Untersuchungen zum Aktiv/Passiv-Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Medien Teil 1. Computerunterstützte Unterbrechertechnik zur Erfassung des Ohmschen Spannungsabfalls

Investigations into the active/passive transition of 304 stainless steel in organic media containing water and hydrogen chloride Part 1. IR drop measurement by a computer-assisted Interrupter technique An interrupter technique with computer-assisted calculation of the ohmic potential drop UΩ is described. It allows to measure UΩ while the working electrode is under potentiostatic control. As an additional result the differential double layer capacity C_D is calculated, which includes some informations about the state of the working electrode surface. Experiments with the presented interrupter technique show, that just using a Luggin capillary is absolutely insufficient to avoid IR drop, especially in low conductive media: steady state current-potential curves will yield erroneous results of the kinetic parameters of the electrode reactions if the IR drop is not carefully eliminated. The described interrupter technique gives the same results for the ohmic resistance RΩ as eg AC techniques or potential pulse measurements. The value of RΩ measured is always associated with the primary current distribution.     

Untersuchungen zum Aktiv/Passiv Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Median Teil 2: Einfluß des Wassergehalts auf die aktive Auflösung und Passivierung

Investigation into the active/passive transition of 304 stainless steel in organic media containing water Part 2: Influence of water on the active dissolution and passivation The influence of water on the corrosion and passivation of type 304 stainless steel has been studied in deaerated ethanolic solutions containing hydrogen chloride. The potentiostatic polarisation curves show that both, critical current density for passivation and passivation potential, strongly depend on the water content of the solution. A passivation model based on the assumption of a reversible primary passivation potential explains this influence of water and allows the determination of other kinetic parameters. The propounded passivation model also explains the result that the part of “passivated” surface at the maximum current density is the same for all water contents examined. The dominating influence of the water content is shown more clearly when measurements from methanolic and propanolic solutions are concerned. The passivation potential only depends on the water content and not on the type of alcohol.     

Wasserstoffinduzierte Korrosion von niedriglegierten Stählen in wäßrigen Medien

Hydrogen induced corrosion of low alloy steels in hydrous media The absorption of hydrogen from media can lead to cracking in steels with a ferritic structure. The interaction of absorbed hydrogen and steel is governed by the hydrogen activity and by stress- and structure-dependent threshold values. Different types of hydrogen-induced corrosion are illustrated by means of tests performed on lowalloy structural steels. Surface blistering and internal cracking can occur on the one hand without external stresses. In the case of hydrogen-induced stress corrosion cracking on the other hand a differentiation is made between corrosion systems in which constant loading displays already cracking and systems in which the cracking is tied to slow strain rates within a critical range.     

Die Bestimmung der Reaktionsarbeit von Korrosionsreaktionen in wäßrigen Medien

Determination of the gibs free energy of corrosion reactions in aqueous media The Gibbs free energy is the sum of some partial contributions, the most important of which can be calculated from the solubility constant of the solid corrosion product and the standard potential of the particular metal. Since these two quantities describe the position of heterogeneous ionic equilibria the author presents methods suitable for the investigation of sch equilibria. Depending on the standard state selected, i.e. pure water (or aqueous solutions at “infinite” dilution or solutions of inert salts of constant ionic strength) one obtains thermodynamic or stoichiometric equilibrium constants; with the latter it is possible to use, instead of activities, the respective concentrations. Using the system Zn/H₂O/CO₂ the author points out possible corrosion products and experimental methods. The methods used enable an approximately exact determination to be made of solubility constants and free energies of formation of simple stoichiometric corrosion products at room temperature, not, however, at higher temperatures or when corrosion products are present as solid solutions.     

Untersuchungen zum SwRK-Verhalten des unlegierten Stahles Ck 45 in chloridionenhaltigen wäßrigen Medien

Corrosion fatigue behaviour of steel Ck 45 in aqueous solutions containing chloride ions The fatigue behaviour of carbon steel Ck 45 (comparable to AISI 1045x) is investigated for cyclic tension and rotating bending load with a frequency of 25 Hz. The fatigue limits under cyclic tension load in air are 410 N/mm² for smooth specimens and 290 N/mm² for notched specimens. For rotating bending load a value of 200 N/mm² is found for smooth specimen. The fatigue limits for N = 10⁷ in two different environments (0.3% and 3% NaCl-solution) reach only 37–57% of the according values in air. Cathodic protection of smooth specimens causes an improvement to 95% of the air fatigue limit. The evaluation of the free corrosion potential for all corrosion fatigue tests and the appearence of fracture show typical attributes of corrosion fatigue in the active state.     

Untersuchungen über das Korrosionsverhalten von Zirkoniumlegierungen I. Verhalten gegenüber wäßrigen Medien

Investigations into the corrosion behaviour of zirconium alloys I. Behaviour in the presence of aqueous media Extensive corrosion research covering 5O Zr alloys have shown that the good corrosion resistance of pure Zr cannot be improved to any greater extent by alloying. Pur Zr as well as most of its alloys have a high corrosion resistance; even in boiling mineral acids (20% H₂SO₄, HNO₃, and HCl) the corrosion rate is below 0.5 g/m²·d and no attack can be recognized in neutral salt solutions. The metal also resists attack by boiling 2O% KOH. Strong attack, however, it caused bv Fe-III-chloride. The metals do not resist attack by HF, while neutral KF attacks but little, however produces pitting. The effect of alloying additions is due to the structure produces by them: pure grain boundaries or very fine heterogeneous structures show good corrosion resistance. Ti, Hf, Sn and small amounts of Nb have little influence, Mo, W, larger amounts Nb and small pt and Re additions give rise to Local cells, while Fe, Cr and Ni have practically no influence.     

Korrosionsreaktiorren von elementarem Schwefel mit unlegiertem Stahl in wäßrigen Medien

Corrosion reactions between elemental sulphur and plain carbon steel in aqueous media Plain carbon steels are rather severely attacked by elemental sulphur at room temperature in the presence of aqueous media. The corrosion occurs preferentially at the places where the two solid substances iron and sulphur are in contact with each other and results in shallow pit formation. At the same time the pH is also decreased slightly and small amounts of H₂S and sulphate ions are formed. Neutral salts stimulate the corrosion process whereas phosphates inhibit it and the alkaline media such as Na₂CO₃ and ethylamine with pH > 12 prevent it completely. The latter can be made use of for corrosion protection. At high salt concentrations (c > 1 mol/l) the corrosion rate, however, decreases with increasing salt concentration. The corrosion rate may increase with increasing flow velocity of the medium, but the corrosion takes place uniformly. The results of electrochemical investigations show that the reduction of sulphur occurs at the corrosion product FeS and is the rate controlling step. No sulphur reduction is observed on platinum electrodes when no FeS is present. It is assumed that the starting reaction to initiate corrosion in the system Fe/S/H₂O is a slight disproportionation of S to H₂SO₄ resulting in the formation of FeS.     

Die Ermittlung der Beständigkeit gegen Spannungsrißkorrosion (SpRK) von hochlegierten Sonderstählen in chloridhaltigen wäßrigen Medien

Determination of resistance to stress corrosion cracking (SCC) of high-alloy special steels in chloride-containing aqueous media The 18 Cr 10 Ni(Mo) based stainless steels have been continually improved by raising the Cr, Ni and Mo contents. The behavior of these high-alloy steels towards SCC was determined in test media generally used in practice since the question of the resistance to stress corrosion cracking (SCC) had still remained unanswered to a large extent. SCC tests on U-bend samples in boiling 62% CaCl₂ solution showed a good differentiation depending on the Ni and Mo contents. With increasing Ni content, the susceptibility of special high-alloy steels to SCC is shifted towards longer service lives, alloys containing ≧ 42% by weight of Ni being resistant. High-Mo special alloy steels are more resistant to SCC than low-Mo special alloy steels. These results could be confirmed by tests carried out on circular cross section samples in boiling 62% CaCl₂ solution under constant load and potentiostatic control. The free corrosion potentials recorded for 25% Ni special alloy steel and Ni-based alloys are within the potentiostatically determined range of insusceptibility to SCC. The high-Mo special alloy steel X 2 NiCrMoCu 25 20 6 (1.4529) shows the same critical SCC potential on the anodic side as the Ni alloy NiCr21 Mo (2.4858). Superferrit X 1 CrNiMoNb 28 4 2 (1.4575) and austenitic ferritic steel X 2 CrNiMoN 22 5 (1.4462) showed that the SCC behavior was unsatisfactory in both tests as in the case of steel X 10 CrNiMoTi 18 10 (1.4571). Tests in boiling 4 m NaCl showed no SCC, not even under the aggrevated test conditions in the test set-up. The great influence of the oxygen content was demonstrated in tests carried out in the autoclave with defined oxygen and chloride concentrations. The resistance of the steels to SCC decreases under air-saturated conditions (8 … 10 ppm O₂) whereas the chloride concentration (200 and 2000 ppm Cl^?) does not exercise an important influence. U-bend samples should be given preference to Erichsen samples for SCC tests. SCC break characteristics could be determined metallographically and by scanning electron microscope.     

Klärung der Einzelmechanismen bei der Schädigung von Kupfer unter Kavitationsbeanspruchung in wäßrigen Medien

Study of the individual mechanisms giving rise to the destruction of copper under cavitation in aqueous media An experimental method developed by the authors enables the overall destruction mechanism during cacitation corrosion to be subdivided into the two main components “mechanical destruction” and “corrosion”. With the aid of impressed current it is then possible in a defined way largely to control the ratio of the two components of attack. Mechanical sollicitation due to the successive formation and collapse of bubbles in the liquid corresponds to the pattern found in corrosion fatigue; the authors have indeed found the phenomenological pattern of corrosion fatigue in some systems (grain boundary attack by electrochemical action, glide line attack by mechanical sollicitation).     

Die Wirkung von reibungsmindernden Zusätzen auf die Metallkorrosion in wäßrigen Medien

Effects of friction reducing additives on the corrosion of metals in aqueous solutions It is known that small additions of certain chemical agents to water can significantly alter its flow behaviour (“drag reducing additives”). Such solutions show also different mass transfer properties as compared with pure water. Since corrosion reactions are frequently mass transfer controlled it might be expected, that drag reducers will affect the metal corrosion as well. In order to get more insight into the subject, various experiments with several substances known as drag reducing agents were performed (hexadecyltrimethyl-ammoniumsalicylate and two commercially available cationic tensides). The experimental techniques used were rotating disc electrodes and corrosion tests with segmented tube as a corrosion sample. The results obtained indicate that the products tested indeed reduce the rate of corrosion of different materials, this due to partly interfering with the mass transfer of oxygen, or/and acting directly as a corrosion inhibitor. In addition, a synergic effect with other conventional water treatment chemicals was observed. Certain problems with respect to the practical use of such additives (e.g. in district heating systems) are discussed.     

Mößbauer- und ESCA-Untersuchungen zur Bildung oxidischer Eisenphasen in wäßrigen Medien unter der Einwirkung organischer Korrosionsinhibitoren

Mössbauer and ESCA investigations on the formation of oxidic iron phases in aqueous solution under the influence of organic corrosion inhibitors Corrosion layers on steel grown in water of well defined hardness and chloride concentration were studied by Mossbauer and ESCA spectroscopy with particular emphasis on the influence of added organic inhibitors. Relatively thick layers were found with an unexpectedly small iron content (as FeOOH). The layers contain a remarkable amount of constituent ions from the solution and fragments of the inhibitors. The latter seem to be decomposed by the corrosive medium: It is assumed that the whole organic molecule determines the kind of transportation of the inhibitor to the iron metal, but that the inhibition itself is due to functional groups only.     

Korrosion und Korrosionsinhibierung von Aluminiumpigmenten im alkalisch wäßrigen Medium

Corrosion and inhibition of corrosion of aluminium pigments in alkaline aqueous medium Flakelike aluminum pigments (Al-content > 99.5%) were corroded in an aqueous alkaline mixture of water and butyl glycol in the ratio 9: 1. Chelating agents like citric acid or polyacrylic acid inhibit this corrosion reaction. The temporal progress of the corrosion reaction can be determined by volumetric analysis of the evolved hydrogen and furthermore, with addition of the corrosion inhibitors, by measuring the electrical conductivity of the solution. The corrosion reaction consists essentially of two steps:

• 1 In the latency period no or only little corrosion takes place; during this, time the protective layers (normal oxide layer or layers reinforced by reaction products of inhibitor and aluminum) were dissolved.
• 2 After that the actual corrosion reaction takes place.

It was proved that changes of the pH-value or addition of corrosion inhibitors only influence the duration of the latency period; after that period the rate of the corrosion reaction is nearly independent from the examined conditions.     

Saccharide und deren Derivate als Korrosionsinhibitoren für Aluminiumpigmente in wäßrigen Medien

Saccharides and their derivatives as corrosion inhibitors for aluminum pigments in aqueous media Aluminium pigments (Al-content >99,5%) were corroded in aqueous, slightly alkaline metallic coating compositions. The temporal progress of the corrosion can be determined easily by volumetric analysis of the evolved hydrogen. As simple model systems for testing mono-, di- and polysaccharides and their derivatives as corrosion inhibitors two water/butyl glycol-mixtures were chosen the pH of which was elevated to 10 to accelerate the corrosion reaction. In addition it was tried to establish structure-efficiency-relations by comparison with model compounds. Two classes of compounds were active:

• 1. Reducing mono- and disaccharides were inhibiting the corrosion reaction presumably by their in the equilibrium existing endiol structure element.
• 2. The examined hydroxycarboxylic acids are forming chelate complexes which seem to be the corrosion inhibitors.

The most effective of all tested compounds was the strong reducing agent ascorbic acid.     

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.     

Zum Einfluß eines wäßrigen Mediums auf die Simulation von Erosion und Erosionskorrosion

The influence of an aqueous medium on the simulation of erosion and erosion-corrosion

1 Die Arbeiten wurden am Zentralinstitut für physikalische Chemie in Berlin-Adlershof durchgeführt. Das ehemals zur Akademie der Wissenschaft der DDR gehörige Institut wurde entsprechend Einigungsvertrag zum 31.12.1991 aufgelöst. Die Autoren sind inzwischen Mitarbeiter der Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87, W-1000 Berlin 45:

For the investigation of the role of a medium in erosive-corrosive treatment it is necessary to keep comparable the mechanical component of the attack of both the erosive and the erosivecorrosive mode of treatment. However, some recent results pointed to changes in the regime of erosive attack by addition of an aqueous medium to the particle stream. The aim of the present paper is to analyse by different methods the influences of the aqueous medium on the angle of particle drop-out, the geometry of the particle stream and the particle distribution therein and the impulse of impingement.     

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.     

Chloridinduzierte Korrosion von nichtrostenden Stählen in Schwimmhallen-Atmosphären Teil 2: Einfluß von Hypochloriten

Chloride induced corrosion on stainless steels at indoor swimming pools atmospheres Part 2: influence of hypochlorite The work was started on the occasion of failures of stainless steel components at the indoor swimming pool atmosphere in Uster (Switzerland). Highly-alloyed stainless steels were tested at defined mechanical and environmental conditions. Therefore Ubend specimens with salt spots were examined at 40°C and 35 and 70 % rel. humidity respectively. The results of the experiments with the electrolyte magnesium-chloride (30 %) were presented in part 1. The high nitrogen alloyed steels 1.4529 and 1.4565 showed the best corrosion resistance. The highest corrosion attacks were observed at the steels 1.4401, 1.4462, 1.4439 and 1.4539. Stress corrosion cracking (SCC) was determined at the steels 1.4401, 1.4462 and 1.4439 at 35 % rel. humidity by metallographic methods. A distinctive relationship was observed between the pitting resistance equivalent and the kind of corrosion attack. Furthermore, short time experiments (< 1000 h) can be used to define a qualitative judgement over the occurrence of the kind of long time corrosion attacks at stainless steels.     

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.     

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.     

Anwendung der Warburg-Technik zur Untersuchung der Korrosion von Nickel in sauerstoffhaltigen,wäßrigen Salzsäurelösungen

Application of the Warburg technique to the examination of the corrosion of nickel in aqueous hydrochloric acid solutions containing oxygen By applying the Warburg technique to corrosion phenomena, it is possible to determine the type and extent of the participation of a gaseous reagent in the reaction, provided that it is not less than 10^?8 Mols. By improving the measuring photocells, it was possible to eliminate disturbances to the stationary equilibrium caused by the measuring technique. In the present research effort, the Warburg technique has been used for determining the corrosion rate of pure nickel in aqueous hydrochloric acid solution containing oxygen, as a function of the acidity of the solution. The correlation thus found has been qualitatively discussed. It was shown that, under the prevailing conditions (Po₂ = 1 atm; O ? p_H ? 7), there is no hydrogen evolution, but always an oxygen consumption. The oxygen has been quantitatively reduced to hydrogen peroxide, dissolving the equivalent quantity of nickel.