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.     

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 Mo,V, Nb,Ti, Zr und deren Karbiden auf die Korrosion und Wasserstoffaufnahme des Eisens in Schwefelsäure

Effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in sulphuric acid The effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in 1 M H₂SO₄/N₂/25°C were investigated by electrochemical and surface analytical methods using binary and ternary Fe-Me alloys with about 0.2 at.-%Me. Due to the experimental conditions, no protective surface layers formed. The transition metals were enriched at the iron surface only as a carbide or oxicarbide. Mo or Zr were not markedly enriched. The corrosion current density and the hydrogen activity were decreased only by Mo or Zr. Hydrogen permeation measurements were analysed in terms of the trapping theory. The average binding energy of shallow traps for hydrogen increases with increasing atomic radius of the substituted alloying elements in the order V, Mo, Ti, Nb and Zr. Correspondingly, the hydrogen diffusion coefficient (after saturation of the deep traps) decreases and the hydrogen solubility increases. The steadystate hydrogen permeation rate remains almost unaffected. The total hydrogen content is determined by the density of deep traps and thus mostly independent of the external hydrogen activity. The trapping effect of iron is strongly increased by dissolved Zr or Nb or in the presence of fine dispersed carbides as VC_x. The effect of coarse carbide particles at a lower density is small.     

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.     

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.     

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 Kinetik der Sauerstoffreduktion bei der Korrosion von Nickel in sauerstoffhaltiger Schwefelsäure

Kinetics of oxygen reduction during the corrosion of nickel in oxygen containing sulfuric acid The rate of oxygen reduction was determined for the corrosion of nickel in sulphuric acid containing oxygen. The rate depends upon the oxygen concentration and the acidity of the electrolyte. For a second order reaction with a preceding diffusion of oxygen and protons the values of the rate constant and the thickness of the diffusion layer were calculated. It could be shown that in In sulphuric acid the reduction rate is control led by the oxygen diffusion while in the weaker acids the rate controlling step is a proton transfer to the oxygen, adsorbed at the metal surface. Measurements of the dependance of the corrosion in In sulphuric and hydrochloride acid confirm these results.     

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 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 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.     

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.     

Einfluß der chemischen Zusammensetzung auf die Kinetik der atmosphärischen Korrosion von Stählen

The influence of the chemical composition on the kinetics of the atmospheric corrosion of steel A total of 19 different structural steels were exposed to urban, rural and industrial atmospheres for periods up to three years. The evaluation of the weight losses determind at certain intervals permitted the establishment of a linear relationship which can be used for assessing, within certain limits of concentration, the influence of specific alloying elements. The tests showed that the effects of the different alloys varies with the type of atmosphere and with the degree of corrosion, and may well shift from “favourable” to “detrimental”. The results are also influenced by the quantitative ratios of certain components. The corrosion behavior of the steels is consistently governed by P, S and Cu and, to a lesser extent, by C and Mn.     

Säureinhibitoren. III. Einfluß von quartären Ammoniumsalzen auf die Wasserstoffaufnahme von unlegiertem Stahl in H2S-freier und H2S-gesättigter Ameisensäure

Acid inhibitors. II. Influence of quarternary ammonium salts on the hydrogen absorption of mild steel in H₂S-free and H₂S-saturated formic acid The influence of quaternary ammonium compounds (benzylpyridinium chloride, benzylquinolinium halides, p-dodecylbenzylquinolinium chloride, benzylisoquinolinium chloride, tri-n-butylbenzylammonium halides, benzyldabcolinium chloride) on the rate of metal dissolution and absorption of corrosion hydrogen in the metal was studied at mild steel in chloride containing 10% formic acid at 25 °C in the presence and absence of H₂S. Generally the same effects were observed like in the system steel/hydrochloric acid. Depending on the kind of inhibitor the presence of H₂S increased or decreased the efficiency of inhibitors to block the H-absorption. Regardless of the presence of H₂S p-dodecylbenzylquinolinium chloride and benzylquinolinium iodide exhibited the best efficiencies in decreasing the rate of metal dissolution and inhibiting the H-absorption in the metal.     

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 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.     

Zum Einfluß der Oberflächenbearbeitung und der chemischen Zusammensetzung auf die Korrosion von Dental-Amalgamen

Influence of mechanical treatment of the surface and of chemical composition on the corrosion of dental amalgams The influence of mechanical treatment of the surface and of chemical composition of amalgams on the corrosion behaviour is examined in artificial saliva by determination of useful characteristical dates using potentiodynamical polarization measurements on little (0.03 V min^?1) and high (0.6 V min^?1) polarization rate. In the case of the amalgams the corrosion resistance increase with smaller roughness of the surface and smaller concentration of γ₂-phase. The concentration of γ₂-phase is influenced by the chemical composition and to a certain extent by the mechanical treatment of surface. Therefore dental amalgam with little or without γ₂-phase and polishing of amalgam are required for therapy of filling.     

Über den beschleunigenden Einfluß von Schwefeldioxid und Wasser auf die atmosphärische Korrosion von verrosteten Eisen

The accelerating effect of sulphur dioxide and water on the atmospheric corrosion of rusty iron The atmospheric corrosion process of rusty steel was observed in the laboratory in an atmosphere with 1, 10 and 100 p.p.m. SO₂, respectively, at a temperature of 30°C. It was found that the correlation of the corrosion rate with humidity can, in the range between critical humidity and nearly 100 per cent. Relative humidity, be represented by a rising quadratic parabolic equation. The differences in the three SO₂ concentrations had no influence on the corrosion kinetics, which is explained by the fact that, under the testing conditions, the rust was fully saturated with SO₄^2?. On the strength of these and earlier results, the authors submit a new working theory concerning the atmospheric corrosion of already rusty steel, introducing the theories of Heusler and Florianovitsch-Kolotyrkin into the sphere of atmospheric corrosion.     

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.     

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.

     

Über den Mechanismus der Zunderbildung auf Metallen und binären Legierungen

The mechanism of scale formation on metals and binary alloys The scale formed during high-temperature oxidation of many metals consists of two layers which clearly differ from each other morphologically yet occur in the same phase of the reaction product. The inner layer in direct contact with the metal is porous whilst the outer layer is compact. Experiments have shown that the inner layer is formed as a result of secondary processes connected with the influence of the metal consumption zone on the progress of the oxidation process. As the reaction product cannot fully adapt itself to the continually shrinking metal core, a gap occurs between the metal and the reaction product. At the boundary between this gap and the scale, the reaction product begins to decompose. The oxygen atoms liberated as a result of this decomposition react with the metal core, thus giving rise to the formation of the inner layer of scale in the metal consumption zone. The dissociation of the outer layer thus has an anisotrope character so that, in the originally compact layer of scale, a porous consumption zone is formed. The singlephase reaction product is actually composed of three layers, viz. a compact outer layer, a porous transition layer, and a porous inner layer. The porous consumption zone may even comprise the entire originally compact layer of scale so that the oxygen is able to diffuse towards the metal core through fissures and gaps in the reaction product.