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.     

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.     

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 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.     

Effect of different thermochemical treatments on the electrochemical behaviour of a hot working steel

The X 37CrMoV5 1 KU hot working steel has been quenched and tempered, overcarburised, nitrided and nitrocarburised. The surface layers constituted after the thermochemical treatments were characterised by means of X‐ray diffraction (XRD) analyses, optical and scanning electron microscope (SEM) observations, micro‐ and macrohardness indentations. The electrochemical behaviour was evaluated in two different aggressive environments: 0.1 M Na₂SO₄ and 0.1 M NaCl solutions. While the overcarburised and the nitrocarburised samples exhibit a poor electrochemical behaviour, the presence of a nitrided surface layer enhances the corrosion resistance. This is attributable to the protective action of the nitrided layer that hinders the anodic dissolution reaction of the matrix, whereas the presence of complex carbides in the overcarburised or nitrocarburised samples causes the preferential dissolution of the ferritic matrix due to galvanic coupling phenomena. Finally, the poor corrosion resistance exhibited by the nitrocarburised sample can be ascribed to the presence of a large amount of ferrite together with little ε solid solution in the surface layer.     

Investigations into the stress corrosion cracking of stainless steel alloyed with nitrogen

The resistance to stress corrosion of austenitic stainless steel alloyed with nitrogen (ASS N25) was determined in comparison with AISI 316L steel. The research into stress corrosion cracking was performed by using corrosion elongation curves in a 50% CaCl₂ solution, at 100°C, under axial tensile loadings defined as various percentages of Rp_0,2, and with anodic polarization at a current density of 1.0 mA/cm². The increased resistance of the nitrogen‐alloyed steel to stress corrosion is based on the occurrence of NH₄⁺ ions, and the repulsive action of the negatively charged nitrogen atoms towards the Cl⁻ ions on the interface between the passive layer and the corrosion medium. Additionally, the interstitially dissolved nitrogen is able to accelerate the local deformation hardening of the material at the crack tip, for which reason the critical stress needed for stress corrosion to occur, or for further growth of the crack if it has already occurred, is increased.     

Electrochemical corrosion behavior of X80 pipeline steel in a near‐neutral pH solution

In this work, the electrochemical corrosion behavior of X80 pipeline steel was investigated in a near‐neutral pH solution using electrochemical impedance spectroscopy (EIC) and photo‐electrochemical (PEC) measurements as well as X‐ray photo‐electron spectroscopy (XPS) technique. The effects of hydrogen‐charging and stress were considered. The results show that the steel is in an active dissolution state, and a layer of corrosion product is formed and deposited on the electrode surface, which is subjected to further oxidation to form ferric oxide and hydroxide. Photo‐illumination enhances anodic dissolution of the steel when it is under anodic polarization due to destroying of the corrosion product film. When the steel is under cathodic polarization, the cathodic current density decreases upon laser illumination due to the photo‐oxidation of hydrogen atoms generated during cathodic reactions, which behaves as an anodic reaction to offset the cathodic current density. Hydrogen‐charging and stress decrease the corrosion resistance of the steel and enhance the dissolution rate of the steel.     

The Effect of Nanocomposite Superhydrophobic Coating on Corrosion and Kinetics of Electrode Processes on Steel in 0.5 M NaCl Solution

The corrosion and kinetics of partial electrode reactions on St3 carbon steel with a superhydrophobic coating based on methoxy fluorobutylsilane derivatives in 0.5 M NaCl solution were studied. This influence of in situ duration τ of the medium’s corrosive action (0.5–144 h) on its protective effect is considered. This allows reduction of overall corrosion rate by 23 ± 3 times, when the stationary state is reached. The kinetics of cathodic reduction of dissolved oxygen and steel anodic ionization is also considered. In the initial stages of corrosive action of neutral aerated chloride medium (until steady-state characteristics are reached), steel at potential E near E_cor corrodes in the active state, whereas, when τ is ≤0.5 h, it corrodes under anodic control conditions, and then the control is replaced with a cathodic one.     

Plasma nitriding of plastic mold steel to increase wear- and corrosion properties

In this study, the wear- and corrosion resistance of the layers formed on the surface of a precipitation hardenable plastic mold steel (NAK55) by plasma nitriding were investigated. Plasma nitriding experiments were carried out at an industrial nitriding facility in an atmosphere of 25% N₂ + 75% H₂ at 475 °C, 500 °C, and 525 °C for 10 h. The microstructures of the nitrided layers were examined, and various phases present were determined by X-ray diffraction. Wear tests were carried out on a block-on-ring wear tester under unlubricated conditions. The corrosion behaviors were evaluated using anodic polarization tests in 3.5% NaCl solution.The findings had shown that plasma nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma nitriding of NAK55 mold steel produced a nitrided layer consisted of a compound layer rich in ε-nitride and an adjacent nitrogen diffusion layer on the steel surface. Increasing the nitriding temperature could bring about increase in the thickness of the nitrided layer and the nitride volume fraction. Plasma nitriding improved not only surface hardness but also wear resistance. The anti-wear property of the steel was found to relate to the increase in the thickness of the diffusion layer. Corrosion study revealed that plasma nitriding significantly improved corrosion resistance in terms of corrosion potential and corrosion rate. Improvement in corrosion resistance was found to be directly related to the increase in the nitride volume fraction at the steel surface.     

Corrosion properties of plasma nitrided AISI 410 martensitic stainless steel in 3.5% NaCl and 1% HCl aqueous solutions

Samples of an AISI 410 martensitic stainless steel were plasma nitrided at a temperature of 420 °C, 460 °C or 500 °C for 20 h. The composition, microstructure and hardness of the nitrided samples were characterised using a variety of analytical techniques. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarisation tests in 3.5% NaCl solution and immersion tests in 1% HCl acidic water solution. The results showed that plasma nitriding produced a relatively thick nitrided case consisting of a compound layer and a nitrogen diffusion layer on the 410 stainless steel surface. Plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the martensitic stainless steel. In the immersion test, nitrided samples showed lower weight loss and lower corrosion rate than untreated one. In the electrochemical corrosion tests, the nitrided samples showed higher corrosion potentials, higher pitting potentials and greatly reduced current densities. The improved corrosion resistance was believed to be related to the iron nitride compound layer formed on the martensitic stainless steel surface during plasma nitriding, which protected the underlying metal from corrosive attack under the testing conditions.     

Long-term anticorrosion behaviour of polyaniline on mild Steel

Anticorrosion performances of polyaniline emeraldine base/epoxy resin (EB/ER) coating on mild steel in 3.5% NaCl solutions of various pH values were investigated by electrochemical impedance spectroscopy (EIS) for 150 days. In neutral solution (pH 6.1), EB/ER coating offered very efficient corrosion protection with respect to pure ER coating, especially when EB content was 5-10%. The impedance at 0.1 Hz of the coating increased in the first 1-40 immersion days and then remained constant above 10⁹ Ω·cm² until 150 days, which in combination with the observation of a Fe₂O₃/Fe₃O₄ passive film formed on steel confirmed that the protection of EB was mainly anodic. In acidic or basic solution (pH 1 or 13), EB/ER coating also performed much better than pure ER coating. However, these media weakened the corrosion resistance due to breakdown of the passive film or deterioration of the ER binder.     

Study of plasma nitriding and nitrocarburising of AISI 430F stainless steel for high hardness and corrosion resistance

ABSTRACT

In order to improve both the hardness and corrosion resistance properties of AISI 430F stainless steel, plasma nitriding (PN) and nitrocarburising processes were carried out at different temperatures ranging from 350 to 500°C for 4?h. After PN, the nitrided layer was found to be thicker compared to that obtained by plasma nitrocarburising process. There was an increase in microhardness values by a factor of six to seven compared to the plasma nitrided and nitrocarburised specimens respectively, treated at 500°C. The electrochemical corrosion behaviour of the plasma nitrided and nitrocarburised AISI 430F specimens show that the plasma nitrided and nitrocarburised specimens treated at 400°C for 4?h showed better corrosion resistance and higher surface hardness than the untreated AISI 430F stainless steel specimens. This is mainly attributed to the presence of nitrogen in the modified layer existing as a solid solution in the ferrite phase.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Steam oxidation and surface hardness of power plant valve materials under the ultra super critical steam condition

Current supereritical steam power plants operate at 3,600 psi and 1,000°F. If the steam temperature is raised from 1,000 °F (538 °C) to 1,150 °F (621°C), the efficiency increases by 2%. Therefore, study on the high temperature corrosion of power plant materials under ultra-superciritical conditions (USC) is necessary to protect the plant from corrosion. In this study, valve materials of 17% Cr martensitic steels (17Cr steel), Incoloy 901 (1901) and their surface nitrided specimens were exposed to USC of 621 °C and 3600 psi (255 kg/cm²) steam for 200 °C, 400 °C, and 800 h. The oxidation of both 17Cr steel and 1901 under the USC for 800 h is very small due to the formation of a protective thin oxide layer formation on the surface. The USC oxidation of both nitrided specimens were increased due to the decomposition and formation of active nitrogen from the non protective nitrides such as Fe₄N, Fe_2–3N, and CrN. The oxidation of nitrided 17Cr steel (n17Cr steel) is about two times higher compared to nitrided 1901 (n1901). The surface hardness is improved by more than two times near the surface by nitriding, and the degradation of hardness by USC oxidation is rapid for n17Cr steel, but slow for n1901.     

Microbiologically influenced corrosion of X60 carbon steel in CO2‐saturated oilfield flooding water

The corrosion behavior of X60 carbon steel in CO₂‐saturated oilfield flooding water inoculated with sulfate‐reducing bacteria (SRB) was investigated using polarization curves and electrochemical impedance spectroscopy (EIS). With the propagation of SRB in the flooding water, the pH values of flooding water increased quickly in the initial 2 days and remained relatively steady during the later stage. Polarization curves showed that the corrosion current density decreased during the first 10 days due to the protection of corrosion products and SRB‐biofilms, and then increased possibly due to the partial detachment of the corrosion products and the biofilms. EIS analysis also showed that the charge transfer resistance increased initially and then decreased with exposure time. In the beginning of corrosion, the anodic dissolution of X60 steel was dominated by CO₂. After the formation of SRB‐biofilms, part of FeCO₃ corrosion products was converted to incompact FeS precipitates by SRB bio‐mineralization. Thus, the dispersed iron sulfide in SRB‐biofilms and X60 steel base may constitute a galvanic couple, accelerating the localized corrosion of the steel base in the flooding water.     

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.     

Comparative corrosion resistance of plasma-based low-energy nitrogen ion implanted austenitic stainless steel

A high nitrogen face-centered-cubic phase (γ_N) was obtained on the nitrided surface of 1Cr18Ni9Ti austenitic stainless steel by plasma-based low-energy nitrogen ion implantation. No pitting corrosion for the γ_N phase was confirmed by electrochemical polarization measurement in 3% NaCl solution. The protective passive film with a duplex character, iron hydroxide/oxides in the outer region and chromium hydroxide/oxides and iron oxides accompanying chromium and iron nitrides in the inner region, was by 2-3 times thicker than that of original stainless steel. The thick iron hydroxide/oxides region formed on the chromium hydroxide/oxides region due to the increase of alkalinity in the solution, leading to barrier against penetration of localized attack of the aggressive ions. The equivalent general corrosion resistance for the γ_N phase was observed in 0.5 mol/l H₂SO₄ solution relative to the original stainless steel. The passive film formed on the γ_N phase in 0.5 mol/l H₂SO₄ solution was similar to that of original stainless steel. The different role of nitrogen was proposed in pitting corrosion resistance and general corrosion resistance of austenitic stainless steel.     

Study of an alternative phosphate sealer for replacement of hexavalent chromium

This study evaluates the possibility of replacing the hexavalent chromium passivation treatment used as a sealer after phosphating of carbon steel (SAE 1010) by a treatment with niobium ammonium oxalate (Ox). Samples of carbon steel (SAE 1010) after being phosphated in a zinc phosphate bath (PZn + Ni) were immersed in solution of niobium ammonium oxalate (250 mg L⁻¹ of Nb) either at pH 3.0 or pH 8.0. A passivation treatment with a solution with CrO₃ (200 mg L⁻¹ of Cr⁶⁺) was also used for reference.The corrosion resistance of the phosphated samples after passivation treatments was analyzed in a NaCl 0.5 mol L⁻¹ solution using electrochemical impedance spectroscopy (EIS) and anodic polarization curves. Salt spray tests were also performed to evaluate their corrosion resistance. The results showed that the highest corrosion resistance was obtained by passivation in a solution with (250 mg L⁻¹ of Nb) at pH 8.0.     

Improvement of the Corrosion Resistance of High Alloyed Austenitic Cr-Ni-Mo Stainless Steels by Solution Nitriding

Characteristic features of austenitic steel grades combine a good corrosion resistance with a low hardness, wear resistance and scratch resistance. An interesting possibility for improving the wear behaviour of these steels without loss of their corrosion resistance lies in enriching the near surface region with nitrogen. The process of a solution nitriding allows the rise of the solution of nitrogen in the solid phase. On this state nitrogen increases the corrosion resistance and the tribilogical load-bearing capacity. The aim of the study was, to investigate the improvement of the pitting corrosion behaviour by solution nitriding. A special topic was to observe the effect of nitrogen by different molybdenum content. So austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.06 to 3.6%) had been solution nitrided. The samples could be prepared with various surface content of nitrogen from 0.04 to 0.45% with a step-by-step grinding. The susceptibility against pitting corrosion of these samples had been tested by determination of the stable pitting potential in 0.5M and 1M NaCl at 25℃. For the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen. The influence of nitrogen to all of the determined parameters can be described well by PRE = Cr 3.3 * Mo 25 *N. XPS analysis of the sample surfaces support the results of the pitting corrosion tests.Additionally surface investigations with an acid elektolyte (0.1M HCl 0.4M NaCl) were performed. In this case the passivation effective nitrogen content increases markedly with rising molybdenum concentration of the steel.Obviously an interaction of Mo and N is connected with a strongly acid electrolyte.     

低碳钢在模拟酸雨大气条件下的锈蚀演化

利用干湿循环加速腐蚀实验和电化学极化曲线法研究了低碳钢在模拟工业酸雨大气条件下的锈蚀演化过程.结果表明:实验初期锈蚀速度随干湿循环次数的增加而增大,随后转为随干湿循环次数的增加而降低;带锈低碳钢的干湿循环下的腐蚀产物促进阴极过程抑制阳极过程;在干湿循环加速腐蚀进程中低碳钢表面铁锈的化学组成、结构变化表现为在锈蚀初期α-FeOOH含量较低,锈层疏松,锈蚀速度随干湿循环次数增加呈上升趋势;后期随α-FeOOH含量的增加和锈层变得更加致密,腐蚀速度转变为随干湿循环次数增加而下降.     

The co‐operative effect of chloride ions and some natural extracts in retarding corrosion of steel in neutral media

Effect of lupine and damsissa extracts on the corrosion of steel in 0.5 M Na₂SO₄ solution free from and containing 0.01 or 0.1 M NaCl were examined by potentiodynamic and electrochemical impedance spectroscopy techniques. Potentiodynamic polarization curves measurements indicated that damsissa and lupine extracts behave as anodic type inhibitors. Nyquist impedance plots showed type of distorted semicircle whose size increases with increasing concentrations of lupine or damsissa extract. Increasing chloride ion concentration in the solution led to increase of the inhibition efficiency of the extract and this behaviour was explained on the basis of co‐operative mechanism of adsorption. Kinetic–thermodynamic model was used to clarify the nature of adsorption and showed good fit to the experimental data. The results indicated that lupine extract is more effective as corrosion inhibitor for steel in neutral media containing chloride ions than damsissa extract.