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.     

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.     

Investigations on the Corrosion Behaviour and Structural Characteristics of Low Temperature Nitrided and Carburised Austenitic Stainless Steel

The wear resistance of austenitic stainless steels can be improved by thermo-chemical surface treatment with nitrogen and carbon. However, it is possible that the corrosion resistance will be impaired by the precipitation of chromiumnitrid or -carbide. The present contribution deals with investigations of the corrosion behaviour and structural characteristics of a low temperature nitrided and carburised austenitic stainless steel.The material investigated was AISI 316L (X2CrNiMol7-12-2) austenitic stainless steel. A commercial plasma-nitriding unit (pulsed dc) was used for the nitriding and carburising process. Additional samples were treated by the gasoxinitriding process for a comparison between plasma- and gasoxinitriding. The nitrided and carburised layer of austenitic stainless steel consists of the nitrogen or carbon S-phase (expanded austenite), respectively. X-ray diffraction investigations show the typical shift of the peaks to lower angles, indicating expansion of the fcc lattice. Also the X-ray diffraction technique was employed to study the residual stresses in the nitrogen and carbon S-phase. The corrosion behaviour of surface engineered samples was investigated with electrochemical methods. Anodic potentiodynamic polarisation curves were recorded for testing the resistance against general corrosion (in H2SO4) and pitting corrosion (in NaCl).     

酸洗和固溶处理对不锈钢抗点蚀性能的影响

研究了酸洗和固溶处理对８种不锈钢筋电阻对焊接头抗点蚀能力的影响。研究发现,酸洗和因咱都有效地改善和恢复不锈钢电阻对焊接头的抗点蚀性能。对于普通双相钢,固溶处理比酸洗能更有产地改善接头的抗点蚀性能,并无晶粒粗化。对于奥氏体钢和超纸奥氏体风,固溶处理的效果不及酰洗,且会引起母材晶粒粗化。对含Ｍｏ含Ｎ较高钢种的对焊接头,两者的更佳,尤其是两者综合处理后的接头,其临界点蚀温度ＣＰＴ其至高于对应母材经酸洗以     

酸洗和固溶处理对不锈钢抗点蚀性能的影响

研究了酸洗和固溶处理对 8种不锈钢钢筋电阻对焊接头抗点蚀能力的影响。研究发现 ,酸洗和固溶处理都能有效地改善和恢复不锈钢电阻对焊接头的抗点蚀性能。对于普通双相钢 ,固溶处理比酸洗能更有效地改善接头的抗点蚀性能 ,并无晶粒粗化现象 ;对于奥氏体钢和超级奥氏体钢 ,固溶处理的效果不及酸洗 ,且会引起母材晶粒粗化。对含Mo含N较高钢种的对焊接头 ,两者的效果更佳 ,尤其是两者综合处理后的接头 ,其临界点蚀温度CPT甚至高于对应母材经酸洗以后的CPT。文章对比分析了母材和固溶处理前后接头的显微组织 ,讨论了引起上述抗点蚀性能变化的原因。文章还探讨了酸洗和Mo、N元素可以改善不锈钢抗点蚀性能的机理。     

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。     

The influence of Mn compared to that of Cr,Mo and S on the resistance to initiation of pitting and crevice corrosion in austenitic stainless steels

A large number of production and laboratory heats in grades AISI 304 and 316 with normal and extremely low managanese and sulphur contents and a number of production heats in more highly alloyed austenitic stainless steels have been studied with regard to their resistance to initiation of pitting and crevice corrosion at various temperatures. The criteria for resistance to initiation was the potentiodynamic pitting potential in 0.1 M NaCl and synthetic seawater and the time to attack initiation for crevice corrosion in 0.5 and 5% NaCl solutions. A large number of production and laboratory heats in grades AISI 304 and 316 with normal and extremely low managanese and sulphur contents and a number of production heats in more highly alloyed austenitic stainless steels have been studied with regard to their resistance to initiation of pitting and crevice corrosion at various temperatures. The critieria for resistance to initiation was the potentiodynamic pitting potential in 0.1 M NaCl and synthetic seawater and the time to attack initiation for crevice corrosion in 0.5 and 5% NaCl solutions. The main aims of the study were to examine both the effect of manganese relative to that of chromium, molybdenum and sulphur and the effect of heat treatment and sulphide composition on steels with low manganese contents. Mathematical models for calculation of the pitting potentials have been constructed and multiple linear regression analysis gave the equations and their reliabilities. Lowering of the Mn content in austenitic stainless steels to 0.2% gives rise to a material of interest for constructions where pitting or crevice corrosion are judged to be the only potential types of attack, where operational disturbances leading to greatly increased corrosivity do not occur, where attack can not be tolerated, and where steel with normal managanese content has not exhibited fully satisfactory corrosion resistance. If the above conditions are fulfilled the low manganese content can be said to correspond to the same positive effect as is obtained by an addition of the least 1.5% Mo.     

Investigation on Pitting Corrosion of Nickel-Free and Manganese-Alloyed High-Nitrogen Stainless Steels

Pitting corrosion behavior of three kinds of nickel-free and manganese-alloyed high-nitrogen (N) stainless steels (HNSSs) was investigated using electrochemical and immersion testing methods. Type 316L stainless steel (316L SS) was also included for comparison purpose. Both solution-annealed and sensitization-treated steels were examined. The solution-annealed HNSSs showed much better resistance to pitting corrosion than the 316L SS in both neutral and acidic sodium chloride solutions. The addition of molybdenum (Mo) had no further improvement on the pitting corrosion resistance of the solution-annealed HNSSs. The sensitization treatment resulted in significant degradation of the pitting corrosion resistance of the HNSSs, but not for the 316L SS. Typical large size of corrosion pits was observed on the surface of solution-annealed 316L SS, while small and dispersed corrosion pits on the surfaces of solution-annealed HNSSs. The sensitization-treated HNSSs suffered very severe pitting corrosion, accompanying the intergranular attack. The addition of Mo significantly improved the resistance of the sensitization-treated HNSSs to pitting corrosion, particularly in acidic solution. The good resistance of the solution-annealed HNSSs to pitting corrosion could be attributed to the passive film contributed by N, Cr, and Mo. The sensitization treatment degraded the passive film by decreasing anti-corrosion elements and Cr-bearing oxides in the passive film.     

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.     

Die Lochkorrosion austenitischer Chrom-Nickel- und Chrom-Nickel-Molybdän-Stähle in schwefelsaurer Bromidlösung und ihre Inhibition durch Nitrationen

Pitting corrosion of austenitic chromium nickel and chromium nickel molybdenum steels in sulfuric acid containing bromides, and its inhibition nitrate ions In acidified bromide solution CrNi steels are attacked under pitting when a certain critical potential has been exceeded; this potential is higher than in the case of chloride containing solutions. Bromides are, consequently, less active than chlorides, but the pit density is considerably higher under idential corrosion conditions. While the pitting corrosion in chloride solutions can be considerably reduced by molybdenum addition to the steel, this effect is but little pronounced in the case of bromide solutions (with Mo additions up to 4% the potential is displaced by 0.2 V toward positive values). Mo additions around 2% are even dangerous since the pitting density is considerably increased in that range. Similar to the conditions in chloride solutions corrosion in bromide solutions is inhibited by nitrate additions; the potential limit is considerably higher in the bromide solution; this phenomenon points to stronger adsorption of bromide ions at the metal surface.     

Corrosion resistance properties of glow-discharge nitrided AISI 316L austenitic stainless steel in NaCl solutions

Glow-discharge nitriding treatments can modify the hardness and the corrosion resistance properties of austenitic stainless steels. The modified layer characteristics mainly depend on the treatment temperature. In the present paper the results relative to glow-discharge nitriding treatments carried out on AISI 316L austenitic stainless steel samples at temperatures ranging from 673 to 773 K are reported. Treated and untreated samples were characterized by means of microstructural and morphological analysis, surface microhardness measurements and corrosion tests in NaCl solutions. The electrochemical characterization was carried out by means of linear polarizations, free corrosion potential-time curves and prolonged crevice corrosion tests. Nitriding treatments performed at higher temperatures (>723 K) can largely increase the surface hardness of AISI 316L stainless steel samples, but decrease the corrosion resistance properties due to the CrN precipitation. Nevertheless nitriding treatments performed at lower temperatures (?723 K) avoid a large CrN precipitation and allow to produce modified layers essentially composed by a nitrogen super-saturated austenitic metastable phase (S-phase) that shows high hardness and very high pitting and crevice corrosion resistance; at the same polarization potentials the anodic current density values are reduced up to three orders of magnitude in comparison with untreated samples and no crevice corrosion event can be detected after 60 days of immersion in 10% NaCl solution at 328 K.     

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.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.     

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.     

The wear and corrosion properties of stainless steel nitrided by low-pressure plasma-arc source ion nitriding at low temperatures

Low pressure plasma arc discharge-assisted nitriding of AISI 304 austenitic stainless steel is a process that produces surface layers with useful properties such as a high surface hardness of approximately 1500 Hv_0.1 and a high resistance to frictional wear and corrosion. The phase composition, the thickness, the microstructure and the surface topography of the nitrided layer, as well as its properties, depend essentially on the process parameters. Among them, the processing temperature is the most important factor for forming a hard layer with good wear and corrosion resistance. Nitriding austenitic stainless steel at approximately 420°C for 70 min can produce a thin layer of 7–8 μm with very high hardness and good corrosion resistance on the surface. The microstructure was studied by optical microscopy and both glancing angle and conventional Bragg–Brentano (θ–2θ) symmetric geometry X-ray diffraction (XRD). The formation of expanded austenite was observed. Measurements of the wear depths indicated that the wear resistance of austenitic stainless steel can be improved greatly by nitriding at approximately 420°C using low-pressure plasma-arc source ion nitriding.     

改性纳米SiC粉体强化铸造奥氏体不锈钢耐点蚀性能的研究

在生产条件下采用冲入法制备了改性纳米SiC粉体强化奥氏体不锈钢试样,用化学浸泡和电化学检测两种方法研究了纳米SiC粉体对不锈钢耐点蚀性能的影响。结果表明,经改性纳米SiC粉体强化处理后的不锈钢组织明显细化,成分偏析引起的铁素体析出减少;当纳米SiC粉体加入量为0.1 mass%时,不锈钢的点蚀速率降低了 16%,电极电位提高了3倍。能谱分析表明,经强化处理,不锈钢中的Cr成分偏析减轻,有效改善了晶界等易发生点蚀部位的贫Cr现象。     

氮、铌添加对304奥氏体不锈钢电化学腐蚀行为的影响

采用极化曲线和电化学动电位再活化技术（EPR）研究了不同含量氮（N）、铌（Nb）元素的添加对304奥氏体不锈钢的耐点蚀和耐晶间腐蚀性能的影响。结果表明：N元素的添加可以显著提高材料的耐点蚀性能,但对于晶间腐蚀性能的影响却有不同的机制。少量的N会降低材料的耐晶间腐蚀性能,但含量增加到0.2%时,却可以提高耐晶间腐蚀性能;Nb元素的添加会明显增加材料的耐晶间腐蚀性能,但会降低其耐点蚀性能。基于以上结果,确定了N和Nb添加的最佳含量,并给出上述微量元素改变材料耐腐蚀性能的作用机制。     

Effect of p H on the Electrochemical Behaviour and Passive FilmComposition of 316L Stainless Steel

The effect of pH on the electrochemical behaviour and passive film composition of 316 L stainless steel in alkaline solutions was studied using electrochemical measurements and a surface analysis method. The critical pH of 12.5 was found for the conversion from pitting corrosion to the oxygen evolution reaction(OER). OER was kinetically faster than pitting corrosion when both reactions could occur, and OER could postpone pitting corrosion. This resulted in pitting being initiated during the reversing scan in the cyclic polarization at the critical pH. According to the X-ray photoelectron spectroscopy analysis, the content of Cr and Mo decreased with pH, while Fe content increased. This induced the degradation of the passive film, which resulted in the higher passive current densities under more alkaline conditions. The selective dissolution of Mo at high p H was found, which demonstrated that the addition of Mo in austenitic stainless steels might not be beneficial to the corrosion resistance of 316L in strong alkaline solutions.     

Improving the surface properties of A286 precipitation-hardening stainless steel by low-temperature plasma nitriding

Plasma nitriding over a wide range of treatment temperatures between 350 and 500 °C and time from 5 to 30 h on A286 austenitic precipitation-hardening stainless steels has been investigated. Systematic materials characterisation of the plasma surface alloyed A286 alloy was carried out in terms of microstructure observations, phase identification, chemical composition depth profiling, surface and cross-section microhardness measurements, electrochemical corrosion tests, dry sliding wear tests and corrosion-wear tests. Experimental results have shown that plasma nitriding can significantly improve the hardness and wear resistance of A286 stainless steels owing to the formation of nitrogen supersaturated S-phase; the surface layer characteristics (e.g. microstructure, case depth and hardness) of the plasma surface alloyed cases are highly process condition dependent and there are possibilities to provide considerable improvement in wear, corrosion and corrosion-wear resistance of A286 steel.     

Forthcoming corrosion events

Abstract

Pitting potentials have been measured and some gravimetric testing has been carried out on a series of experimental austenitic stainless steels with varying chromium, molybdenum and nitrogen contents. All three of these elements were found to contribute to the resistance to the initiation of pitting corrosion, and synergistic effects have been noted. The effect of nitrogen is especially potent in a steel with 22% chromium and 3% molybdenum.