Chemische und elektrochemische Reaktionen von Eisensulfid und Mangansulfid in sauren und neutralen Lösungen

Chemical and electrochemical reactions of iron sulfide and manganese sulfide in acid and neutral solutions The reactions which occur upon corrosion of massive iron sulfide and manganese specimens in perchloric acid and in neutral sodium chloride solution were elucidated by measurements of current-potential curves and by coulometric and analytical investigations on the processes. In acids the sulfides are dissolved by prevailing chemical reaction under evolution of H₂S. Upon applying anodic overpotentials electrochemical reactions occur simultaneously, however, with such low velocity that the contribution to corrosion of the sulfides is insignificant. Upon applying cathodic overpotentials some hydrogen discharge is observed on iron sulfide but not on manganese sulfide. In 3% sodium chloride solution both sulfides corrode very slowly upon anodic polarization, forming elementary sulfur according to MeS = Me²⁺ + S + 2e^? (Me = Fe or Mn). At high anodic potentials additional oxidation reactions occur in which three-valent iron and tetravalent manganese ions as well as sulfite and sulfate ions are formed. Iron sulfide and manganese sulfide inclusions can he isolated from steels only by electrochemical dissolution in neutral or weakly basic electrolytes, the potential during electrolysis must not be more positive than the corrosion potential of the sulfides.     

Electrochemical properties of iron silicocarbide and cementite in acidic and neutral environments

Voluminous specimens of individual Fe₅SiC iron silicocarbide and Fe₃C iron carbide phases were produced by mechanical alloying with subsequent pressing and considered as models of nonmetallic inclusions in carbon steels and siliceous cast irons. In an acidic sulfate solution, silicocarbide is highly active in hydrogen reduction and iron ionization in the active dissolution range. Therefore, the corrosion resistance of silicocarbide in acids is lower compared to iron and cementite, which is caused by the peculiarities of its crystal structure. In a neutral borate solution both on silicocarbide and cementite, two anodic peaks are observed that are caused by the dissolution of the corresponding phase and the additional oxidation of the passive film, which is characterized by the heightened defectiveness because of the carbon accumulation. Silicocarbide has lower dissolution currents compared to cementite and a higher resistance to the local activation and depassivation, which is caused by the presence of a superficial layer enriched in SiO₂.     

Untersuchungen zum Korrosionsmechanismus von unlegiertem Stahl in sauerstofffreien Kohlensäurelösungen – Teil II. Kinetik der Eisenauflösung

Investigations into the Corrosion Mechanism of Mild Steel in Oxygen-Free Carbon Dioxide Solutions. Part II. Kinetic of Iron Dissolution Measuring the potential dependence of the anodic iron dissolution at mild steel in O₂-free CO₂-containing 0,5 M sodium sulfate solutions at temperatures between 25 and 75 °C an anodic Tafel-slope of b_a = 40 ± 1 mV was found. This means that the active iron dissolution follows the uncatalyzed Bockris-mechanism. The corrosion rates at the free corrosion potential were measured in dependence of temperature and CO₂ concentration by extrapolation of anodic Tafellines and by polarisation resistance measurements. From these results the activation energy of the iron dissolution was calculated to be 50 to 100 kJ/mol depending on pH. Based on all experimental results a complete corrosion mechanism is formulated.     

Untersuchungen zum Korrosionsmechanismus von unlegiertem Stahl in sauerstofffreien Kohlensäurelösungen. Teil I. Kinetik der Wasserstoffabscheidung

Investigations into the corrosion mechanism of unalloyed steel in oxygen-free carbonic acid solutions. Part I – Kinetics of hydrogen evolution Polarisation measurements with rotating disc electrodes showed that the corrosion of mild steel in ogygen-free carbon dioxide solutions is not limited by transport processes but is controlled by cathodic activation polarisation. The reason, why oxygen-free carbon dioxide solutions are more corrosive than diluted solutions of mineral acids at the same pH, is explained by the findings that the cathodic reaction is controled by the reduction of both hydrogen ion and adsorbed undissociated carbonic acid, thus stimulating the Volmer-reaction. Electrochemical hydrogen permeation measurements showed that in carbon dioxide solutions the uptake of hydrogen in mild steel is approximately twice as high as in comparable sulfuric acid solutions. Considering the results of comparing experiments with carbon monoxide the possibility of hydrogen embrittlement of mild steel in condensates under high partial pressure of carbon dioxide must be taken into account.     

Die Inhibitorwirkung von Phenyl-trithioniummethosulfat auf die Korrosion des Eisens in Salzsäure

The inhibition effect of phenyl-tri-thionium-metho-sulphate on the corrosion of iron The following measurements were made in order to test the inhibitor effect of phenyltrithioniummethosulphate as an example of a film-producing inbibitor: determination of galvanostatic polarization curves, oscillographic cut-off measurements, impedance determinations at very small measuring voltage (8 mV), determination of the dissolved iron quantity per time unit by chemical analysis, polarographic test of the inhibitor. The measurements have shown:

• 1 The a/m substance is definitely a filmproducing inhibitor.
• 2 The inhibition in an air-saturated solution is relatively better than in deaerated solutions; however in deaerated acides the absolute dissolution velocities are consistently smaller than in aerated acides.
• 3 The covering of the iron surface with an inhibitor film causes a strong increase in the slope of the cathodic Tafel line; the better is approximately proportional to the logarithm of the inhibitor concentration. A hypothesis is developed in order to explain this phenomenon.
• 4 The Ohmic resistance of the inhibitor film is very small.
• 5 The dissolution velocities determined by means of the polarization curves show in a deaerated acid a good correspondence with the data found by chemical analysis. No such correspondence exists in air-saturated acid.

     

Electrochemische Untersuchungen zur Hochtemperaturkorrosion von Chromstählen in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of chromium steels in alkali sulfate melts Electrochemical and corrosion-chemical investigations have been carried out with scaling resistant chromium steels, iron, chromium and platinum in a eutectic (Li, Na, K)₂-SO₄. It has turned out that sufficiently exact data concerning corrosion reactions can be obtained only from mass losses, not, however, from current density. The corrosion behaviour depends from potential. Comparable to the conditions in aqueous solutions potential ranges exist with passive and transpassive corrosion and with a rupture potential which depends from the chromium content of a steel. Protective oxide layers exist in the passive range where the mass loss becomes almost constant after a certain in cubation period. In the transpassive range corrosion follows an almost parabolic law with formation of an inner sulfide layer and a thicker external oxide layer where chromium is enriched. These layers are largely formed by oxidizing media carried to the metal surface via sulfur oxides; during this reaction inert marks in the steel remain unchanged. Sulfur oxides may be formed as secondary consecutive products by reactions between metal ions and sulfates. The solubility of metal ions in the sulfate melt is an important parameter for corrosion rates. Oxide ions (as reduction products of O₂) act as inhibitor on the anodic partial reaction, while SO₃ and ferric ions have a large lating effect, so that the anodic dissolution is autocatalyzed. Chlorides, too, act as stimulators in the transpassive range. Corrosion at the free corrosion potential is largely controlled by ferric ions which act as anodic and cathodic stimulators in acid melts. In neutral melts under oxygen an 18% chromium steel is passive.     

Mechanisms of stress corrosion cracking for iron base alloys in hot Ca(NO3 )2 solution

Abstract

The stress corrosion cracking (SCC) susceptibilities of a series of iron base alloys were evaluated in a 2M Ca(NO₃ )₂ solution at a temperature of 90°C. The inclusion distributions in each material were established using quantitative metallography and energy dispersive X-ray analysis (EDX). Electrochemical and microcorrosion measurements were also performed. Cracks were predominantly associated with pits that initiated at ruptures in the oxide film at potentials more anodic than -25 mV or by dissolution at sulphur containing inclusions at potentials more cathodic than -25 mV. Reducing the inclusion content and/or the activity of the inclusions in iron base alloys moves the minimum potential for cracking to more anodic potentials but does not eliminate SCC. The addition of sulphate to the test solution increased the susceptibilities of the alloys.     

Über den Einfluß von Schwefelwasserstoff auf die Korrosion des Eisens in sauren Lösungen

The influence of hydrogen sulphide on the corrosion of iron in acid solutions A considerable change of the polarisation behaviour of iron electrodes in acidic perchlorate and sulphate solutions was observed if they where saturated with hydrogen sulfide gas. H₂S markedly increased the anodic current densities and increases the corrosion rates by a factor of ten. The anodic Tafel slopes d log i_A/dE are only about half as steep as those in the absence of hydrogen sulphide. It is probable that the SH^?-ions behave as catalysts of the iron dissolution reaction in a similar manner as the OH^?-ions, but are adsorbed almost to saturation.     

Electrochemical behavior of nickel in HNO<Subscript>3</Subscript> and the effect of chloride ions

The electrochemical behavior of nickel in HNO₃ solutions of varying concentrations was examined using the cyclic voltammetry and potentiodynamic anodic polarization techniques. The anodic branch of the cyclic voltammogram is characterized by one anodic dissolution peak and a passivation region before oxygen evolution. The cathodic branch shows only one cathodic reduction peak corresponding to the reduction of HNO₃. Analysis of the anodic polarization data shows features of both reversible and irreversible reactions pointing to the complexity of the system. CT⁻ ions enhance the active dissolution of nickel in HNO₃ due to the adsorption on the bare metal surface and cause destruction of the passive film and initiation of pitting corrosion.     

Kinetics and mechanism of corrosion of iron in phosphoric acid

A study of the anodic and cathodic reactions involved in the corrosion of iron in phosphoric acid shows that the rate determining steps for the two reactions are similar to those reported for corrosion of iron in sulphuric acid. However, the dependance of i_corr and E_corr on time and solution parameters shows that an additional factor in the corrosion of iron in phosphoric acid is the slow coverage of the iron surface with phosphate resulting in a decrease of corrosion rate with time.     

Reproducibility of corrosion parameters for the acidic dissolution of pure iron: potentiostatic polarisation

Polarisation curves were determined potentiostatically for pure polycrystalline iron corroding in oxygen-free 0.5 M H₂SO₄. Four different working electrode pre-treatments (abrasion/polishing, pre-polarisation and time to establish E_corr) were employed and the reproducibility of E_corr and calculated corrosion parameters (i_corr, Tafel slopes and i₀) for each treatment was determined. Electrode pre-treatment effects changes in working electrode catalytic activity with subsequent variation in the reproducibility of polarisation curves and measured and calculated corrosion parameters. A method incorporating abrasion/polishing followed by anodic/cathodic pre-polarisation resulted in general in improved parameter reproducibility and cathodic and anodic Tafel slopes close to those predicted by the reaction mechanisms.     

Study on localized corrosion mechanism of 2195 Al‐Li alloy in 4.0% NaCl solution (pH 6.5) using a three‐electrode coupling system

Localized corrosion morphologies of 2195 Al‐Li alloy with various heat treatment in 4.0% NaCl solution (pH 6.5) were investigated, and its corrosion mechanism was studied using a three‐electrode coupling system of α (Al) substituting for the precipitate‐free zone (PFZ), simulated bulk θ′ (Al₂Cu) and T1 (Al₂CuLi). θ′ acts as cathodic zone in the alloy. At the initial stage, T1 phase is active with respect to θ′ and α (Al), and endures the main anodic current, indicating that anodic dissolution occurs on T1. However, its potential moves to positive direction with immersion time, due to dealloying of Li from T1. As a result, the main anodic dissolution occurs on α (Al) at a later stage. At this stage, as only T1 and α (Al) are coupled, T1 is cathodic to α (Al). In real 2195 alloy, T1 phase is very tiny, and anodic dissolution of T1 and PFZ occurs alternately. These results show that its intergranular corrosion or intersubgranular corrosion is caused by alternate anodic dissolution of T1 phase and PFZ along grain and subgrain boundaries.     

Corrosion-electrochemical properties of nanocomposites of α-Fe+Fe<Subscript>3</Subscript>C+VC in acidic and alkaline sulfate solutions

This work studies corrosion-electrochemical properties of three-phase composites of α-Fe+ Fe₃C + VC in acidic and alkaline sulfate solutions. Nanosize carbide inclusions are characterized by a high level of activity in the reaction of cathodic hydrogen evolution and result in a high rate of composite dissolution in acidic solutions. The presence of carbides and carbon has a negative effect on passivation of the iron matrix. The resistance of vanadium carbide inclusions to anodic oxidation increases upon a decrease in the amount of carbon in the carbide.     

Corrosion properties of Co-based cemented carbides in acidic solutions

The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H₂SO₄ solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H₂SO₄ solution. The specimen in H₂SO₄ solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution.     

Über das korrosive Verhalten von Eisen in Essigsäure

The corrosion behaviour of iron in acetic acid Experimental investigation with Armco iron in oxygen-free sodium acetate and sodium chloride acidified with acetic and hydrochloric acid respectively; the reaction is assumed to be under cathodic control. According to results obtained so far both acids may function as proton donators; the activation energy for proton formation from acetic acid is probably lower. Proton donations by acetic acid is further enhanced by adsorption of acid molecules to the metal surface. On the other hand the adsorption effects may be compensated by too low a conductivity of the solution. Complex formation by acetate ions seems to play a minor role. As to the mechanism of the corrosion studied the Volmer reaction is assumed to be the partial reaction subject to the most pronounced deceleration; this reaction is divided into two steps, a dehydratation of the proton with pronounced activation hindrance, and subsequent extremely fast breakthrough reaction.     

Modellversuche zur pH-Wert-Änderung des Elektrolyten in Anrissen verschiedener Stähle während der Korrosion

Pilot experiments for pH-changes of the electrolyte in cracks of corroding steels pH-values and concentrations of metallic ions during anodic dissolution of pure iron, Fe 1 Cr, Fe 5 Cr, Fe 18 Cr 9 Ni in 3 TMKCl in a model of crevice in oxygen-free conditions have been studied. For pure iron pH-values less than 4.65 have not been recorded. For chromium steels it has been found that with increase of chromium ions concentration in crevice solution pH-value decrease to almost zero. This dependence cannot be explained by simple hydrolysis of chromium ions.     

Inhibition of mild steel corrosion in phosphoric acid solution by triazole derivatives

Corrosion inhibition by triazole derivatives (n-PAT) on mild steel in phosphoric acid (H₃PO₄) solutions has been investigated by weight loss and polarization methods. The results indicate that these compounds act as mixed-type inhibitors retarding the anodic and cathodic corrosion reactions with emphasis on the former and do not change the mechanism of either hydrogen evolution reaction or mild steel dissolution. Some kinetic parameters are obtained.     

On the Inhibition of the Carbon Dioxide Corrosion of Steel by Carboxylic Acids

The inhibitive effects of aliphatic carboxylic acids on steel corrosion in the liquid and vapor phases of carbonate media were studied. It was shown that the acids become more effective with an increase in their hydrophobicity and can inhibit both the cathodic and anodic processes on steel because of their high adsorbabilities. Lauric acid was found to be the most effective inhibitor of carbon dioxide corrosion among the carboxylic acids studied. Although its molecule includes a relatively long alkyl fragment (C₁₁H₂₃), this acid is rather volatile and can be used as VCI. Caprylic acid in a concentration of 3.7 mmol/l inhibits steel dissolution in the temperature range from 30 to 100°C and increases the apparent energy of corrosion activation.     

Korrosionselektrochemisches Verhalten von Eisen-Nickel-Legierungen in sauren Sulfidlösungen

Electrochemical corrosion behaviour of iron-nickel alloys in acidic sulphide solutions Using the polarization resistance method and potential dynamic polarsation measurements the influence of hydrogen sulfide on the electrochemical corrosion behaviour of iron nickel alloys in sulfuric acid media has been studied. It has been found that hydrogen sulfide has a stimulating effect on corrosion rates. This effect depends on the solution pH and on the composition and structure of the alloys and is not announced in the case of alloys with higher nickel content. It has been shown that the presence of hydrogen sulfide in the solution influences the anodic metal dissolution reaction as well as the cathodic hydrogen evolution reaction. The differences in the electrochemical behaviour of the two FeNi alloys (having α- and γ-phase structures) can be observed only in the case of the cathodic reaction. According to the results of AAA it has been found that the presence of hydrogen sulfide in sulfuric acid media does not give rise to the selective dissolution of the alloy components.     

Film formation on stainless steel in a solution containing chromic and sulphuric acids

The paper discusses the kinetics of the spontaneous reaction occurring during the formation of a coloured interference film on stainless steel in hot CrO₃ + H₂SO₄ aqueous solution. The film behaves as a porous membrane, permitting liquid phase diffusion of the dissolution product of the steel through it. Film growth is by precipitation at the film/solution interface of a spinel oxide to which metal ions formed by both anodic dissolution of the steel and reduction of chromic acid—the accompanying cathodic process—contribute. The kinetics of the reaction follow an exponentially increasing growth law, which is unusual and possibly novel among film-forming reactions.