Untersuchungen zur Korrosion von metallischen Werkstoffen in hochkonzentrierten,siedenden Salzlösungen

Investigations into corrosion of metallic materials in highly concentrated boiling salt solutions Container for the disposal of radioactive waste in carnallite formations can be exposed to corrosion by highly concentrated hot salt solutions. Previous investigations of the corrosion resistance of candidate materials have been supplemented by measurements of the current density-potential curves of iron-silicon, cobalt-silicon, titanium-nickel and nickel-molybdenum alloys and of tantal. The electrolyte was a concentrated MgCl₂-NaCl-KCl-MgSO₄ solution at its boiling point of 170°C. The measurements give useful information on the stability of the corrosion resistance of these materials.     

Untersuchungen über den Einfluß der Grundbeschichtung auf Elementbildung und Blasenanfälligkeit von mit Teerpech-Epoxidharz beschichtetem Stahl in Salzlösungen

Investigations into the influence of the priming coat on galvanic couple formation and bubble formation susceptibility of steel coated with tar-epoxy coating in salt solutions Unalloyed steel sheets with the surface conditions grit blasted, hot-dip and metal-spray galvanized, coated with 4 different primers have been top coated with coal tar-epoxy resin of 4 different thickness values. The specimens have been immersed in NaCl solutions for two years. Potential and coating resistance values as well as cell currents between the specimens and external steel electrodes have been measured. Finally the corrosion amount of the steel electrodes and of artificial holidays have been determined. The resistance values measured after 3–6 months are found essentially for the degree of corrosion protection. With decreasing resistance values the formation of blisters within the coating as well as the cell activity increase. Thus, steel at holidays and unprotected steel in contact with coated steel are anodically attacked. Excellent corrosion protection is given by coatings with coating thickness > 400 m?m as well as with Zn-epoxy or Zn-ethyl silicate primers, whereas sodium silicate is unsuitable.     

Zur Lochkorrosionsneigung von austenitischen CrNiMo-Stählen in konzentrierten Ammoniumrhodanidlösungen

Pitting corrosion of austenitic CrNiMo-steels in concentrated ammoniumrhodanide solutions Quasipotentiostatic and potentiokinetic polarisation measurements at various 18 Cr-10 Ni steels with molybdenum contents up to 4,3% were performed in 25 and 45% ammoniumrhodanide solutions. It was found that pitting corrosion is caused by incomplete passivation in the potential range of –300 to +250 m V _H. At these potentials the formation of stable passive layers is hindered by the formation and local oxidative dissolution of sulfidic layers. Above +250 m V _H rhodanide ions act in these weak acidic ammoniumrhodanide solutions as agents which destroy passive layers, comparable with chloride ions. The limiting potentials for stable pitting corrosion, obtained from potentiostatic experiments, are shifted from –300 to –150 m V _H with increasing molybdenum content of the steel. The least tendency of pitting corrosion was found for that steel with the highest molybdenum content.     

Der Spannungskorrosionsbruch von vergütetem Stahl bei Angriff von NH4SCN-Lösungen

The stress corrosion fracture of heat-treated steel exposed to NH₄ SCN solutions The paper deals with the stress corrosion testing of heat-treated steels in NH₄ SCN solutions. The corrosion process differs but little from that in hot lyes. There is proportionality with the concentration, there is the same activation energy, and the stress/time curves have a similar shape. But whilst, in lyes, the fracture ist brought about by the chemical destruction of the grain boundaries, the fracture in NH₄ SCN solutions is caused by the mechanical stress after embrittlement and flake formation caused by the hydrogen which has penetrated into the steel. The influence of the absorbed quantity of hydrogen on embrittlement and fracture has been determined. The influence is apparent in the fracture from the appearance of flakes which surround one or more pores. The reduction in ultimate strength is due to the fact that the fracture takes place in two phases, viz (1) the flaking and (2) the remaining fracture. Further tests showed that embrittlement caused by hydrogen is akin to the recrystallisation due to cold working between stages of heat treatment. As the mobility of the hydrogen atoms in the atomic structure is much higher than that of the nitrogen atoms, the hydrogen process is extremely rapid. That is why the hydrogen-laden steel bursts during the static tensile test without expansion and constriction. On the other hand, such steel can be beaten into a sharp 90° angle without breaking.     

Zum Korrosionsverhalten von unlegierten und niedriglegierten Stählen in Kohlensäurelösungen

Corrosion of unalloyed and low alloyed steels in carbonic acid solutions In long period experiments unalloyed steels were found to corrode in O₂-free CO₂-saturated 0,5 M sodium sulfate solutions at 25° C with flow independent corrosion rates of 0,05–0,1 mm/a. Cold-work at unalloyed steels as well as higher contents of phosphorus, copper and chromium in unalloyed and low alloyed steels increased the corrosion rate to 0,2 to 1,4 mm/a. In O₂-free CO₂-saturated distilled water unalloyed steel showed a decreased resistance affording corrosion rates of 0,4 mm/a. Traces of oxygen as introduced by CO₂ containing 100 ppm O₂ increased the corrosion rate only above pH 4,2. All investigated unalloyed and low alloyed steels showed pitting corrosion after long induction periods. The shortest induction periods were observed in O₂-free CO₂-saturated distilled water. Sulfate ions and traces of oxygen seem to inhibit pitting corrosion.     

Korrosion von Chemieemail in alkalischen Lösungen

Corrosion of chemical service glass-enamel in alkaline solutions Chemical service glass-enamels are high resistant, multi component silicate glasses fused onto steel yielding a compound material of mechanical and chemical stability as well. The resistance of these glass-enamels against alkali attack is investigated in laboratory tests involving gravimetric evaluation of totally glass-lined steel samples after contact with various alkaline media. Thus the influence of the following technically relevant parameters on the glass corrosion rate becomes perceptible: agitation and renewal of the medium, volume-to-surface ratio, exposure time, temperature, alkali content, glass quality; special attention is paid to inhibition effects originating e. g. from calcium(II) ions. These results may help to conceive on a laboratory scale a corrosion test for glass-linings which simulates in good approximation service conditions. In consequence, the corrosion situation in glass-lined equipment becomes more evident and appropriate measures may be considered to improve its economy.     

Das Korrosionsverhalten von Hartmetall-Verbundwerkstoffen in chloridhaltigen wäßrigen Lösungen

Corrosion behaviour of metal matrix composites in chloride containing solutions Two experimental set-ups on the basis of rotating disks have been used to investigate the corrosion processes on nickel and cobalt matrix tungsten carbide composites. The influence of corrosion parameter such as chloride concentration, temperature and flow rate have been tested. It is shown that by combination of cathodically active WC with an anodically unstable metal matrix oxygen corrosion occurs which is designated as “internal bimetallic corrosion”. Such an internal bimetallic corrosion can by described by a pore model consisting in its base of the metal matrix, on the wall of WC and within the pore of corrosion products. An evaluation of corrosion rates on the basis of this model exhibits rates similar to systems in practice.     

Anodische Polarisationskurven von Messinglegierungen in Ammoniumchloridlösungen

Anodic polarization curves of brass alloys in ammonium chloride solutions Potentiodynamic investigations have revealed, that curves traced during the low rate polarization characterize the anodic behaviour of brasses. Such curves show that copper-rich alloys behave in a way analogous to that of copper. It is consequently possible from the polarization curves of copper to derive the behaviour of such alloys. Zinc- rich alloys behave like zinc at the beginning but during the dissolution process a new copper-rich phase is probably formed and the behaviour is then comparable to that of copper-rich alloys. Alloys of h&h copper content can be passivated, but passivation current and potential grow as the zinc con- tent is increased.     

Die Korrosion von Messing in Ammoniumchloridlösungen

Corrosion of brass in ammonium chloride solutions The corrosion behaviour of brass (Cu77Zn21A/2, with and without addition of As) has been investigated in 0.01 to 1 M ammonium chloride solution of PH 4 and 2, and in solutions 1 M in chloride ion, with variable Na⁺ and NH⁺₄ concentrations (total concentration 1 M in each case), at pH 4 and 2. While arsenium-free brass was attacked by dezincification in most cases, intercrystalline corrosion was found on As-containing brass. The latter type of corrosion is found already at As contents as low as 0.01%. In a medium 1 M in chloride ion intercrystalline corrosion is found at PH 2, irrespective of the ammonium ion concentration, while at PH 4 intercrystalline corrosion is found only at higher ammonium ion concentrations (practically 1 M). The intercrystalline corrosion was found to be due to the segregation of Zn and As respectively at the grain boundaries and to the influence of As on the stability of the CuCl^?₂ formed by the anodic reaction. The influence of the solution composition on the progress in time of the intercrystalline corrosion is explained.     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

Ein Beitrag zum Korrosionsverhalten von austenitischem 17-12-2 CrNiMoTi-Stahl in siedenden Essigsäurelösungen - Betriebserfahrungen und Laborversuche

A contribution concerning the corrosion behaviour of austenitic 17-12-2 CrNiMoTi stainless steel in boiling acetic acid solutions - Service experience and laboratory tests Service experience of acetic acid processing industry shows that X 6 CrNiMoTi 17-12-2 stainless steel (similar to AISI type 316 Ti) can be used as construction material for handling concentrated boiling acid without facing corrosion problems if, in the medium, dissolved oxygen and small amounts of water are present, and if chloride ions are virtually absent. In laboratory tests performed on X 6 CrNiMoTi 17-12-2 stainless steel in acetic acid of 118°C containing variable extents of impurities, the steel exhibited passive behaviour in cases where the chloride ion concentration was less than 1 ppm and, at the same time, water amounts to some few tenths of a percent. The exact value of the water content necessary to achieve passivity depends on the concentration of formic acid the presence of which, at least in traces, is unavoidable. In anhydrous acetic acid, the steel corrodes in its active state, and dissolved oxygen increases the corrodes in its active state, and dissolved oxygen increases the corrosion rate. If chloride ion concentration is 1 ppm or higher the otherwise passive steel undergoes localized corrosion and the attack mostly follows crystallographic orientations. In aerated boiling mixtures of acetic acid and anhydride, the steel is boiling mixtures of acetic acid and acetic anhydride, the steel is severely corroded until the portion of anhydride reaches 70%. Beyond this, corrosion rate steeply decreases. Practically no corrosion could be found in 97% acetic anhydride.     

Nicotinsäure als Inhibitor der Korrosion von Aluminium-Magnesium-Legierungen in salzsauren Lösungen

Nicotinic acid as an inhibitor of the corrosion of aluminium-magnesium alloys in hydrochloric solutions Research has been carried out into the inhibiting effect of nicotinic acid on the corrosion of pure aluminium, 98,8 pC Aluminium, Al-Zn and Al-Cu alloys in hydrochloric acid. The inhibitting effect of graduated quantities of nicotinic acid on the solubility in hydrochloric acid of an Al-Mg-Mn alloy as a function of hydrochloric acid concentration and of the duration of the test is discussed in detail A comparison of cathode polarisation and weight loss shows that this alloy can be given effective cathodic protection.     

Loch- und Spaltkorrosion von Chrom- und Chromnickelstählen in chloridhaltigen Lösungen

Pitting and crevice corrosion of stainless steels in chloride solutions In practice stainless steels in chloride containing waters are found to be susceptible to crevice corrosion and pitting. Corrosion tests were carried out on AISI 304 L stainless using a simulated crevice and the compositions of the electrolyte in the crevice determined. Long term potentiostatic tests were used to determine the critical potentials for crevice corrosion (U_S), for various steels in sodium chloride solutions at different concentrations and temperatures. The steels studied were 22 CrMo V 121, X 22 CrNi 17 and AISI 304 L. Like the critical pitting potential (U_L), U_S was found to have a strong dependence on the chloride content of the external solution. At higher concentrations the two potentials were similar. At lower concentrations the U_S was lower than U_L. The knowledge of these critical potentials together with well known rest potentials for a steel in an electrolyte of known concentration, allows conclusions to be drawn about its susceptibility to pitting and crevice corrosion. The method is suitable also for other passive metals.     

Inhibierung der Korrosion von Kupfer in Natriumhydroxidlösungen

Inhibition of copper corrosion in sodium hydroxide solutions The authors have investigated the effect of 30 organic substances on the corrosion of copper in aqueous NaOH solutions (0.2, 0.5 and 1.0 n). The following substances were found to be effective inhibitors: resorcin, phloroglucin, pyragallol, tanni, β-naphthom, m and p-aminophenol, glucose, furfural, cyclohexanon, 8-oxychinolin, hydrazine sulphate, chinalizarine, hydrochinon, sodium diethyldithiocarbamate, sodium rhodizonate, gallocyanine and acriflavine. In most cases, the inhibition effect is due to a covering layer of reaction products of metal, inhibitor and hydroxide. The decisive factor is the stability of the covering layer with the different lye concentrations.     

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.     

Wasserstoff-induzierte Spannungsrißkorrosion von Stählen durch dynamisch-plastische Beanspruchung in Promotor-freien Elektrolytlösungen

Hydrogen induced stress corrosion cracking of steels subjected to dynamic loading involving plastic deformation in promotor free electrolytic solutions Plain carbon steels and low alloy steels suffer internal cracking and a relatively high embrittlement when they are subjected to dynamic loading involving plastic deformation in any type of electrolytic solution where there is a simultaneous cathodic hydrogen evolution. These conditions can be encountered in service in the case of cathodic polarisation and free corrosion in acids if the mechanical stresses lead to plastic deformation, e.g. at notches. There is an upper limit to the potential range in which internal cracking occurs. This limit is independent of the yield strength (300 to 500 N mm^?2) of the materials tested and lies at U_H = ?0.5 V in oxygen free salt waters. It is more negative in oxygen bearing electrolytic solutions or in alkalaine media. Materials containing hard transformation products such as martensite and bainite are more susceptible to cracking in the regions of these hard transformation products. There is only a small decrease in susceptibility with increasing temperature. Ultra high strength, quenched and tempered steels with yield strengths > 1000 N mm^?2 undergo hydrogen induced stress corrosion cracking even when they are subjected to static loading in the elastic region. The critical potential is very negative and shifts to less negative values with increasing strength of the material. Stainless steels with stable austenitic microstructure are resistant to this type of corrosion. However, if the mechanical deformation can lead to the formation of martensite surface cracking and brittle fractures occur. The hydrogen induced damage decreases with increasing temperature.     

Inhibition der Lochkorrosion von Eisenwerkstoffen in neutralen Lösungen durch organische Phosphonsäurederivate

Inhibition of pitting corrosion of Iron and steel in neutral solutions by organic phosphonic acid derivatives Pitting corrosion and inhibition by organic phosphonic acid derivatives were investigated for pure iron, mild steel, 99,5% nickel and annealed austenitic steel (AISI 304) electrodes in aerated, not stirred and chloride ions containing borate buffer solution of pH = 8.0. Inhibition efficiency is different for the electrode materials investigated. Best results were obtained for nickel, were the inhibition efficiency decreases following the row: benzole-1.4-diphosphonic acid, hexane-l.6-diphosphonic acid-monohydrate, propane- 1.3-diphosphonic acid-monohydrate, amino-tris(methylene-phosphonic acid) and 1-hydroxyethane-1.1-diphosphonic acidmonohydrate. For iron and mild steel the last two substances do not show any inhibition efficiency. For stainless steel none of the compounds investigated can be used as inhibitor for pitting corrosion. For all materials inhibition depends significantly on the ratio of the inhibitor to the chloride concentration. Potentiodynamic measurements as well as cathodic discharging experiments of passive electrodes show that the inhibiting molecules are not incorporated into the passive layers but work by the adsorption mechanism. Results of Photopotential- and AES-investigations are in agreement with this statement.     

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.     

Inhibierung der Korrosion von Kupfer in salzsauren Lösungen

Inhibition of copper corrosion in hydrochloric solutions Furfural is one of the best corrosion inhibitors of copper in hydrochloric acid. The inhibition effect ranges from 90 to 100 per cent. With increasing acid concentration, the quantity of furfural required for effective corrosion inhibition becomes smaller. By means of polarisation measurements, it can be found that, in 0.2 n and 1.0 n HCl, the inhibitor has an essential anodic effect and the cathode becomes depolarised whilst, in 2.0 n and 3.0 n HCl, the anodic zone is virtually unchanged and there is a considerable polarisation in the cathodic zone. The protective effect of furfural is probably due to its resin forming capacity.