Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.     

Oxidation resistance of sintered stainless steels: effect of yttria additions

The porosity of sintered stainless steels modifies their oxidation behavior, as compared to that of wrought stainless steels. This work studies the oxidation behavior of three sintered stainless steels: one ferritic (AISI 434L) and two austenitic (AISI 316L and 304L). 304L with yttria additions is also been studied to explore the possibility of reducing the oxidation rate of austenitic stainless steels by using this reactive element. The results demonstrate the influence of the formation of NiFe₂O₄ on the high-temperature behavior of sintered austenitic stainless steels and the effectiveness of yttria additions in increasing the oxidation resistance at 800 °C.     

Improving the corrosion behaviour of powder metallurgical 316L alloy by prepassivation in 20% nitric acid

The electrochemical improvement of PM 316L stainless steels by pre-alloyed powder prepassivation in 20% nitric acid in comparison with as-received specimens has been studied in this work. For comparison purposes a simultaneous study was carried out on similar composition wrought AISI 316L steels. Corrosion resistance was studied using evolution of the corrosion potential vs. time, anodic polarisations curves and Zero Resistance Ammeter technique. Reductions of the corrosion rate (i_corr) were observed in prepassivated specimens in neutral chloride media. Crevice resistance was higher for prepassivated specimens and for higher densities and annealing as post-heat treatments.     

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.     

Effect of ageing heat treatments on the microstructure and intergranular corrosion of powder metallurgy duplex stainless steels

The influence of ageing heat treatments (675 and 875 °C for 1.5 to 48 h) on the microstructure and intergranular corrosion resistance of sintered in nitrogen duplex stainless steels was investigated. The materials were obtained by sintering mixtures of austenitic AISI 316L and ferritic AISI 430L powders. Corrosion behaviour was evaluated by using electrochemical techniques. The beneficial effect of nitrogen on corrosion behaviour of solution annealed samples was established. During ageing, secondary phases were precipitated and the intergranular and transgranular corrosion resistance significantly decreased though repassivation was observed in specimens aged at 875 °C for times up to 8 h.     

Microstructure and pitting corrosion of similar and dissimilar stainless steel welds

Abstract

Pitting Corrosion behaviour of similar and dissimilar metal welds of three classes of stainless steels, namely, austenitic stainless steel (AISI 304), ferritic stainless steel (AISI 430) and duplex stainless steel (AISI 2205), has been studied. Three regions of the weldment, i.e. fusion zone, heat affected zone and unaffected parent metal, were subjected to corrosion studies. Electron beam and friction welds have been compared. Optical, scanning electron microscopy and electron probe analysis were carried out to determine the mechanism of corrosion behaviour. Dissimilar metal electron beam welds of austenitic–ferritic (A–F), ferritic–duplex (F–D) and austenitic–duplex stainless steel (A–D) welds contained coarse grains which are predominantly equiaxed on austenitic and duplex stainless steel side while they were columnar on the ferritic stainless steel side. Microstructural features in the central region of dissimilar stainless steel friction welds exhibit fine equiaxed grains due to dynamic recrystallisation as a result of thermomechanical working during welding and is confined to ferritic stainless steel side in the case of A–F, D–F welds and duplex stainless steel side in the case of D–A welds. Beside this region bent and elongated grains were observed on ferritic stainless steel side in the case of A–F, D–F welds and duplex stainless steel side in the case of D–A welds. Interdiffusion of elements was significant in electron beam welding and insignificant in friction welds. Pitting corrosion has been observed to be predominantly confined to heat affected zone (HAZ) close to fusion boundary of ferritic stainless steel interface of A–F electron beam and D–F electron beam and friction weldments. The pitting resistance of stainless steel electron beam weldments was found to be lower than that of parent metal as a result of segregation and partitioning of alloying elements. In general, friction weldments exhibited better pitting corrosion resistance due to lower incidence of carbides in the microstructure.     

Corrosion behaviour of Lotus-type porous high nitrogen nickel-free stainless steels

Corrosion behaviour of three austenitic Lotus-type porous high nitrogen Ni-free stainless steels exposed to an acidic chloride solution has been investigated by electrochemical tests and weight loss measurements. Polarization resistance indicates that the corrosion rate of Lotus-type porous high nitrogen Ni-free stainless steels is an order of magnitude lower than that of Lotus-type porous 316L stainless steel in acidic environment. The localised corrosion resistance of the investigated high nitrogen Ni-free stainless steels, measured as pitting potential, E_b, also resulted to be higher than that of type 316L stainless steel. The influences of porous structure, surface finish and nitrogen addition on the corrosion behaviour were discussed.     

Effect of Geobacter sulfurreducens on the microbial corrosion of mild steel, ferritic and austenitic stainless steels

The influence of Geobacter sulfurreducens was tested on the anaerobic corrosion of four different steels: mild steel 1145, ferritic steel 403 and austenitic steels 304L and 316L. Within a few hours, the presence of cells induced a free potential (E_oc) ennoblement around +0.3 V on 1145 mild steel, 403 ferritic steel and 304L austenitic steels and slightly less on 316L. The kinetics of E_oc ennoblement depended on the amount of bacteria in the inoculum, but the final potential value depended essentially on the nature of the material. This effect was due to the capacity of G. sulfurreducens to create a direct cathodic reaction on steel surfaces, extracting the electrons directly from material. The presence of bacterial cells modified the corrosion features of mild steel and ferritic steel, so that corrosion attacks were gathered in determined zones of the surface. Local corrosion was significantly enhanced on ferritic steel. Potential ennoblement was not sufficient to induce corrosion on austenitic steels. In contrast G. sulfurreducens delayed the occurrence of pitting on 304L steel because of its capability to oxidize acetate at high potential values. The electrochemical behaviour of 304L steel was not affected by the concentration of soluble electron donor (acetate, 1–10 mM) or the amount of planktonic cells; it was directly linked to the biofilm coverage. After polarization pitting curves had been recorded, microscopic observations showed that pits propagated only in the surface zones where cell settlement was the densest. The study evidenced that Geobacter sulfurreducens can control the electrochemical behaviour of steels in complex ways that can lead to severe corrosion. As Geobacteraceae are ubiquitous species in sediments and soils they should now be considered as possible crucial actors in the microbial corrosion of buried equipment.     

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.     

Electrochemical behavior of sintered oxide dispersion strengthened stainless steels

The electrochemical behavior of powder metallurgy (P/M) oxide dispersion strengthened stainless steels (SS) (316L and 434L) have been compared with standard 430 and 316 wrought samples in 0.05 mol/l sulfuric acid. The effects of sintering temperature and yttria addition on the electrochemical behavior have been studied. The behaviour of the dispersion strengthened SS was comparable to that of the straight P/M samples. The straight P/M samples sintered at 1400 °C exhibited better corrosion resistance compared to the samples sintered at 1250 °C and this has been correlated to sintered densities. The P/M austenitic SS were superior to the P/M ferritic SS. Pitting resistance, studied by cyclic polarization experiments in 3.56 wt.% NaCl, of the P/M samples were comparable to the wrought samples. The addition of Y₂O₃ did not affect the pitting resistance.     

节镍型不锈钢的耐腐蚀性能比较

通过3.5%NaCl溶液中动电位极化曲线测定和中性盐雾试验,对200系列奥氏体不锈钢和400系列铁素体不锈钢两类节镍型不锈钢与304不锈钢的耐腐蚀性能进行了对比研究。结果显示,400系列铁素体不锈钢的耐点蚀性能优于200系列奥氏体不锈钢,两种节镍型不锈钢的耐点蚀性能均不如304不锈钢好;200系列奥氏体不锈钢的耐均匀腐蚀性能最差,443不锈钢耐均匀腐蚀性能与304不锈钢相当,439不锈钢比304不锈钢耐均匀腐蚀性能稍差。201、202、304、439和443不锈钢在3.5%NaCl溶液中的点蚀电位分别为(vs.SCE)-32 mV、-22 mV、312mV、165 mV和227 mV,腐蚀速率分别为0.0071 mm/a、0.0062 mm/a、0.0026 mm/a、0.0038 mm/a和0.0024mm/a。     

Pitting Potential of stainless steels in artificial sweat

The pitting potentials of 12/12, 316 PX, AISI 303, 304, 316F and 316L austenitic stainless steels were determined in artificial sweat (perspiration) at room temperature. Two compositions of sweat were used: the BAM composition which contains two malodorous organic acids and the composition proposed in an ISO standard which does not contain them. The quasi-potentiostatic method (10 mV potential steps per min), potentiodynamic technique (1 V/min linear sweep) and scratch test were used on mechanically polished and HNO₃-passivated surfaces. The aggressivity of the two artificial sweats with respect to stainless steels was found to be practically the same. The pitting potentials and the classification of the stainless steels according to their pitting potential values were found to depend on the surface preparation and, to a lesser extent, on the test method. The experimental techniques were often complementary and a combination of two or three methods in conjunction may increase the confidence with which the conclusions can be applied. The lowest pitting corrosion resistance was found for AISI 303 and the highest for AISI 316L. As expected stainless steels with the lowest concentration of nonmetallic inclusions (sulfides and oxides) exhibited the best pitting corrosion resistance.     

典型不锈钢晶间腐蚀敏化温度的研究

用电化学动电位再活化（EPR）法、硫酸—硫酸铜法及扫描电镜研究了典型的202、304奥氏体不锈钢与409、430铁素体不锈钢在不同敏化温度下晶间腐蚀的敏感性。结果表明,奥氏体与铁素体不锈钢敏感温度区间不同,奥氏体不锈钢诱发晶间腐蚀的敏感温度约为650℃,铁素体不锈钢诱发晶间腐蚀的敏感温度约为950℃。研究结果为正确地评判不锈钢晶间腐蚀敏感性及优化生产工艺提供了科学依据。     

Oxidation Behavior at 900°C of Austenitic,Ferritic, and Duplex Stainless Steels Manufactured by Powder Metallurgy

This study examines the resistance to cyclic oxidation at 900°C of two commercial P/M stainless steels—one austenitic (AISI 316L), the other ferritic (AISI 434L)—and of three duplex stainless steels of different compositions. The test results show that the porosity of these materials leads to notable differences between their high-temperature oxidation behavior and that of conventional stainless steels of similar composition. In the case of P/M materials, the resistance to oxidation and the chemical composition of the oxides formed are strongly influenced not only by the degree of porosity of each material, but also by the concentration of Ni in the base metal. In the conditions adopted in the study, ferritic stainless steel was found to provide the best high-temperature oxidation resistance.     

General and local corrosion of as-cast 25cr-20ni steel in molten sulphates: The influence of Si and W additions

The corrosion of as-cast austenitic stainless steels 25Cr-20Ni (AISI 310) in alkaline sulphate melts was studied at 700°C. General corrosion occurred on the austenitic matrix and local corrosion in the interdendritic zones. Local corrosion depended on the type of solidification precipitates (carbides). A Cr-depleted zone was found around M₇C₃ type carbides. Electrochemical investigations with alloys and sintered carbides showed anodic behaviour of this Cr-depleted zone in relation to the austenitic matrix or M₇C₃ carbides. The influence of silicon and tungsten on general and local corrosion is discussed.     

Sintered stainless steels: Fatigue crack propagation resistance under hydrogen charging conditions

Monophasic and multiphasic (two and three phases) sintered stainless steels were prepared both considering premixes of AISI 316LHC and AISI 434LHC stainless steels powders and using a prealloyed duplex stainless steel 25% Cr, 5% Ni, 2% Mo powder. Their fatigue crack propagation resistance was investigated both in air and under hydrogen charging conditions (0.5 M H₂SO₄ + 0.01 M KSCN aqueous solution; applied potential = −700 mV/SCE), considering three different stress ratios (R = 0.1; 0.5; 0.75). Fatigue crack propagation micromechanisms were investigated by means of fracture surface scanning electron microscope (SEM) analysis.For all the investigated sintered stainless, fatigue crack propagation resistance is influenced by hydrogen charging and an increase of crack growth rates dependent on the steel microstructure is obtained. Experimental results also allow to identify the sintered stainless steel obtained from the prealloyed 25% Cr, 5% Ni, 2% Mo powder as the most resistant to fatigue crack propagation in air and under hydrogen charging conditions.     

Study of corrosion protection of different stainless steels by nanocrystalline plasma electrolysis

The effect of nitrogen and carbon bombardment on the corrosion behavior of three different stainless steels (AISI 304, AISI 316L and AISI 430) is discussed in this work. Nitrogen and Carbon was entered into the lattice in order to generate a rich region near the surface by a relatively new method called plasma electrolysis. Different ion bombardment doses under different applied voltages and thus different electrical fields (230, 245, 260 volts) at 330 mS/Cm of electrical conductivity of specific electrolyte have been tested to optimize the ion bombardment doses for each steel. The corrosion measurements were carried out in sodium chloride solution by using potentiodynamic scanning (PDS) and electrochemical impedance spectroscopy (EIS). The experimental results showed that the effect of N and C ion bombardment mainly depends on the microstructure and/or composition of the stainless steels. Less compact structures and/or less amount of alloying elements (as occurs with the body centered cubic, ferritic AISI 430) achieve bigger changes with this modification, whereas on stainless steels with a larger amount of alloying elements and / or more compact structures (like the face centered cubic, austenitic AISI 316L) ion bombardment slightly modifies the corrosion behavior. Original Russian Text ? M.Kh. Aliev, A. Sabour, T. Shahrabi, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 4, pp. 432–437. The text was submitted by the authors in English     

Neue Erkenntnisse auf dem Gebiet der ferritischen rostfreien Stähle

Recent developments in ferritic stainless steels The pitting resistance of ferritic stainless steels in HCl is visibly improved by Mo, in particular in the case of vacuum-melted material. In this context the ratio Cr:Mo = 25:2 is superior ta Cr:Mo = 17:3; addition of Mo prevents, beyond that, crevice corrosion. Ti increases resistance in the Strauß test but not in the Huey test, while Nb turns out to have a positive effect in either test. Steels containing Cr: Mo = 17:l are certainly still susceptible to pitting, but no longer to stress corrosion cracking in boiling MgCl₂, solution; stress corrosion cracking is not observed in 55% boiling Ca(NO₃)₂, and 25% boiling NaOH, but after annealing at 980 °C intercrystalline corrosion takes place. The test duration required for establishing cracking susceptibility is considerably shorter with ferritic than with austenitic steels (100 and 1000 to 2000 hours respectively).     

Investigation of the diffusion kinetics of borided stainless steels

In this study, the kinetics of borides formed on AISI 420, AISI 304 and AISI 304L stainless steels was investigated. Boronizing treatment was carried out using Ekabor-II powders at the processing temperatures of 1123, 1173 and 1223 K for 2, 4 and 6 h. The phases of the boride layers of borided AISI 420, AISI 304 and AISI 304L stainless steels were FeB, Fe₂B, CrB and NiB, respectively. The thickness of the boride layer formed on the borided steels ranged from 4.6 to 64 μm depending on the boriding temperature, boriding time and alloying elements of the stainless steels. Depending on the chemical composition, temperature and layer thickness, the activation energies of boron in AISI 420, AISI 304 and AISI 304L stainless steels were found to be 206.161, 234.641 and 222.818 kJ/mol, respectively. The kinetics of growth of the boride layers formed on the AISI 420, AISI 304 and AISI 304L stainless steels and the thickness of the boride layers were investigated.     

Effect of temperature on sintered austeno-ferritic stainless steel microstructure

The influence of temperature on microstructural changes of sintered austeno-ferritic steels has been investigated. PM stainless steels have been obtained by sintering mixtures of austenitic and ferritic stainless steel powders. Only temperature-induced phase transformation was observed in austenite, as a result of elements interdiffusion between both phases. Microstructural characterization was completed with atomic force microscopy (AFM) and micro- and nano-indentation test, it is revealed an increase in the hardness with respect to the solutionized materials.