Einflüsse von Mo,V, Nb,Ti, Zr und deren Karbiden auf die Korrosion und Wasserstoffaufnahme von Eisen in H2S-haltigen Lösungen

Effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in H₂S-solutions Effects of the transition metals Mo, V, Nb, Ti, Zr and their carbides as well as of phosphorous on the corrosion and hydrogen uptake of iron in acid to weakly acid NaCl solutions with and without H₂S are discussed. Investigations were carried out on binary, ternary and quaternary iron based alloys, using electrochemical and surface analytical methods. No specific effect of one of the alloying elements or the carbides on the corrosion or hydrogen uptake is observed. Due to the experimental conditions, sulphur and oxygen enriched surface scales form, by which the kinetics of the corrosion processes are determined. The alloying elements are enriched on the iron surface only as a carbide. Phosphorous is enriched as a phosphide at low pH and as a phosphate at higher pH. H₂S and phosphides increase the corrosion rate and hydrogen uptake. In pure iron or low strength iron alloys, at the very high H₂S affected hydrogen activities new lattice defects are induced permanently resulting in extremely high hydrogen concentrations.     

Einflüsse von Mo,V, Nb,Ti, Zr und deren Karbiden auf die Bindungszustände des Wasserstoffs in Eisen und das Bruchverhalten der Eisenlegieungen

Effects of Mo, V, Nb, Ti, Zr and their carbides on the binding states of hydrogen in iron and the fracture behaviour of the iron alloys Effects of Mo, V, Nb, Ti, Zr and their carbides on hydrogen permeation, diffusion, solubility and its distribution on different binding states in iron and iron based alloys are studied by use of the electrochemical hydrogen permeation technique. The results are analysed and described in terms of the trapping theory. The fracture behaviour of the alloys affected by hydrogen in different binding states are tested under constant elongation rate conditions. No essential specific effects of one of the alloying elements or their carbides are observed. Hydrogen diffusion and solubility, the total hydrogen content as well as the fracture behaviour are affected by the various microstructures of the alloys, thus only indirectly by the alloying elements. Crack initiation and fracture progress depend first of all on the external hydrogen activity and the grain size of the material, not on the total hydrogen content. Fine crystalline iron alloys of higher strength prove to be less sensible to hydrogen damage than coarse grain weak structures.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil II: Korrosion und Deckschichtbildung

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid Part II: Corrosion and Formation of Surface Layers The effects of C, S, P, Mn, Si, Cr, Ni, Sn and Cu on the formation of surface layer and hydrogen uptake of iron during corrosion in 1 M H₂SO₄/N₂ were investigated using AES, XPS, SEM and electrochemical permeation techniques. Cu, Sn, P and C are enriched on the surface of iron during corrosion in H₂SO₄. Cu is enriched in the metallic form. P forms a phosphate and phosphide containing surface layer. Ni is not enriched. Cr is preferentially dissolved. Cu, Sn and Ni inhibit the dissolution of iron and thus decrease the hydrogen activity. S, P and Mn (MnS) increase the corrosion and hydrogen activity. Cr forms traps in iron which increase the hydrogen uptake.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil III: Kinetik der Abscheidung und Aufnahme von Wasserstoff an binären Eisenlegierungen

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid – Part III: Kinetics of Proton Discharge and Hydrogen Uptake at Binary Iron Alloys The effects of C, S, P, Mn, Si, Cr, Ni, Sn und Cu on the kinetics of hydrogen uptake of iron in 1 M H₂SO₄ at 25°C are investigated by potentio-kinetic and steady state current density-potential measurements accompanied by hydrogen permeation measurements. The anodic dissolution of iron is increased mainly by S and P, and decreased by Ni, Sn and Cu. Hydrogen uptake is enhanced by S, P and Sn, and inhibited by Ni and Cu. The kinetics of hydrogen evolution on the surface of iron alloys investigated are controlled by a coupled discharge – chemical recombination mechanism.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil I: Permeation,Diffusion und Löslichkeit von Wasserstoff in binären Eisenlegierungen

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid Part I: Permeation, Diffusion and Solubility of Hydrogen in Binary Iron Alloys The influence of elements in common steels like C, S, P, Mn, Si, Cr, Ni, Sn, and Cu on hydrogen uptake and permeation through iron was studied for various binary iron alloys using the electrochemical permeation method. In order to characterize the effects of the alloying additions on the hydrogen activity at the metal surface as well as on the hydrogen transport in the bulk, the hydrogen permeation coefficients, diffusion coefficients and solubilities have been determined in the temperature range of 10 to 80° C. Steady state hydrogen permeation is significantly decreased only by Si and Cr. Si reduces mainly the solubility of hydrogen in iron and decreases only slightly hydrogen diffusion. Cr additions produce strong traps for hydrogen in iron, with an average binding energy of ? 58 kJ/mol. Thus, hydrogen diffusion is retarded. The total hydrogen content is increased by Cr. Permeation measurements of commercial steels showed mainly the Si-effect.     

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.

     

Einfluß von Silber und Thallium auf die anodische Korrosion der Bleilegierungen in Schwefelsäure

Influence of silver and thallium on the anodic corrosion of lead alloys in sulfuric acid Experimental alloys (Pb, up to 4.5% Ag; Pb, up to 10% Sb, Ag; Pb, up to 11% Sb, Tl; Pb, up to 0.3% Tl, As) were anodically oxidized in 7 N sulfuric acid at 25° C. The corrosion rate decreases as the Ag content increases; increasing Sb-content has only little effect. The influence of Tl is considerably smaller and is largely suppressed by Sb; addition of As results in corrosion rates exceeding those obtained with the Tl-free metal.     

Zur Korrosionsreaktion des Eisens in Salzsäure,Schwefelsäure und Perchlorsäure

Contribution to the corrosion reaction of iron in hydrochloric, sulfuric and perchloric acids At pH values between 0 and 4, even under circulation, a linear relationship results between log j_corr and the pH value, the slope of the corrosion straight line being between ?0.23 and ?0.27. The dissolution process is predominantly under cathodic control by hydrogen ion discharge; at pH values above 3 the diffusion inhibition of the hydrated proton is the rate controlling step. The corrosion potential in sulfuric and perchloric acids decreases by about 50 mV as the pH is increased, while a different behaviour is observed in hydrochloric acid; this behaviour may be explained in terms of the chloride ion adsorption. In the acids of higher concentration the activation energy is higher than in those of lower concentrations; this phenomenon may eventually be ascribed to a change in the mechanism of the corrosion reaction.     

Einflüsse von Silicium und Kohlenstoff auf die Sulfidierung von Eisen

Effects of silicon and of carbon on the sulfidation of iron The corrosive attack of steels by H₂S under FeS formation is impeding the use of heat exchangers in processes in which sulfidizing gases occur at low oxygen pressures – the project was aimed at finding ways and means to retard or suppress the FeS growth. Thermogravimetric investigations in H₂? H₂S at 400 °C showed that during the first 100 h of sulfidation a transition takes place from the linear kinetics controlled by the phase boundary reaction H₂S ? S (in FeS) + H₂ to the parabolic kinetics controlled by solid state diffusion in the corrosion products. During this transition the linear constant decreases with time and increasing sulfur activity a_s at the FeS surface (k₁ ? 1/a_s). Upon sulfidation of Fe-6%Si an internal Fe₃Si layer is formed te sulfidation is retarded since for long time ( 100 h) the slow surface reaction at high a_s is rate determining. Carbon deposits formed in carburizing atmospheres (a_c 1) on the iron surface, have a negligible effect on the sulfidation. Only graphitization of the iron surface at 700 °C after preceding carbon saturation at 1000 °C in CH₄? H₂ has an initially retarding effect. This study demonstrated possibilities of retarding the H₂S corrosion by Si or C which, however, are rather limited.     

Einflüsse von Werkstoff und Verarbeitung auf die Spannungsrißkorrosion von Spannstählen

Influence of material and processing on stress corrosion cracking of prestressing steel In prestressed concrete constructions the highstrength prestressing steels perform essential bearing effects. The alkaline layer of concrete or mortar protects the steels against corrosion and guarantees a permanent load capacity. If the corrosion protection as a result of poor workmanship is not guaranteed from the beginning, or is lost because of lacks of construction in the course of time, or the steels are predamaged during handling, stress corrosion cracking and failure of steel and construction may occur. Also an application of unsuitable materials (prestressing steel, injection mortar, concrete) can alone or in combination with the before mentioned influences favour stress corrosion cracking. In the contribution the correlations and typical failures are discussed.     

Einfluß von Kohlenstoff,Stickstoff und Phosphor auf die interkristalline Korrosion und Spannungsrißkorrosion des Eisens

Effects of carbon, nitrogen and phosphorous on the intergranular corrosion and the stress-corrosion cracking of iron Experiments without stress, with constant load, or with constant strain rate were performed in boiling 55% Ca(NO₃)₂ solution (115°C) to elucidate the effects of carbon, nitrogen and phosphorous on the intergranular corrosion (IC) and stress-corrosion cracking (SCC) of iron. The original material contained 20 to 40 ppm C, 17 ppm N, and 20 ppm P. One batch of this material was normalized only, a second batch was also decarburized up to under 10 ppm C prior to normalizing, and a third batch was decarburized, and then nitrided up to 140 to 220 ppm N prior to normalizing. Some of the specimens were tempered at 550°C for one month. All normalized specimens showed no susceptibility to IC at applied potentials from 800 to 1000 mV (SHE). In contrast, specimens tempered at 550°C were very susceptible to IC, which was observed even at 200 mV after decarburization. Auger-spectra of intergranular fracture surfaces of a tempered specimen produced at liquid-nitrogen temperature revealed grain boundary segregation of P (about 10 atomic %). The as-normalized specimens fractured intergranularly within 1 to 2 h, when held at constant load of 0.8 and 0.9 of ultimate tensile strength (UTS), and 0 mV. By contrast, under identical conditions, the decarburized specimens and the nitrided specimens did not fracture in 30 d. The fracture energy tested with the constant strain-rate method at 0 mV, as compared with experiments in oil, was strongly lowered in the as-normalized specimens and significantly lowered in the nitrided specimens. The values of fracture energy obtained for the decarburized specimens were mostly similar to those in oil at 115°C. Only some of the decarburized specimens showed substantial decreases in fracture energy. However, the decarburized and tempered specimen showed an even larger decrease in fracture energy, with intergranular fracture mode.     

Einflüsse des Phosphors auf die Interkristalline Spannungsrißkorrosion von Kohlenstoffstählen

Influence of phosphorus on the intergranular stress corrosion cracking of carbon steels The effects of phosphorus on the intergranular stress corrosion cracking were studied for steels with 0.15% C and 0.4 or 2% Mn, the phosphorus contents were 0.003, 0.03 and 0.05% P. Constant strain rate tests were conducted at constant potentials in 55% Ca(NO₃)₂ at 75°C, in 5 N NH₄NO₃ at 75°C, and in 33% NaOH at 120°C. The strain rate was 10^?6/s. Different grain boundary concentrations of phosphorus were established by varying the annealing time at 500°C, they were determined by fracturing the samples in UHV and analyzing intergranular fracture faces by Auger-electron spectroscopy. In the nitrate solutions the toughness, i.e. resistance against stress corrosion cracking of the steels is somewhat decreased with increasing P-content within the range of the corrosion potential, that is ?300 mV_H to ?50 mV_H in NH₄NO₃ and ?80 mV_H to ?50 mV_H in Ca(NO₃)₂. It is shown that this effect is caused by phosphorus segregated at the grain boundaries. At potentials above ?50 mV_H the relative toughness is very low for all tested steels, also without stress intergranular attack is observed. In NaOH the steels are most sensitive against stress corrosion cracking at ?700 mV_H, here the phosphorus content or segregation shows no effect. At potentials varied in both directions the toughness increases and a somewhat negative effect of phosphorus becomes visible. In all tested electrolytes the effect of phosphorus is restricted to a small potential range. There is no effect of phosphorus in the range of the minimal toughness, thus the steels are sensitive anyway, with and without phosphorus segregation. Therefore low phosphorus contents in carbon steels cannot guarantee resistance against stress corrosion, most decisive are the potential and the electrolyte.     

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.     

Zur Korrosion von binären CuAl-Legierungen in Schwefelsäure

On the corrosion of binary CuAl alloys in sulfuric acid The corrosion of binary CuAl-Alloys with Al-concentrations up to 16.5 w/o was investigated after different heat treatments in 1N sulfuric acid at 25°C. The corrosion behaviour of the various phases was characterized by potentiodynamic polarization curves and by long term potentiostatic tests combined with metallographic investigations of corroded electrodes. Within a limited potential range the results confirm the excellent corrosion resistance of the α-phase and the effect of dealuminization of all other phases. The origin of these differences in corrosion behaviour is discussed.     

Untersuchungen zum Einfluß von Sulfid- und Sulfationen auf die Korrosion des Eisens in alkalischer Lösung

Investigation into the influence of sulfide and sulfate ions on the corrosion of iron in alcaline solution The corrosion behaviour of Armco iron as well as that of a technical high-strength steel was investigated in sulphide - and sulphate-containing electrolytes of pH ～ 12.6. Current density - potential curves show that iron, after successful passivation, remains passive and protected against corrosion in a solution saturated with CaS, CaSO₄ and Ca(OH)₂ at potentials up to 800 mV(EH). Passivation experiments conducted by changing from the active region to various anodic potentials yielded a critical potential region around 300 mV(EH), above which corrosion appeared. Up to E_H = 200 mV complete passivation was observed. The same critical potential region was observed in experiments of repassivation after mechanical damaging of a protective passive layer. Additional tensile stresses of 85%· σ_0,2 caused no stress corrosion cracking at a potential of 500 mV(EH). After damaging the passive layer, through scratching of the stressed wires fracture occured at and above potentials of 300 mV(EH). At 200 mV(EH) repassivation was observed even under tensile load. In a sulphate-free Na₂S/Ca(OH)₂ solution of the same sulphide ion concentration no stress corrosion cracking of the samples occured at any potential up to 800 mV(EH) even after scratching the steel samples. The observed stress corrosion in sulphide - and sulphate-containing electrolytes is to be ascribed to the action of sulphate and not to sulphide ions.     

Korrosion von Kupfer,Nickel, Blei und Aluminium in 1 n Schwefelsäure durch Wechselstrom

Corrosion of copper, nickel, lead and aluminium in 1 N sulfuric acid by alternating current In view of the possibility that alternating current might flow to underground structures as a stray current the corrosion behaviour of steel in 1 N sulfuric acid has already been investigated. The study has new been extended to the effect of alternating current (50 cycles) on the behaviour of Cu, steel, Ni, Pb and Al. In this study it has been found, that the corrosion of these metals is increased by the current to 3 to times the normal corrosion, depending on the nature of the layer eventually formed by corrosion products and on the polarizability of the metal. Aluminium forms a protective layer having insulating properties.     

Einflüsse von HCl und N2 auf die Hochtemperaturkorrosion in Atmosphären mit niedrigem Sauerstoffdruck

Influence of HCl and N₂ on high temperature corrosion in atmospheres with low oxygen pressures The effects of HCl and N₂ on the high temperature corrosion in atmospheres with low oxygen pressures were investigated. Investigations of the alloy Fe-20Cr in He-H₂-H₂-O-HCl atmospheres at 1173 K under conditions, where no condensed chlorides could be formed, show the simultaneous growth of Cr₂O₃ and evaporation of CrCl₂. The kinetics of the reactions can be described by growth of Cr₂O₃ according to the parabolic rate law and transport of CrCl₂ in the gas flow following a linear rate law. During heating of the samples in He-H₂-H₂O-HCl condensed chlorides are formed which decrease the adherence of the oxide layers and cause oxide spallation during cooling. This can be avoided either by preoxidation in H₂-H₂O or heating in dry hydrogen. The porosity of Cr₂O₃ layers is not enhanced when HCl is added to the gas atmosphere. The oxidation and evaporation is enhanced in N₂-H₂-H₂O-HCl atmospheres compared to He-H₂-H₂O-HCl atmospheres. The Cr₂O₃ scales are coarse grained and poros. The enhanced porosity is caused by the nucleation and growth of chromium nitrides in the transient state of oxidation.     

Einflüsse kleiner Zusätze von Niob oder Cer auf Korrosion und Kriechen von Incoloy 800 in Co-H2O-H2-Atmosphären

Effects of small alloying additions of niobium or cerium on the corrosion and creep of Incoloy 800 in CO-H₂O-H₂-atmospheres In oxidizing and carburizing atmospheres at high temperatures Fe-Ni-Cr-alloys are carburized under creep conditions by carbon transfer through cracks in the oxide layer. In creep experiments in CO-H₂O-H₂ atmospheres at 1000 °C with several alloys based on Incoloy 800 the carburization could be related to the strain of the specimens. Alloying additions of Nb in the range 0.2 to 1% caused changes in the creep rate, a decrease for >0.35% and an increase for >0.35% Nb. The creep resistance for the high Nb concentrations could be improved by solution annealing at high temperatures (1200 °C). Niobium strongly decreases the carburization - this effect can be explained by the formation of an internal layer of Al- and Nb-oxides beneath the outer Cr₂O₃ layer. An alloying addition of Ce (0.06%) also has beneficial effects on the creep resistance and carburization resistance of Incoloy 800.     

Untersuchungen über die Einflüsse auf Stromdichte-Potential-Kurven und ihre Reproduzierbarkeit

Investigations into factors having a bearing on current density vs. potential curves and their reproducibility The shape of potentio-kinetic current density/potential curves enables conclusions concerning the corrosion behaviour of a material to be drawn only when the reactions taking place at the surface of the material are known, and when initial potential, holding time prior to potential variation and potential feed are taken into account. Experimental work with Pt in tap water, 0.5 N KCl and 5% H₂SO₄ has revealed primarily a relation between initial potential and current density in the regions of hydrogen ionisation and adsorption. The holding time preceding potential variation comes to bear in water and H₂SO₄ primarily in the hydrogen adsorption region, in KCl, however, in the oxygen adsorption region. Increasing potential feed results in increasing limiting diffusion current, the trend being parallel to that of current density changes.     

Der Einfluss von Oxidationsmitteln auf die Korrosionsbeständigkeit von Nicrofer 3127 hMo in niedrig‐ und mittelkonzentrierter Schwefelsäure

The influence of oxidants on the corrosion resistance of Alloy 31 in low and medium concentrated sulfuric acid The isocorrosion lines < 0.1 mm/y were determined for Nicrofer 3127 hMo (1.4562, Alloy 31) by weight loss measurements after 24 h immersion in areated sulfuric acid of technical and p.a. grades in the concentration range up to 90% – both without and with 1 g/l chloride. The free corrosion potential indicates that the superior corrosion resistance of Nicrofer 3127 hMo in the technical acid grade can be attributed to the content of oxidants in the acid. The presence of oxidants leads to metastable passivity in technical sulfuric acid with concentrations ≥ 20% at temperatures that can be 10–50 K higher than in p.a. acid depending on the acid concentration. For sulfuric acid concentrations up to 10% the content of oxidants in the technical acid is too small to improve the corrosion resistance of Nicrofer 3127 hMo as compared with that in p.a. acid. Similar behaviour was observed for Nicrofer 3127 hMo in sulfuric acid containing chlorides. However the activating effect of the chlorides limits the region of corrosion resistance to lower temperatures in comparison with the chloride free‐acid.