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.     

Einfluß der Kationenzusammensetzung von Chloridlösungen beim Modellieren von Korrosionseinflüssen der Küstenatmosphären

Influence of the cation composition of chloride solutions in modelling corrosion effects due to caoastal atmospheres Pure sodium chloride solutions cannto be used for accelerated weathering tests in artificial sea atmosphere. A spray liquid suitable for such tests is artificial seawater containing (grams/litre) 27 NaCl, 6 Mg chloride, 1 Ca chloride and 1 KCl. The corrosion behaviour of the two metals studied – iron and zinc – is characterized by typical differences, in particular variations depending on the salt quantity applied. While the corrosion of iron at first increases with the salt concentration applied and achieves a maximum at about 1000 mg/m²d, the losses of zinc increase just to a salt quantity of 200 mg/m² d and are then practically constant up to about 1000 mg/m²d; at higher concentrations there is again a slight diminuation of corrosion. The corrosion in artificial seawater is more pronounced than in sodium chloride solutions; this fact can be attributed to the hydrolysis of the chlorides in seawater, which gives rise to the formation of anodic and cathodic regions on the metal surface; as a consequence, pH differences are created and corrosion is enhanced. In order to simulate natural conditions as closely as possible the application of the salt spray should be interrupted by extended dry periods.     

Zum Einfluss der kathodischen Reaktion auf die chloridinduzierte Bewehrungskorrosion

Investigations on cathodic control of chloride‐induced reinforcement corrosion Regarding the mechanisms of reinforcement corrosion there are still contradictions with respect to the controlling/rate‐determining factors of the corrosion process. It is often discussed, that the electrolytic resistance of the concrete is the controlling factor and that the corrosion rates can subsequently be calculated from concrete resistivity. However, extensive research carried out by the authors has clearly demonstrated, that instead of concrete resistivity the resistance to cathodic polarisation normally is the controlling factor in the case of chloride‐induced macrocell‐corrosion. Generally, cathodic control can be related to restricted oxygen diffusion or activation control. In the present paper, these relationships are discussed in detail by results of numerous tests on the cathodic polarisation behaviour of passive reinforcement. For simple defined geometrical conditions simulating practical cases it is shown by a numerical analysis that the resistance to activation is normally the controlling factor for the corrosion rate and that oxygen diffusion has only to be taken into account, when the concrete is permanently water saturated or extremely dense. To verify, whether it is correct to estimate corrosion rates from resistivity data, tests should be carried out to check the influencing parameters on concrete resistivity and cathodic activation of passive steel surface areas.     

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.     

A peculiar cathodic process during iron and steel corrosion in sulfate reducing bacteria (SRB) media

Features related to the cathodic reduction of iron sulfides precipitation on iron surface during its exposure to SRB culture were studied. Electrochemical measurements were performed with pure iron and platinum electrodes plated with a thin iron film in de-aerated SRB culture. The study reveals that iron sulfide precipitation is being cathodically reduced just below a potential of −0.1 V_SCE, and if iron corrosion process occurs at potentials below that threshold potential, then the reduction of iron sulfide may provide an alternative cathodic depolarization mechanism in SRB. This cathodic process can maintain iron and corrosion at potentials above RHE potential.     

Untersuchungen über den Einfluß der Mikrostruktur auf die interkristalline und Kornflächenkorrosion von reinen Aluminium-Zink-Magnesium-Legierungen in 1 M Natriumchloridlösung

Investigation into the influence of the microstructure on the intercrystalline and grainphase corrosion of pure aluminium-zinc-magnesium alloys in an 1 M sodium chloride solution Potentiostatic polarisation tests of homogeneous pure aluminium-zinc-magnesium 2- and aluminium-zinc-magnesium 3-alloys as well as of a pure aluminium zinc-magnesium 3-alloy with two step ageing and quench-interruption treatments were carried out in an airsaturated 1 M sodium chloride solution at a temperature of 303 K. The pure aluminium-zinc-magnesium 3-alloy contents (weight percent): zinc 4,76; magnesium 3,00: impurities 0,08; balance aluminium, the pure aluminium-zinc-magnesium 2-alloy: zinc 4,60; magnesium 2,00; impurities 0,011; balance aluminium. The more negative breakdown potential in the homogeneous aluminium-zinc-magnesium-alloy than in the pure aluminium is caused by the zinc. According to the heattreatment the aged aluminium-zinc-magnesium 3-alloy shows above the breakdown potential grainphase corrosion and/or intercrystalline corrosion. In contrast to specimens with G.P.-zones or ν-precipitates in the matrix the specimens with ν′-matrix precipitates are grain-phase corroded along deep parallel streaks. This observation is attributed to the favoured formation of the ν′-precipitates on the (111)-planes in the matrix. The intercrystalline corrosion is more marked in specimens with a wider precipitate free zone, i.e. with a higher concentration of zinc and magnesium in the precipitate free zone.     

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.     

Untersuchungen zur Korrosion und zum Korrosionsschutz von meerwassergekühlten Rohrkondensatoren aus Kupferbasislegierungen-Versuche mit rotierenden Proben

Investigations on the corrosion and corrosion protection of seawater cooled condensor tubes of copper-base alloys - Experiments with rotating samples Experiments have been done on rotating disc and cylinder electrodes of CuZn20Al2, CuNi10Fe and CuNi30Fe in 3 w/o sodium chloride solution and in artificial sea water at 25 and 40 ° C. Erosion corrosion of sea water cooled tube condensors should be simulated and studied. The corrosion mechanism of copper base alloys in clean and polluted sea water was intensely studied with particular consideration to the influence of ammonia, sulfides and chlorine. The effectiveness of iron sulfate dosing was tested. With the rotating samples under test no erosion corrosion could be detected on free corroding specimens.     

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.     

Untersuchung der Reaktionsmöglichkeiten von Sulfiden bei der Einwirkung von Kohlensäure und Sauerstoff auf sulfidhaltigen Zementstein

Investigation into potential reactions of sulfides during the action of carbonic acid and oxygen on sulfide containing concrete A possible sulfide content in concrete cannot be considered to be a potential source of stress corrosion cracking, it is true that during carbonate formation a small part of the sulfide is set free as hydrogen sulfide, but, on the other hand, oxygen from the air gives rise to rapid oxidation with formation of sulfur or thiosulfate. To study potential corrosion under such conditions the saturated calcium sulfide solution has shown to be unsuitable since in this case the modifications due to atmospheric oxygen are disregarded.     

Untersuchungen zum Einfluß von Inhibitoren auf die Wasserstoffversprödung von unlegiertem Stahl in Salzsäure

Investigations on the influence of inhibitors on hydrogen embrittlement of mild steel in hydrochloric acid The influence of four commercially available inhibitors as well as their main effective components on the corrosion behaviour of mild steel in 16 and 32% hydrochloric acid was investigated by potentiokinetic polarisation measurements, electrochemically controlled measurements of hydrogen permeation and by estimating the rate of mass loss. The inhibitors showed different, yet positive effects on the inhibition of the permeation of cathodically produced hydrogen. The best results were found with benzylchinolinium chloride as inhibitor. Particularly in acids containing hydrogen sulfide propargylalcohol turned out to be a poor inhibitor of hydrogen permeation. Under these conditions the addition of propargylalcohol to quart. ammonium salts even decreased their good effectiveness as single components. The fact that the effectiveness for the inhibition of hydrogen permeation often diminished in the course of short periods was not due to a simultanous increase of the corrosion rate.     

Zum Einfluß von Mangan auf die Korrosionseigenschaften von austenitischen 18.10-CrNi-Stählen

Influence of manganese on the corrosion properties of austenitic 18.10-CrNi stainless steels The influence of manganese in the range of 0.25 to 1.5 mass-% on the passivation and pitting corrosion behaviour of unstabilized and Tistabilized austenitic 18/10 CrNi stainless steels is examined by determination of useful characteristical electrochemical dates using potentiodynamical polarization measurements in H₂SO₄-acidic and neutral model electrolytes. In the case of Ti-stabilized steels, a trend to an improved ability to passivation and to an increased pitting corrosion resistance is signified with increased Mn-content. This is in agreement with the austenite stabilizing effect of manganese. In the case of unstabilized steels, a significant deterioration of the passivation and pitting corrosion behaviour is observed, if the Mn-content of the steel is increased from less than 0.7 to more than 1.0 mass-%. These observations are discussed in the viewpoint of segregation of Mn-rich sulfide inclusions in the steel, which are essentially influenced by the presence of titanium in the steel.     

Untersuchungen über die Lochfraßkorrosion des passiven Eisens in chlorionenhaltiger Schwefelsäure

Investigation into pitting corrosion of passive iron in sulphuric acid containing chloride ions Pitting corrosion of metallic materials is generally connected with presence of a surface layer giving rise to a local differentiation of the electrochemical behaviour of the metal surface. The pitting corrosion by halogen ions on passive metals is investigated using passive iron in chloride ion-containing sulphuric acid as the model system. Quantitative data are presented concerning the mechanism and kinetics of the individual processes giving rise to pitting corrosion in a chloride ion concentration range covering three powers of ten, and in the whole potential range of iron passivity, from the Flade potential to the transpassive breakthrough potential. Pit formation normally follows a linear kinetic law, the rate depending in particular from the chloride ion concentration and from the thickness of the passive layer. The growth of pit diameters follows a linear kinetic law, too; the dissolution current density in the pits depends from the chloride ion concentration. Comparative investigations carried out on active iron, and potential distribution as measured in the pits show that the metal is active in the pits, too. The heterogeneous mixed electrode condition — active pit/passive metal surface — is stabilised by resistance polarisation. The investigations so far do not permit any statement concerning the specific effect of the chloride ions.     

Einfluß von Eisen auf die Korrosionsbeständigkeit von Nickelbasislegierungen als mögliche Werkstoffe für SCWO-Reaktoren

Effect of iron on the corrosion behaviour of nickel based alloys for SCWO plants An experimental set up to study corrosion at high temperatures and high pressures and able to simulate the conditions of supercritical water oxidation is described. On the basis of the alloys AC 66, 45 TM, G-3 and 601 H the influence of iron on the corrosion behaviour of nickel base alloys in aqueous solution under high pressures containing oxygen and chloride is shown and discussed. The corrosion resistance of the nickel base alloys is decreased with increasing iron concentration in the whole temperature range (100 °C–420 °C). The corrosion started at lower temperatures and the weight loss increased rapidly with increasing iron content. The corrosion morphology changed from uniform corrosion for alloys with less iron content to deep pits for alloys with high iron concentrations.     

Corrosion inhibition of carbon steel in the near neutral media by blends of tannin and calcium gluconate

The efficiency of tannin/calcium gluconate blends, as corrosion inhibitors for carbon steel in near neutral water media, have been studied. Synergistic behaviour of two components blend of different concentrations has been demonstrated in solutions prepared with distilled and a tap water. The anodic character of the inhibitor was recognised by the shift of corrosion potential to more positive values and a decrease of the passivation current on potentiodynamic polarisation curves. The cathodic inhibition was also recognised by slowing down the cathodic current of the polarisation run. Soluble iron tannate chelate has been observed as a dark blue flux flowing down from the metal surface into solution of tannin in the tap water, resulting in a corrosion process. Tannin decreases the pH of tap water. Calcium gluconate added in tannin solutions increases the pH maintaining it in the near neutral region enabling the precipitation of a low soluble protective iron tannate and metal gluconate.     

Über den Einfluß des Gefüges von Zinklegierungen auf die Schutzfähigkeit von Zinkanoden in Seewasser

Influence of the structure of zinc alloys on the protective capacity of zinc anodes in sea water The use of zinc for cathodic protection duties in sea water is restricted because of the considerable polarisation of zinc. The behaviour is due in particular to cathodically active additions of lead, copper and iron. The microcouples then formed give rise to considerable zinc dissolution and to the formation of a thick and dense passivation layer. Since, however, any reduction of the iron content in zinc involves high cost it is more economical to add small quantities of Al, Cd, Si, Hg, Sn or Mn. These additions produce a considerable grain refinement and thus reduce the tendency to form microcouples even in the presence of iron.     

Inhibitoren der Korrosion 25(1) – Über die Rolle der Kationen in Neutralsalzen bei der Korrosion von Eisenwerkstoffen in Gegenwart von Sauerstoff

Corrosion inhibitors 25(1) – The role of the cation in a neutral salt in the corrosion of iron in the presence of oxygen The rate of uptake of oxygen, under standard conditions, in the corrosion of 500 mg DAB6 iron powder is dependent on the nature of the cation in a neutral salt (chloride). In the presence of alkali, quaternary ammonium, quaternary phosphonium and alkali-earth chlorides, the rate of oxygen uptake is comparable. With zinc, cadmium, nickel and cobalt chlorides a marked decrease in the oxygen uptake is observed between pH5 and pH7. This is probably due to a block in the cathodic area in the iron boundary layer through precipitation of the rather insoluble basic hydroxides of zinc, cadmium, nickel and cobalt.     

Simulation eines Korrosionselementes bestehend aus einer beschichteten Stahlelektrode mit einer Verletzung

Simulation of a corrosion element consisting of a coated steel electrode with a damage Cathodic blisters are generated on samples with an anodic electrocoating bearing no conversion layer to simulate the corrosion element coated steel electrode/bar steel. Therefore potentiostatic polarisation experiments were carried out in 3 wt.% sodium chloride solution. The coating pigmented with titanium dioxide consists of oil-free polyester, melamin resin and polyalcohol. In our investigations we varied the polarisation potential, the time of corrosion loading and in a restricted manner the coating thickness of the coating. The polarisation potential ranged between the limiting values of the free corrosion potential of a coated electrode and that of a bar steel electrode. The galvanic current and the delaminated area consisting of the number of blisters as well as the growth-rate of blisters are measured. There exists a constant current density at the delaminated areas, because for the galvanic current and the delamination has been found the same time behaviour. The current density depends on the polarisation potentials in a non linear way. This shows that the ohmic resistance of the coating does not determine the rate of the electrochemical reactions at the phase boundary exclusively. Blister generation and blister growth-rate increase linearly with time of corrosion loading, but only the blister growth-rate is influenced by the polarisation potential.     

Aqueous solution corrosion behaviour of “poor man” high manganese-aluminum steels

The corrosion behaviour of high manganese (19.8 – 32.5%)aluminum (7.1 – 10.2%) austenitic steels in different aqueous solutions has been examined. Potentiodynamic tests in 1 N sulphuric acid solution show high current densities either in the active or in the passive region and the formation of a thick film which separates in active/passive transition zone. Potentiodynamic tests in neutral buffer solution show a behaviour similar to that of the other ferrous alloys. Potentiodynamic tests in 1 N sodium hydroxide basic solution show a corrosion current density in the passive region related to the different manganese oxides higher than that of pure iron. The corrosion resistance of these steels is always worse than that of conventional grades austenitic stainless steels.     

Improved Corrosion Resistance in AZ61 Magnesium Alloys Induced by Impurity Reduction

The effect of impurity element Fe on corrosion behavior of AZ61 magnesium alloys in various states has been investigated by immersion test and hydrogen evolution measurements in 3.5% sodium chloride solution.The corrosion rate is found to relay on the impurity Fe concentration in the alloys and decreases with decreasing Fe content.When Fe content drops from 150 ppm to 10 ppm,the corresponding corrosion rates under as-cast and solution treatment conditions are reduced from 8.54 mm/a and 8.61 mm/a to 2.54 mm/a and 0.21 mm/a,respectively.The corrosion pattern of the AZ61 alloys is the localized corrosion,and the galvanic couples are formed among the impurity particles,second-phase particles and the matrix.The Fe impurity particles tend to act as main cathodic to form micro-galvanic cell with the α-Mg matrix,which is harmful for corrosion resistance of AZ61 alloy.