Effects of dissolved oxygen on passive behavior of stainless alloys

A potential-control circuit has been devised to measure passive-current (corrosion rate) on stainless alloys in conditions simulating the usual noble drift of the open-circuit corrosion potential in the presence of passivating dissolved oxidizers. Open-circuit corrosion potential and passive-current decay were measured with and without dissolved oxygen in dilute sulfuric acid solutions as a function of time on austenitic stainless steel and a Ni-base Cr-Mo-W alloy. The results confirm the long-standing assumption that constant-potential potentiostatic tests give a good approximation of passive-film growth kinetics in the presence of passivating dissolved oxidizers. Mott-Schottky measurements showed that passive films on each alloy have similar semiconducting properties whether formed potentiostatically or by dissolved oxygen. In both cases, the passive film on stainless steel seems to be composed of an inner n-type barrier layer covered with an outer p-type layer. The passive film on the nickel-base alloy is primarily n-type with only weak evidence of p-type behavior in an outer layer. Results are in general agreement with predictions of the point-defect passive-film model proposed by Macdonald and coworkers.     

Corrosion behaviour of steel in the presence of Y(III) salts: Kinetic and mechanistic studies

The corrosion resistance of carbon steel after deposition of yttrium rich (hydr)oxide films on its surface was evaluated as a function of the Y(III) concentration in the solution, the immersion time and the presence of oxygen. During film formation in an aerated solution, two different cathodic reactions are involved: oxygen reduction and water reduction.The rate of oxygen reduction is lowered, whereas water reduction is activated in the presence of Y(III) in the solution. In the absence of oxygen, activated water reduction gets the main cathodic reaction, leading to polarization resistance values which are lower in the presence than in the absence of Y(III) ions.     

Role of environmental variables on the stress corrosion cracking of sensitized AISI type 304 stainless steel (SS304) in thiosulfate solutions

The stress corrosion cracking (SCC) behavior of sensitized AISI type 304 stainless steel (SS304) has been studied in dilute thiosulfate solutions as a function of thiosulfate concentrations and applied potentials. The susceptibility to SCC was observed to increase with thiosulfate concentrations and applied potentials. The addition of boric acid produced the reverse effect. A critical potential was found to exist, below which no SCC took place. Potential fluctuations, as recorded in the tests under open circuit conditions, appeared to be correlated with crack initiation and propagation during SCC. Current fluctuations observed in the controlled potential tests also gave indications of crack nucleation; however, at higher applied potentials such fluctuations were absent. The formation and presence of martensite in the specimens seemed to have a minor role in the overall SCC process. The aggressiveness of the thiosulfate concentration was also an important factor in determining the degree of susceptibility to SCC. The results obtained in the slow strain rate tests under open circuit as well as under potential-controlled conditions suggested a film ruptureanodic dissolution type of mechanism operative during SCC of sensitized SS304 in thiosulfate solutions.     

Probing into the effects of a magnetic field on the electrode processes of iron in sulphuric acid solutions with dichromate based on the fundamental electrochemistry kinetics

The effects of an applied magnetic field on the electrode processes of iron in sulphuric acid solutions with dichromate have been investigated by electrochemical measurements. Open circuit potentials, cathodic and anodic polarisation curves, values of polarisation resistance were measured in the presence or absence of a 0.4 T horizontal magnetic field (HMF). A potentiostatic polarisation plus magnetic field perturbation technique was used to study the effect of the magnetic field on open circuit corrosion. Cathodic reaction rates at open circuit potentials for iron in sulphuric acid solutions containing dichromate ions are controlled by both the electron-transfer process and the diffusion process. A magnetic field made the open circuit potential move in the positive direction, and changes of the open circuit potentials due to the magnetic field increase with increasing dichromate concentration. When iron was potentiostatically polarised at open circuit potentials in the absence of a magnetic field, a cathodic current was observed after a magnetic field was imposed. Such cathodic currents induced by the magnetic field increases with increasing dichromate concentration. The positive shifts of open circuit potential, the decrease of polarisation resistance, and the occurrence of cathodic currents induced by the magnetic field are caused by the accelerating effect of magnetic field on the cathodic diffusion process. Measured current densities showed lower, equal, or higher values in the presence of the magnetic field than those in the absence of a magnetic field at certain anodic potentials. This effect of the magnetic field is related to the contribution of the cathodic and anodic reactions to the measured current and the types of rate-determining steps for each reaction at certain potentials. The applied magnetic field significantly decreased the polarisation resistance. The experimental results in this paper are formulated based on the fundamental electrochemistry kinetics after introducing a coefficient term of the magnetic field effect on the mass transfer process of reactive ions.     

The corrosion and wetting behaviour of copper in the presence of some thiol reagents

The corrosion and wetting behaviour of copper in aqueous solutions of potassium diethyldithiophosphate and sodium diethyldithiocarbamate has been studied by measurement of the limiting current density for oxygen attack of the metal and contact angles. In dilute solutions of the reagent, bulk precipitation of the metal-reagent salt is observed; there is no effect on the limiting current density; and the contact angle is zero. In concentrated solutions there is no bulk precipitate; the limiting current density decreases with increasing reagent concentration; the contact angle is finite. Finite contact angles appear to result from the presence of a hydrophobic metal-reagent salt on the surface. The behaviour of the system, which could not be explained on the thermodynamic basis, is explained in terms of a simple model that takes into consideration kinetic factors.     

The corrosion of copper as a tool in the investigation of the reaction kinetics

The copper electrode reaction kinetics have been studied in acid CuSO, solutions. Dissolved oxygen was used to remove the reaction intermediate Cu⁺ from the electrode. This shifted the equilibrium potentials of the two partial reactions in an opposite direction allowing a somewhat unusual interpretation of impedance measurements. The charge transfer resistance of the Cu²⁺ → Cu⁺ reaction was always higher than that of the Cu⁺ → Cu reaction. In the neighbourhood of the open circuit potential an additional “surface step” in the latter reaction made it rate-controlling. The slow diffusion of cuprous ions was manifested in an inductive impedance component in this region.     

Passivity and pitting of corrosion resistant pure metals Ta,Nb, Ti,Zr, Cr and A1 in chloride solutions

The relative passivity and pitting resistance of pure metals Ta, Nb, Ti, Zr, Cr and A1 in chloride solutions were studied through quasi-stationary potentiokinetic anodic polarization technique. Studies were conducted in both non-aqueous methanolic and aqueous chloride solutions containing 0.4% HC1 with either hydrogen or oxygen saturation, in order to determine the role of water and oxygen in the passivity of these metals. Ta and Nb are passive in all the solutions, Zr and Cr require sufficient water to become passive, Ti needs both water and oxygen for stable passivity while A1 dissolves actively in all the solutions. The pitting resistance of the above metals decreases in the order Ta > Nb > Ti > Cr > Zr > Al. These passive metals are prone to pitting in chloride solutions in the absence of sufficient water which is the essential passivating agent.     

Cathodic inhibition of the corrosion of mild steel in phosphate,tungstate, arsenate and silicate solutions containing Ca2+ions

Abstract

It was found, usinig the potentiokinetic methed of cathodic polarisation of steel and platinum, that in the presence of Ca²⁺ Ions In dilute aerated solutions containing hydrogen-phosphate, -arsenate, -silicate and -tungstate ions, the cathodic reaction of the oxygen reduction is inhibited. This inhibition is due to the formation on the cathode of a layer of sparingly soluble calcium salts. Corrosion rate measurements with steel confirmed the beneficial influence of the addition of Ca²⁺ ions on the inhibitive efficiency of the above solutions. The best results were obtained with calcium phosphate and calcium tungstate.     

Corrosion inhibitor control in aqueous solutions evaluated with galvanic couples

An electrochemical method was evaluated for obtaining measurements to determine the effectiveness and the type of control exerted by corrosion inhibitors in galvanic systems. Anode and cathode potentials and corrosion currents were measured on zinc-platinum and iron-platinum galvanic couples in test solutions before and after addition of potassium chromate as an inhibitor. The measurements were then plotted as galvanic polarization diagrams which indicate a comparative corrosion rate and the type of inhibitor control, as anodic, cathodic, or mixed. Some of the results were also compared with weight loss data. Experimental variables such as inhibitor concentration, sodium sulfate concentration, corrosion current density, spacing between the anode and the cathode of the couple, and external resistance of the test circuit were investigated.The method gave a good evaluation of the behavior and the type of control of an inhibitor in galvanic systems in dilute aqueous solutions. The results show that it possibly could be used to identify the inhibitor behavior in ordinary corrosion provided the corrosion current generated by the galvanic couple is adjusted to approximate that generated by local cell action. The inhibiting effectiveness of chromate ions was lowered by the presence of the other anions such as the sulfate ion. When the initial corrosion current started at low values, the chromate tended to develop greater anodic polarization and better inhibition than higher initial corrosion currents.     

Study of the inhibition of the acid corrosion of steel by auger electron spectroscopy

The Auger spectra for mild steel specimens immersed in deoxygenated uninhibited and inhibited acid solutions gave clear peaks for iron, oxygen and carbon. A comparison between the height of the oxygen and carbon peaks in an acid medium and in an acid medium containing 1-phenyl thio-semicarbazide indicated that the height of the oxygen peak becomes less in the presence of the inhibitor while the carbon peak becomes larger: this behaviour was attributed to the adsorbed molecules of the inhibitor.From the Auger sputtering depth profile for mild steel immersed in H₂SO₄ solutions in the absence and presence of the inhibitor, the thicknesses of the oxide film formed were found to be similar in both cases, i.e. the inhibitor does not appear to change the composition of the corrosion product film.     

Mechanism of the corrosion inhibition of Zn Anode in NaOH by gelatine and some inorganic anions

The quantity of electricity required for passivation of Zn anode in NaOH solutions has been determined in absence and presence of different concentrations of gelatine, phthalic acid, sodium sulphide, sodium silicate or potassium chromate. The results obtained for gelatine in dilute alkali conform with a simple Langmuir model, whereas deviations are observed for the rest of the additives. The concentrations needed to establish a certain inhibiting efficiency increase wth increase in alkali concentration. The results are interpreted in the light of dissolution-precipitation mechanism.     

Zur Thermodynamik des Systems Mangan/wäßrige Lösung in der Abwesenheit und Anwesenheit von gelöstem Sauerstoff

Contribution to the thermodynamics of the system Mn/aqueous solution in the absence and presence of dissolved oxygen Using the thermodynamical criterion of the priority of certain reactions within a given system of possible reactions the authors consider the stability ranges of the different possible oxidation products of Mn in aqueous solutions. According to this study there is a limit at pH 8.5: below that value Mn²⁺-ions predominate, while above that value Mn(OH)₂ is the predominating substance. Under the influence of the oxygen dissolved in the water these primary oxidation products are transformed into products of increasing insolubility, the formation priorities of these products being different in alkaline and in acid media.     

Corrosion Behavior of Mild Carbon Steel in Ethanolic Solutions

Electrochemical evaluation of ASTM A36 steel was performed in ethanolic solutions containing small concentrations of water ranging from 0 to 10 vol.%. Electrochemical techniques such as open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization were utilized to analyze corrosion parameters. A fixed concentration of chloride, as per the ASTM specification for fuel grade ethanol, was added to increase the conductivity of the solutions. The effects of water and oxygen on the corrosion behavior of steel in these solutions have been discussed. Pitting corrosion of the steel specimens in these solutions was evaluated using scanning electron microscopy (SEM) and pitting analysis. This investigation was performed to establish a baseline for the microbiologically influenced corrosion (MIC) of steel in ethanolic solutions.     

Comparative study of the behaviour of AISI 304 SS in a natural seawater hopper, in sterile media and with SRB using electrochemical techniques and SEM

A comparative study of the electrochemical behaviour of stainless steel AISI 304 plates immersed in a natural seawater hopper and, under lab conditions, in sterile natural seawater, were carried out over seven weeks. The behaviour of these plates was, likewise, studied in seawater plus sulphate reducing bacteria (SRB) isolated from the natural seawater medium at the end of the seven week period. Electrochemical studies which measured the voltage over open circuit conditions (E_corr), linear polarisation and electrochemical impedance spectroscopy (EIS) were also carried out. The results have allowed us to profile the varying behaviour of the plates’ rusting in the natural seawater hopper, where a complex bio-film characteristic of natural environments is formed, as opposed to the processes of spreading in zones of high frequency due to the presence of voltage carrier species produced as the result of the transformation of sulphates into sulphides and to the lab conditions where a passive layer is formed, with capacitating behaviour, in the absence of bacteria, and a bio-film with sulphide inclusions is formed in the presence of the sulphate reducing bacteria (SRB) in an inductive semi-circle. The least protective passive layer appears in the presence of the SRB since the iron sulphide increases the susceptibility of steel to corrosion.     

Investigation of the major reduction reaction occurring during the passivation of galvanized steel rebars

The corrosion behaviour of zinc plates immersed in calcium hydroxide saturated solutions in the presence of oxygen, air and nitrogen, respectively, was studied. The corrosion resistance was monitored by potentials and corrosion rate measurements. The results obtained were confirmed by further data collected on galvanized rebars embedded in concrete and stored in different curing environments. The results clearly show that oxygen is the main oxidizing agent and indeed not water for the rapid formation of an effective passivation layer on zinc coating.     

Effects of Cathodic Potential, Bicarbonate, and Chloride Ions on SCC of X70 Pipeline Steel

This is an investigation on the stress corrosion cracking (SCC) behavior of X70 pipeline steel in high pH carbonate-bicarbonate solutions with different concentrations of bicarbonate and chloride ions and at cathodic potential of ?1100 mV versus saturated calomel electrode (SCE) using slow strain rate testing. Electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy were used to investigate the electrochemical behavior of X70 pipeline steel in solutions with different concentrations. X70 pipeline steel fracture surface morphology in these different solutions was also studied by scanning electron microscopy (SEM). The results suggested that the susceptibility to SCC for X70 pipeline steel decreased in the most dilute carbonate-bicarbonate solution in the absence of the chloride ion. Also, at potential of ?1100 mV versus SCE, all fracture surfaces showed semi-brittle behavior with transgranular cracks.     

Conductivity of aqueous suspensions of alumosilicates

When studying the electrochemical properties of diluted suspensions of natural and modified bentonite, the correlation between the inductive component of the system’s resistance and the temperature has been established. It confirms the hypothesis on the presence of paramagnetic particles in the system, whose solubility decreases with the growth of the temperature. Active forms of oxygen, as generated in the near-electrode space, can be such particles. It is shown that, in model systems, hydrogen peroxide is formed in this space due to the reversible dissociative adsorption of oxygen on platinum in the presence of a suspension electrode with proton conductivity.     

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.     

Zum einfluß des borat-anions auf die elektrochemischen eigenschaften des eisens in Wäßrigen Lösungen

Measurements of potential—time and current density—potential curves of iron were carried out in air-saturated and argon-flushed sodium borate solutions of different concentrations and pH to determine whether borate anions possibly influence the electrochemical behaviour of iron even in the absence of oxygen. The critical minimum pH necessary for passivating iron in air-saturated borate solutions has been established. Using these results and those taken from the literature we discuss the role of the borate anion in possibly stimulating the process of oxygen reduction and/or in influencing the process of metal oxydation, respectively. The latter influence, leading to a decrease of the critical current density, does not absolutely require the presence of oxygen. However, with the formation of hydrogen as the cathodic reaction it cannot give rise to the passive state of iron. The relation between the potential of the passive iron electrode in air-saturated borate solutions and the pH is explained by Fe(III)-borohydroxo-complexes possibly being formed on the solution side of the passive layer.     

Corrosion behaviour of pure Mg,AS31 and AZ91 in buffered and unbuffered sulphate and chloride solutions

Abstract

The low corrosion resistance of magnesium limited its application in industrial affairs. The main corrosive anions for magnesium are sulphate and chloride. This paper deals with their effect at low concentrations on the corrosion behaviour of Mg and Mg based alloy (AS31 and AZ91) in presence/absence of buffer solutions (pH 8). The electrochemical measurements used are open circuit potential and potentiodynamic polarisation. The results exposed that in the absence of a protective layer, general corrosion is observed to occur at greater rate than when sulphate is present; however, the general corrosion is transformed to localised corrosion faster in presence of chloride. In the presence of a protective layer, the localised corrosion is perceived in the case of chloride more than sulphate. The presence of aluminium in the alloys has two contradictory actions. One increases the passivity of the formed layer and the other increases the localised corrosion. The percentage of aluminium in the alloy controls these actions.