Intergranular corrosion of Ti-stabilized 11 wt% Cr ferritic stainless steel for automotive exhaust systems

Intergranular corrosion (IGC) of type 409L ferritic stainless steel (FSS) was investigated. A free-exposure corrosion and a double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to examine IGC of the FSS. IGC occurred in the specimens aged at the temperature range of 400–600 °C that has the sensitization nose located around 600 °C. The critical I_r/I_a value was determined to be about 0.03 above which IGC occurred. Based on the analysis of the intergranular precipitates by an energy dispersive spectroscopy (EDS) and a transmission electron microscopy (TEM), IGC was induced by the Cr depletion zone formation due to Cr segregation around intergranular TiC.     

Effect of high temperature heat treatment on intergranular corrosion of AlMgSi(Cu) model alloy

Copper containing 6000-series aluminium alloys may become susceptible to intergranular corrosion (IGC) as a result of improper thermomechanical processing. Effect of cooling rate after solution heat treatment on the corrosion behaviour of a model AlMgSi(Cu) alloy of nominal composition (wt%) 0.6 Mg, 0.6 Si, 0.2 Fe, 0.2 Mn and 0.1 Cu was investigated. Slow cooling rates were simulated by isothermal treatment for predetermined times in lower temperature baths immediately after solution heat treatment. Treatment for 10-100 s at temperatures below 400 °C introduced susceptibility to IGC. Longer heat treatment at the same temperatures introduced susceptibility to pitting. A corrosion resistant time zone was found between the zones of IGC and pitting at temperatures lower than 350 °C. Quenching in water after solution heat treatment prevented IGC. IGC was related to microgalvanic coupling between the noble Q-phase (Al₄Mg₈Si₇Cu₂) grain boundary precipitates and the adjacent depleted zone. Pitting was attributed to coarse particles in the matrix. Possible mechanisms causing the corrosion resistant intermediate zone are discussed. The results indicate possible methods for obtaining increased corrosion resistance of similar alloys by proper thermal processing.     

Temperature and water vapour effects on the cyclic oxidation behaviour of Fe-Cr alloys

Binary Fe-Cr alloys were subjected to cyclic oxidation at 600, 700 and 950 °C in flowing gases of Ar-20O₂ and Ar-20O₂-5H₂O (vol.%). The minimum chromium concentration required to achieve protective scale growth decreased as temperature increased from 600 to 700 °C. This change is attributed to faster chromium diffusion at higher temperature. Conversely, this minimum chromium level increased when the temperature was raised from 700 to 950 °C. This is attributed to faster scale growth, leading to its rapid mechanical failure, along with formation of slow-diffusing austenite. Water vapour accelerated scaling, leading to a need for higher chromium concentrations to resist breakaway oxidation.     

Discontinuous oxidation and erosion-oxidation of a CeO2-dispersion-strengthened chromium coating

A CeO₂-dispersion-strengthened chromium coating was developed on a carbon steel using a two-step process: prior electrodeposition of a Ni-CeO₂ nanocomposite film and subsequent chromization using a conventional pack cementation method. Compared to the CeO₂-free coatings prepared on the carbon steel without and with pre-electrodeposition of a pure Ni film, the CeO₂ dispersed chromium coating offered profoundly improved discontinuous oxidation resistance at 900 °C in 5% O₂ + N₂ and in 5% O₂ + 1000 ppm SO₂ + N₂, and erosion-oxidation resistance in a laboratory-scale fluidized-bed combustor (FBC), mainly because of the development of a denser, less wrinkled and more adherent chromia scale.     

Microstructure and intergranular corrosion resistance of UNS S17400 (17-4PH) stainless steel

UNS S17400 or 17-4PH is a precipitation hardening martensitic stainless steel with many industrial applications. Quite different mechanical properties can be produced in this material by varying the aging temperature. In this work, the influence of aging temperature on the intergranular corrosion susceptibility was evaluated by electrochemical and metallographic tests. The microstructural features were investigated by X-ray diffraction, optical and scanning electron microscopy. Intergranular chromium carbide precipitation occurs in specimens aged at high temperatures, although NbC carbides were also observed. The results obtained by double loop electrochemical potentiodynamic reactivation tests (DL-EPR) show that the susceptibility to intergranular corrosion resistance increases with the increase of aging temperature. Healing due to Cr diffusion in the 600-650 °C range was not observed by DL-EPR tests.     

Pourbaix diagrams for chromium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Cr-Br⁻-H₂O system at 25 °C were developed in 400, 700, 850, and 992 g/L (4.61, 8.06, 9.79, and 11.42 M) LiBr solutions, common concentrations in different parts of absorption devices. The diagrams were compared with the simple Cr-H₂O system at 25 °C. Equilibria for the Cr-Br⁻-H₂O system at 25 °C were determined for bromide ion activities of 15.61, 194.77, 650.06, and 2042.65, which corresponded to the 400, 700, 850, and 992 g/L LiBr solutions, respectively. Activities of all the dissolved species containing chromium were plotted for 10⁻⁶, 10⁻⁴, 10⁻², and 10⁰. Comparison of the simple Cr-H₂O system at 25 °C with the diagrams for Cr-Br⁻-H₂O system at 25 °C showed that the dominant aqueous Cr(III) species in acid solutions was Cr⁺³ for Br⁻ activities of 15.61, 194.77, and 650.06, whereas it was CrBr⁺² for Br^- activity of 2042.65. Aqueous CrBr⁺² formed at a Br⁻ activity higher than 943.05. The chromium solubility range in the acid area of the diagrams extended slightly to higher pH values with increasing Br⁻ activity and decreasing water activity, as a result of destabilization of Cr₂O₃.     

Synthesis of chromium carbide by reduction of chromium oxide with methane

In the present work, production of the chromium carbide was investigated by reduction of chromium oxide with methane-containing gas mixture. The experiments were conducted on the chromium oxide powder and methane gas at different temperatures, times, and gas mixtures. X-ray diffraction (XRD) analysis was used to characterize the products at different stages of reduction. The morphology of the starting chromium oxide powder and Cr₃C₂ were studied by electron microscopy technique. The results showed that the minimum temperature and time for carbide formation in 30%-methane gas mixture is about 850 °C and 20 min, respectively. X-ray diffraction analysis showed that Cr₃C₂ is the only carbide product. The formation of chromium carbide in 30%-methane gas mixture was completed at 1000 °C and 60 min.     

The effect of manganese and chromium on surface oxidation products formed during batch annealing of low carbon steel strip

Surface oxides formed at ferrite grain boundaries of low carbon steels annealed at 700 °C in 5% hydrogen 95% nitrogen atmosphere were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two different oxides (Fe,Mn)O and MnCr₂O₄ are observed at the grain boundaries and the former is five-fold coarser than the latter. It was found at the annealing temperature of 700 °C that the mean particle size of the (Fe,Mn)O depends on the manganese content, and the mean particle size and distribution of the MnCr₂O₄ dependent on chromium, but independent of manganese. It is unlikely the coarse (Fe,Mn)O precipitates pose any potential risks to the electrolytic tin coating quality as they will be removed by the pickling operation prior to tinning. The potential risks posed by the MnCr₂O₄ to the quality of the electrolytic tin coating of tinplate products can be minimized by restricting the chromium content of the steel.     

Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys

Certain 6000-series extrusions may develop susceptibility to intergranular corrosion (IGC) by improper heat treatment, especially if copper is present as an alloying element. Although occurrence of IGC in such cases is documented, the underlying mechanisms are not adequately explained. We present corrosion data for two model alloys, having different Cu content and Mg:Si ratio, showing that the susceptibility to IGC depended primarily on the Cu content and secondly on thermal processing. Low Cu samples (0.0005 wt.% Cu) were essentially resistant to IGC. High Cu samples (0.12 wt.% Cu), which were air cooled after extrusion, exhibited significant IGC. However, IGC susceptibility was reduced significantly as a result of artificial aging to peak strength. Water quenched high Cu samples were essentially resistant to IGC. However, slight IGC susceptibility was introduced after aging. Electron optical characterisation revealed Al₄Mg₈Si₇Cu₂ (Q-phase) grain boundary precipitates on all the variants susceptible to IGC. The susceptibility was attributed to microgalvanic coupling between Q-phase grain boundary precipitates (noble) and the adjacent depleted zone (active).     

Effect of lamellar microstructure on oxidation kinetics of Fe3Al sintered by hot isostatic pressing

The present paper focuses on the investigation of the relationship between microstructure of Fe₃Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe₃Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe₃Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al₂O₃ stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al₂O₃. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe₃Al samples are supposed to be the main factors that determine observed peculiarities after Fe₃Al oxidation at 900 °C and 1000 °C.     

Investigation on ultra-pure ferritic stainless steel 436L susceptibility to intergranular corrosion using optimised double loop electrochemical potentiokinetic reactivation method

The susceptibility of 17Cr ferrite stainless steel to intergranular corrosion (IGC) was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test and the microstructural characterisation. The results show that the optimised DL-EPR test condition for 17Cr ferrite stainless steel is l?M H₂SO₄?+?0.006?M KSCN solution with a scan rate of 0.1?V?min^?1 at 25°C The severe IGC occurs in the temperature range of 500–650°C and the nose temperature locates at approximately 600°C. The I_r/I_a value rises up to 16.99% when heat treated at 600°C for 30?min. After aging treatment, M₂₃C₆ precipitates are detected using transmission electron microscopy, resulting in the presence of the Cr depletion zone. However, the degree of IGC for 436L is extremely limited due to the Cr element diffusion form the matrix despite prolonging the aging time to 5?h.     

Tuning DOS measuring parameters based on double-loop EPR in H2SO4 containing KSCN by Taguchi method

In the present work, a higher resolution than that of standard DL-EPR technique was obtained by tuning the degree of sensitisation (DOS) measuring parameters. SS304 with different sensitisation conditions was chosen. Acid and depassivator concentration, temperature, scan rate and reverse potential were defined as key factors and Taguchi was applied. The optimal condition was found to be 1 M H₂SO₄, 0.02 M KSCN, 40 °C, 30 mV/min scan rate and 200 mV reverse potential. Main factors were acid concentration, scan rate, depassivator concentration. An increase in DOS from 18 to 42.2 in the 60 min sensitised sample was observed.     

Evaluation of aged duplex stainless steel UNS S32750 susceptibility to intergranular corrosion by optimized double loop electrochemical potentiokinetic reactivation method

Optimized double loop electrochemical potentiokinetic reactivation (DL-EPR) in combination with microstructure observation were successfully applied to study the sensitization of super duplex stainless steel UNS S32750 aged at 900 °C for different duration to intergranular corrosion (IGC). The results indicated that the degree of sensitization increased gradually with the aging time increased from 10 min to 4 h due to the sigma phase precipitation, and I_r:I_a value reached the maximum value of 17.1%. However, further increasing aging time slightly decreased the sensitization due to the healing effect incurred by the diffusion of chromium from adjacent grains to chromium-depleted zone.     

Recession behavior of Yb2Si2O7 phase under high speed steam jet at high temperatures

Recession behavior of Yb₂Si₂O₇ phase was examined under high speed steam jet environment between 1300 °C and 1500 °C. Yb₂SiO₅ phase was formed on the bulk surface by the decomposition of Yb₂Si₂O₇ phase and the elimination of silica component at elevated temperatures. The phase ratio of Yb₂SiO₅/Yb₂Si₂O₇ increased up to 1400 °C and then decreased above 1400 °C. The relative intensity of 2 2 0 peak for Yb₂Si₂O₇ phase increased with increasing the temperatures. Fine grains were generated on the bulk surface at 1300 °C. The phase decomposition caused on the grain interior. A porous structure was formed on the bulk surface during the test at 1400 °C. Surface cracks were generated for 1400 °C test sample. A smooth surface was generated on the surface of 1500 °C test sample. The triple points of the grains were bridged with a glassy phase.     

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.     

The influence of solution treatment temperature on microstructure and corrosion behavior of high temperature ageing in 25% Cr duplex stainless steel

In 25% Cr duplex stainless steels, the effect of prior-solution treatment temperature (STT) on the microstructure and corrosion behavior with ageing at 750 °C and 850 °C was investigated. The results revealed that the precipitation rate of σ-phase was fast in the early stage of ageing for 80 min, and then got slower with ageing time up to 330 min. The σ-phase formation was effectively suppressed by raising STT from 1060 °C to 1230 °C especially for ageing at 750 °C. Consequently, the corrosion rate of specimen was dependent on the amount of σ-phase precipitation, and was lowered due to higher STT, and more σ-phase precipitation can lead to the transition from metastable to stable pitting with ageing at 750 °C up to 330 min. Pitting occurred easily around coarse σ precipitates and caused selective dissolution in ferrite. The longer ageing time increased intergranular corrosion (IGC) susceptibility, whereas higher STT contributed to better resistance to IGC.     

Assessment of intergranular corrosion of heat treated austenitic stainless steel (AISI 316L Grade) by electron microscopy and electrochemical tests

The resistance of austenitic stainless steel to intergranular corrosion (IGC) varies during the process of aging at temperatures between 500 and 700°C. This follows the well-known phenomena of precipitating of M₂₃C₆ chromium carbides and intermetallic phases (η, σ, χ). Consequently, this leads to significant Cr-depletion zones at grain boundaries responsible for material sensitization to IGC. The assessment of the sensitivity to IGC from the Strauss or equivalent tests requires cutting a sample off the material, which can be harmful to the integrity of the structure in service. Such a sampling is in essence only qualitative and insufficiently sensitive to the low widths of Cr-depletion at the beginning of precipitation. The DL-EPR method (Double-Loop Electrochemical Potentiodynamic Reactivation test) is known to be a non-destructive and quantitative test method of detecting relatively mild degrees of sensitization in austenitic stainless steel. The current ratios Ir/Ia > 1% (sensitization criteria) and, as a consequence, the electric charge ratios Qr/Qa > 1% of the degree of sensitization (DOS) to intergranular corrosion can be considered as good parameter values to differentiate materials with only difference in the DOS and to detect the fine precipitation responsible for the depletion in the elements of an alloy. This criteria is also valid for the detection of desensitization during againg for longer periods of time. The text was submitted by the authors in English.     

High temperature oxidation of metallic chromium exposed to eight different metal chlorides

The influence of eight different chlorides (BaCl₂, CaCl₂, KCl, LiCl, MgCl₂, NaCl, PbCl₂, and ZnCl₂) on the oxidation of metallic chromium powder was studied at four different temperatures (400 °C, 500 °C, 550 °C, and 600 °C) under dry conditions in synthetic air by using a DTA/TG-apparatus. BaCl₂, CaCl₂, and MgCl₂ did not react with chromium at any of the studied temperatures. ZnCl₂ evaporated already before the air was introduced. KCl, LiCl, NaCl, and PbCl₂ were all found to be reactive and to accelerate the oxidation of chromium. LiCl reacted only at 600 °C, whereas the other three chlorides mentioned above reacted from 500 °C upwards.     

Lignin terpolymer for corrosion inhibition of mild steel in 10% hydrochloric acid medium

Lignin terpolymer has been obtained by grafting copolymerization of both dimethyl diallyl ammonium chloride (DMDAAC) and acrylamide (AM) onto lignin. The corrosion inhibition properties of the terpolymer were tested. The results showed that the highest corrosion inhibition percentage was over 95% in 10% HCl acid medium at 25 °C and 80 °C. The lignin terpolymer inhibitor adsorption followed Temkin isotherm at 25 °C and 80 °C, and the adsorption capability was in reverse proportion to the temperature according to −ΔG_ads. The effects of corrosion inhibition are the comprehensive synergistic effect through the graft reaction among lignin, AM and DMDAAC.     

Evolution of intergranular corrosion resistance for HR3C heat-resistant austenitic stainless steel at elevated temperature

An attempt has been made to investigate the evolution of intergranular corrosion (IGC) resistance of HR3C heat-resistant austenitic stainless steel. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to evaluate the IGC resistance of HR3C steel. The results show that the side peaks can be observed in active–passive region of DL-EPR curves due to inhomogeneous distribution of the chromium dissolved in matrix. The variation of the degree of sensitisation (DOS) during sensitising at 800°C can be divided into three regions (sensitisation dominant region, desensitisation dominant region and equilibrium region). When HR3C steel is sensitised at 800°C for over 24?h, the equilibrium state in DOS can be reached. Simultaneously, the IGC morphologies after DL-EPR tests can correlate well with the DOS. On the basis of the equilibrium state in DOS, proper heat treatments and corrosion protection measures can be taken to avoid IGC of HR3C steel.