Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

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.     

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.     

Corrosion behavior of nickel alloys in wet hydrofluoric acid

Wet hydrofluoric acid at concentrations below approximately 60% is highly corrosive to glass, reactive metals, carbon steel and stainless steels. Nickel alloys offer moderate corrosion resistance over a wide range of acid concentration and temperature. The corrosion behavior of eleven commercial alloys was quantified through laboratory testing. Variables that were studied included testing time, acid concentration, temperature, vapor and liquid phases and the presence of residual stresses. Results show that the corrosion rate of a Ni‐Cu and a Ni‐Cr‐Mo‐Cu alloy increased with the acid concentration and the temperature. However, both for increasing acid concentration and temperature, the corrosion rate of the Ni‐Cu alloy increased faster than the corrosion rate of the Ni‐Cr‐Mo‐Cu alloy, especially in the vapor phase. Even in unstressed coupons, nickel alloys showed internal penetration in presence of wet HF; the mode of this internal penetration varied from alloy to alloy. Considering all the studied variables that influence corrosion, the highest ranked material for wet HF service was a Ni‐Cr‐Mo‐Cu alloy.     

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.     

Einfluß der Legierungselemente auf die Passivierung und die Korrosionsbeständigkeit der Legierungen auf Eisen-Chrom-Basis

Influence of alloying elements on the passivation and the corrosion resistance of iron chromium base alloys The authors investigated the influence of cathodic alloying (with 0.1–0.5% Pd) on the passivation and the corrosion resistance of alloys of the system FeCr(25–100% Cr) and 25% Cr steels containing Mn, Ni, Mo and N; the experiments have been carried out in hot concentrated sulfuric acid and diluted hydrochloric acid solutions. It has been found that the addition of Pd as a cathodically active component considerably increases the autopassivation tendency as well as the corrosion resistance of the alloys under the particular conditions. Mn gives rise to improved autopassivation of austenitic and austenitic ferritic CrNi and CrNiMo steels, since it is catodically active, too. Cr steels cathodically alloyed with Pd acquire resistance to hydrochloric acid, too.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle (Teil II)

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels (Part II) Austenitic stainless steel (18% Cr, 12% Ni, Mo gradation between 0,06 to 3,6%) had been solution nitrided. By step‐by‐step removing, the samples could be prepared with various surface contents of nitrogen from 0.04 to 0.42%. In two test series the influence of nitrogen had been determined. The susceptibility against pitting corrosion of these samples had been tested by the chronopotentiostatical method. For the investigated steel composition and the used corrosion system there is no infuence of molybdenum on the effectiveness of nitrogen. The effectiveness of nitrogen can be described by the factor 25 in the PRE. By the investigation of the surfaces with the XPS analysis, it could be shown that the passivation and the pit nucleation is influenced by nitrogen. In these ranges NO_x, NH_x, and NH_z‐spectra have been detected. Bound Mo was found in steels containing molybdenum. It is assumed that the repassivation mechanisms of N and Mo work independently of each other. With the results efforts are supported to improve the pitting corrosion resistance also at molybdenum poor steels by surface nitriding or nitrogen alloying. The achieved results justify the assumption that the observed positive effect of the nitrogen may be extented to even higher nitrogen contents. A prerequisite for this is avoiding secondary phases in the matrix. The adverse influence of small particles is known well.     

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.     

Erfahrungen mit zwei hochlegierten,hochkorrosionsbeständigen Stählen in der chemischen Industrie

Experience from the chemical industry of the use of two high-alloy, highly corrosion resistant steels The steels SANDVIK 2RK65 and 2RE69 described in the article are fully austenitic and have the following nominal compositions:

• 2RK65 = Steel A' ? 0,020 C, 19, s Cr, 25 Ni, 4.5 Mo, 1,5 Cu,
• 2RE69 = Steel B ?0,020 C, 25 Cr, 22 Ni, 2,1 Mo, 0.12 N

Steel A has mechanical properties corresponding to most austenitic stainless steels, whereas Steel B, which is nitrogen alloyed, has somewhat higher strength. Besides very good resistance to non-oxidizing acids such as sulphuric acid and phosphoric acid and to all organic acids, Steel A has excellent resistance to pitting and good resistance to stress corrosion cracking. From the use of tubes of Steel A in highly corrosive environment in various chemical plants, considerable experience is now available, some of which is reported in this article. Steel B is a new steel up to now mainly used as stripper tubes in urea plants. Steel B is characterized by outstanding resistance in oxidizing acid solutions also in the presence of chlorides. As in the case of standard austenitic steels, neither of these two steels offer any fabrication problems.     

Corrosion of austenitic and ferritic-martensitic steels exposed to supercritical carbon dioxide

Supercritical carbon dioxide (S-CO₂) is a potential coolant for advanced nuclear reactors. The corrosion behavior of austenitic steels (alloys 800H and AL-6XN) and ferritic-martensitic (FM) steels (F91 and HCM12A) exposed to S-CO₂ at 650 °C and 20.7 MPa is presented in this work. Oxidation was identified as the primary corrosion phenomenon. Alloy 800H had oxidation resistance superior to AL-6XN. The FM steels were less corrosion resistant than the austenitic steels, which developed thick oxide scales that tended to exfoliate. Detailed microstructure characterization suggests the effect of alloying elements such as Al, Mo, Cr, and Ni on the oxidation of the steels.     

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.     

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.     

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.     

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.     

Warmverformbarkeit und Korrosionswiderstand von Stählen der Type 1Cr18Ni12Mo2Ti

Hot workability and corrosion resistance of 1Cr18Ni12Mo2Ti type steels Increasing the Mo content to the level required for corrosion resistance has a negative effect on the hot workability of austenitic steels. Addition of 2% Mo requires an increase in the proportion of austenite-forming elements (in particular Ni), while the Cr content must be reduced to the lowest admissible value for corrosion resistance. As has been shown by experimental work in a redox system and in a pulp digester 16–20% Cr can be considered sufficient (corrosion loss 0,01 mm/y). In bleach liquors of the textile industry, however, there is the danger of pitting corrosion. Heat treatment and hot working have a very pronounced bearing on corrosion resistance.     

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.     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Erweiterte Diskussionsbemerkungen zum Einfluß von Silicium und Molybdän auf die Spannungsrißkorrosionsempfindlichkeit austenitischer und austeno-ferritischer Chrom-Nickel-Stähle

Amplified discussion of the influence of Silicon and Molybdenum on the stress corrosion cracking proneness of austenitic and austeno-ferritic chrome-nickel steels Tests have been carried out in boiling magnesium chloride, calcium chloride, NaCl, water (150–200°C) and hot steam (500°C); in the two last-named cases, the tests were carried out with and without the addition of chloride and oxygen. The following materials were tested: steels with (per cent.) 17 and 17.5 Cr, 12–15.5 Ni, 0 and 2.5 Mo, 0 and 4 Si, less than 0.03 C, as well as steels with 20 and 21 Cr, 8—10 Ni, 0 and 2.5 Mo, 0 and 3 Si, 0, 1.5 CU, less than 0.05 C. Silicon has a favourable effect on the stress corrosion cracking behaviour in magnesium chloride and also in calcium chloride, but a detrimental effect in NaCl and water. Its probable effect is to inhibit the extension of the crack by crystallographic obstacles and, later, by the formation of a surface film of poor conductivity (where Mo also plays a part). The attempt is made to interpret this behaviour on the strength of electro-chemical investigations; in this connection, renewed critical reservations are made in respect of the test in magnesium chloride.     

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.     

Ein neuer hochlegierter Nickel-Chrom-Molybdän-Stahl für den Chemie-Apparatebau

A new high-alloyed nickel-chromium-molybdenum steel for the chemical process industry A new austenitic nickel-chromium-molybdenum steel (Nicrofer 3127 hMo/VDM alloy 31) is combining the advantages of highchromium alloyed materials, i.e. an excellent resistance to corrosive attack by oxidizing media, with a molybdenum content of more than 6%. The nickel content is comparatively low. This way, it was possible to create a highly corrosion resistant material in using only small amounts of expensive alloying elements. The new material exhibits an excellent resistance to pitting and crevice corrosion in neutral and acid aqueous solutions which makes it superior to the known 6% Mo stainless steels and nickel-base alloys like G-3. Therefore, it lends itself to applications as e.g. flue gas desulfurization, concentrating of diluted acid, pulp & paper, phosphoric acid, acetic acid and hot seawater as a promising new material for the future. The new alloy is readily weldable.