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 alloy element contents on caustic stress corrosion cracking of several stainless steels

The stress corrosion cracking behavior in caustic solutions (200 g/l sodium hydroxide, 10 g/l sodium chloride) of three austenitic (18Cr-10Ni-2.5Mo, 20Cr-25Ni-4.5Mo, 27Cr-31Ni-3.5Mo) and three duplex (23Cr-4Ni, 22Cr-5Ni-3Mo, 25Cr-7Ni-4Mo-N) stainless steels was examined. U-bend and Slow Strain Rate (SSR) tests were performed at 200–250°C. The negative influence of nickel in the lower range content for the 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo has been shown; when the nickel content is significantly increased (>30%), as in the case of the steel 27Cr-31Ni-3.5Mo, an increase of SCC resistance has been detected. The negative effect of molybdenum, mainly on the behaviour of duplex stainless steels, has also been evidenced. The duplex stainless steels show better caustic SCC resistance than austenitic stainless steels type 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo. The best behaviour has been found for the less-alloyed steel 23Cr-4Ni.     

New stainiess steels for sea water applications. Part. 1: Corrosion and mechanical properties of ferritic stainless steels

Two experimental ELI ferritic stainless steels (22 Cr – 2.5 Ni – 3 Mo and 22 Cr – 2.5 Ni – 3 Mo – Ti) prepared in laboratory and a commercial one (21 Cr – 3 Mo – Ti) were investigated. Electrochemical and laboratory exposure tests were carried out to define the localized corrosion resistance (pitting and crevice) of such steels in chloride solution. Intergranular and stress corrosion resistance was also evaluated. Room temperature tension tests and impact tests were performed. 22 Cr – 2.5 Ni – 3 Mo – Ti and 21 Cr – 3 Mo – Ti steels are immune to intergranular corrosion whatever temperature they are heat treated at and have the same pitting corrosion resistance as a function of temperature; crevice corrosion of 22 Cr – 2.5 Ni – 3 Mo is decidely better than in the commercial 21 Cr – 3 Mo – Ti. The experimental steels were immune to stress corrosion in hot chloride environment.     

合金元素对铸造Fe-Cr铁素体不锈钢耐浓硫酸腐蚀性能的影响

采用电化学技术、浸泡腐蚀及能谱分析等研究了合金元素对铸造Fe-Cr铁素体不锈钢耐浓硫酸腐蚀行为的影响。结果表明:随铬含量的增加,Fe-Cr合金的耐浓硫酸腐蚀性能增强,单一的铬合金化不能使Fe-Cr合金在60℃,98%H2SO4中自钝化;钼能促进Fe-Cr25-Mo合金的钝化和自钝化,随钼含量的增加,Fe-Cr25-Mo合金的耐浓硫酸腐蚀性能增强;辅助合金元素镍、铜可促进Fe-Cr25Mo2合金的钝化和自钝化,而钛、铌的影响不大。     

Nitrogen in austenitic stainless steels

Nitrogen alloyed in austenitic stainless steels improves austenite stability, mechanical properties and corrosion resistance. Steels supersaturated with nitrogen (“super-nitrogen steels”) have been investigated, which rival the latest ferritic steels in strength but have potentially greater toughness.     

Electrochemical synthesis of ferritic stainless steels alloyed with 1 wt-%Ti,V and Nb in sulphuric acid solution

Abstract

Corrosion behaviours of stainless steel alloys containing corrosion resistant elements were investigated. Ferritic stainless steel electrodes were synthesised by the application of a scan rate of 1 mV s^–1. Stainless steels were unalloyed and alloyed with approximate 1 wt-% Ti, V and Nb elements. The samples were obtained from casting and forging. The steels were exposed to different heat treatments. Heat treatment was not applied to the first group of samples. The second and the third group of samples were rapidly cooled after annealing at 1100°C for 30 and 180 min respectively. The corrosion performances of ferritic stainless steels were investigated in 0·1 M H₂SO₄ solution, by use of electrochemical impedance spectroscopy. Scanning electron microscopy (SEM) investigations were performed. Scanning electron microscopy micrographs showed generalised pitting on the surface. Corrosion resistance was calculated by Stearn-Geary equation. It was determined that titanium has the best effect on the corrosion resistance of ferritic stainless steels homogenised for 180 min.     

Effect of nickel alloying on crevice corrosion resistance of stainless steels

The crevice corrosion behaviour of stainless steels containing 25 mass% Cr, 3 mass% Mo and various amounts of Ni was investigated in natural seawater. The results showed that ferritic steels containing nickel were more resistant to corrosion than both ferritic steels without nickel and austenitic steels. The superiority of the Ni bearing ferritic steel over the other steels was in close agreement with the depassivation pH of those steels in acidic chloride solutions. The results showed that the addition of Ni to ferritic steel was effective in decreasing the depassivation pH and the dissolution rate in acidic chloride solutions at crevices.     

Effect of chlorination on stainless steels in seawater

The effect of chlorination on the corrosion resistance of different stainless steels in North Sea water has been studied, the sea-water exposures being performed at two Norwegian seawater laboratories. In one laboratory test specimens, simulating various components used in a seawater cooling system, were immersed in the water. In the other laboratory a pipe system containing real components was studied. Seawater temperature, chlorine concentration and chlorination procedure have been varied at both laboratories. The steels investigated include highly alloyed grades like UNS S31254 (AVESTA 254 SMO), UNS S44635 (Avesta Monit) and UNS NO8028 (Sandvik Sanicro 28). The investigation shows that continuously chlorinated seawater is considerably more aggressive than unchlorinated or intermittently chlorinated seawater and that high temperature increases the risks of localized corrosion at the same chlorine concentration. The highest alloyed steel grades were very resistant to crevice corrosion even in continuously chlorinated water but certain kinds of metal-metal crevices have to be avoided. When welded, duplex UNS S31803 was sensitive to pitting corrosion in chlorinated water. Contrary to crevice corrosion, the risk for galvanic corrosion decreases considerably if the seawater is chlorinated.     

High-temperature oxidation and thermal cycling of aluminum-electroplated stainless steels

An alumina coating, produced from the oxidation of an aluminum-electroplated deposit, improved the oxidation resistance in air of a ferritic, AISI-type 446 stainless steel, Fe-24Cr-1.2Al containing 0.15% of mischmetal, and an austenitic AISI 321 stainless steel containing 0.53% Ti, at least up to 1100°C. In thermal-cycling tests from 1000°C to room temperature, the alumina coating was adherent on the ferritic and austenitic steels, for at least 1000 and about 700 cycles, respectively. The addition of rare earths to the ferritic steels and titanium to the austenitic, provided good adhesion between the coating and substrate. The porous nature of the coating was found to be very beneficial by causing the coating to be more resistant to thermal and growth stresses. Oxidation mechanisms are discussed in the light of results obtained from the thermogravimetric tests and metallographic observations by SEM-ED analysis.     

Corrosion resistance and mechanical properties of nickel-bearing ferritic stainless steels

Additions of nickel to ferritic steels containing 25–28% Cr and 2–4% Mo increased the impact toguhness especially when more than 2% Ni was present. The effect of nickel content increased up to 4% Ni, the largest addition studied. Steels stabilized with niobium had lower transition temperatures then did corresponding steels stablizied with titanium. Steels containing 4% Ni required annealing at 1050 C to avoid intermetalic compounds. It was also noted that nickel reduced the upper shelf energy in the Charpy impact test and eliminated a sharp transition from ductile to brittle behaviour. No definite effect of nickel on pitting potential was pound but steels in the series 25Cr-3.5 Mo-Ni-Ti consistenly had more noble pitting potentials and greater resistance to crevice corrosion than the 28 Cr-2Mo-Ni-Ti steels. Nickel contents of 1 or 2% tended to improve crevice corrosion resistance while larger nickel contents were somewhat ditrimental. Nickel strongly reduced critical current densities for passivity both in l N H₂SO₄ and in l N HCL and yielded corresponding increases in resistance to corrosion by these acids. Although 1% Ni or more caused the annealed steels to be susceptible to stress corrosion cracking in MgCl₂ boiling at 140 C, while the as-Welded steels containing 4% Ni did not crack in boiling 25% Nacl at pH 1.     

Corrosion behavior of various steels in the simulated fluidized bed boiler environment

The corrosion behavior of various austenitic stainless steels and high-alloy steels has been studied in simulated fluidized bed boiler environment to develop a new corrosion resistant austenitic stainless steel for the superheater tube. The superheater is usually not installed within the bed position, which is different from the evaporator installed within the bed position. Therefore, the superheater tubes are exposed to an oxidizing environment; but it is also necessary to estimate the corrosion resistance of the steels in a reducing environment. It is already known that the high temperature corrosion behavior in conditions where CaSO₄ is coated on the steels is more important than the erosion of the superheater tubes. The main results in this present study are as follows: The Nb bearing steels and low C steels showed good resistance to high-temperature corrosion in CaSO₄/CaO, e.g. 347, 304L and HR3C. The corrosion rate of all steels used increased with increase in temperature, particularly at temperatures higher than 650°C. Internal penetration was not detected at temperatures lower than 550°C, but it was detected at temperatures higher than 600°C, in particular, higher than 650°C. The corrosion thickness loss was almost the same as the internal penetration depth at 700 and 750°C in the 300 series steels placed in CaSO₄/CaO, including the fine grained 347 steel, while the internal penetration depth was larger than the corrosion thickness loss in high-alloyed materials such as Alloy 800 and 310 steels. At temperatures higher than 800°C, the same result was also obtained for the fine grained 347 steel. The corrosion during exposure to oxidizing or reducing gases without CaSO₄/CaO or CaS was slight, but when the test specimens were placed in CaSO₄/CaO or CaS, the corrosion rate sharply increased, regardless of the atmospheric gas composition. Cr, Si, Mn (less than 5 %), Mo and Nb are beneficial elements while C, Cu and Al are harmful elements. From the above results, the following steel was developed for high temperature corrosion resistance in CaSO₄/CaO: low C-22/25Cr-17/25Ni-3/5Mn-(2Mo)-Nb-0.08/0.2N-Al-(B).     

Influence of heat treatments on microstructure and pitting corrosion resistance of 15%Cr supermartensitic stainless steel

Supermartensitic is a new class of stainless steels in development and consolidation as commercial products. Significant changes on chemical composition of conventional martensitic stainless steels, such as the reduction of the carbon content to <0·03 wt-%, and the addition of Ni and Mo, marked the development of this new group of alloys. New grades containing higher amounts of Cr and small additions of Ti and/or Nb were developed recently. As a result, supermartensitic steels offer an interesting combination of high strength, toughness, weldability and corrosion resistance. In this work, the pitting corrosion resistance of a 15Cr supermartensitic steel with Ni, Mo and Cu additions was studied. It is well known that the mechanical properties and corrosion resistance of martensitic steels are adjusted by the final tempering treatment. Several single tempering treatments in the 300–650°C range and double tempering treatments were performed in order to obtain different microstructures. The pitting corrosion resistance was investigated by electrochemical test in 3·5%NaCl solution. It was found that the pitting potential slightly decreased with the increase in temperature and time of tempering. The analysis of pits formed during the corrosion tests showed that ferrite islands are more corrosion resistant than the martensitic matrix due to the higher Cr and Mo contents.     

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).     

基于d电子合金理论的新型节镍双相不锈钢设计及性能研究

依据d电子合金理论,设计了新型节镍00Cr19Mn6Ni0.5N0.2(DSSs)系列双相不锈钢,同时研究了W、Mo、Cu单独添加对该系列双相不锈钢显微组织、力学及腐蚀性能的影响。结果表明:Cu可以显著扩大奥氏体相区,Mo和Cu能够适当提高材料的腐蚀性能,W和Mo可以不同程度的提高室温拉伸性能,该系列新型双相不锈钢的屈服强度、抗拉强度和延伸率分别达到360、880MPa和35%以上。实验合金中没有发现sigma相的析出,与理论设计结果基本一致。     

铁素体不锈钢铸坯凝固特性及等轴晶率控制

具有较高等轴晶率的铁素体不锈钢连铸坯可以显著避免冷成型加工过程中的皱折缺陷。采用了低倍组织观察、第二相特征分析以及凝固模型计算的方法研究了含Ti、Nb双稳定化超纯铁素体不锈钢的凝固特性以及等轴晶率控制机理。结果表明,当Ti和Nb加入到铁素体不锈钢中,连铸的等轴晶率显著提高;模型表明铸坯凝固参数的变化规律基本一致,然而钢中第二相粒子的成分及形貌发生显著变化,第二相粒子的非均质形核能力显著增强。本研究建立的凝固模型可以较好预测连铸坯的CET转变,同时指出稳定地提高铁素体不锈钢连铸坯的等轴晶率需要对钢中微量元素含量、核心形核效果以及冶炼过程进行合理设计。     

Resistance of Mo-bearing stainless steels and Mo-bearing stainless-steel coating to naphthenic acid corrosion and erosion-corrosion

Naphthenic acid corrosion (NAC) and erosion-corrosion (NAEC) behavior of Mo-bearing (0-7.0 wt.%) stainless steels have been investigated in laboratory to evaluate the essential role of Mo on their NAC and NAEC resistance. The NAC and NAEC resistance of a high-velocity-oxygen-fuel (HVOF) thermal sprayed Mo-bearing stainless-steel coating was also investigated in both laboratory and an oil refinery. It was found that increasing the Mo content remarkably enhanced the NAC and NAEC resistance of stainless steels. The Mo-rich areas or phases in the steels played an importance role in resisting the NAEC. The HVOF coating showed excellent NAC and NAEC resistance in laboratory tests. The 700-day field test in an oil refinery revealed that the coating can effectively prevent the carbon-steel substrate from erosion-corrosion during long-term exposure in oil-refining environment. The corresponding NAC and NAEC mechanisms of the stainless steels and the beneficial role of Mo were discussed by taking account of inherent susceptibility of metal elements to corrosion, possible effects of Mo on surface films, and Mo-induced change in microstructure and microhardness.     

AVESTA 654 SMO™ – A new nitrogen-enhanced superaustenitic stainless steel

The properties of a new, very highly alloyed stainless steel, Avesta 654 SMO, are compared with those of other stainless steels and nickel-base alloys. The new steel has a very high strength but still possesses a high elongation. The corrosion properties have been investigated in accelerated laboratory tests and in environments representing the expected main application areas, sea water, pulp bleaching and flue gas cleaning. The tests clearly show that the new steel has appreciably better corrosion resistance than the superduplex and superaustenitic 6 Mo steels and that the resistance in many cases matches that of the best nickel-base alloys.     

经济型铁素体不锈钢焊接接头组织与耐蚀性能

采用ER-309焊丝焊接了TCS345,T4003,Nirosta 4003,JFE410RW四种铁素体不锈钢,用金相方法分析了四种母材、接头的显微组织,通过电化学极化曲线测量,对母材和焊接接头的耐腐蚀性能进行了评价.结果表明,TCS345和T4003铁素体不锈钢与Nirosta 4003和JFE410RW铁素体不锈钢的主要区别是钛和锰含量偏低,尤其是钛含量少,导致TCS345和T4003铁素体不锈钢的晶粒长大倾向较Nirosta 4003和JFE410RW铁素体不锈钢的明显.铁素体不锈钢焊接接头存在较大的热影响区,热影响区的组织与母材晶粒相比,其晶粒明显粗大;TCS345铁素体不锈钢的晶粒长大严重,Nirosta 4003和JFE410RW铁素体不锈钢的铁素体晶粒尺寸比TCS345略小,晶粒长大不明显.在1mol/L Na2SO4溶液中,TCS345和JFE410RW不锈钢母材的腐蚀性能优于T4003和Nirosta 4003.     

The influence of plastic straining on localized and general corrosion of stainless steels

Stress corrosion cracking of stainless steels is dependent upon different factors: (a) plastic deformation, the dissolution rate of (b) passive and (c) depassivated unfilmed metal, and (d) the repassivation rate. The particular concurrence of these four processes creates the possibility of cracking. The kinetics of these phenomena seem to be the determining factors for cracking to occur. Stainless steels of several different compositions and structures (ferritic, austenitic and duplex), as well as steels of varying Mo and Si composition, have been examined by observing the influence of straining rates on depassivation, and on rates of dissolution and repassivation. Average crack propagation rates have been obtained. Structure exerts an important influence, as deformation modes are very different for alpha, gamma and duplex stainless steels. In certain ferritic steels, containing Ni and/or Cu, deformation modes such as mechanical twinning may cause very strong localized depassivation, which initiates cracking. In environments other than MgCl₂, such as dilute solutions of NaCl, repassivation kinetics seem to prevent crack propagation in spite of initial depassivation. It seems nearly impossible to define a standard stress corrosion test applicable to any metal/solution couple. A safer approach would be to define a methodology for investigating stress corrosion, putting to use the different experimental techniques relevant to each of the four processes listed above, on the alloy immersed in the actual environment, or in an environment of very similar properties.     

An X-ray photo-electron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steels in HC1

X-ray photo-electron spectroscopy has been used to investigate the correlation between composition of surface films and the beneficial effects of molybdenum addition to high purity, 30Cr ferritic stainless steels in improving the corrosion resistance properties in HCI. It has been found that the passive films formed consist mainly of hydrated chromium oxy-hydroxide and the composition of the films on 30Cr and 30Cr-2Mo stainless steels is essentially the same, except for the existence of a small amount of hexavalent molybdenum on the latter steel. The surface film formed in the active region contains a large amount of hexavalent molybdenum. The beneficial effects of molybdenum have been interpreted as follows: molybdenum eliminates the active surface sites through the formation of molybdenum oxy-hydroxide or molybdate on these site, on which it is difficult to form the stable passive film. This leads to the appearance of a homogeneous steel surface and to the formation of a homogeneous passive film.