Das anodische und kathodische Verhalten des Eisens in sulfathaltigen Elektrolyten. Chaire d'Electrochimie,Faculté des Sciences,Strasbourg, France

The anodic and cathodic behaviour of iron in sulphate containing electrolytes The formation of Fe₂(SO₄)₃ on passive iron at p_H = 1 appears probable from a thermodynamical point of view. At high SO₄^2? concentrations the equilibrium system contains but low concentrations of Fe³⁺, and no Fe²⁺ ions, a fact showing the relatively elevated stability of the Fe₂(SO₄)₃ layer on passive iron. In slightly acid solution (p_H = 4) the passivity of the iron is determined by iron oxide layers. The formation of FeSO₄ from metallic iron and sulphate ions is restricted to the transpassive zone (p_H 4 to 7), in alkaline solutions even to the active zone. In the p_H region 2 to 14 the passive layer on iron has about the same composition in the systems Fe|H₂O + SO₄^2? and Fe|H₂O.     

The behaviour of homogeneous AlCu4 and AlZn3 in weakly acid and in O2-saturated chloride solutions

The electrochemical behaviour of a homogeneous binary Al–4wt.%Cu and of a homogeneous binary Al–3wt.%Zn alloy in weakly acid chloride solutions, in O₂-saturated chloride and citrate buffer solution and in Fe²⁺/Fe³⁺-containing citrate buffer solution has been examined using a rotating disc electrode. In weakly acid and in O₂-containing solutions the behaviour of the AlCu and of the AlZn alloy differs from the behaviour of pure Al. H⁺ -and O₂-reduction at these alloys take place at a much higher rate. Under certain conditions, the reactions are even under transport control. Obviously, this is caused by the preceding selective dissolution of Al. Clusters from residual Cu and Zn, respectively, can pierce the passive layer and act as cathodic sites. In an electrolyte, where the passive layer is not dissolved, the AlZn and the AlCu alloy behave like pure Al. This is shown with the Fe²⁺/Fe³⁺ redox system in a neutral buffer solution.     

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.     

Electrochemical Behaviour of Nickel-Containing Alloys in Phosphoric Acid Solutions

Abstract

The electrochemical behaviour of a number of nickel-containing alloys has been studied in solutions of pure phosphoric acid, and the effect of varying the acid concentration and the alloy composition determined.

The potential/current density curves show active, passive and transpassive regions, but the corrosion appears to take place mainly under conditions of active dissolution. The active regions of the curves are complex and show as many as four distinct current maxima. The appearance of these maxima is related to the corrosion behaviour of the alloys, and to the effect that compositional changes have upon this behaviour, which is in turn attributed to the formation of various orthophosphate and pyrophosphate complexes.     

Technical note: Transpassive anodic dissolution of nickel in sulphuric acid

Abstract

Anodic dissolution behaviour of nickel in 3 and 10M H₂SO₄ solutions in the transpassive region has been investigated. Nickel dissolution current efficiency has been determined by splitting potentiostatic polarisation curves into oxygen evolution and nickel dissolution curves by carrying out solution analysis. Nickel dissolution current efficiency is minimum at 1800 mV(SCE) in 3M H₂SO₄ and primary passivation is not observed in 10M H₂SO₄, To study the stability fo the passive film formed on the surface of nickel in 3 and 10M H₂SO₄ solutions, potential decay curves have been recorded from various anodic potentials.     

Electrochemical impedance spectroscopy of anodic processes on Co<Subscript>2</Subscript>Si electrode in sulfuric acid solutions

Frequency dependences of the impedance of Co₂Si electrode in 0.5 M H₂SO₄ and 0.05 M H₂SO₄ + 0.45 M Na₂SO₄ solutions are measured in the ranges of the active dissolution, active-passive transition, passive state, and transpassivity. There are potential values in the active range when the impedance can be interpreted as the impedance of the selective cobalt dissolution. The impedance spectra in the passive range and in the beginning of the transpassive potential range are described with the use of an equivalent circuit taking into account the existence of an oxide layer on the electrode surface.     

Electrochemical impedance spectroscopy of anodic processes on cobalt disilicide in sulfuric acid solutions

The frequency dependences of the impedance of a CoSi₂ electrode in solutions of 0.5 M H₂SO₄ and 0.05 M H₂SO₄ + 0.45 M Na₂SO₄ are measured in the range of potentials of the active-passive transition, passive state, and transpassive state. Impedance spectra are described using an equivalent circuit with regard to the presence of an oxide layer on the electrode surface. The dependence of equivalent circuit parameters and time constants on the electrode potential is analyzed. It is found that the high anodic stability of cobalt disilicide in sulfuric acid solutions is due to the barrier properties of the oxide film. The variations in the thickness of the oxide layer with increasing electrode potential in the range of passive dissolution are estimated. It is revealed that the time constant related to the oxide layer increases before the transpassive transition.     

Electrolyte and temperature effects on pitting corrosion of type 316LN stainless steels

Abstract

The influence of electrolyte composition and temperature on the pitting corrosion resistance of nitrogen bearing (0·015, 0·198, and 0·56%N) type 316L stainless steels has been investi gated. Anodic polarisation curves were determined in neutral chloride solution at room temperature and at elevated temperatures of 308, 318, and 333 K. Similar polarisation studies were also conducted at room temperature in 1N H₂ SO₄ and in an acidic chloride solution containing 1N H₂ SO₄ and 0·5M NaCl. The results show that the critical pitting potential E_pp in neutral chloride and acidic chloride media increases as the nitrogen content of the alloy increases, indicating that resistance to pitting increases with the addition of nitrogen. However, in 1N H₂ SO₄ solution, the transpassive potential was almost independent of the nitrogen content. As the temperature of the neutral chloride medium was increased, the E_pp values decreased, irrespective of nitrogen content. The decrease in E_pp is attributed not only to the temperature induced modification of the passive oxide film but also to chloride induced activity at the passive film/solution interface. Nevertheless, the alloy con taining 0·56% nitrogen showed better resistance to pitting at 333 K than did the alloy with 0·015% nitrogen at room temperature. SEM examination of the pitted specimens showed clear evidence of pitting for the alloy with 0·015% nitrogen, but insignificant pitting attack for the alloy with 0·56%N. However, the alloy with 0·56%N displayed some pitting attack when the experiments were conducted at 333 K.     

Anodisches und kathodisches Verhalten des Chroms in salzsaurer Lösung

Anodic and cathodic behaviour of chromium in hydrochloric solution The electrochemical behaviour of chromium in aqueous solutions has been investigated with the aid of the electrochemical affinity (A) and overpotential (Δ U) diagrams in the p_H range from ?1 to 4 in the presence and absence of Cl^? ions. In the Cr/H₂O, Cl^? system, the chromium passivation is neutralized by the formation of CrCl₃. In the transpassive zone, CrCl₂ is formed. A passive layer with the composition Cr(OH)₂ was found at the Flade reference potential and under self-passivation conditions in the systems Cr/H₂O and Cr/H₂O, Cl^?.     

Semiconductor properties and protective role of passive films of iron base alloys

Semiconductor properties of passive films formed on the Fe-18Cr alloy in a borate buffer solution (pH = 8.4) and 0.1 M H₂SO₄ solution were examined using a photoelectrochemical spectroscopy and an electrochemical impedance spectroscopy. Photo current reveals two photo action spectra that derived from outer hydroxide and inner oxide layers. A typical n-type semiconductor behaviour is observed by both photo current and impedance for the passive films formed in the borate buffer solution. On the other hand, a negative photo current generated, the absolute value of which decreased as applied potential increased in the sulfuric acid solution. This indicates that the passive film behaves as a p-type semiconductor. However, Mott-Schottky plot revealed the typical n-type semiconductor property. It is concluded that the passive film on the Fe-18Cr alloy formed in the borate buffer solution is composed of both n-type outer hydroxide and inner oxide layers. On the other hand, the passive film of the Fe-18Cr alloy in the sulphuric acid consists of p-type oxide and n-type hydroxide layers. The behaviour of passive film growth and corrosion was discussed in terms of the electronic structure in the passive film.     

Investigation of the SCC behaviour of alloy 2024 using the slow strain rate technique

The stress corrosion cracking (SCC) behaviour of 2024 plate in T351 and T851 tempers was investigated in short transverse direction performing accelerated tests under constant deformation, constant load and slow strain rate conditions. Corrosive media used were: aqueous 3.5% NaCl solution, an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH 3 (according to LN 65666), an aqueous solution of 3% NaCl + 0.3% H₂O₂, and substitute ocean water according to ASTM D1141. Alternate immersion tests in 3.5% NaCl solution indicated the low SCC resistance of the alloy 2024-T351 as well as the improved SCC behaviour due to aging to T851 condition. Similar results were obtained from constant load tests under permanent immersion conditions in the acidified chloride-chromate solution, in 3% NaCl solution with peroxide, and in substitute ocean water, whereas no SCC failure was observed with specimens which were permanently immersed in 3.5% NaCl solution. Using the slow strain rate method, 3% NaCl + 0.3% H₂O₂ and substitute ocean water were found to be effective synthetic environments. The other two electrolytes did not promote severe stress corrosion cracking with alloy 2024-T351. The SCC behaviour of 2024-T851 was difficult to determine employing the slow strain rate technique. Large scatter in data, observed even in inert environment, and the low elongation of the aged material, exacerbated by a further degradation of ductility due to pitting and intergranular corrosion, precluded an evaluation.     

Polarisationsmessungen an niedriglegierten Stählen in konzentrieten Nitratlösungen

Polarisation measurements on low alloy steels in concentrated nitrate solutions Current density potential curves were determined by potentiostatic measurements involving constant potentials and stepwise potential change for six low alloy steels in Ca(NO₃)₂ and NaNO₃ solutions. The instationary curves vary markedly with the polarisation technique and do not characterize the anodic behaviour of the materials. For Ca(NO₃)₂ solutions the polarisation measurements do not show any correlation with stress corrosion cracking behaviour. Similar results were also obtained in NaNO₃, solutions. Contrary to the situation in Ca(NO₃)₂ solutions, a potential range for resistance against intergranular corrosion types exists at about U_H = 0,5 V. This range is characterized by relatively high stationary current densities. For materials without this range of resistance, the stationary current densities are significantly smaller. The electrochemical properties of the system steel/nitrate solution are extremely complicated. The free corrosion potential varies between the potential ranges for passive and transpassive corrosion. For anodic polarisation, the potential ranges of susceptibility as well as resistance against intergranular corrosion types follow each other.     

Complete protection of a passive film on iron from breakdown in a borate buffer containing 0.1 M of Cl by coverage with an ultrathin film of two-dimensional polymer

An ultrathin film of two-dimensional polymer was prepared on a passivated iron electrode by modification of a 16-hydroxyhexadecanoate ion self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. This film prevented passive film breakdown examined by potentiodynamic anodic polarization of the coated electrode in the borate buffer solution containing 0.1 M of Cl⁻. Neither current spikes nor the pitting potential was observed in the passive and transpassive regions of polarization curve. The anodic current density was decreased in these regions markedly, implying hindrance to permeation of Cl⁻ and water through the film. Structure of the film was clarified by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement with a drop of water. Electron-probe microanalysis of the passivated surface coated with the film after anodic polarization scanning up to the transpassive region revealed that the polymer film prevents pit initiation by an attack on the passive film with Cl⁻.     

Effect of citrate ions on the electrochemical behaviour of low-carbon steel in borate buffer solutions

The electrochemical behaviour of cold-rolled low carbon steel was studied on both active and passive potential regions in borate buffer solutions with and without the addition of sodium citrate (NaCit). In the active region anodic charges increased significantly and R_CT values decreased with citrate, due to the formation of soluble complexes. In the passive potential region the film formed at +0.4 V in borate buffer solution with and without 0.010 M NaCit is probably enriched by Fe₃O₄ oxide, while films formed at +0.8 V are probably enriched by γ-Fe₂O₃. The equivalent circuit [R_s(R_CTQ)] fitted all experimental impedance data.     

Electrochemical corrosion behavior of bulk ultra-fine grained Fe-Ni-Cr alloy

The corrosion behavior of bulk ultra-fine grained (UFG) Fe-Ni-Cr alloy prepared by equal-channel angular pressing technique was investigated in 0.25 mol/L Na₂SO₄+0.05 mol/L H₂SO₄ solution by electrochemical measurements. As compared to the coarse grained (CG) counterpart, the UFG alloy exhibits an acceleration of the active dissolution and a shrunk passive region with a higher passive current. The Mott-Schottky analysis in conjunction with the point defect model indicates that the donor diffusion coefficient in the passive films of the UFG sample increases greatly to one magnitude order higher and the donor density is slightly lower than that of the CG sample.     

Corrosion of Construction Materials,Ti 125, Ti 260 304 L,Uranus 65, in Contact with Platinum Catalysts in Nitric Acid Production

Abstract

With the object of finding a material more resistant than stainless steel for use in the catalytic production of nitric acid, a study was made of the corrosion of stainless steels (304 L, URANUS 65) and titanium alloys (Ti 125, Ti 260) In contact with platinum-rhodium alloy in nitric acid solutions. The corrosion potential of stainless steel, when in contact with Pt-Rh, shifts towards more positive values, i.e., it passes from the passive into the transpassive region, whereby the corrosion rate of stainless steel is greatly Increased. The corrosion potential of titanium alloy, in contact with platinum-rhodium alloy in nitric acid solutions, also shifts towards more positive values, but here the corrosion rate of the alloy is not increased, since the system Ti/Pt-Rh alloy remains in the passive region.     

Elektrochemische Untersuchungen der Korrosion hitzebeständiger Fe- und Ni-Basislegierungen in eutektischer Alkalisulfatschmelze bei 700 °C

Electrochemical Investigation of the Corrosion of Heat Resistant Fe- and Ni-Base-Alloys in Eutectic Alkali Sulphate Melt at 700 °C Eight Fe-Cr-Ni-alloys with (mass-%) I5-29 Cr, 0-76 Ni, Fe balance have been corroded in eutectic alkali sulphate melt at 700 °C, I bar O₂, potentiostatically for 23 h. The corrosion behaviour at different potential in the active, passive and transpassive range was investigated by mass loss measurements and metallographically. Cr was found to have a markedly effect on the passive/transpassive behaviour and for the corrosion resistance at all. But there is no further improvement at > 20% Cr. The effect of Ni was found to be negative or indifferent in the active range. The greater effect was found with the “minor” elements, e. g. Al and Si, which can support protective layers formation in the active range, whereas in the transpassive range they can enhance the corrosion process. Ti was found to have a negative effect due to local sulfide formation. Heat resistant alloys with high contents of Cr and Ni have been found to have necessarily no increased corrosion resistance to sulphate attack. In the range of 20-30% Cr the resistance can alter markedly showing the greater effect of minor elements.     

Impedance of NiSi electrode in sulfuric-acid solution in the region of passive and transpassive states

The behavior of an NiSi electrode in 0.5 M H₂SO₄ in the region of passive and transpassive state (from 0.50 to 2.1 V (versus SHE)) is studied. The conclusion is made about formation of oxide film close to SiO₂ by composition on the electrode surface in the passivation process. Equivalent electric circuit modeling passive and transpassive state of nickel silicide is proposed. Thickness of oxide film and its specific resistance depending on electrode potential are calculated basing on the impedance data.     

Electrochemical behavior of Ni3Al-based intermetallic alloys in NaOH

The corrosion behavior of Ni₃Al-based intermetallic alloys in a 0.5 M NaOH solution was studied at 25 °C. The open circuit potential, cathodic and anodic potentiodynamic polarization, Tafel plots and linear polarization resistance measurements were used to characterize the corrosion behavior. For the Ni₃Al(B, Zr) alloy, potentiodynamic polarization curves showed a wide passive region that can be found between about ?0.220 V_SCE and 0.520 V_SCE. On the other hand, a narrow passive region, in the range of potentials from about ?0.180 V_SCE to 0.180 V_SCE, was observed for the Ni₃Al(B, Zr, Cr, Mo) alloy. Chromium, as an alloying element in the Ni₃Al(B, Zr, Cr, Mo) alloy, contributes to transpassive dissolution of the passive film at much lower anodic potentials and remarkably reduces the passivation region. The experiments indicated also that damaged passive films on alloys repairs itself and pits do not initiate. The surface of both alloys and passive films possess extremely high corrosion resistance in a studied solution. However, Tafel and linear polarization tests revealed that freshly exposed surfaces of the Ni₃Al(B, Zr) alloy exhibited better corrosion resistances than the Ni₃Al(B, Zr, Cr, Mo) alloy. Both methods, used for the determination of corrosion rates gave very similar results. The calculated corrosion rates are about 2.8 ·10^?3 and 6.0·10^?3 mm year^?1 for the Ni₃Al(B, Zr) alloy and B, respectively.     

Potentialabhängigkeit der Korrosion Mo-freier und Mo-haltiger Stähle in Calciumnitrat-Lösung und Natronlauge

Effect of potential on corrosion of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The effect of potential on the corrosion behaviour of three low alloy steels with different carbon and molybdenum contents (0.08 C, 0.01 Mo; 0.08 C, 0.98 Mo; 0.18 C, 1.10 Mo) was investigated in boiling 60 wt.% Ca(NO₃)₂ solution (DIN 50 915) and in boiling concentrated NaOH solutions (20 and 35 wt.% NaOH) by potentiodynamic and chronopotentiostatic polarization measurements (i/E curves) and chronopotentiostatic mass loss measurements (corrosion rate v vs. potential E curves). In Ca(NO₃)₂ solution, i/E measurements give no information about the effect of potential on the anodic dissolution. For the materials investigated, v/E measurements indicate the existence of potential ranges with pronounced differences of the corrosion response. It can be differentiated between active, passive, and transpassive ranges, and also a potential range of secondary passivity was established. Transpassivity and secondary passivity are markedly pronounced with the molybdenum bearing steels but not with the steel free from molybdenum. There are no hints to the occurrence of intergranular attack in the specimens which are free from of internal and external mechanical stresses, whereas such hints could be derived from the shape of the i/E curves. Nevertheless, under mechanical stresses (constant load, CERT conditions) the materials are susceptible to intergranular SCC. The conception that intergranular SCC of low alloy steels in Ca(NO₃)₂ solution is connected with a break-through potential of grain boundary corrosion and hence is to be interpreted as an intergranular attack which, under mechanical stresses, runs in a modified form as SCC with intergranular crack path, cannot be maintained in such general terms. A pronounced active/passive behaviour is observed in NaOH. The effect of potential on anodic metal dissolution which is derived from v/E curve is also established by i/E measurements. For molybdenum bearing steels, the active potential range is somewhat extended to more positive potentials. Manifestations of localized attack, e.g., intergranular corrosion, do not occur. From the investigations, no hints to the cause of the deterioration of the resistance to intergranular SCC in caustic solutions by molybdenum can be derived.