Influence of cold working on the pitting corrosion resistance of stainless steels

This paper addresses the influence of cold rolling and tensile deformation on the pitting corrosion resistance of AISI 304 and AISI 430 stainless steels, investigated using some electrochemical techniques specifically designed for the different pitting stages to be analyzed separately. Cold work is shown to act differently depending on the pitting stage under consideration. (i) The pit initiation frequency shows a maximum after 20% cold-rolling reduction or 10% tensile deformation. This maximum is also observed on the ferritic grade, contradicting the hypothesis of a direct effect of strain-induced martensite, and is more likely related to the dislocations pile-ups. (ii) The pit propagation rate increases monotonously with cold rolling reduction, and pit repassivation ability decreases (leading to a larger number of stable pits), suggesting that the overall dislocation density is the controlling factor in these stages. Last, the significance of pitting potential measurements is discussed in the light of the effect of the cold-rolling reduction on the measured values.     

Influence of chemical composition on the pitting corrosion resistance of non-standard low-Ni high-Mn-N duplex stainless steels

The pitting corrosion resistance of a new family of duplex stainless steels has been evaluated. These non-standard duplex stainless steels are characterised by low Ni content and high N and Mn levels. Potentiodynamic polarisation scans in NaCl solution have been carried out to determine pitting potentials. A crevice-free cell has been used to perform the electrochemical tests.An exponential equation is obtained in the regression analysis between the pitting potential and chemical composition which allows an estimate of the pitting resistance of these new duplex stainless steels.     

Role of nitrogen on the corrosion behavior of austenitic stainless steels

The role of nitrogen in the mechanisms of localized corrosion resistance and repassivation were electrochemically investigated using a nitrogen-bearing austenitic stainless (SUS316L) steel in 0.1 and 0.5 M Na₂SO₄ solutions and a 3.5% NaCl solution. Almost 100% of the nitrogen compounds dissolved into the bulk solution after crevice corrosion were transformed into NH₃. That is, the mole amount of ammonia in the solution was approximately equivalent to the mole amount of nitrogen dissolved in the steel. This suggests that NH₄⁺ consuming H⁺ in the pit controlled the local decrease of pH and promoted the repassivation. NO₃-N and NO₂-N were not detected by chemical analyses in the high potential and thermodynamically stable zone as NO₃⁻ in the potential-pH diagram. The repassivation in nitrogen-bearing SUS316L steel in a 0.1 M Na₂SO₄ solution was studied using the scratching electrode technique, which measured the partially destroyed passivation films on the steel. This technique showed that nitrogen dissolved in the steel has a strong repassivation capacity.     

On the pitting corrosion resistance of nitrogen alloyed cold worked austenitic stainless steels

Pitting corrosion studies were carried out on cold worked (5%, 10%, 15%, 20%, 30% and 40%) nitrogen-bearing (0.05%, 0.1% and 0.22% N) type 316L austenitic stainless steels in neutral chloride medium. Potentiodynamic anodic polarisation study revealed that cold working up to 20% enhanced the pitting resistance, and thereafter a sudden decrease in pitting resistance was noticed at 30% and 40% cold working. Increase in nitrogen content was beneficial up to 20% cold work in improving the pitting corrosion resistance, beyond which it had a detrimental effect at 30% and 40% cold working. The role of nitrogen in influencing the deformation band width and dislocation configuration is explained based on the results of transmission electron microscopy investigations. Scanning electron microscopy observation of the pitted specimens indicated decreasing size and increasing density of pits, along the deformation bands with increasing nitrogen for 40% cold worked specimens. The macrohardness values increased as the cold working increased from 0% to 40%. X-ray diffraction studies revealed the increased peak broadening of austenite peak {0 2 2} with increase in cold working. The relationship between pitting corrosion and deformation structures with respect to nitrogen addition and cold working is discussed.     

Effect of austenite stability on the pitting corrosion resistance of cold worked stainless steels

The influence of cold rolling on the pitting resistance of stainless steels is investigated with respect to the austenite stability. A maximum in pitting initiation frequency for 20% reduction is confirmed whatever the austenite stability, but the value of this maximum fairly correlates to the amount of deformation-induced martensite. A direct influence of dislocations piling-up and an indirect role of martensite as a pile-up stabilizer is postulated. In the potentiostatic regime, cold work is shown to lower the repassivation ability and to increase the number of stable pits. Then, addition of alloying elements (such as copper) affects the pitting resistance not only for chemical but also for mechanical reasons (such as their effect on the staking fault energy and austenite stability), confirming the decisive role of microstructure in pitting corrosion of industrial alloys.     

Crevice corrosion behaviour of superaustenitic stainless steels: Dynamic electrochemical impedance spectroscopy and atomic force microscopy studies

Potentiodynamic anodic polarisation and dynamic electrochemical impedance spectroscopic (DEIS) measurements were carried out for 316L SS, alloy 33 and alloy 24 in natural sea water in order to assess their crevice corrosion resistance. DEIS measurements were performed from open circuit potential to dissolution potential. It was shown that the impedance measurements in potentiodynamic conditions allow simultaneous investigation of changes in passive layer structure. The impedance spectra of various potential regions were also discussed. The surface morphology of the alloys after crevice corrosion studies were studied using optical microscope and atomic force microscopy (AFM).     

固溶温度及时间对高氮不锈钢耐蚀性能的影响

使用真空感应炉+电渣重熔炉在0.08 MPa下制备了氮含量0.54%的高氮无镍奥氏体不锈钢,热轧后分别在800、900、1000、1100、1200 ℃下保温不同时间,研究在不同固溶工艺下试验钢的显微组织和耐蚀性。采用动电位极化曲线研究不同固溶工艺下高氮不锈钢在3.5%NaCl溶液中的耐蚀性能,并在6%FeCl₃溶液中浸泡8 d后计算其质量损失率和腐蚀速率。结果表明,固溶对高氮不锈钢组织及耐蚀性能的影响很大,经1000、1100 ℃热处理后的试验钢为单一的奥氏体组织;未经热处理和经800、900 ℃热处理的试验钢组织中存在析出相Cr₂N;经1200 ℃热处理的试验钢从奥氏体中析出了铁素体组织;1100 ℃下保温1 h的试验钢耐蚀性最好,腐蚀速率仅为1.35×10^-5 g·cm^-2·h^-1;800 ℃保温3 h后试验钢的耐蚀性最差,腐蚀速率高达8.18×10^-4 g·cm^-2·h^-1;而316L不锈钢的耐蚀性能介于两者之间,腐蚀速率为1.24×10^-4 g·cm^-2·h^-1。     

The effect of passive layer stability on electrochemical noise signals arising from pitting corrosion of stainless steels

Electrochemical noise (EN) enables corrosion research and monitoring in real time and with high sensitivity. In the case of submicroscopic nucleation events of pitting corrosion it has been observed that the cathodic process plays a decisive role in the mechanism as well as in the origin of EN signals, which depend not only on the metallic dissolution reaction but also on the electron‐consuming process. EN signals arising from the nucleation process of localized corrosion on stainless steels can only be recorded due to the inhibition effect on the cathodic process achieved by the spontaneous formation of the passive layer. In consequence, passive layer stability becomes a significant factor in influencing EN signals. The way in which the passive layer stability affects the acquisition and analysis of EN signals arising from pitting corrosion on stainless steels is discussed in detail.     

N含量对13Cr超级马氏体不锈钢耐蚀性的影响

对13Cr超级马氏体不锈钢进行氮合金化,采用淬火-配分热处理工艺,研究了不同N含量对13Cr钢的微观组织及电化学腐蚀行为的影响。结果表明:随N含量增加,板条马氏体组织表现出明显的细化行为,奥氏体含量增加,且有VN生成,从而防止N与Cr结合生成Cr₂N,促使Cr在材料表面形成以Cr₂O₃为主的致密腐蚀产物膜,提高材料耐蚀性;试验钢表面以局部腐蚀为主,试样表面有点蚀发生。随着N含量的增加,形成腐蚀产物膜的孔隙度减小;表面钝化膜为双电层结构,增加了钝化膜的稳定性,点蚀坑数量明显减少且变小;提高N含量有利于试样耐点蚀性能的改善,0.35%N试验钢表面腐蚀产物附着牢固,平整且致密,晶粒大小均匀,可起到良好的保护作用。     

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.     

Uniform corrosion and intergranular corrosion behavior of nickel‐free and manganese alloyed high nitrogen stainless steels

The uniform and intergranular corrosion behavior of two kinds of nickel‐free and manganese alloyed high nitrogen stainless steels (HNSSs) were investigated. A type of 316L stainless steel (316L SS) was also included for comparison purpose. Both solution annealed (SA) and sensitization treated (ST) steels were examined. It was found that the SA HNSSs had much weaker resistance to uniform corrosion compared to the SA 316L SS. The addition of molybdenum, to some extent, improved the uniform corrosion resistance of the HNSSs. The sensitization treatment had little influence on the uniform corrosion resistance of all the steels. The HNSSs showed an obvious susceptibility to intergranular corrosion, in particular the ST HNSSs. The intergranular corrosion rates of the sensitized HNSSs were much higher than that of the sensitized 316L SS. The degree of interganular attack for the ST HNSSs was much more serious than that for the 316L SS. The addition of molybdenum obviously improved the resistance of the ST HNSSs to intergranular corrosion. The double loop electrochemical potentiokinetic reactivation tests also proved that the HNSSs were rather susceptible to the sensitization treatment compared to the 316L SS. The relatively weak resistance of the HNSSs to uniform and intergranular corrosion may be due to high manganese promoted anodic dissolution. The improvement of uniform and intergranular corrosion resistance caused by the addition of molybdenum could be attributed to the synergistic effects of molybdenum and nitrogen in the HNSSs on the formation and stability of passive film.     

粉末冶金高氮超级奥氏体不锈钢的开发

采用气雾化法制备高氮超级奥氏体不锈钢粉末,利用热等静压成形。结果表明,热等静压后,材料完全致密,而σ及Cr2 N两相的析出导致材料塑性、韧性及耐蚀性显著下降。材料经1200℃×1 h固溶处理后,力学性能及耐蚀性能大大提高,抗拉强度Rm为1050 MPa、屈服强度Rp0.2为735 MPa,伸长率A为57.0%,自腐蚀电位Ecorr为0.946 V。     

Effect of nitrogen on microstructure and corrosion resistance of Cr15 super martensitic stainless steel

ABSTRACT

To investigate the influence of nitrogen on structure and corrosion resistance of Cr15 super martensitic stainless steels (SMSS), two types (N-free and N-0.12%) of specimens were quenched at 1050°C and tempered at different temperatures, and then, optical microscope, transmission electron microscopy, X-ray diffraction, potentiodynamic polarisation, immersion experiments and Kelvin Probe Force Microscope were used to characterize its microstructures and corrosion properties. The experimental results show that the microstructure in the N-free Cr15 super martensitic stainless steel is a biphasic tissue with alternating martensite and austenite distribution while quenched at 1050°C and tempered between 600 and 700°C. The nitrogen addition increases the content of austenite, and changes the austenite morphology significantly into the coarse block and strip distribution. What’s more, micro-galvanic corrosion is formed between austenite and martensite, which deteriorates the corrosion resistance of the SMSS.     

Influence of sulphide ions on corrosion resistance of special austenitic stainless steels in phosphoric acid

Abstract

The influence of sulphide ions on the corrosion resistance of some special stainless steels in 40 wt-% H₃PO₄ with 330 ppm SO^2?₄ and 1000 ppm Cl^? at 80°C has been investigated using electrochemical (polarisation curves) and spectroscopic (infrared, SIMS) techniques. The behaviour of ZI NCDU 25–20 and ZI CNDU 25–25 commonly used in the phosphoric acid industry is compared with that of ZI CN 25–20 to estimate the role of the alloying elements. The corrosive effect of sulphide ions is confirmed: they shift the corrosion potential to more negative values and increase the anodic current in the active and passive states. In order of increasing corrosion rate the three steels are ranked as follows: ZI CNDU 25–25, ZI NCDU 25–20, ZI CN 25–20. The corrosion products consist largely of magnetite in a more or less oxidised state with chromium and nickel substituted for iron. The film formed on the ZI CNDU 25–25 alloy is more protective because of the high chromium content of the mixed oxide and the effect of molybdenum. The film retards the diffusion of corrosive ions through to the metal andfacilitates the development of passivity. However, the presence of a sufficiently high concentration of sulphides in solution prevents the establishment of passivity.     

Possible mechanisms for the improvement by vanadium of the pitting corrosion resistance of 18% chromium ferritic stainless steel

Vanadium additions increase the pitting potential of 18% chromium ferritic stainless steel. Artificial-pit measurements showed that this is not related to any change in the dissolution kinetics in the pit anolyte, in contrast with the effect of molybdenum which lowers the dissolution rate strongly. A decrease in the nucleation rate of metastably growing pits is found for vanadium-alloyed steels, but the effect is smaller than for molybdenum alloying. The salt film in artificial pits in vanadium-alloyed steels was found to dissolve more slowly than for the molybdenum-alloyed steels; the slower dissolution is expected to contribute to the pitting resistance of the vanadium-alloyed steels.     

Effects of copper addition on the formation of inclusions and the resistance to pitting corrosion of high performance duplex stainless steels

To elucidate the effects of copper addition on the formation of inclusions and the resistance to pitting corrosion of alloys, potentiodynamic and potentiostatic polarization tests, a SEM-EDS analysis of inclusions, and thermodynamic calculations of the formation of inclusions were conducted. The addition of copper to the base alloy increased the number and area of numerous (Mn, Cr, (Al), (Fe)) oxides and oxy-sulfides due to an increase in the activity of chromium and resulted in decreased pitting resistance. The thermodynamic prediction of the formation of inclusions was in good agreement with the experimental results.     

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials (E_p) and sites of corrosion damage initiation. The variations in E_p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.     

Novel strategies for assessing the pitting corrosion resistance of stainless steel surfaces

Pitting corrosion is one of the most common mechanisms of surface damage on stainless steels. Electrochemical methods have been preferentially applied for the evaluation of the pitting corrosion resistance of stainless steels in the laboratory. Nevertheless, some of them are not reliable enough and in general the application of electrochemical methods in the field becomes difficult because of required deep understanding of corrosive phenomena and measurement technology. Therefore, new approaches for the evaluation of the pitting corrosion susceptibility of stainless steel surfaces in the laboratory as well as in the field are necessary. In the present paper two novel strategies including electrochemical noise measurements under anodic polarization for laboratory testing, and an indicator test to assess the susceptibility of stainless steel surfaces to pitting corrosion in the field are introduced. Experimental results concerning the influence of surface treatments on the pitting corrosion resistance on stainless steels have confirmed that final surface condition has a significant effect on their future pitting corrosion susceptibility. In addition, the pitting corrosion resistance of stainless steel surfaces was observed being specifically dependent on the achieved surface topography and in some cases independent on the roughness parameters of the surface.