and their dependence on chloride and hydroxide ion concentration was investigated. The pit nucleation potential U_np is a linear function of the logarithm of the anion concentration. The value being determined by the adsorption equilibrium of the anions on the passivated metal surface.The critical pitting potential U_cp depends on the pH of the solution only. Above pH 6-5 U_cp decreases with increasing pH. On the basis of the mechanism for crevice corrosion this behaviour may be explained by the influence of OH^?-ions being known to take part in the ionization of the metal atoms.     

Initiation and Inhibition of Pitting Corrosion in Alkali Solutions under potentiostatic polarization conditions

The variation of the critical pitting potential of a zincalloy was studied in aerated NaOH solutions as a function of the concentration of the aggressive ions, Cl^?, Br^? and I^?. Curves with segmented nature were obtained when E_pitting was plotted versus logarithm of the halogen ion concentrations. Initiation of pitting corrosion was discussed on the basis of formation of complex halo-compounds with the oxides/hydroxides that constitute the passive surface film. Addition of chromate, phosphate and carbonate ions to the halogen-containing solutions causes the shift of the critical pitting potential in the noble direction, accounting for increased resistance to pitting attack. Nitrite-ion additions contribute with the halogen ions in the destruction of the passive film. Sufficient concentrations of the carbonate ions cause complete inhibition of pitting corrosion.     

Electrochemical behaviour of nickel anode in H2SO4 solutions and the effect of halide ions

The electrochemical behaviour of pure nickel in H₂SO₄ solutions has been potentiodynamically investigated. The effects of the following factors on the anodic dissolution and passivation of the metal are discussed: potential scan rate, successive cyclic voltammetry and progressive additions of Cl^?, Br^? and I^? ions. Increasing the potential scan rate increases the critical current density i_cc, denoting that the active dissolution of nickel in H₂SO₄ is a diffusion controlled process. Cyclic voltammetry shows that the reverse excursion does not restore the anode to its active state. On successive cycling, the height of i_cc decreases; this could be attributed to the decrease in the reduction efficiency of passivating oxide film during the cathodic half cycles. The presence of the halogen ions below a certain concentration specific to each anion inhibits the anodic dissolution both in the active and passive states. The inhibitive action of these additives decreases in the order I^?, Br^?, Cl^?. Beyond the specific concentrations, the halogen ions accelerate the anodic dissolution and shift the active passive transition to more positive values. The aggressiveness of these anions decreases in the sequence Cl^?, Br^?, I^?, Further increase in the halogen ion concentrations can lead to breakdown of the passive film and initiate pitting. The susceptibility of nickel to pitting attack enhances with increasing H₂SO₄ concentration.     

Regeneration and decontamination of a nickel citrate complex in spent electroless nickel plating solutions

The possibility of citrate precipitation by excess metal ions in alkaline solutions suggests the use of Ni(II) together with OH^? to precipitate a Ni(II)-citrate complex from spent electroless plating solutions. After treatment of the precipitate with acid, excess Ni(II) can be removed in the form of insoluble Ni(OH)₂. The solution of the Ni(II)-citrate complex can then be reused for electroless nickel plating. During this procedure the additive adipate is not regenerated. For decontamination of spent electroless nickel plating solutions Fe(III) can be used as Ni(II)-citrate complex precipitant.