Beurteilung spezieller Fragen zur Beständigkeit hochlegierter Werkstoffe mit dem EPR‐Versuch (Číhal‐Test)

The EPR‐technique to evaluate special questions on the corrosion resistance of high‐alloyed materials The electrochemical potentiokinetic reactivation (EPR‐)test is a very useful tool to reveal microstructural peculiarities of high‐alloyed materials and especially the sensibility to intergranular corrosion even if only very localised; it is possible to quantify the results referring to a standard. As this test does not only react on chromium depletion due to carbide formation a metallographic examination of the tested surface is indispensable. Due to it's very high sensitivity this technique is highly qualified not only for research tasks but also for the evaluation of components and for failure case analysis. To our opinion this method seems less recommendable or suitable for general purposes of acceptance testing as the results may be ambiguous and, in addition, generally accepted standards are missing up to now.     

Intergranular corrosion of welded joints of austenitic stainless steels studied by using an electrochemical minicell

An intergranular corrosion study of welded joints of austenitic stainless steels (AISI 304 and 316L) has been addressed. A specific small-scale electrochemical cell (minicell) has been used. Four different weldment zones have been studied. The electrochemical methods applied were the electrochemical potentiokinetic reactivation test and electrochemical potentiokinetic reactivation double loop test. These techniques showed that the HAZ was the most critical zone to intergranular corrosion for both materials. The weld metal was susceptible to interdendritic corrosion and the fusion line showed a mixture of intergranular and interdendritic corrosion. The effect of pre- and post-welding heat treatments for AISI 316L was analyzed. The HAZ was again the most critical zone in every heat treatment condition. The results were correlated to the microstructural features of the materials.     

Korrosionsuntersuchungen an chemisch beständigen Stählen mit Hilfe von stationären Reaktivierungsmessungen (Umschaltversuchen)

Corrosion investigations on chemical resistant steels by means of steady stade electrochemical reactivation tests (change over tests) A method to quantify the degree of sensitization on selective corrosion behaviour is the electrochemical potentiokinetic reactivation method (EPR‐Method). Due to the potentiokinetic character, however, the application of the EPR‐testing method leads to some difficulties. The corrosion is influenced by kinetic effects and electrolytic secondary reactions and shows a huge dependence on time. Therefore a steady state electrochemical reactivation method to quantify the degree of sensitization on selective corrosion behaviour would lead to a reduction of the mentioned problems and the interpretation, comparability and reproducibility of the achieved results would be more simple and clear. This paper presents the first measurements and results which point to a suitable and promising steady state electrochemical testing method.     

A new approach for DL-EPR testing of thermo-mechanically processed austenitic stainless steel

Standard test methods such as the electrochemical potentiokinetic reactivation test (EPR) and double-loop EPR test (DL-EPR) are commonly used to characterise sensitisation behaviour in austenitic stainless steels and nickel-based alloys. In this study, the DL-EPR test is augmented by large-area image analysis (IA) to characterise and quantify the networks of attacked grain boundaries. A new analysis approach that is based on a grain boundary cluster parameter is proposed to describe the network of corrosion susceptible grain boundaries, which may be estimated from electron backscatter diffraction (EBSD) data. This method may provide a better assessment of the relative DOS of different heats of austenitic stainless steels.     

Electrochemical methods for testing the intergranular corrosion susceptibility of stainless steels

In addition to the standardized intergranular corrosion testing methods to detect sensitization in austenitic stainless steels, electrochemical test methods, i.e. the potentiokinetic reactivation test (EPR) and the anodic polarization and constant potential etching methods in perchloric acid solution containing sodium chloride, have been developed. The results of these electrochemical measurements are compared with those obtained from copper sulfate-sulfuric acid tests.     

Korrosionsschäden am Förderband eines Ofens zur Reinigung von mit organischen Chlorverbindungen kontaminiertem Erdboden

Corrosion damage to the conveyor belt of a furnace for the removal of soil from organic chlorine compounds This paper reports the corrosion behaviour of metallic materials which were used in components of a conveyor belt of a decontamination furnace or of which specimens were attached to the belt for testing purposes. The furnace was used for decontaminating soil containing organic chlorine compounds. A total of 1 ferritic and 8 austenitic steels, 9 nickel base alloys, 2 east steels with 15 % Si and 15 % Si + 5 % Cr respectively, and 3 aluminised steels were tested and afterwards investigated by metallographic and micro-analytical methods. Most of the materials failed due to external and/or internal corrosion, preferentially along grain boundaries. The 2 cast steels revealed fairly good corrosion resistance. The material 2.4061 (LC-Ni 99.6) displayed surprisingly good behaviour, however it remains to be seen whether the inward migration of O and C along grain boundaries causes embrittlement. The materials 2.4610 and 2.4831, containing 14-17 % Mo and 8.5-9,5 % Mo respectively, showed good resistance. The reason for this good corrosion resistance could not be clarified. One reason may be the relatively high Mo content but other materials with a comparable Mo content, like materials 2.4856 and 2.4663, were significantly attacked. It appears necessary to check the long-term behaviour of these two materials.     

Korrosionsverhalten hochchromhaltiger,ferritischer, chemisch beständiger Stähle

Corrosion behaviour of high chromium ferritic stainless steels Ferritic steels developed for seawater desalination and containing 20 to 28% chromium, up to 5% Mo and additions of nickel and copper have been tested with respect to their corrosion behaviour, in particular in chloride containing media. The materials in the sensibilized state were tested for inter-crystalline corrosion susceptibility in the Strauß-, Streicher-, nitric acid hydrofluoric acid- and Huey-Tests. No intercrystalline corrosion was encountered in the case of the steels with 28% Cr and 2% Mo. The resistance to pitting was assessed on the basis of rupture potentials determined by potentiokinetic tests. The resistance of the steels with 20% Cr and 5% Mo or 28% Cr and 2% Mo is superior to that of the molybdenum containing austenitic types. Addition of nickel yields a significant increase in crevice corrosion resistance; the same applies to resistance in sulfuric acid. In boiling seawater all the materials tested are resistant to stress corrosion cracking. No sign of any type of corrosion was found on nickel containing steels after about 6000 hours exposure to boiling 50% seawater brine even under salt deposits.     

Uniform corrosion and intergranular corrosion behavior of nickel‐free and manganese alloyed high nitrogen stainless steels

The uniform and intergranular corrosion behavior of two kinds of nickel‐free and manganese alloyed high nitrogen stainless steels (HNSSs) were investigated. A type of 316L stainless steel (316L SS) was also included for comparison purpose. Both solution annealed (SA) and sensitization treated (ST) steels were examined. It was found that the SA HNSSs had much weaker resistance to uniform corrosion compared to the SA 316L SS. The addition of molybdenum, to some extent, improved the uniform corrosion resistance of the HNSSs. The sensitization treatment had little influence on the uniform corrosion resistance of all the steels. The HNSSs showed an obvious susceptibility to intergranular corrosion, in particular the ST HNSSs. The intergranular corrosion rates of the sensitized HNSSs were much higher than that of the sensitized 316L SS. The degree of interganular attack for the ST HNSSs was much more serious than that for the 316L SS. The addition of molybdenum obviously improved the resistance of the ST HNSSs to intergranular corrosion. The double loop electrochemical potentiokinetic reactivation tests also proved that the HNSSs were rather susceptible to the sensitization treatment compared to the 316L SS. The relatively weak resistance of the HNSSs to uniform and intergranular corrosion may be due to high manganese promoted anodic dissolution. The improvement of uniform and intergranular corrosion resistance caused by the addition of molybdenum could be attributed to the synergistic effects of molybdenum and nitrogen in the HNSSs on the formation and stability of passive film.     

Einfluss der Halbzeugform auf die Korrosionsbeständigkeit hochlegierter Ni‐Cr‐Mo‐Werkstoffe

Effect of semi‐finished products on the corrosion resistance of high‐alloyed Ni‐Cr‐Mo materials The corrosion resistance of different semi‐finished products of six superaustenitic steels and nickel based alloys in the condition of delivery was investigated in some typical standard corrosion tests. The resistance of sheets, plates, strips, seamless tubes and welded tubes to intercrystalline corrosion was tested according to ASTM G 28 methods A and B, as well the resistance to pitting corrosion according to ASTM G 48 method C. The nickel based alloys 625, C‐276 and alloy 59 are resistant to the FeCl₃‐test according to ASTM G 48 method C and therefore a differentiation of these types in regard to their localized corrosion resistance was achieved only in the more aggressive ‘Green‐Death’‐solution. The laboratory experiments confirmed that the corrosion resistance is identical for all semi‐finished products and that it shows only a slight dependence of the surface condition of the materials tested. Additionally, some typical industrial and practical applications of the six high performance materials are presented to demonstrate the excellent corrosion resistance in the manufactured condition.     

On the substitution of conventional corrosion tests by an electrochemical potentiokinetic reactivation test

Nickel-based alloy 625 was investigated by means of a conventional corrosion test (Streicher-test according to ASTM G28A [1]) and an electrochemical potentiokinetic reactivation test (according to ASTM G108 [1], with some modifications). Susceptibility to intergranular corrosion after various heat treatments was examined by using both test methods and the results were compared. Dependent on the type of heat treatment applied, both methods showed partially different results. Possibilities and limitations of replacing Streicher-test with EPR-test are pointed out and commentated. In conclusion, the EPR-test presents itself as a promising and quick investigation technique for detecting susceptibility to intergranular corrosion in nickel-based alloys.     

Anticipated Degradation Modes of Metallic Engineered Barriers for High-Level Nuclear Waste Repositories

Metallic engineered barriers must provide a period of absolute containment to high-level radioactive waste in geological repositories. Candidate materials include copper alloys, carbon steels, stainless steels, nickel alloys, and titanium alloys. The national programs of nuclear waste management have to identify and assess the anticipated degradation modes of the selected materials in the corresponding repository environment, which evolves in time. Commonly assessed degradation modes include general corrosion, localized corrosion, stress-corrosion cracking, hydrogen-assisted cracking, and microbiologically influenced corrosion. Laboratory testing and modeling in metallurgical and environmental conditions of similar and higher aggressiveness than those expected in service conditions are used to evaluate the corrosion resistance of the materials. This review focuses on the anticipated degradation modes of the selected or reference materials as corrosion-resistant barriers in nuclear repositories. These degradation modes depend not only on the selected alloy but also on the near-field environment. The evolution of the near-field environment varies for saturated and unsaturated repositories considering backfilled and unbackfilled conditions. In saturated repositories, localized corrosion and stress-corrosion cracking may occur in the initial aerobic stage, while general corrosion and hydrogen-assisted cracking are the main degradation modes in the anaerobic stage. Unsaturated repositories would provide an oxidizing environment during the entire repository lifetime. Microbiologically influenced corrosion may be avoided or minimized by selecting an appropriate backfill material. Radiation effects are negligible provided that a thick-walled container or an inner shielding container is used.     

Versuche zur Natriumkorrosion austenitischer Werkstoffe bei 1000 °C

Investigations into sodium corrosion of austenitic materials at 1000 °C A series of austenitic materials with sufficiently high strength for the use at high temperatures were examined in respect to their compatibility with liquid sodium at 1000 °C. Austenitic steels with nickel contents of 12% showed the highest stability, but materials with 32% nickel and 20% chromium suffered also moderate losses of weight, however, additional internal corrosion effects. Nickelbase alloys were most severely attacked. They are less applicable for the use with sodium of very high temperatures in spite of their high temperature strengths.     

Die Lochkorrosionsbegrenzung bei hochlegierten Sonderedelstählen und NiCrMo‐Legierungen in Chloridlösung – Einflußfaktoren und Ausmaß

Limit of pitting corrosion at high‐alloyed special steels and NiCrMo alloys in chloride solution The phenomenon of the limit of pitting corrosion in direction to positive potentials is studied by potentiokinetic polarization after a jump in the transpassive range and by potentiostatic tests at technical wrought materials and at model alloys of the systems NiCrMo and NiMo in CaCl₂ solution in the concentration range 1 to 9 mol/l chloride at pH‐values of 1 to 9 at temperatures of 30 to 110°C. Surface‐analytical investigations gives in connection with knowledges from anodic polarization studies directions to the mechanism of the limit of pitting corrosion. Ranges of the limit of pitting corrosion are obtained at materials with a Mo content above 6.5% and contents of chloride of the media above 2 mol/l chloride. Increasing temperatures, increasing contents of chloride and sulfate shift the potential of the limit of pitting corrosion being always above 0.2 V (SCE) at potentiostatic determination to noble direction. There are indications that the mechanisms of limit of pitting corrosion is resulting from an inactivation of pitting nuclei by the formation of hardly soluble molybdenum chlorides in the potential range of limit of pitting corrosion.     

Potentiokinetic Criteria relevant to the pitting corrosion of stainless steels in chloride solutions

The adequacy of 7 parameters of potentiokinetic curves for evaluating the resistance to pitting corrosion in sodium chloride solutions was quantitatively assessed by reference to immersion testing in ferric chloride solutions for 11 type CrNi (MoTiCu) stainless steels on the basis of 314 experimental runs.     

Werkstoff- und Korrosionsprobleme in wäßrigen Lösungen bei hohen Temperaturen und Drücken

Materials and corrosion problems in aqueous solutions at high temperatures and pressures The present report, based on papers and discussions of an international conference on electrochemistry at high temperatures and pressures forms a survey of materials and corrosion problems at high temperatures and pressures. Among the topics of the conference there were, among others, corrosion problems in power stations with particular emphasis on high alloy chromium nickel steels and nickel base alloys which are preferentially used in nuclear power stations. Unalloyed and low alloy steels were lower on the scale of interests. A matter of particular interest, however, were investigations and findings on corrosion cracking failures, which may occur on chromium nickel steels and nickel chromium iron alloys under rather different conditions of heat treatment of the materials (solution annealed or sensitized) and of the corrosive medium. With reference to pertaining literature data a summary is presented of the present state of knowledge concerning the various type of cracking corrosion which may occur on the materials of interest, i. e. chromium nickel steels of the 18 8 type, Incoloy 800 und nickel base alloys of the Inconel 600 type.     

Evaluation of Alloy 146, 279, 900, and 926 sensitization to intergranular corrosion by means of electrochemical methods and image analysis

Properties of stainless steels are affected by improper heat treatment. When that happens, the material can become sensitive to intergranular corrosion. This loss of properties is due to a microstructural changes. In this paper, electrochemical potentiokinetic reactivation tests (single and double loop) were used to evaluate steel sensitization to intergranular corrosion. Four alloys, Alloy 146 (UNS 1.4331) and 279 (UNS 1.4429) (austenitic stainless steels), Alloy 900 (UNS 1.4462) (duplex stainless steel), and Alloy 926 (UNS N08926) (high alloyed stainless steel), were employed as working electrodes. The specimens were sensitized at different temperatures (600–825 °C) for periods of 1 or 2 h in argon atmosphere. Electrochemical tests were conducted using the electro-optical devices P-200002525 and P-200002526. These devices permit to obtain images of the electrode surface and electrochemical data simultaneously. In addition, these results were compared to standardized etching with oxalic acid (ASTM A-262), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Two alloys showed sensitization to intergranular corrosion, Alloy 900 and Alloy 926 at 725 and 825 °C. Additionally, the single and double loop methods showed different passivation results. In the double loop test, the formation of a layer over the entire electrode surface was visible during the process of activation. The film remained until the end of experiment. However, in the single loop method no significant variations were observed on the electrode surface during passivation process, but if material is sensitized a film was formed during reactivation branch.     

Schadgaskorrosion von Werkstoffen in der Befestigungstechnik

Corrosion due to contaminated atmosphere of materials for fastening systems Different materials (mainly stainless steels) for fastening systems were tested under extremely contaminated conditions like in road tunnels and multi-storey car-parks. The atmospheric constituents and climatic conditions of the Gotthard-tunnel were taken as a basis for the experiments to have an example from real life. The materials were tested in a contaminant and climatic chamber. The acid gas concentrations, temperature, humidity and water-soluble chlorides were increased in the testing device whereas the climatic periods with conditions for condensation and following drying were reduced. Thus in a very short period of time materials showing a sufficient corrosion resistance could be chosen for the extremly contaminated atmosphere of the Gotthard-tunnel. Experiences gained by prior field tests were taken to define a factor for time-reduction. The results of 2 years lasting field tests were achieved in less than one week, although dust consisting soot was not taken into account. The only material passed all series of experiments without corrosion was alloy 24. The high Molybdenum steels 904hMo, 254 SMO and 317LN also attained very good results. The Duplex-steel specimens 318LN and material 316L showed pitting corrosion in places. All further low alloyed stainless steels had strong pitting and crevice corrosion as well as a strong extensively harm. The tested copper and aluminium alloys also showed a high surface corrosion. The stainless steels type 304 and 316 Ti which are registered in Germany as material for building and construction industry [10] took also part in the tests. In extremely high contaminated atmospheres like simulated, these materials didn't show any sufficient corrosion-resistance at all. A linear correlation between the PRE-equivalent and the corrosion-resistance of the non-corroding steels has been clearly prooved. Different batches of the same materials were involved in these tests. The results showed a testing of different batches is necessary to ensure a sufficient corrosion-resistance. Further experiments with climatic and harmful conditions of other contaminated environments can be carried out in the testing device. By this the resistance of different materials and the influence of separate parameters on corrosion can be tested.     

Effect of laser surface melting on intergranular corrosion behaviour of aged austenitic and duplex stainless steels

In the present study, the effect of laser surface melting (LSM) on intergranular corrosion behaviour of aged austenitic stainless steels (UNS S30400, S31603, S32100 and S34700) and aged duplex stainless steels (UNS S31803 and S32950) were investigated. LSM of the aged stainless steels was carried out using a 2.5 kW CW Nd:YAG laser. The microstructure of the aged stainless steels after LSM depends on their compositions. After LSM, the aged austenitic stainless steels mainly contain austenite (γ) with some ferrite (δ) as the minor phase, but the carbide phases are completely eliminated. For the aged duplex stainless steels after LSM, δ becomes the major phase and the δ/γ phase balance is disturbed, whereas the sigma (σ) phase is eliminated. The degree of sensitization (DOS) and corrosion morphology of the aged stainless steels before and after LSM were determined by the double loop electrochemical potentiokinetic reactivation (DL-EPR) using a potentiostat and SEM observation, respectively. Desensitization of the aged stainless steels has been successfully achieved by LSM and their intergranular corrosion resistance is found to be significantly enhanced as reflected by the decrease in DOS due to dissolution of the carbides or σ phase, which reduced Cr depletion or the possibility of solute segregation at the grain or phase boundaries, despite the presence of δ and disturbance of δ/γ phase balance.     

Einfluß von Chrom,Molybdän und Stickstoff auf die Korrosionsbeständigkeit des Ni-freien,austenitischen Edelstahles Macrofer 2515MoN (W.-Nr.: 1.4653)

Influence of chromium, molybdenum and nitrogen on the corrosion resistance of the Ni-free, austenitic stainless steel Macrofer 2515MoN (German Alloy No. 1.4653) Nitrogen alloyed, Ni-free, austenitic stainless steels comprising of more than 1 wt.-% nitrogen are a new group of alloys with promising properties. They show a very interesting combination of high strength and toughness with a high corrosion resistance. This combination of properties make the alloys not only suitable for fasteners but also for parts for medical and dental applications. This work shows the influence of chromium, molybdenum and nitrogen on the corrosion resistance of Fe25Mn-alloys in media typical for the above mentioned applications. According to these results Fe25Mn-alloys with appr. 20 wt.-% chromium, about 3 wt.-% molybdenum and appr. 1,3 wt.-% nitrogen have an excellent corrosion resistance in Ringer solution, artificial saliva and artificial sweat. The critical pitting temperature (CPT) as well as the critical crevice temperature (CCT) with 61°C respectively 37°C tested according ASTM G 48A provided significantly higher temperatures when compared to the commercially well established Ni-austenite X6CrNiMoTi17-12-2 (German Alloy No. 1.4571).     

Allgemeine Beständigkeit von Schweißnähten unter Berücksichtigung der Spannungs- und Schwingungsrißkorrosion

General resistance of weld seams with a view to stress corrosion cracking and corrosion fatigue The corrosion of welds is due to thermal effects during welding which give rise to structural changes and, frequently, compositional changes in the transition zone. The welded material is rapidly cooled and may thus be heterogeneous and may present residual stresses resulting in increased susceptibility to selective and stress corrosion. The manganese content is of high importance in low alloy steels, as well as residual martensite or austenite embedded in a ferrite matrix. Low ferrite contents are generally beneficial because they counteract high temperature cracking; however, ferrite contents should be hept below 10% in order to prevent the formation of a continuous network giving rise to selective corrosion. Corrosion susceptibility may also be produced by carbide or carbonitride precipitation in austenitic and ferritic steels and nickel base alloys. Weld zones in aluminium alloys are attacked in rare cases (e.g. by HNO₃) and the susceptibility of Ta, Zr and Ti depends from the properties of the protective atmospheres.