Untersuchungen über einen Lochfraßindikatortest an Chrom- und Chrom-Nickel-Stählen in chlorid- und bromidhaltigen Lösungen

Investigations into a pitting corrosion indicator test on chromium and chromenickel steels in solutions containing chloride or bromide An investigation has been carried out into the validity of the results of an indicator test for the examination of the pit corrosion proneness of stainless ferritic chromium steels and austenitic Cr-Ni and Cr-Ni-Mo steels in neutral solutions containing chloride, at room temperature. Suitable indicators are K₃(Fe(CN)₆) and equimolar mixtures of K₃(Fe(CN)₆) and K₄(Fe(CN)₆). The latter are preferable because their redox potential has a better constancy over time, because of the pH independence of this potential in neutral and alkaline solutions, and because of the position of the redox potential in the vicinity of the potential assumed by stainlèss steels in neutral aerated solutions. But the position of the redox potential depends on the concentration of the complex salts. The added indicatior inhibits pit corrosion at low chloride contents by shifting the pit corrosion potential. In diluted solutions, the indicator test is therefore only suited for comparative investigations on different steels, and not for the determination of the absolute value of the pit corrosion potential. Investigations carried out at higher temperatures show that, above 30° C, the age-resistance ranges of the steels examined are largely independent of steel composition and temperature. At room temperature, the well-known favourable influence of increasing molybdenum contents is apparent. In solutions containing bromide, pit corrosion was found to be generally less pronounced with solutions containing chloride.     

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.     

Einfluß von Natriumchlorid auf die Oxidation von hochlegierten Chrom- und Chrom-Nickel-Stählen

Effects of sodium chloride on the oxidation of high alloy Cr- and Cr-Ni-steels The effects on the oxidation were investigated of solid NaCl deposits on the oxide scales of the technical steels X10 CrMoV 12 1, X15 CrNiSi 20 12 and X10 CrNiSi 25 20 at 700°C. Independent of the alloy composition the presence of NaCl(s) initiated a markedly accelerated Fe₂O₃ growth on the surface of preoxidized samples, under formation of voluminous, nonprotective layers. Below these scales on the metallic matrix in all cases chloride was detected. The oxides grow according to the mechanism of active oxidation in which chlorine plays a catalytic role. The presence of the chloride at the oxide/metal interface reduces the adhesion of the oxide scale and leads to spalling upon cooling to room temperature. The effects observed are independent of the alloy composition, however, the thickness of the oxide scale is decisive which means the diffusion distance for the gaseous iron chloride.     

Untersuchungen über den interkristallinen Kongrenzenangriff von austenitischen Mangan-Chrom-Stählen durch Wasser und wäßrige Lösungen

Research on intercrystalline grain boundary corrosion of austenitic manganesechrome steels caused by water and aqueous salt solutions Stressed bar specimens of welded and tempered austenitic manganesechrome steel such as as X 40 MnCr 18 and X 22 MnCrNi 885, exposed to sea water, tap water or destilled water, are liable to suffer intercrystalline stress corrosion cracking. The occurrence of intercrystalline corrosion is not necessarily dependent on mechanical tensil stress. In particular, even unstressed specimens exposed to tap or distilled water are liable to show marked intercrystalline corrosion. Additional tensile stress merely has the effect of speeding up the inter-crystalline corrosion. The cause for the intercrystalline corrosion must be seen in the segregation of chrome-rich carbids at the grain boundaries and the consequent reduction in the chrome content within the grain boundary zone. It is possible to prove the segregation of a chrome-rich carbide of the structure M₇C₃ at the grain boundaries of steel X 40 MnCr 18, and the aggregation of a chrome-rich carbide of the structure M₂₃C₆ at the grain boundaries of steel X 22 MnCrNi 885. Through contact with zinc, it is possible to obtain a complete corrosion protection with the manganese-chrome steels investigated. A short circuit with passive austenitic chrome-nickel steels increases the general or selective corrosion rate through anodic polarisation. By adding chromate to the sea water, the corrosion rate is retarded. In tap water, an addition of chromate result in complete corrosion protection.     

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.     

Anodisches Verhalten und Korrosion von Titan in methanolischen Lösungen

Anodic behaviour and corrosion of titanium in methanolic solutions The anodic behaviour of titanium in methanolic solutions containing halides or water in various concentrations has been investigated and the result were compared with the result obtained by corrosion tests performed on ?U”? bend specimens in similar solutions. Polarization curves can predict the susceptibility to titanium to stress corrosion cracking as in the case the metal surface is not passivable. These conditions can be obtained in the presence of low water content (i.e. 0.1% H₂O) and of activating ions as Cl^? and Br^? even in very small concentration (10^5?, 10^4? M). Presence of higher water contents and/or of F^? or I^? ions can to some extent produce passivation of the metal surface and cracking does not occur. A lower water amount in the solution (i.e. 130 ppm) eliminates any possibility of passivation of the metal surface, so that cracking can occur even in the absence of chlorides or bromides. High Cl^? concentration in the solution. (i.e. 0.1 M NaCl + 0.1% H₂O) can produce intergranular attack even in the absence of applied stress.     

Kritischer Chloridgehalt – Untersuchungen an Stahl in chloridhaltigen alkalischen Lösungen

Critical chloride content – Investigations of steel in alkaline chloride solutions The question of the critical corrosion-inducing chloride content of steel in concrete still incites controversial debates. In a first series of trials, which will be outlined as part of the present article, the reduced corrosion system of steel/concrete pore solution was investigated. The results suggest that a log-log linear relationship exists between the molar concentrations of corrosion-inducing and passivating ions that can be expressed by the following equation: log c(Cl¯)_krit = 1.5 · log c(OH¯) - 0.245. On the basis of this relationship the results compiled in the bibliographical search were discussed and the significance with respect to the pitting potential was explained. Supplementary investigations into the time of chloride addition and the composition of the pore solution were conducted.     

Korrosion von hochdispersen,aus Eisen-, Chrom- und Kobaltsalzlösungen reduzierten Metallpulvern

Corrosion of highly dispersed metal powders obtained by reduction of iron, chromium and cobalt salt solutions The corrosion of highly dipersed iron powder has been studied in moist air at 25, 50 and 70 °C by determination of the weight increase over 4 months. In addition, the magnetic properties of the powders have been measured before and after the investigation. The powders were obtained by reduction of ferrous sulfate with NabH₄ in aqueous solutions containing various amounts of cobalt sulfate and chromium salts. The investigations have shown that the powders have good corrosion resitance; after an irregular initial period the weight increases linearly with the square root of time. The weight change increases as the cobalt contents increase and decreases as the chromium contents increase. The saturation magnetization decreases decreases ut little during storage at 25 °C, but is reduced by 20% at 70 °C, while the coercive strength remains constant or slightly increases. Annealing at 4900 °C has a beneficial effect.     

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.     

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.     

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.     

Die aktive und passive Molybdän-Elektrode in sauren Lösungen

The active and passive molybdenum electrode in acid solution It follows from the anodic and cathodic thermodynamic polarization curves in the system molybdenum/aqueous solution (p_H 0 to 10) that the favourable behaviour of molybdenum in acid solutions is determined by the extension of the immunity region becomes smaller, but without simultaneous passivation of molybdenum; this fact explains the active behaviour of molybdenum in alcaline solution. The passivation of molybdenum starts as soon as the half cell reaction sets in, where The p_H-dependence of the cathodic passivation potential in acid solutions follows the equation      

Kritischer korrosionsauslösender Chloridgehalt – Untersuchungen an Mörtelelektroden in chloridhaltigen alkalischen Lösungen

Critical chloride content – Investigations of mortar electrodes in alkaline chloride solutions Although the penetration of chloride ions into the concrete is obstructed by numerous protective mechanisms, the concrete thus a complex multi‐barrier protective system represents, the chloride ions nevertheless succeed it to break through the protecting passive oxide layer of the steel, like the numerous chloride‐induced corrosion damage from building practice clearly document. In a first part results have been outlined concerning the investigations of the reduced corrosion system of steel/concrete pore solution [1]. The present article comprises results of investigations into the critical corrosion‐inducing chloride content that have been obtained by electrochemical investigations of mortar electrodes in alkaline chloride solutions. Finally the findings gained are explained with a view to the relevance they imply for building practice, and particularly for the design of new buildings to be constructed and the assessment of existing structures.     

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.     

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.     

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.     

Zur Korrosion von Eisen und Aluminium in wässerigen Lösungen von Ammoniak und Kohlensäure – 2. Mitteilung: Elektrochemische Untersuchungen

Contributionto the corrosion of iron and aluminum in aqueous solutions of ammonia and carbonic acid. 2nd Communication: Electrochemical investigations Corrosion potential-time curves and current density-potential curves as well as polarization resistances and other electrochemical methods have been used to characterize the corrosion behaviour of aluminum and iron base materials in an aqueous solution of 11% ammonia and 7% carbon dioxide (leaching solution applied in hydrometallurgical nickel raffination) at 50 °C. For comparisonal purposes the behaviour of the same materials has been investigated in ammonia, sodium carbonate and sodium hydroxide solutions at constant pH. Despite of the relatively elevated pH value aluminum is protected against corrosion in the ammonia-carbon dioxide-water-mixture through selfpassivation (fast repassivation of mechanically destroyed passive layer). In contrast, the stability of iron and mild steel in the same solution is seriously impaired by easy activation. Permanent passivation is only achieved through anodic protection by means of an external current source.     

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.     

Die potentiodynamische Polarisationskurve des Kupfers in Lösungen von Chloriden

The potentiodynamic polarization curve of copper in chloride solution The potentiodynamic curve obtained under conditions of high rate polarization does not only represent the dissolution of copper but also the redox reactions taking place in this rather complex system. The first part of the curve characterizes the anodic reaction resulting in the formation of CuCl^?₂ and CuCl; it can therefore be used for studying spontaneous corrosion. In addition, the fast polarization enables the influence of various factors to be assessed and fast reactions to be recognized which are not registered under conditions of slow polarization. It should be possible on this basis to study e.g. the influence resulting from structural changes in copper alloys.     

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.