Beitrag zum Spannungsrißkorrosions-Verhalten der AlZnMg-Legierungen

The stress corrosion cracking behaviour of aluminium-zinc-magnesium alloys The stress corrosion cracking of AlZnMg3 is a two-phase process where the preparation period may also take place without tensile stress. With decreasing pH, the service life is reduced, the reduction being confined to the preparation period. The same applies to anodic connection associated with heavy pitting corrosion where the fracture consists of stress corrosion cracking zones and transcrystalline force fracture. Even with cathodic connection, stress corrosion cracking is encountered. Since, in this case, the current density does not decrease prior to fracture, it must be assumed that the propagation of the crack may have nothing to do with the electro-chemical dissolution at the tip of the crack. Even minor quantities of water — e.g. in carbon tetrachloride — are sufficient to cause stress corrosion cracking, probably through adsorption of atomic hydrogen which has the effect of reducing the surface energy.     

Beitrag zum Mechanismus der Lochfraßkorrosion des Nickels

A contribution to the mechanism of the pitting corrosion of nickel The pitting corrosion of nickel has been studied as a function of time, pH, potential and concentration in various salt solutions. It has been revealed that the number of pits as a function of time depends from the salt concentration and this dependence may be linear or parabolic. The potential dependence of the number of pits, on the other hand, follows an exponential law. From the salts included in the present study the chlorides in particular give rise to pitting; their effect is in part rather stimulated by nitrates. Sulfates, too, affect the action of chlorides, but their effect is different, depending on their concentration On the basis of the experimental results the author discusses possible mechanisms of pit formation and pit growth.     

Ursachen und Mechanismen der Spannungsrißkorrosion

Causes and mechanisms of stress corrosion cracking The paper provides a synopsis of the causes and mechanisms of stress corrosion cracking discussed at the present time. The discussion covers: selective metal dissolution (formation of cover layers, influence of lattice imperfections on the anodic metal dissolution, crack spreading rate, anodic hydrogen embrittlement); adsorption and brittle fracture (energy balance, atomistic and electro-chemical aspects); metallurgy (plastic deformation, alloying influence, sliding condition); fracture mechanics (crack formation and growth). Particularly important is the formation of cover layers where stress corrosion cracking depends on a certain ratio of the rate at which the cover layer is reformed and the rate at which surfaces free from cover layer are formed at the bottom of the crack. At high crack expansion rates, the adsorption-brittle fracture mechanism may play a part, as it tends to lower the cohesion at the bottom of the crack. As the crack represents an ideally sharp notch, it is possible to apply the linear elastic fracture mechanics to the crack expansion phenomena.     

Die Bedeutung einer Deckschichtbildung bei der Spannungsrißkorrosion

The importance of the formation of a cover with stress corrosion cracking Many metals and alloys, when exposed to corrosive agents, from covering layers which have a considerable influence on the corrosion process. This applies, in particular, to covering layers with protective effect. The significance of the formation of electro-positive cover films in stress corrosion cracking is illustrated by the example of the formation of cracks in unalloyed steels in nitrate solutions and alkali solutions as well as in austenitic steels in solutions containing chloride and in sulphuric acid. As with pit corrosion, the incomplete or disturbed formation of such layers is the cause of the occurrence of localized corrosion where, under the influence of tensile stresses, the local anodes become the points where the cracks originate. Tension crack corrosion must therefore be regarded as a special problem of the corrosion of metals with incomplete cover films.     

Untersuchung der Spannungsrißkorrosion von Baustählen in flüssigem Zink

Investigation into stress corrosion cracking of unalloyed steels in liquid zinc This investigation has been done in order to get further informations of stress corrosion cracking in galvanized steel. The samples were made of unalloyed steel with increased contents of carbon (up to 0,24%), silicon (up to 0,30%) and copper (up to 0,4%). Welded and notched samples have been equally tested. One batch of the samples was preloaded with 70, 80, 90 and 100% of yield point and then dipped into hot zinc (450°C). Another batch was tested in liquid zinc with constant strain rates of 5 · 10^?4, 5 · 10^?5, and 5 · 10^?6 · s^?1 during 250 h. None of the tested samples have been destroyed by stress corrosion cracking.     

Einflüsse bei der Prüfung der Spannungsrißkorrosion

Influence factors in the testing of stress corrosion cracking The results of the tests of stress corrosion cracking of austenitic stainless steels in MgCl₂ solution are significantly dependent on the type and method of the test. The testing method used in the Research Institute of the Schoeller-Bleckmann Steelworks Ltd. is discussed, and attention is drawn to the influence of a number of test conditions such as the preparation of the MgCl₂ solution, the preheating of the test vessel with water, the surface condition of the specimen, the diameter of the specimen, the covering of its shaft, the time at which the load is applied, and the advance potential applied. The correlation of the service life with stress, concentration and temperature is used as an example in order to show that the test method is apt to affect not only we service life but also the extent and, type of these correlations. It is therefore only in conjunction with the exact test conditions that the test results can be regarded as fully significant.     

Stand der Erkenntnisse auf dem Gebiet der klassischen Spannungsrißkorrosion

State of knowledge in the field of classical stress corrosion cracking A summary is given of the most important results obtained using classical test methods to determine the susceptibility and resistance of unalloyed and low-alloy steels, austenitic chromium-nickel steels and certain aluminum and titanium materials to stress corrosion cracking. Particular attention is paid to the existence of the limit of critical potentials and limit of critical stresses or K_ISCC values. The results presented in this paper are intended to indicate stress corrosion properties which are common to the various metals and alloy systems. The paper concludes with a discussion of the major influential factors and the causes and requirements surrounding stress corrosion cracking and its initiation, the main issues discussed being the mechanical effects and their influence on the stress corrosion process.     

Zur Frage des Mechanismus der Lochfraßkorrosion

The question of the pitting corrosion mechanism It is concluded from the time, concentration, temperature, PH and potential dependences of the corrosion current densities in the pits, that it is not justified always to suppose parallelity between the metal dissolution rates in the active pits and in the active region of the polarization curve. This is due to the fact, that in some cases diffusion is not the rate controlling factor of pitting corrosion, and that the metal dissolution in the pits and in the active states follows different mechanisms.     

Spannungsrißkorrosion an Kupferlegierungen

Stress-corrosion cracking of copper alloys Stress-corrosion cracking (SCC) occurs if three factors are simultaneously present: a susceptible material, a specific corrosive medium, and tensile stresses. All copper alloys and copper itself are susceptible to SCC – but in a different extend. The most susceptible alloys are brasses with copper contents below 80%. For a long time only ammonia and its derivatives were considered to cause SCC of copper alloys. Only in recent years other mediums have been reported to produce SCC – especially nitrites. With the exception of some rare SCC-cases of copper and zinc-free copper alloys SCC-failures of brasses prove the highest importance in practice. This is due to the high susceptibility and the large use of brass. Besides clear failures by ammonia and nitrite increasing cases of SCC influenced by outdoor environment - mainly industrial or urban atmosphere - can be stated. The reason could be the general atmospheric pollution by sulphur and nitrogen oxides. Failures of this kind may appear after long times of service - e.g. after several years. The risk of SCC can be reduced by minimizing tensile stresses or by choosing other materials than brass.     

Zur Frage der Spannungsrißkorrosion durch flüssiges Ammoniak

Contribution to stress corrosion cracking by liquid ammonia Accidents experienced in connection with ammonia shipment in the fertilizer industries of the USA for about 15 years were, at least in part, attributable to the deleterious effect of contaminations by air and carbonic acid. In certain cases it is possible to achieve inhibition by addition of water, but in other cases the use of mild steel is being preferred. High strength steels require a thermal treatment, in particular after welding, because corrosion (inter-or transcrystalline) appears preferentially in the weld seam. By contrast to experiences with shipping containers no such accidents have been experienced so far with storage containers.     

Autoklaven-Untersuchungen der Spannungsrißkorrosion von Fe-Cr-Ni-Legierungen in NaCl/Co2/H2S-Medien

Autoclave investigation of stress corrosion cracking behaviour of Fe-Cr-Ni alloys in NaCl/CO₂/H₂S-environment In oil and gas production, the corrosion problems increase as the depth of the reservoirs increases. The oil and gas products contain chloride-rich waters and mixtures of H₂S and CO₂ at high pressures and temperatures. Materials that can be used under these conditions are only high strength high alloy steels and nickel base alloys. These materials must be assessed for corrosion resistance under these conditions. The environment contain chloride ions and hydrogen sulphide, which are known to be critical components for SCC. With the aid of autoclave experiments, the fields of corrosion resistance for the materials no. 1.4462, 1.4563 and 2.4618 were determined as a function of temperature and hydrogen sulphide pressure. The base environment was a 5 Molar sodium chloride solution at 20 bar carbon dioxide. While the corrosion resistance of the duplex steel, material no. 1.4462, decreases markedly as the strength of the material and the hydrogen sulphide pressure increase, the two austenitic materials are completely resistant up to 300 °C and hydrogen sulphide pressure of 15 bar. Only at 300 °C and high partial pressures of hydrogen sulphide the material no. 1.4563 did fail, when stressed to stress levels higher than the YS. The crack path was predominantly transgranular with minute fractions of intergranular cracking. The microstructure appears to have no effect. All results indicate that a mixed mechanism of hydrogen- and chloride induced SCC is operting, while a corrosion enhancement due to interaction of both critical components takes place.     

Beurteilung der Streuung der Standzeit bei der Prüfung auf wasserstoffinduzierte Spannungsrißkorrosion

Assessment of the scatter of the time-to-failure during hydrogen-induced stress corrosion cracking testing Usually the resistance of different steelgrades to hydrogen induced stress corrosion is evaluated by their threshold stress values. The reliability of these values is difficult to assess due to a lack of information regarding their deviation. Assuming there exists a well defined connection between stress and time to failure, it should be possible to derive the deviation of stress values from the deviation of time to failure, thus getting the deviation of threshold stress values. An evaluation of data concerning time to failure, having already been published or obtained in various laboratories, shows that because of their small sample size no definite identification of the kind of distributions is possible. In particular, one cannot distinguish between a lognormal and a Weibull distribution, especially since one seems to encounter a mixed distribution rather frequently. An analysis of various effects on the size of the deviation showed that maximal deviation is frequently found in those ranges of stress, where – often far below the 0.2% creep limit – minor plastic deformations take place. Inspite of these restrictions, in an additional paper it will be attempted to derive an estimation of the deviation of the threshold stress from the deviations of the time to failure.     

Passivierungsverhalten und Spannungsrißkorrosion von Eisen-Mangan-Chrom-Legierungen in Natriumchlorid-Lösung

Passivation behaviour and stress corrosion cracking of iron-maganese-chromium alloys in sodium chloride solution Electrochemical experiments with MnCr steels (20–28% Mn, up to 12% Cr) in 3% NaCl solution. High Mn contents reduce the passivation tendency, while increasing Cr contents broaden the range of passivity. The formation of surface layers is due primarily to a direct reaction with the solution (good adhesion, high protective value) and, secondarily, to precipitation from the solution (porosity, low protective value). The tendency to form secondary layers increases as the Cr content is reduced. In oxygen containing solution there is a pronounced corrosion in the pitting range. At low Cr contents, stress corrosion cracking is mostly transcrystalline, at higher Cr contents (8–12%) it is intercrystalline, in particular when Cr carbide precipitations are present at the grain boundaries. In the range of transcrystalline corrosion the susceptibility to selective corrosion extends beyond the pitting potential. At higher Cr contents there may be pitting without any indication of stress corrosion cracking.     

Spannungsrißkorrosion von Stählen in flüssigem Ammoniak

Stress corrosion cracking of steels in liquid ammonia The apparatus developped for the investigation of stress corrosion cracking of steel in liquid ammonia under controlled electrochemical conditions is described. The parameters of the experiments were set by a computer which also stored and correlated the experimental data. Cylindrical samples of the welding steel W. Nr. 1.0143 and of the steel STE 355 (W. Nr. 1.0562) in liquid ammonia containing ammonium chloride or lithium perchlorate as the electrolyte developped cracks only at negative electrode potentials in the regions of active dissolution and hydrogen deposition. Other parameters including contamination of the solution by air were unimportant. The results are explained by hydrogen induced stress corrosion cracking. No embrittlement was observed with passive samples. However, ultimate tensile strengths in liquid ammonia were clearly lower than at air also for samples breaking without formation of cracks. Experiments with notched sheets resulted in sharp, essentially transcrystalline cracks. Passivation of these samples was difficult in the region of the notch indicating the danger of anodic stress corrosion cracking.     

Prüfung auf Spannungsrißkorrosion

Stress corrosion testing The authors present a survey of the different methods used in stress corrosion testing of materials of construction. After weighing the advantages and shortcomings of the individual methods the requirements are explained which should be met by an optimum method for plant use. By using screw-type springs as charging elements these requirements can be met to a great extent. Some examples are used to illustrate the progress of elongation over the test duration as well as the influence of aggressive medium, surface treatment and specimen shape on specimen life under stress corrosion conditions.     

Dehnungsinduzierte Spannungsrißkorrosion in Flüssigmetallen

Strain-induced stress corrosion cracking in liquid metals Intercrystalline attack of steels may be caused by liquid metals (LME = Liquid Metal Embrittlement) as well as by electrolytes. LME-cracks were observed on both ferritic and austenitic stainless steels and on low-alloy or unalloyed steels. Especially, steel-constructions under tensile-stresses are endangered, for instance during brass-soldering and hot-dip-galvanizing. Considerable safety-risks and economical losses are connected with cracks caused by LME of steel-vessels for lead-raffination and galvanizing-purposes, because high quantities of liquid metal may flow out. A hypothesis for crack-behaviour is discussed. No possibilities seem to exist avoiding LME-damages by measures aside the composition of the liquid metals or the steels. Special investigations shall give results about critical figures for tensile-stress and strain-rate. These might be a base for constructor and shop-manager to create rules for building and handling of endangered steel-equipment to avoid LME.     

Über den Mechanismus der Lochfraßkorrosion bei Aluminium

On the mechanism of pitting corrosion of aluminum In an extensive study the dependencies of the pit growth of aluminum on time, potential and electrolyte conductivity have been quantitatively analysed. From this analysis it could be concluded that the pit growth kinetics is governed by the ohmic potential drop inside the pit. This explains the square-root time law experimentally found for the pit growth and the low activation energy of 3.14 kcal/mole. Moreover this analysis led to the conclusion that pitting is caused by a primary change of the properties of the pit surface due to the adsorption of chloride ions, since high local composition changes are prevented by the gas bubbles generated in the pits increasing the mass transport enormously, and since the total ohmic potential drop inside and outside the pits can not be larger than the difference between the actual potential and the pit growth limiting potential. Additional statements refer to the reaction order of the chloride ions, to the minimum chloride concentration for pitting and to the influence of pH and motion of the electrolyte.     

Spannungsrißkorrosion an Spannstählen

Stress corrosion cracking of prestressing steels During the investigation of a post-tensioned bridge structure incipient cracks of the prestressing steels of the transverse prestressed members were observed. Defects related to non-injected ducts or the presence of corrosion inducing substances could not be detected. The prestressing steel used is a quenched and tempered steel, strength class St 140/160, which was produced in the former GDR. The cause for the cracks is the susceptibility of this type of steel to hydrogen-induced stress corrosion cracking as could be shown in laboratory tests. Under unfavourable conditions cracks can be initiated before grouting. Additional magnetic particle tests at selected areas of the longitudinal prestressed members did not indicate any signs for incipient cracks.