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.     

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.     

Applications and experience with a Superaustenitic 7Mo stainless steel in hostile environments

A new superaustenitic stainless steel is described in terms of chemical composition. mechanical properties and corrosion resistance. Results obtained by using this highly alloyed stainless steel, EN 1.4652, in a number of hostile environments are reported. Applications include neutral and acid chloride containing environments such as those existing in e.g. – pulp bleach plants, – ventilation systems of pharmaceutical plants, – equipment for flue gas cleaning in power plants and municipal waste incineration plants, – equipment for recovery of chlorinated hydrocarbons, – sea water piping, – nuclear power plant condenser tubes. The paper also contains laboratory corrosion test results positioning this steel in relation to other highly alloyed stainless steels and nickel base alloys, mainly in terms of its resistance to chloride induced localised corrosion.     

Seawater resistance of a second generation superaustenitic stainless steel

The first generation of superaustenitic 6Mo steels like 254 SMO® has been further developed into a new steel–654 SMO®–containing very high amounts of those alloying elements most beneficial for the resistance to localized corrosion. 654 SMO is compared with other stainless steels and nickel-base alloys in tests performed in various kinds of seawater. The new steel is much superior to the 6Mo steel under all conditions investigated and compares favourably with Alloy C–276 in most tests. The most corrosive condition among those investigated represents a seawater system where aerobic and anaerobic areas exist simultaneously.     

Corrosion performance of selected stainless steels in brine from reverse osmosis desalination plants

Abstract

The corrosion performances of some commercial stainless steel alloys in the brine reject solution from a reverse osmosis sea water desalination plant was studied in terms of their pitting susceptibilities (investigated under aerated conditions at ambient temperature using a cyclic polarisation technique) and crevice corrosion resistances (evaluated in the plant over a 3 month exposure period using multiple crevice test assemblies). The alloys used were four austenitic steels, UNS S31603, UNSS 31703, UNS N08904, UNS S31254, a ferritic steel UNS S44635, and a duplex steel UNS S32550. Cyclic polarisation studies show that the pitting or breakdown potentials for S31603 and S31703 occurred at more active values than for N08904, S31254, S44635, and S32550 alloys, and indicated a reduced resistance to pitting corrosion. The multiple crevice tests show that the alloys S31603, S31703, and N08904 do suffer crevice corrosion in the brine reject solution at ambient temperature, while the S44635 S32550, and S31254 alloys showed considerably higher crevice corrosion resistance.     

Electrochemical behaviour of duplex stainless steels in caustic environment

Recent studies have shown that duplex stainless steels can be susceptible to general corrosion and stress corrosion cracking in high pH caustic environments. This difference in the corrosion resistance can be attributed to changes in the electrochemical behaviour of steels. The present study has shown that the corrosion rates of duplex stainless steels increase with an increase in temperature and sulphide addition to caustic environments. Moreover, alloying Fe with Cr and Ni helps to raise the corrosion potential and lower critical current density of DSS in an alkaline environment whereas Mo can be detrimental to the corrosion resistance of the steel.     

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

Laboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic-austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits-crevices were found under the corrosion crust deposits on the duplex stainless steel.     

Influence of dissolved oxygen content on the bacteria‐induced ennoblement of stainless steels in seawater and its consequence on the localized corrosion risk

The ennoblement of stainless steel (e.g., the increase of open circuit potential [OCP]) is associated with bacterial colonization. This increases the risk of localized corrosion as the critical pitting/crevice potential can be overcome, especially for lower grade stainless steel. In this study, we assessed the influence of dissolved oxygen content (DOC) on the crevice corrosion of duplex and super duplex stainless steels. In addition, we used DNA amplicon sequencing to identify the bacteria most likely associated with the ennoblement. Above approximately 100 parts per billion (ppb) of dissolved oxygen, the ennoblement of OCP was observed leading to an increased risk of localized corrosion. Below approximately 100 ppb of dissolved oxygen, no ennoblement occurred and the risk of localized corrosion was reduced. We identified certain hydrocarbon‐degrading bacteria whose presence correlated with the ennoblement of super duplex stainless steel at saturated DOC. The role of these bacteria is not clear yet, but their distribution indicates a possible involvement in stainless steel ennoblement in seawater.     

Enhancing the localised corrosion resistance of 316L stainless steel via FBR-CVD chromising treatment

The resistance of stainless steels to localised corrosion can be adversely affected by environmental and metallurgical heterogeneities existed in complex industrial infrastructures such as seawater desalination plants exposed to aggressive evnironments. It is therefore critical to enhance the localised corrosion resistance and understand the corrosion behaviour of stainless steels in complex and aggressive industrial environmental conditions. In this work, the localised corrosion resistance of chromised stainless steel 316L (SS316L) in simulated seawater desalination systems has been investigated by electrochemical and surface analytical techniques. It has been found that chromising processes have improved the localised corrosion resistance of SS316L by reducing its susceptibility to pitting, crevice, and welding zone corrosion in simulated seawater desalination environments. This increased corrosion resistance has been explained by electrochemical polarisation studies and surface analysis showing that the chromising treatment at 1050°C resulted in a continuous and stable chromium-enriched layer on the SS316L surface.     

New stainless steels for sea-water applications. Part II: Comparison of corrosion and mechanical properties of laboratory and commercial ELI ferritic and superaustenitic stainless steels

This paper represents a follow-up to the first part of the work on new stainless steels for sea-water service. Four laboratory ELI (Extra Low Interstitial) ferritic stainless steels (types 25 Cr-4 Ni-4 Mo), two commercial ELI ferritic stainless steels (types 25 Cr-4 Ni-4 Mo? Ti and 26 Cr-2.5 Ni-3 Mo? Ti) and two highly alloyed austenitic stainless steels (types 20 Cr-25 Ni-4.5 Mo? Cu and 20 Cr-18 Ni-6 Mo? N) have been investigated. With a view to establish the performance of these new alloys in chloride containing environments, systematic electrochemical and laboratory exposure tests have been carried out to define how various factors affect its susceptibility to intergranular, pitting, crevice and stress corrosion. Tension tests were also performed. From the comparison of the localized corrosion resistance and mechanical properties it has been concluded that the laboratory Ti, Ti + Nb or Nb stabilized ELI ferritic stainless steels and the commercial type 25 Cr-4 Ni-4 Mo? Ti of analogous composition could be a valuable alternative to the more expensive highly alloyed stainless steel type 20 Cr-25 Ni-4.5 Mo? Cu which has been especially developed and already used for industrial sea-water applications.     

新型海水管系材料HDR双相不锈钢的腐蚀和电化学性能

针对新型海水管系材料HDR双相不锈钢的研制和开发，通过冲刷腐蚀，砂侵蚀以及点蚀和缝隙腐蚀电化学试验，研究了其耐海水腐蚀性能和电化学性能，与TUP，B10，B30的耐蚀性能进行了对比，结果表明，HDR很耐流动海水冲刷腐蚀和砂侵蚀，耐点蚀，缝隙腐蚀性能良好，适合作为海水管系材料。     

High Temperature Aqueous Corrosion Research at the National Physical Laboratory

Abstract

The susceptibility of stainless steels and high nickel alloys to crevice corrosion severely restricts their use for marine engineering applications. The development of crevice corrosion test procedures for alloy evaluation and development purposes is reviewed. As susceptibility to crevice corrosion is an inherent weakness of this type of alloy, it is suggested that it might in future be more rewarding to determine the design parameters required for the prevention of crevice corrosion by galvanic protection, rather than to pursue the continuing present investment in the quest for the development of an immune alloy.     

Corrosion behaviour of carbon S-phase created on Ni-free biomedical stainless steel

Following the success of forming a carbon S-phase (expanded austenite) surface layer on medical grade Ni-free austenitic stainless steel by DC plasma carburising, the established commercial carburising process Kolsterising® was performed on both Ni-containing (AISI 304) and Ni-free austenitic stainless steels. While the Ni-containing stainless steel responded very well to Kolsterising®, the Ni-free alloy did not. The carbon absorption and the hardness of the Kolsterised® Ni-free alloy are inferior to Kolsterised® AISI 304 Ni-containing stainless steel, however, the hardness of the untreated Ni-free alloy was doubled by Kolsterising®. The response of both Kolsterised® Ni-free and Ni-containing alloys to pitting, crevice corrosion and intergranular corrosion resistance was similar. From this work it can be concluded that the Kolsterised® austenitic stainless steels do not suffer from intergranular corrosion but are susceptible to intragranular pitting when tested in boiling sulphuric acid and copper sulphate solution. It was also observed that Kolsterising® improves significantly the pitting and crevice corrosion resistance of the alloys used in this study.     

Improved Ferritic–Austenitic Stainless Steel

Abstract

For the efficient operation of chemical process plant it is of paramount importance to ensure as far as possible that the periods of closurefor maintenance and repair are minimised.

The arduous conditions encountered in many chemical plants as a result of progressive trends towards. higher yields, etc., have therefore highlighted the need for greater strength and pitting resistance than can be obtained in conventional austenitic stainless steels. This paper describes the development of ferritic/austenitic high chromium duplex cast stainless steels and demonstrates how, by addition of nitrogen and close control of composition, certain inherent drawbacks of duplex alloys can be overcome. The nitro gen-containing steel has an attractive combination of mechanical properties, corrosion resistance, ductility, weldability and castability, and has received application in a wide range of aggressive environments.     

Korrosionsverhalten von verzinktem und nichtrostendem Stahl sowie von Nickelbasis-Legierungen in Schwimmhallenluft

Corrosion behaviour of galvanized and stainless steel as well as nickelbase-alloys in the atmosphere of indoor swimming pools U-bend specimens of a galvanized steel, nine different stainless steels and two nickel-base alloys were exposed to the chloric atmosphere of the reservoir for overflow water of a swimming pool for max. 68 months. The visual and metallographic examination of their corrosion performance has shown that none of the metals remained completely stainless. Only the galvanized steel and the stainless steels W.-Nr. 1.4529, 1.4539 and 1.4575 remained relative free from rust and absolutely free of pitting and crevice corrosion. None of the metals showed stress corrosion cracking.     

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.     

不锈钢缝隙腐蚀的微区电化学行为及缝内钝化膜厚度的变化

利用微电极和椭圆偏振仪研究了2Cr13不锈钢在3.5%NaCl 溶液中的缝隙腐蚀历程。实验结果表明,不锈钢的缝隙腐蚀经历了三个阶段一孕育期,发展期和稳定期。缝内 pH 值下降和 Cl~-浓度增加均促进了钝化膜的溶解和加速了缝隙腐蚀。从孕育期向发展期的过渡中,存在着使合金钝化膜破裂的临界缝隙溶液(CCS)。在本试验条件下,其组成大约为 pH=3.5,[Cl~-]:1.2N。在孕育期,缝内钝化膜厚度随时间逐渐减薄,直到钝化膜肢裂,此时,缝隙腐蚀即进入发展期。微电极测试技术和椭圆偏振法可应用于不锈钢缝隙腐蚀过程的原位测试。     

Loch- und Spaltkorrosion von Chrom- und Chromnickelstählen in chloridhaltigen Lösungen

Pitting and crevice corrosion of stainless steels in chloride solutions In practice stainless steels in chloride containing waters are found to be susceptible to crevice corrosion and pitting. Corrosion tests were carried out on AISI 304 L stainless using a simulated crevice and the compositions of the electrolyte in the crevice determined. Long term potentiostatic tests were used to determine the critical potentials for crevice corrosion (U_S), for various steels in sodium chloride solutions at different concentrations and temperatures. The steels studied were 22 CrMo V 121, X 22 CrNi 17 and AISI 304 L. Like the critical pitting potential (U_L), U_S was found to have a strong dependence on the chloride content of the external solution. At higher concentrations the two potentials were similar. At lower concentrations the U_S was lower than U_L. The knowledge of these critical potentials together with well known rest potentials for a steel in an electrolyte of known concentration, allows conclusions to be drawn about its susceptibility to pitting and crevice corrosion. The method is suitable also for other passive metals.     

Corrosion behavior of a superduplex stainless steel in chloride aqueous solution

Super duplex stainless steels (SDSS) have been widely used as structural materials for chemical plants (especially in those engaged in phosphoric acid production), in the hydrometallurgy industries, and as materials for offshore applications due to their excellent corrosion resistance in chloride environments, compared with other commercial types of ferritic stainless steels. These alloys also possess superior weldability and better mechanical properties than austenitic stainless steels. However, due to their two-phase structure, the nature of which is very dependent on their composition and thermal history, the behavior of SDSS regarding localized corrosion appears difficult to predict, especially in chloride environments. To improve their final properties, the effect of the partition of the alloying elements between the two phases, and the composition and microstructure of each phase are the key to understanding the localized corrosion phenomena of SDSS. This paper concerns the effects of the SDSS microstructure and heat treatment on the SDSS corrosion resistance in aqueous solutions, containing different amounts of NaCl at room temperature.     

An experimental study of crevice corrosion behaviour of 316L stainless steel in artificial seawater

The effects of applied torque on corrosion behaviour of 316L stainless steel with crevices were investigated using the cyclic potentiodynamic polarization method. Three kinds of crevices (316L-to-polytetrafluoroethylene, 316L-to-fluoroelastomeric and 316L-to-316L) were tested in artificial seawater at 50 °C. Corroded surface morphology was also investigated using scanning electron microscopy. Results indicate similar trends in crevice corrosion susceptibility with increasing applied torque. Among the three crevices, the 316L stainless steel specimen, coupled to the 316L stainless steel crevice former, is the most susceptible to crevice corrosion.