The influence of alloy diffusion processes on the corrosion of ferritic and austenitic stainless steels

Sulphidation has been studied in austenitic steels AISI 310 and 447 and in ferritic steel AISI 446 at temperatures between 550°C and 700°C. The kinetics of mass gain change at about 650°C from a linear, spalling-controlled type at lower temperatures to a diffusion-controlled type above this temperature. At the higher temperatures internal sulphidation is observed. The explanation for the mass gain rate constants measured is that diffusion in the alloy is rate controlling. This is substantiated by a new high diffusivity-path-enhanced diffusion model described in a parallel paper. The high diffusivity paths are shown to be intergranular and intragranular sulphides, in ferritic and austenitic alloys, respectively. These sulphides nucleate on existing carbides. In cases where the aggressiveness of the atmosphere is not high, as is the case here, this alloy diffusion controlled corrosion mechanism can be suppressed by using austenitic as opposed to ferritic matrices and reducing the concentration of relatively unstable carbides like M₂₃C₆ in the material.     

Diffusion behaviour of hydrogen in nitrogen containing austenitic alloys

Electrochemical permeation technique and thermal desorption spectroscopy were used to evaluate the hydrogen diffusion, solubility, and trapping behaviour in nitrogen containing austenitic alloys. The hydrogen diffusion of all alloys obeyed Arrhenius relationships in the studied experimental temperature range. The apparent diffusion coefficients were determined and the relevant activation energies were estimated. Nitrogen at higher concentration (>0.38 wt.%) as well as chromium decrease the hydrogen diffusion whereas nickel has opposite effect. The thermal analysis of all alloys except Cr18Mn18N0.57 shows a main single peak at different heating rates indicating no significant hydrogen interaction with microstructural defects. The alloy Cr18Mn18N shows a shoulder in addition to the peak which is attributed to nitrogen–hydrogen interaction. The activation energy estimated from the thermal analysis is nearly equal to the activation energy for diffusion obtained from the Arrhenius plots of the corresponding alloys.     

Influence of aluminium content on retarding hydrogen transport in Fe–Al binary alloys

The electrochemical hydrogen permeation measurement was utilized to determine the effective diffusivity, permeation rate and apparent solubility of hydrogen in a series of Fe–Al binary alloys at 25°C. The experimental results reveal that both the hydrogen effective diffusivity and permeation rate in these alloys are significantly dropped when Al content is more than 15–20 at.%. The hydrogen apparent solubility in these alloys is reduced with the increment of Al content, even though their lattice parameters and strain fields in these alloys are increased. For these Fe–Al binary alloys, the retardation factor of hydrogen is mainly dependent upon Al content. Both oxide film and the degree of structural order are the major retardation factors on hydrogen transport in Fe–Al binary alloys.     

Einflüsse von Mo,V, Nb,Ti, Zr und deren Karbiden auf die Bindungszustände des Wasserstoffs in Eisen und das Bruchverhalten der Eisenlegieungen

Effects of Mo, V, Nb, Ti, Zr and their carbides on the binding states of hydrogen in iron and the fracture behaviour of the iron alloys Effects of Mo, V, Nb, Ti, Zr and their carbides on hydrogen permeation, diffusion, solubility and its distribution on different binding states in iron and iron based alloys are studied by use of the electrochemical hydrogen permeation technique. The results are analysed and described in terms of the trapping theory. The fracture behaviour of the alloys affected by hydrogen in different binding states are tested under constant elongation rate conditions. No essential specific effects of one of the alloying elements or their carbides are observed. Hydrogen diffusion and solubility, the total hydrogen content as well as the fracture behaviour are affected by the various microstructures of the alloys, thus only indirectly by the alloying elements. Crack initiation and fracture progress depend first of all on the external hydrogen activity and the grain size of the material, not on the total hydrogen content. Fine crystalline iron alloys of higher strength prove to be less sensible to hydrogen damage than coarse grain weak structures.     

Hydrogen uptake in austenitic stainless steels by exposure to gaseous hydrogen and its effect on tensile deformation

Hydrogen solubility and diffusivity of Fe–Cr–Ni austenitic stainless steels were measured through exposure to gaseous hydrogen at a pressure of 10 MPa over the temperature range 110–235 °C. The hydrogen solubility depended on the alloy compositions, whereas the diffusion coefficients were nearly identical at a given temperature. Hydrogen uptake in the stable austenitic steels by exposure to high-pressure gaseous hydrogen led to some loss of ductility, while their fracture surfaces showed evidence of plastic deformation. This was attributed to the enhanced inhomogeneity of plastic deformation in the presence of hydrogen and the increased stress for plastic instability with increasing hydrogen concentration.     

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.     

Oberflächenbehandlung als Korrosionsschutzmaßnahme von nichtrostenden Stählen

Surface treatment as corrosion protection measure of stainless steels The pickling behaviour of several stainless austenitic steels and of one steel with ferritic/austenitic grain structure were investigated in pickling solutions of different compositions based on hydrofluoric acid. Because of uncertainties in practical applications, the influence of temperature, time and acid content on the mass loss is of high interest. In another series of experiments, aqueous solutions of citric acid were tested for their suitability as pickling chemicals for the materials X 6 CrNiTi 18 10 (AISI 321) and X 6 CrNiMoTi 17 12 2 (AISI 316 Ti). Finally, the pickling procedures based on nitric acid/hydrofluoric acid mixtures were compared with mechanical cleansing methods and with pickling procedures based on aqueous citric acid solutions as well, to elucidate their influence on the corrosion resistance of the treated materials. The valuation followed a pitting corrosion test in sodium chloride solutions of different concentrations after Herbsleb and Schwenk. Pickling with hydrofluoric acid solutions is superior to other cleansing procedures, if corrosive environments are present. The ecologically beneficial citric acid solutions are only able to remove the annealing colours from stainless steels.     

Stress-corrosion cracking of 15cr-15ni-2.2mo titanium-modified austenitic stainless steels

The stress- corrosion cracking (SCC) behavior of two alloys of titanium- modified austenitic stainless steels with different TiJC ratios in the 20% cold worked condition was studied in 45% boiling magnesium chloride (BP427 K) using the constant- extension rate testing (CERT) technique. The SCC susceptibility of the two titanium-modified alloys was assessed using the ratios of the values of ultimate tensile strength (UTS) and percent elongation in magnesium chloride and liquid paraffin, the susceptibility index (I), crack propagation rates (CPR), and stress ratios at different values of plastic strains. The results obtained on these alloys were compared with AISI type 316 stainless steel. It was observed that the two titaniummodified austenitic stainless steels had better SCC resistance than type 316 stainless steel, mainly due to their higher nickel content and, to a lesser extent, to the presence of titanium. Increasing the value of the TiJC ratio led to increased SCC resistance due to the availability of more free titanium in the solid solution. Fractography of the failed samples indicated failure by a combination of transgranular SCC and ductile fracture.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil I: Permeation,Diffusion und Löslichkeit von Wasserstoff in binären Eisenlegierungen

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid Part I: Permeation, Diffusion and Solubility of Hydrogen in Binary Iron Alloys The influence of elements in common steels like C, S, P, Mn, Si, Cr, Ni, Sn, and Cu on hydrogen uptake and permeation through iron was studied for various binary iron alloys using the electrochemical permeation method. In order to characterize the effects of the alloying additions on the hydrogen activity at the metal surface as well as on the hydrogen transport in the bulk, the hydrogen permeation coefficients, diffusion coefficients and solubilities have been determined in the temperature range of 10 to 80° C. Steady state hydrogen permeation is significantly decreased only by Si and Cr. Si reduces mainly the solubility of hydrogen in iron and decreases only slightly hydrogen diffusion. Cr additions produce strong traps for hydrogen in iron, with an average binding energy of ? 58 kJ/mol. Thus, hydrogen diffusion is retarded. The total hydrogen content is increased by Cr. Permeation measurements of commercial steels showed mainly the Si-effect.     

Hydrogen permeation behavior of nickel electroplated AISI 4340 steel

The role of the electroplated nickel layer on hydrogen permeation through AISI 4340 steel was investigated by a electrochemical hydrogen permeation test. The permeation test, composed of three steps, was conducted to measure the hydrogen diffusivity and surface hydrogen concentration. A constant current of 20 mAcm^-2 and a constant potential of -100 mV vs. Ag/AgCl electrode were applied to the hydrogen entry and exit cells, respectively. The thickness of the electroplated nickel layer on AISI 4340 steel increased in a linear fashion with an increase in electroplating time. The nickel coated layer contributed to a decrease in the hydrogen permeability of nickel coated AISI 4340 steel specimens. This is due to the fact that the surface hydrogen concentration and hydrogen diffusivity in nickel coated layer were lower than those of AISI 4340 steel substrate. Especially, low hydrogen diffusivity decreased significantly with hydrogen permeability. The critical effective hydrogen diffusivity for barrier of nickel electroplated AISI 4340 steel specimens was higher than the hydrogen diffusivity of AISI 4340 steel specimen. It is proposed then that the thin nickel layer on AISI 4340 steel acts as a barrier for hydrogen permeation through AISI 4340 steel.     

Surface nanocrystallization by surface mechanical attrition treatment and its effect on structure and properties of plasma nitrided AISI 321 stainless steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 321 austenitic stainless steel by means of surface mechanical attrition treatment (SMAT). Low-temperature nitriding of SMAT and un-SMAT AISI 321 stainless steel was carried out in pulsed-DC glow discharge. The effect of SMAT pretreatment on the microstructure and properties of the stainless steel were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Vickers hardness tester and UMT-2MT tribometer. The results show that the plasma nitriding of AISI 321 steel can be enhanced considerably by means of SMAT process before nitriding, and a much thicker nitrogen diffusion layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples. In addition, the wear resistance and load capacity of the nitrided layers on the SMAT samples was much higher than that of the un-SMAT samples due to the thicker S phase case and the gradient nitrogen diffusion layer.     

Einfluß von Mo2C und P auf die Wasserstoffdiffusion und Löslichkeit im System Fe-Mo-C-P

Effect of Mo₂C and P on hydrogen diffusion and solubility in the Fe-Mo-C-P system In Fe-Mo-C-P alloys at moderately high temperatures, phosphorus favors the precipitation of Mo₂C particles in the matrix. In the vicinity of these precipitates, the dislocation density is increased and phosphorus is enriched by trapping. The effect of this microstructure on the steady state hydrogen permeation is negligible. The hydrogen solubility is increased and the hydrogen diffusion is decreased according to interactions between hydrogen and the traps which are formed along with the carbides. The average binding energy of the deep traps for hydrogen was found to be about ?54 kJ/mol H. This value corresponds to the interaction energy between hydrogen and the deep traps at dislocations. If phosphorus is enriched in the vicinity of Mo₂C, the average binding energy of the deep traps decreases.     

Dissimilar welding of AISI 310 austenitic stainless steel to nickel-based alloy Inconel 657

The current work was carried out to characterize welding of AISI 310 austenitic stainless steel to Inconel 657 nickel–chromium superalloy. The welds were produced using four types of filler materials; the nickel-based corresponding to Inconel 82, Inconel A, Inconel 617 and 310 austenitic stainless steels. This paper describes the selection of welding consumables for the joint. The comparative evaluation was based on hot-cracking tests (Varestraint test) and estimation of mechanical properties. According to Varestraint tests, Inconel A showed the least susceptibility to hot cracking. In tension tests, all weldments failed in the weaker parent metals (i.e., Inconel 657). Moreover, Inconel A weldment had the highest strength and total elongation. On the other hand, the weld metals failed by ductile fracture except Inconel 617, which exhibited mixed fracture mode. At last, it was concluded that Inconel A filler material offered the best compromise for the joint between Inconel 657 and 310 stainless steel.     

Mössbauer Effect Study of Room Temperature Cathodic Polarization of AISI321SS Austenitic Stainless Steel

Room temperature hydrogen charging by cathodic polarization of cold rolled AISI 321SS austenitic stainless steel in appropriate electrolytic medium leads to its decomposition to structural defects and a ferromagnetic α′-martensitic phase. The degree of decomposition, and hence the resulting products depends on hydrogen charging time with martensitic transformation yielding up to 14-22% martensite for charging periods of 30 and 96 h, respectively. Based on M?ssbauer spectroscopy measurements, the magnetically split portion of the spectra corresponding to the α′-martensite phase was resolved in terms of one Fe-site with internal magnetic field in the range of 260-265 ± 10 kOe. Both the uncharged and retained (after hydrogen charging) austenitic phases were resolved similarly at ambient and sub-ambient cryogenic temperatures. The austenitic phase in both the uncharged and charged states remained stable from ambient down to 4.2 K, where they exhibited singlet broadening suggesting weakly ferro/antifero-magnetic ordering.     

Zur Korrosionsbeständigkeit von Vanadium und Vanadiumlegierungen als Strukturwerkstoffe in einem lithiumgekühlten Fusionsreaktor

Corrosion resistance of vanadium and vanadium alloys as structural materials in a lithium cooled fusion reactor Liquid lithium has a potential as a blanket fluid of a D-T fusion reactor. Its effects upon solid materials are physical solution and chemical exchange processes. The exchange reactions are favoured by the fact that the solubility and the free enthalpies of formation of compounds of oxygen, nitrogen, carbon, and hydrogen are relatively high. Vanadium and its alloys are not affected by physical solution as is the case with nickel. However, oxygen can be removed by lithium, carbon can be exchanged between the melt and the solid metal, and nitrogen will be picked-up by the vanadium alloys. Procedures to decrease the extent of exchange are discusses. If the exchange reactions can be widely suppresses, vanadium alloys will be potential materials for the application in a D-T-Li fusion reactor up to high temperatures.     

Corrosion Testing of some Austenitic Stainless Steels and of Inconel Alloy at 25°c in Acidic Chloride Solutions

Abstract

The corrosion of austenitic stainless steels types AISI 304, 310 and 316, and of Inconel alloy, was studied at 25°c, in 5% NaCl solution at an initial pH value of 2·5, and in 5% FeCl³ at pH 1·2. The resistance of the alloys in both corrosive environments was in the order: 310 > 316 > 304 > Inconel. Pre-treatment of the specimens with bubbling chlorine gas increased the subsequent corrosion rates of the alloys. Intermittent bubbling of gas mixtures such as Cl₂, N₂, and/or H₂S, increased the corrosion rate of Inconel alloy when Cl₂ was present, but decreased the corrosion rate when H₂ was present. Heat treatment of austenitic stainless steels increased the subsequent corrosion rates, whereas 16% pre-straining of annealed specimens slightly reduced the rates. Addition of trisodium phosphate to the corrosive solution reduced the corrosion rates and pitting tendency for all three types of austenitic stainless steel.     

The chloride corrosion of austenitic stainless steels and of an inconel alloy in hot acidic media

Open circuit corrosion testing of austenitic stainless steels, AISI types 304, 310 and 316, and of an Inconel alloy in boiling 5% NaCl solution at pH 2.5 was carried out. The influences of aeration and the introduction of Cl₂ and/or H₂S were also examined. Cl₂ accelerated corrosion, pitting and crack formation whereas H₂S had an inhibiting effect on the corrosion rates and pitting but induced hydrogen attack, which appeared in the form of blisters on the surface specimen. H₂S in the absence of oxygen resulted in the growth of corrosion product on the specimen surface, instead of dissolution, but the cracking tendency also increased due to hydrogen penetration. Addition of trisodium phosphate to the corrosive solution markedly reduced pitting and lowered the corrosion rate by approximately half. Throughout the various tests it was found that the resistance of different alloys to hot chloride corrosion was in the order 310 > 316 > 304 > Inconel.     

The Influence of Temperature on Oxide-Scale Formation during Erosion--Corrosion

Four steels (Fe2.25Cr1Mo, Fe9Cr1Mo, AISI 304, 353 MA) and one Ni-based superalloy (Inconel 625) were exposed in an erosion--corrosion test rig at the temperatures 20, 350, 550 and 700°C for 1 week. The atmosphere was air and the particle velocity 1.2 m/s. The composition and thickness of the developed surface layers were determined by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The ferritic and Ni-based alloys investigated show a minimum in wastage rate around 350°C due to the development of a particle strengthened/toughened composite layer on the surface. The greatest wastage rates were measured at 700°C. Rapid diffusion paths created in the oxide from the particle bombardment results in the growth of oxide nodules at the oxide/metal-interface causing protruding oxide flakes which are chipped away. At this temperature the ferritic steel Fe9Cr1Mo is degraded to a larger extent than the austenitic steels.     

Untersuchungen zum Einfluß des Stahlgefüges auf die instationäre Wasserstoffpermeation

Investigations on the influence of microstructure of steels on unsteady state hydrogen permeation The effect of microstructure of iron and a low alloyed steel on hydrogen permeation at room temperature is studied by means of the electrochemical permeation method. Hydrogen transport through these materials during charging or effusion processes is affected by microstructural heterogeneities and hydrogen content. The diffusivity as a function of the state of the materials ranges over about five orders of magnitude. In course of non-steady state diffusion dislocations and interfaces decrease the effective diffusivity to values between 10⁻⁷ and 10⁻⁹ cm²/s. At steady state permeation the diffusivity reaches the values of 10⁻⁵ to 10⁻⁴ cm²/s.     

Inhibiting effect of octadecylamine on pitting corrosion behaviour of stainless steel type 1.4541 up to 250 °C

The inhibiting effect of octadecyclamine (ODA) on chloride induced localized corrosion of austenitic stainless steel type 1.4541 (AISI 321) is described at temperatures from 150 °C and 250 °C indeaerated aqueous solutions with chloride concentrations up to 10^{- 1} M. Results of electrochemical (current-potentiAl-curves, impedance spectroscopy) as well as corrosion tests on mechanical loaded samples (constant extension rate test), and surface analytical characterization (Auger electron spectroscopy) showed the inhibiting effect of ODA. The increase of the corrosion resistance of the investigated material is interpreted by an ODA-film on steel surface as a diffusion barrier, and the formation of a compact hot-water oxide layer, caused by the increase of pH resulted from ODA-cracking and dissociation.