Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO₃ + 25%H₂O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel.     

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.     

The effect of heat treatment on mechanical properties and corrosion behavior of AISI420 martensitic stainless steel

Martensitic stainless steels are widely used for their good mechanical properties and moderate corrosion resistance. However, the need for superior properties in specific applications (e.g. steam generators, mixer blades, etc.) led to wide researches on the performance improvement of these steels. Heat treatment was recommended as one of the best ways to this regard hence the effects of astenitizing temperature and time, and tempering temperature on the microstructure, mechanical and corrosion properties of AISI420 have been studied. In the current work the experimental results showed that the austeitizing temperature significantly affects mechanical properties. The increase of tempering temperature led to precipitation of M₇C₃ and secondary hardening in the range of 400-500 °C. SEM micrographs of the fracture surfaces showed a mixed fracture mechanism (brittle and ductile) at 200 °C and 700 °C and brittle mechanism at 500 °C. The Polarization curves were not significantly affected by the increment of austenitizing temperature.     

离子渗氮AISI 420马氏体不锈钢耐蚀行为研究

采用不同温度对AISI 420马氏体不锈钢进行离子渗氮处理.借助光学显微镜和X射线衍射(XRD)技术分析了渗氮层的微观组织结构,利用显微硬度计测试了渗氮层的硬度分布,通过电化学极化曲线测试和盐雾腐蚀试验研究了离子渗氮AISI 420不锈钢在模拟工业环境中的腐蚀行为.结果表明:AISI 420不锈钢350℃低温离子渗氮层由ε-Fe3N和N过饱和固溶体αN相组成,其化学稳定性高,加之固溶Cr元素的联合作用,明显提高了AISI 420不锈钢基材的腐蚀抗力.AISI 420钢经450℃和550℃渗氮处理,渗氮层中的αN分解成了α相和CrN,造成基体贫Cr,降低了基材的耐蚀性能.马氏体不锈钢低温离子渗氮处理不仅可以提高表面硬度,而且可以获得良好的耐蚀性能.     

Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment.The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ?-Fe₃N and α_N formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.     

Corrosion behavior of martensitic and precipitation hardening stainless steels treated by plasma nitriding

Plasma nitriding is a well established technology to improve wear and corrosion properties of austenitic stainless steels. Nevertheless, in the case of martensitic stainless steels, it continues being a problem mainly from the corrosion resistance viewpoint.In this work, three high chromium stainless steels (M340, N695 and Corrax) were hardened by ion nitriding at low temperature, intending to preserve their corrosion resistance.Corrosion behavior was evaluated by CuSO₄ spot, salt spray fog and potentiodynamic polarization in NaCl solution. Microstructure was analyzed by optical microscopy, SEM (EDS) and glancing angle X-ray diffraction. All the samples showed an acceptable corrosion resistance in experiments with CuSO₄, but in salt spray fog and electrochemical tests, only Corrax showed good behavior. The poor corrosion performance could be explained by chromium carbides formed in thermal treatment stage in martensitic steels and chromium nitrides formed during nitriding, even though the process was carried out at low temperature.     

A study of martensitic stainless steel AISI 420 modified using plasma nitriding

We studied martensitic stainless steel AISI 420, modified using glow discharge plasma nitriding. Microhardness measurements, X-ray diffraction (XRD), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to investigate the surface microhardness, crystal structure, microstructure and chemical bonding in the modified surfaces. High surface microhardness (1300 HV) over a case depth of about 60 microns is obtained. Glancing incidence X-ray diffraction (GIXRD) indicates the presence of a predominantly Fe₃N phase with dispersed CrN within 2–5 microns on the surface. In addition, using Bragg–Brentano geometry, we measured the presence of a minor phase of Fe₄N in the case depth. SEM confirms that the microstructure within 2–5 microns of the surface is different from that of the bulk. XPS shows nitride phase formation on the surface. AES measured over the cross-section of the case depth shows a direct relation of the increased surface microhardness to the high nitrogen content.     

Potentiodynamic polarization behaviour of AISI type 316 stainless steel in NaCl solution

Potentiodynamic polarization behaviour of AISI type 316 SS in NaCl solution was investigated in terms of the potential scan rate effect. The critical pitting potential, E_pit, of the stainless steel appeared to be strongly dependent on the potential scan rate. A cumulative anodic electric charge density of the steel was defined as the total charge density from the open circuit potential, E_ocp, and calculated using the potentiodynamic polarization curves. It was found that, plotted as a function of the polarization time, the values of the cumulative charge density consisted of two lines with different slopes. It was confirmed that the deflection of the cumulative charge density vs. time plots corresponded to E_pit and the values of the cumulative charge density at the deflection were little dependent on the applied scan rate. The cumulative charge density at the deflection was defined as a critical electric charge density for the stable pitting. Also, it was suggested that this electric charge density should be associated with the critical condition for the stable pitting and the critical electric charge for stable pitting should be a representative parameter for the pitting resistance of a material.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

Corrosion behaviour of stainless steels in aqueous solutions of methanesulfonic acid

The corrosion behaviour of different grades of stainless steel in five commercial products labeled as 70% aqueous methanesulfonic acid was investigated. Chemical analysis showed that these products contain different amounts of impurities, most probably the consequence of the production procedure. Three austenitic stainless steel grades, 304, 316 and 316Ti, are widely used for storage and transportation vessels for methanesulfonic acid (MSA). Corrosion behavior has been studied by electrochemical techniques, immersion tests and by surface analysis. The selected stainless steels are highly stable in the most purified acid, but the presence of impurities in MSA disrupts the metal’s surface, leading to corrosive attack.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

Low-temperature plasma carburizing of AISI 420 martensitic stainless steel: Influence of gas mixture and gas flow rate

Low-temperature plasma carburizing was studied aiming to determine the effect of the gas mixture and flow rate on the surface properties of AISI 420 martensitic stainless steel samples. Plasma carburizing was carried out for gas mixtures of 20% Ar + 80% H₂ comprising CH₄ contents between 0.25 and 1.00%, and gas flow rates ranging from 1.67 × 10^− 6 to 6.68 × 10^− 6 Nm³ s^− 1. The modified layers were characterized by confocal laser scanning microscopy, X-ray diffractometry and microhardness measurements. The plasma was also characterized by optical emission spectrometry. Results indicate the presence of a hard and thin outer layer and a carbon-enriched martensite diffusion layer. It is shown that gas mixture composition plays an important role in the process kinetics. Spectroscopic characterization of the glow discharge shows that the variation of the CH₄ content in the gas mixture leads to a variation of the emission lines intensity but does not significantly alter the relative peak intensities. It suggests a variation on the plasma density and no significant variation on the active species. It also indicates that, for the studied conditions, the emission spectroscopy cannot be applied as a tool for process control.     

AISI 410钢膏剂渗硼工艺渗层的组织与性能

通过膏剂渗硼工艺对AISI 410钢进行表面强化,分析了渗硼层的组织形貌,并测试了其硬度、耐磨损性能以及耐腐蚀性能。结果表明,渗硼层主要由FeB、Fe₂B相组成,渗硼层/母材的结合界面较平坦;渗硼层外渗区硬度达1400 HV0.5以上,内渗区硬度达1100 HV0.5以上;渗硼处理后试样表面摩擦因数和截面磨痕面积显著减小,耐磨损性能得到改善;在电化学腐蚀试验中,渗硼试样自腐蚀电位大于母材试样、自腐蚀电流密度小于母材试样,说明渗硼处理降低了AISI 410钢表面的腐蚀倾向,其表面耐蚀性得到提高。     

Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 °C and examined in 0.1 M Na₂SO₄ without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.     

Corrosion behaviour of Mg-Cu and Mg-Mo composites in 3.5% NaCl

The corrosion behaviour of pure magnesium, Mg-Cu (0.3, 0.6, and 1 vol.%) and Mg-Mo (0.1, 0.3, and 0.6 vol.%) composites has been studied in 3.5% NaCl solution by weight loss and polarisation methods. Corrosion rates determined by weight loss method were considerably higher than that determined by polarisation method. The corrosion rate increased with increasing volume fraction of reinforcement in Mg-Cu and Mg-Mo composites. At the same volume fraction of reinforcement, molybdenum reinforced composite corroded faster than copper reinforced composite. The galvanic current density between Mg-Cu and Mg-Mo couples has been experimentally measured using zero resistance ammeter technique. The experimentally observed galvanic current densities were in close agreement with those obtained using mixed potential theory analysis. SEM observation of corroded samples confirmed microgalvanic activity at the matrix/reinforcement interfaces. The poor corrosion resistance of composites has been attributed to microgalvanic effects between the matrix and reinforcements and inferior quality of surface films.     

Corrosion performance of HVOF and APS thermally sprayed NiTi intermetallic coatings in 3.5% NaCl solution

Amorphous/nanocrystalline Ni-Ti powders produced by low energy mechanical alloying were used as feedstock to deposit NiTi intermetallic coatings on 316L stainless steel substrate using high velocity oxy-fuel (HVOF) and air plasma spraying (APS) processes. Electrochemical impedance spectroscopy (EIS) and polarization tests indicated that the corrosion performance and passive behaviour of HVOF coating were far better than those of APS coating. The study also showed that the solution had penetrated through the coating microcracks and caused interior corrosion of APS coating, while the HVOF coating was immune from interior corrosion attack and consequently exhibited a good passive behaviour during long-term immersion.     

Corrosion behaviour of low temperature plasma carburised 316L stainless steel in chloride containing solutions

The electrochemical corrosion behaviour of the carburised (expanded austenite) layer on 316L austenitic stainless steel produced by low temperature plasma carburising has been studied in 0.5 M NaCl and 0.5 M HCl + 0.5 M NaCl solutions. The present work focuses on the variation of the corrosion behaviour of the carburised layer with depth from the surface and the effect of carbon concentration on electrochemical behaviour. The results show that the carburised layer has excellent resistance to localised corrosion. There exists a critical carbon concentration, above which the expanded austenite possesses excellent resistance to both metastable pit formation and pit growth.     

Corrosion of nanocrystalline Ni–W alloys in alkaline and acidic 3.5 wt.% NaCl solutions

The corrosion behaviour of reverse-pulse electrodeposited nanocrystalline nickel tungsten alloys (nc Ni–W) in pH 3 and 10 3.5 wt.% NaCl solutions is investigated and analysed as a function of grain size. A potentiodynamic polarisation study reveals that the corrosion rate of nc Ni–W generally increases with the reduction of grain size in alkaline condition, but decreases with the reduction of grain size in acidic environment. Furthermore, for both environments, nc Ni–W alloys exhibit superior localised corrosion resistance than a microcrystalline Ni control specimen. Factors controlling the corrosion behaviour of these materials, including grain size, tungsten content, passivation and crystallographic texture are addressed.     

Corrosion and passivation of low-temperature nitrided AISI 304L and 316L stainless steels in acidified sodium sulphate solution

304L and 316L steels were nitrided at 425 °C for 30 h and examined at various depths in 0.1 M Na₂SO₄ acidified to pH 3.0. In the near-surface region with about 7-14 wt% N, at potentials of active state anodic currents were much higher than those for untreated steels, whereas in deeper regions with <7 wt% N the currents were only slightly increased in comparison with untreated steels or they were even lower in passive and transpassive states. Surface films were composed of oxygen-containing species on top and of Cr-N species in deeper layers. It is suggested that strong corrosion of near-surface regions is associated with nitride precipitates. Beneficial effect of low nitrogen concentrations can be due to initially accelerated corrosion which leads to larger amounts of passivating species and to the accumulation of corrosion resistant chromium nitrides.