Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

Nitridation in NH3‐H2O‐mixtures

Exposures were conducted of iron, nickel, ferritic 1‐18%Cr steels, austenitic 18%Cr‐9%Ni‐ and 20%Cr‐31%Ni‐steels and a 16%Cr‐Ni‐base alloy at 500°C in He‐30%H₂O and 70%H₂O‐30%NH₃, to compare the corrosion behaviour of these materials in water vapor as in conventional power plants with their behaviour in a NH₃‐H₂O mixture, i.e. under conditions of the “Kalina‐cycle”. After 50 h in He‐H₂O generally a dense oxide scale had grown on iron and on the steels, whereas the scale grown in NH₃‐H₂O was porous, due to initial formation of the γ′‐ and ε‐nitrides, which are converted to Fe₃O₄ later. The porous scale allows internal nitridation of the Cr‐steels, nitrogen is transferred into the metal phase and reacts to finely dispersed CrN‐precipitates. This process causes stresses in the material and formation of cracks. The higher the Cr‐content of the material, the worse is the damage of the materials surface. Least corrosion damage occurs for iron and the 1%CrMo‐steel, however, the inward penetration of nitridation is greatest, and after 5 years on the low Cr‐steel a layer of about 15 mm would be embrittled by internal nitridation, formation of γ′ and ε‐nitride layers and external oxidation. Nickel is strongly damaged by intermediate formation of instable Ni₃N, which causes internal stresses and cracking, but also pore formation by its decomposition. The surface region of the 15%Cr‐Ni‐base alloy is also destroyed by internal nitridation and extrusion of Ni‐particles, while for this material the inward penetration of nitridation is relatively slow due to the low solubility and diffusivity of N in Ni and Ni‐alloys.     

Oxide scale formation on Al containing Ni–Cr‐based high temperature alloys during application as flame tube material in recirculation oil burners

The flame tube is an important functional component of burners using the concept of the flame tube stabilised combustion. Under typical combustion conditions the material of the flame tube is exposed to high temperatures (≥900 °C) and to corrosion attack by the combustion gases. Furthermore as the burners are generally operated intermittently, the material suffers from extreme temperature and atmosphere changes. For flame tubes, a lifetime of approximately 8000 h is desired. Predominantly metallic high temperature materials are used. The scope of the present work was to test—under application conditions and for maximum material temperatures exceeding 900 °C—alternative high temperature alloys for use as tube material. The corrosion resistance of the austenitic Ni–Cr‐based alloys (601, 602 CA, 617 and 693) has been investigated in a burner rig at maximum material temperatures of 950 and 1000 °C and with exposure times from 50 to 3000 h. The chromium content of the alloys was between 20 and 30 wt% and that of aluminium between 1 and 3.4 wt%. Metallographic cross‐sections of samples of the alloys were analysed by electron microprobe yielding information about the microstructure and composition of the oxides in the surface zone and variations during exposure time. This study focuses on the observed specific effects of the alloying element aluminium on the development of the oxide scale and on the lifetime of the alloys. At the alloy surface after 500 h exposure time a chromium oxide scale had formed with aluminium oxides underneath predominantly along grain boundaries. For the alloys with the lower aluminium content, the aluminium oxides built up an open network but not a closed layer. For the alloy with the highest aluminium content (alloy 693) after 50 h two different characteristic microstructures at the surface were found. In one case, the grains at the surface were covered with chromium oxide on top and the remaining grain surface was completely enclosed by aluminium oxides. In the other case, the aluminium oxide formed a thin layer directly below the chromium oxide scale. After 500 h exposure time, a significantly thinner chromium oxide scale and massive internal chromium oxides were observed. Catastrophic corrosion, formation of internal oxides and aluminium nitrides started even after 500 h. It will be demonstrated that the early breakdown of alloy 693 is linked to the aluminium oxides which act as a barrier constricting the diffusion of chromium from the alloy matrix towards the surface. Under the conditions of extreme temperature changes given in the burner the aluminium oxide layer on its part did not provide corrosion protection.     

Das Ausscheidungsverhalten von hochlegierten austenitischen Stählen mit 6% Molybdän und sein Einfluß auf die Korrosionsbeständigkeit

Precipitation behaviour of high-alloyed austenitic steels with 6% molybdenum and its influence on the corrosion resistance The high-alloy austenitic steels with 6 to 7% Mo, 20 to 21% Cr and 18 to 25% Ni are increasingly used in seawater and chemical applications. This is due to the excellent resistance to pitting and crevice corrosion in chloride-containing neutral and acidic environments. It is the high chromium and molybdenum content which provides the excellent corrosion behaviour but, at the same time favors the tendency to precipitation of intermetallic phases. Therefore, time-temperature-precipitation diagrams have been established for two steels with 6% Mo, 21% Cr, 25% Ni, 0.14 and 0.19% N and for one steel with 6% Mo, 20% Cr, 18% Ni and 0.21% N. The corresponding time-temperature-sensitization diagrams (in accordance to SEP 1877/II) and time-temperature-pitting diagrams (testing in 6% FeCl₃ solution) have been evaluated as well. Precipitation of intermetallics occurs rapidly especially in the range between 700 and 1000°C. In case of the 18% Ni steel and the 25% Ni/0.14% N steel grain boundaries are covered to a large extent with precipitates after only 15 min at 850 or 950°C. In case of the 25% Ni/0.19% N steel precipitation is considerably slower. The precipitates are interpreted to be chi-phase. After very long annealing times additionally small amounts of Laves phase appear. Neither carbides nor nitrides were observed. In spite of the rapid precipitation, sensitization in terms of the 50 m?m grain boundary penetration criterion is observed not before 0.7 h at 850 °C and not before about 2 h at 800°C in case of the 25% Ni/0.19% N steel. After about the same times of annealing also the critical pitting temperature as observed in the FeCl₃-test is dropping below 50°C. Therefore, when welding according to established rules and recommendations, no deterioration of the corrosion resistance in the heat-affected zone is to be expected. If high heat inputs will occur during manufacturing because of hot forming operations or welding of heavy sections, or if more severe test conditions are a requirement, a steel with 25% Ni and about 0.2% N (UNS N 08925, Cronifer hMo) is recommended due to its retarded precipitation and sensitization behaviour when compared to steels with only 18% Ni (UNS S 31 254). Additionally, the steel with 25% Ni has an increased resistance to general corrosion in acids. Notch impact strength of the materials under consideration is increased by the initial precipitation of the intermetallic phases and decreases only after longer times of annealing below the ductility of the solution annealed material.     

Microstructural investigation of the oxide formed on TP 347H FG during long‐term steam oxidation

The long‐term oxidation behaviour of TP347H FG in ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal‐fired power plant and characterising the oxide layer with reflective light and electron microscopy. Double layered oxide scales formed during steam oxidation. TEM investigations reveal that the inner oxide layer consists of particles of metallic Ni/Fe and Fe? Cr spinel in the interior of the former alloy grains and a compact layer of Fe? Cr spinel and Cr₂O₃ along the former alloy grain boundaries. The morphology suggests that the inner layer grows by internal oxidation of the interior of the alloy grains. The thickness of the inner oxide layer did not change significantly with oxidation time and temperature for exposure times up to 30 000 h. Faster Cr diffusion within the fine‐grained alloy at higher temperatures is held responsible for this observation. This hypothesis is supported by kinetic data. The oxide thickness at low and high temperatures after 58 000 h exposure was higher than expected.     

Pre-oxidation of Inconel Alloys for Inhibition of Carbon Deposition from Heated Jet Fuel

Exposure of superalloy surfaces to jet fuel at elevated temperatures leads to the formation of carbonaceous deposits and metal sulfides. The formation of stable oxide layers on alloy surfaces can reduce the activity of the constituent transition metals that catalyze the dehydrogenation of hydrocarbons and the subsequent carbon deposit growth. The metals Ni, Cr, Fe, Mn, Al, Ti and Nb + Ta form thermodynamically stable oxide layers after oxidation above 800°C under O₂ flow. In this study, we investigated the formation of oxides and spinels on three different Ni-base superalloys (Inconel 600, Inconel 718, and Inconel X750) and their activity towards carbon and sulfur deposit formation from jet fuel (JP-8) thermal stressing at 600°C and 34 atm (500 psig) for 5 hr. Metal oxide formation during pre-oxidation and the behavior of pre-oxidized samples in thermal stressing were found to depend strongly on the minor element composition of these superalloys.     

Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam

Several Fe-Cr-Ni austenitic stainless steels (Cr wt.%: 13–25, Ni wt.%: 15) were oxidized in steam for 1000 hr at 500–900°C. The oxide scales were examined and categorized with respect to the chromium concentration and the grain size of the base metal. Experiments showed three conditions for the critical bulk Cr concentration and the oxidation temperature at which the oxidation behavior changed drastically. Metallographic examination showed that two of these three conditions resulted from the internal-external transition of Cr₂O₃ either on the metal surface or along the grain boundaries of the base metal. Attempts were made to interpret these conditions from the available oxidation theories. Atkinson's treatment was employed with some modification to incorporate the grain-boundary diffusion of Cr in the base metal. The calculation basically explained the internal-external transition for the oxidation of these steels.     

Oxidation von Schweißverbindungen austenitischer Stähle bei hohen Temperaturen

Oxidation of austenitic steel welds at elevated temperatures Two steels (18–21 Cr, 8–11 Ni and 17–20 Cr, 7–10 Ni) welded with filler material of the composition 18 Cr, 11 Ni, have been studied with respect to their resistance to oxidizing atmospheres in the temperature range between 850 and 1000°C, test durations being up to 5000 h. The behaviour has been evaluated in terms of scale properties (compactness, adhesion), structural stability (by metallographic methods), scale composition (with the micro analizer) and investigation of diffusion from the base metal. On the basis of the results obtained it may be concluded that the base metal could be exposed to a maximum temperature of 900°C, while the welded zone is not sufficiently resistant at that temperature; consequently, the service temperature should be limited to a maximum of 850°C.     

Roles of Manganese in the High-temperature Oxidation Resistance of Alumina-forming Austenitic Steels at above 800?°C

A new family of alumina-forming austenitic (AFA) stainless steels is under development for uses in aggressive oxidizing conditions. This paper investigates the effect of manganese additions on the oxidation kinetics and alumina scale formation in two series of AFA steels, i.e., Fe–20Ni–14Cr–2.5Al and Fe–18Cr–25Ni–3Al base. At 800?°C in dry air, the oxidation resistance was moderately degraded with additions of larger than 1 wt% Mn in the AFA steels based on Fe–14Cr–20Ni–2.5Al. At 900?°C in air with 10?% water vapor, however, additions of Mn in these AFA steels based on Fe–18Cr–25Ni–3Al would significantly destabilize the alumina scale formation and degrade the oxidation resistance. Our analysis revealed that additions of Mn stimulated formation of the coarse spinel CrMn_1.5O₄ and Cr₂O₃ oxide and destroyed the continuity of the protective alumina scales, thus worsening the oxidation performance. In addition, it was found that there exists an upper limit for the Mn additions which is decreased with the increase of the service temperatures and presence of aggressive oxidizing agents.     

Long‐term exposure of austenitic steels and nickel‐based alloys in lignite‐biomass cofiring

The aim of this study was to assess the long‐term impact that the addition of biomass provokes on superheater materials exposed to fireside corrosion environments. Alloys covering a broad range of commercially available materials were investigated. Their corrosion kinetics under different corrosive deposits and atmospheres was evaluated, and their corrosion products analyzed to deepen understanding of the underlying corrosion mechanisms. Therefore, three nickel‐based alloys and three austenitic steels containing 20–24 wt.% Cr were tested at 650°C for 7,000 hr. The long‐term exposure shows new mechanistic aspects of Type II hot corrosion that were revealed by accelerated material depletion. The formation of Ni–NiS eutectic and the formation of a Cr depleted zone close to the substrate corrosion product interface are indicative of the breakaway occurrence. Differences in the corrosion behavior are related to the balance of Ni, Mo, Co, and Cr and can serve as the material selection argument. The evaluation concluded with the finding that alloys presenting Mo and Ni might be preferentially used in fireside corrosion in the presence of biomass, whereas the use of austenitic steels suffer less corrosion if no biomass is present in the corrosive atmosphere.     

High temperature corrosion of structural materials under gas‐cooled reactor helium

The Generation IV International Forum has selected six promising nuclear power systems for further collaborative investigations and development. Among these six concepts, two candidates are Gas Cooled Reactors (GCR), namely the Very High Temperature Reactor (VHTR) and the Gas‐cooled Fast Reactor (GFR). The CEA has launched a R&D program on the metallic materials for application in an innovative GCR. Structural GCR alloys have been extensively studied in the past three decades. Some critical aspects for the steels and nickel base alloys resistance under the service conditions are microstructural stability, creep strength and compatibility with the cooling gas. The coolant, namely helium, proved to contain impurities mainly H₂, CO, CH₄, N₂ and steam in the microbar range that interact with metals at high temperature. Surface scale formation, bulk carburisation and/or decarburisation can occur, depending on the atmosphere characteristics, primarily the effective oxygen partial pressure and carbon activity, on the temperature and on the alloys chemical composition. These structural transformations can notably influence the mechanical properties: carburisation may induce a loss in toughness and ductility whereas decarburisation impedes the creep strength. There is a valuable theoretical as well as practical knowledge on the corrosion of high temperature alloys in the primary circuit of a GCR but this past experience is not sufficient to qualify every component in a future reactor. On the one hand, the material environment could be significantly different from the former GCR's, especially regarding the higher temperature. On the other hand, the materials of interest are partly different. Ni‐Cr‐W alloys, for instance, may offer significant improvement in the maximum operating temperature as far as the mechanical properties are concerned. However, their corrosion resistance toward the GCR atmosphere is still unknown. We describe here our first corrosion tests of Haynes 230, a high strength Ni base alloy containing tungsten, under low oxidising helium at 750 °C or 950 °C. The experiments were carried out in a purposely‐designed device that allows controlling low impurity partial pressure. The test duration was up to 1000 h. The corrosion behaviour was assessed through gravimetry and microscopy. Some specimens were investigated using EDS and DRX. The results under low oxidizing potential are compared to the oxidation kinetics in air. Tentative interpretations are proposed based on published models.     

A new austenitic alumina forming alloy: an aluminium‐coated FeNi32Cr20

The FeCrAl alloys owe their low oxidation rate to the formation of a slow growing α‐aluminium oxide scale. Therefore they are used, for example, as a substrate material in metal‐supported automotive catalytic converters. Increasing exhaust gas temperatures mean that, in addition to the oxidation properties, high temperature mechanical properties should also be improved. Compared to the ferritic FeCrAl alloys, austenitic alloys possess the required high mechanical strength at higher temperatures. However for most commercially available materials the oxidation resistance is not sufficient due to a low aluminium content. High aluminium contents are avoided in austenitic alloys, since they cause severe workability problems, even at aluminium contents, which are below the necessary amount to get a pure alumina scale. The newly developed material Nicrofer 3220 PAl (coated FeNiCrAl) consists of an austenitic FeNi32Cr20 alloy coated with aluminium on both sides. It combines the outstanding oxidation resistance of an alumina forming FeCrAl alloy with the advantage of the high temperature strength of an austenitic alloy. Additionally the oxidation is even lower than the oxidation of the commercial grade Aluchrom YHf (FeCr20Al6)—conventional homogenous FeCrAl. Aluminium coated FeNiCrAl can easily be formed into its final shape. Prior to service, an in situ heat treatment is recommended in order to optimize the properties.     

Corrosion characteristics of the wrought Ni‐Cr‐Mo alloys

This paper concerns the wrought, nickel‐chromium‐molybdenum (Ni‐Cr‐Mo) alloys, a family of materials with a long history of use in the chemical process industries. Their attributes include resistance to the halogen acids and resistance to pitting, crevice attack, and stress corrosion cracking in hot, halide salt solutions. The purpose of this paper is to characterize the performance of the Ni‐Cr‐Mo alloys in several key chemicals, using iso‐corrosion diagrams. These indicate the expected corrosion rates over wide ranges of concentration and temperature. Furthermore, the differences between individual Ni‐Cr‐Mo alloys, and their behavior relative to the stainless steels are defined. The data indicate benefits of both a high chromium content and a copper addition, as used in Hastelloy® C‐2000® alloy.     

Optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion

The main objective of the EU OPTICORR project is the optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion, the development of a life‐cycle approach (LCA) for the materials in energy production, particularly for the steels used in waste incinerators and co‐fired boiler plants. The expected benefits of this approach for safe and cost effective energy production are: ‐ control and optimisation of in‐service performance of boiler materials, ‐ understanding of high‐temperature corrosion and oxidation mechanisms under service conditions, ‐ improvement of reliability to prevent the failure of components and plant accidents and ‐ expanding the limits of boiler plant materials by corrosion simulations for flexible plant operation conditions (steel, fuel, temperature etc.). The technical aim of the EU OPTICORR project is the development of modelling tools for high‐temperature oxidation and corrosion specifically in boiler conditions with HCl‐ and SO₂‐containing combustion gases and Cl‐containing salts. The work necessitates thermodynamic data collection and processing. For development and modelling, knowledge about the corrosion mechanisms and exact data are needed. The kinetics of high‐temperature oxidation and corrosion are determined from laboratory thermo‐gravimetric tests (TG) and multi‐sample exposure tests. The materials studied are typical boiler tubes and fin‐steels: ferritic alloys, the austenitic steel T347 and the Ni‐based alloy Inconel 625. The exposure gases are dry air, air with 15 vol‐% H₂O, and with 2000 ppm HCl and 200 ppm SO₂. The salt deposits used are based on KCl‐ZnCl₂ and Ca, Na, K, Pb, Zn‐sulfates. The test temperatures for exposures with deposits are 320 and 420°C and, for gas exposures, 500 to 600°C. At present the tools being developed are ChemSheet based programmes with a kinetic module and easy‐to‐use interface and a more sophisticated numerical finite‐difference‐based diffusion calculation programme, InCorr, developed for prediction of inward corrosion and internal corrosion. The development of modelling tools for oxidation and high‐temperature corrosion was started with thermodynamic data collection for relevant systems and thermodynamic mappings. Further, there are needs to develop the simulation model and tool for salt‐induced hot corrosion based on the ChemSheet approach. Also, the work on modelling and simulating with the InCorr kinetic modelling tool will be continued to demonstrate the use of the tool for various steels and alloys in defined combustion environments.     

Recent advances in understanding metal dusting: A review

Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster, because carbon diffusion is more rapid in α‐Fe than in Fe₃C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr–Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni–Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni–Sn Fe‐base materials facilitates the formation of a Fe₃SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.     

Metal dusting of high temperature alloys

Metal dusting, i.e. disintegration into fine metal particles and carbon, was induced on a selection of chromia forming high temperature alloys in a flowing CO-H₂-H₂O atmosphere in exposures at 650°C, 600°C, 500°, and 450°C. The materials were pretreated by annealing in H₂ at 1000°C and electropolishing, this leads to large grain size and low surface deformation, both is disadvantageous for formation of a Cr₂O₃ scale. The resistance to metal dusting is only dependent on the ability to form a protective Cr₂O₃ scale, thus the high Cr ferritic steels proved to be very resistant, the ferritic steels with 12–13% Cr were less resistant. Due to the lower Cr diffusivity in the austenitic steels, these were very susceptible, especially two alloys with about 30% Ni (Alloy 800, AC 66). The appearance of metal dusting was somewhat different for Ni-base materials but they were also attacked under pitting. The metal dusting is preceded in all cases by internal carburization whereby the chromium is tied up, afterwards the remaining Fe or Fe-Ni matrix can react to the instable intermediate carbide M₃C which decomposes to metal particles and carbon, in case of Ni-base materials a supersaturated solid solution of carbon is the intermediate.     

Gaseous corrosion of alloys and novel coatings in simulated environments for coal,waste and biomass boilers

The reduction of emissions from power generation plants is a key part of the Kyoto Protocol. Reduced emissions per unit of power produced can be achieved via increased thermal efficiency and this can be achieved by increasing steam parameters (i.e. temperature and pressure). Increased steam parameters in turn leads to accelerated corrosion of boiler components. Biomass and solid waste fuels introduce a number of aggressive species into process environments that result in enhanced rates of boiler degradation. This paper reports on studies, both theoretical and experimental, of the corrosion behaviour of high‐alloy steels and Ni‐base alloys as well as coatings for use in high efficiency coal and/or biomass‐ and waste‐fired power plants. Coatings produced within the SUNASPO project have been laboratory tested in gaseous atmospheres representative of coal combustion, biomass combustion and waste incineration. Laboratory tests were carried out mainly in the temperature range 500 °C to 800 °C. Initial results showed the poor performance of traditional uncoated low‐alloy boiler steels P91 (9% Cr) and HCM12A (12% Cr), as well as the higher alloy steel, 17Cr/13Ni. Results show the beneficial effects of coatings containing Al, Si, Al + Si, Al + Ti and Al + B in reducing the rate of corrosive attack. In a combustion product gas containing 100 ppm HCl and 1000 ppm SO₂, aluminizing affords corrosion resistance of low‐alloy steels such as HCM12A and P91 similar to that of Alloy 800 over 1000 h of test. The presence of Al inhibits internal, sometimes localized corrosion by promoting the formation of a protective surface oxide layer even at relatively low temperatures. The results of experiments in simulated coal; biomass and waste atmospheres are presented and discussed in terms of both corrosion kinetics and mechanisms of degradation.     

Weldability and weld performance of candidate nickel base superalloys for advanced ultrasupercritical fossil power plants part I: fundamentals

Abstract

Fossil fuel will continue to be the major source of energy for the foreseeable future. To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary. There are several initiatives worldwide to achieve efficiencies >45% higher heating value (HHV) through an increase in steam temperature (700 to 760°C) and pressure (27.6 to 34.5?MPa). Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures. In order to accomplish this, three classes of materials have been identified: creep strength enhanced ferritic steels, austenitic stainless steels and nickel base superalloys. Although new alloys have been designed and developed to meet this need, welding can have a significant and often detrimental effect on the required mechanical and corrosion resistant properties. Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels. Welding and weldability of nickel base alloys will be discussed in a two part paper. In this paper, the primary focus will be on the fundamentals of welding and weldability of Ni base superalloys.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels Austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.5 to 3.6%) had been gas‐nitrided. By stepwise removal, samples could be prepared with various surface content of nitrogen up to 0.45%. The susceptibility against pitting corrosion of these samples had been tested by two methods: – determination of the stable pitting potential in 0.5 M NaCl at 25°C – determination of the critical pitting temperature in artificial sea water (DIN 81249‐4) The influence of nitrogen to both determined parameter can be described well by PRE = Cr + 3,3 · Mo + 25 · N That means for the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen.