Accelerated corrosion of five commercial steels under a ZnCl2-KCl deposit in a reducing environment typical of waste gasification at 673-773 K

The corrosion of five Fe-Cr commercial steels containing 0-18 wt.% Cr at 673-773 K has been studied in a reducing H₂-HCl-CO₂ atmosphere under a ZnCl₂-KCl deposit typical of waste gasification environments. In comparison with the behavior of the same steels in a similar gas mixture without salt deposit, all steels suffered from accelerated corrosion induced by the salt and formed porous scales with poor adherence to the underlying steels. Some Cl was detected close to the steels/scale interface, indicating that Cl-containing species were able to go through the scale down to the metal matrix. Even though the corrosion rates generally decreased with increasing Cr content, the high-Cr stainless steel SS304 was still unable to provide a good corrosion resistance against the ZnCl₂-KCl deposit. The reaction mechanisms are discussed on the basis of thermodynamic considerations and of the “active oxidation” model.     

Interfacial morphology and corrosion resistance of Fe–B cast steel containing chromium and nickel in liquid zinc

The interfacial morphology and corrosion resistance of low carbon Fe–B cast steels in zinc bath at 520 °C were investigated. The results show Fe–B cast steel containing high Cr and Ni exhibits the best corrosion resistance to liquid zinc. The corrosion layers are composed of Γ-Fe₃Zn₁₀, δ-FeZn₁₀, ξ-FeZn₁₃ and η-Zn. The corrosion behaviour of Fe–B cast steels includes the following processes: the preferential leach and dissolution of Cr and Ni, the formation of Fe–Zn compounds controlled by zinc atom diffusion, and the spalling of borides without the supporting role of α-(Fe, Cr) matrix corroded by liquid zinc.     

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.     

High temperature naphthenic acid corrosion and sulphidic corrosion of Q235 and 5Cr1/2Mo steels in synthetic refining media

The corrosion of Q235 and 5Cr1/2Mo steels in synthetic refining media containing naphthenic acid and/or sulphur compounds was studied to evaluate naphthenic acid corrosion (NAC), sulphidic corrosion (SC), and their interaction. Corrosion dependencies on the test duration, temperature, total acid number (TAN) and content of sulphur compound were assessed. Specimens after NAC and SC tests were characterized by SEM/EDX, and XRD. It is found that in liquid phase of media containing only naphthenic acid and at temperature about 230 °C, 5Cr1/2Mo and Q235 steels have almost the same NAC rate, and above 230 °C, 5Cr1/2Mo has a higher NAC rate than Q235 has due to the higher NAC activation energy (63.2 kJ mol⁻¹) of 5Cr1/2Mo than that of Q235 (54.0 kJ mol⁻¹), indicating that increasing temperature accelerates NAC rate of 5Cr1/2Mo more than that of Q235. In oil containing only dimethyl disulphide, the growth of SC film follows parabolic kinetics, and the film of Q235 grows faster than that of 5Cr1/2Mo while SC rate of Q235 is higher than that of 5Cr1/2Mo. In oil containing both naphthenic acid and dimethyl disulphide, 5Cr1/2Mo has a lower corrosion rate than Q235 has. On the basis of “naphthenic acid corrosion index” (NACI), the benefits of 5Cr1/2Mo over Q235 should ascribe to that the pseudo-passive film for 5Cr1/2Mo has better NAC resistance than that for Q235. This is close related to the existing of additional chromium sulphide (Cr₅S₈) on the pseudo-passive film of 5Cr1/2Mo, in contrast with the pyrrhotite (Fe₇S₈) and troilite (FeS) on the film of Q235.     

Effect of chromium content on intergranular corrosion and precipitation of Ti-stabilized ferritic stainless steels

Intergranular corrosion (IGC) and precipitation of ferritic stainless steels (FSS) were investigated with change in Cr content from 11 wt.% to 17 wt.%. The increase in Cr content improved IGC resistance as temperature and time for the sensitization became higher and longer, respectively, but it did not prevent IGC. The analysis on the intergranular precipitates revealed that Cr segregation around fine intergranular TiC in developed all FSS regardless of Cr content. This Cr segregation is proposed to explain the Cr depletion for the cause of IGC in Ti-stabilized Cr FSS.     

Structural and compositional transformations in the near-surface layers of Fe–Cr based steels exposed to lithium – Effect of alloying and corrosion-assisted substructure coarsening

The corrosion behaviour of RAF/M RUSFER-EK-181 and F/M (EP-823, SUS410) steels was investigated in static “pure” (0.004 wt.% N) and N-added Li (0.5 wt.% N) at 600 °C up to 750 h. The weight losses of samples increased with time and nitrogen content in Li. The corrosion-assisted coarsening was observed in the near-surface layers of RAF/M steels. It is caused by the dissolution of Cr and decomposition of carbides during exposure to both melts. The coarsening mechanism and the influence of alloying on the structural and compositional transformations in steels exposed to Li are discussed.     

Corrosion behavior of three iron-based model alloys in reducing atmospheres containing HCl and H2S at 600 °C

The corrosion of three Fe-xCr alloys (x = 8, 12, 18 wt.%) was examined at 600 °C in reducing atmospheres containing HCl and H₂S with two different H₂S contents and compared with the behavior of the same materials in H₂S-free H₂-CO₂ and H₂-HCl-CO₂ mixtures producing similar oxygen and chlorine pressures. Exposure to the low-H₂S gas mixture had only a reduced effect on the behavior of Fe-8Cr and Fe-18Cr, but increased significantly the corrosion rate of Fe-12Cr. Increasing the H₂S level accelerated the corrosion of all the alloys, but particularly that of Fe-18Cr. In both cases only minor amounts of sulfur and chlorine were present close to the alloy/scale interface. An increase of the Cr content reduced the corrosion rate in both H₂S gas mixtures, especially in the range 8-12 wt.% Cr, due to a larger volume fraction of Cr₂O₃ in the scale. The results have been discussed on the basis of the thermodynamic stability diagrams of the Fe-O-Cl-S and Cr-O-Cl-S systems.     

Corrosion of austenitic and ferritic-martensitic steels exposed to supercritical carbon dioxide

Supercritical carbon dioxide (S-CO₂) is a potential coolant for advanced nuclear reactors. The corrosion behavior of austenitic steels (alloys 800H and AL-6XN) and ferritic-martensitic (FM) steels (F91 and HCM12A) exposed to S-CO₂ at 650 °C and 20.7 MPa is presented in this work. Oxidation was identified as the primary corrosion phenomenon. Alloy 800H had oxidation resistance superior to AL-6XN. The FM steels were less corrosion resistant than the austenitic steels, which developed thick oxide scales that tended to exfoliate. Detailed microstructure characterization suggests the effect of alloying elements such as Al, Mo, Cr, and Ni on the oxidation of the steels.     

The influence of alloying elements on the corrosion behaviour of ferritic steels in simulated combustion atmospheres

The low‐chromium steels (Cr content 0.7–1.4 wt.%) are usually being used in low temperature boiler applications where the environments are not so aggressive as in waste‐fire boilers. To improve their mechanical properties and corrosion resistance several alloying elements have been used. In the present study the new low‐chromium steels modified with Si, Ni and Ce have been investigated. These steels were compared with the common boiler steel – 2.25Cr1M. The multi‐sample exposure tests were made in moist air (8% O₂ + 15% H₂O) with and without 200 ppm SO₂ or 2000 ppm HCl addition at the isothermal temperature of 500°C. Corrosion products were analysed by Scanning Electron Microscope with Energy Dispersive Spectrometer (SEM/EDS) and XRD techniques. The results of the study show that addition of Ni and Si to the steels studied could improve their corrosion resistance, but only in moist air atmospheres. In SO₂‐containing atmospheres the corrosion resistance seems to remain the same, when in HCl‐containing atmospheres Ni and Si additions seem to have a negative effect on the corrosion behaviour. As could by seen from SEM analysis, the scales formed, on the steels with above elements, are more adherent to the metallic core. The cerium was added to the studied materials mainly to improve their mechanical properties. This alloying element seems to increase the corrosion rate of materials, especially in atmospheres containing Cl.     

Effect of Co and W on the corrosion behaviour of 1% Cr steel in flowing synthetic seawater

The aqueous corrosion characteristics of 1% Cr‐steel alloyed with small amounts of Co and W in synthetic seawater was studied by using immersion weight‐loss tests, electrochemical corrosion tests (potentiodynamic tests and electrochemical impedance spectroscopy (EIS) measurements) and analytical techniques. 1% Cr steels containing Co or W from 0.2 to 0.4 wt.% showed higher corrosion resistance than the 1% Cr steel in the immersion and potentiodynamic tests. EIS measurements showed that the Nyquist plot presented one time constant. Furthermore, the Co‐ and W‐bearing steels present higher R_p values than the 1% Cr steel through all the test period. The better corrosion resistance of the Co‐and W‐bearing steels is attributed to the protectiveness of the surface layer. The corrosion products were examined using electron probe microanalysis (EPMA) and X‐ray photoelectron spectroscopy (XPS). The results of EPMA indicated that Cr was concentrated in the inner region of the rust layer, while Co and W were distributed all over the rust layer. XPS results showed that Co existed as a trivalent oxide in the rust layer and W in the rust appeared in the form of a WO₄ compound. These compounds act as a factor for corrosion resistance in aqueous solutions.     

Decarburization of Fe-Cr-C steels during high-temperature oxidation

Isothermal oxidation treatments were carried out on Fe-Cr-C steels. The steels containing 0.08, 0.15, 0.17, 0.88, 1.51, and 12.77wt.% Cr and 0.10, 0.49, 1.19, 0.18, 1.05, and 1.63 wt.% C were oxidized in ambient air at temperatures of 900, 1000, and 1200°C. Steels containing 13.22, 12.90, 12.52, and 12.77wt.% Cr and 0.15, 0.30, 0.50, and 1.63 wt.% C were heated (1100°C/3hr) in a flowing atmosphere of O₂-N₂-He in a SETARAM thermobalance. Evidence of decarburization of the steels is given by metallographic observations, by direct measurements of carbon diffusivities from the decarburization profiles in the oxidized samples, and by the results of kinetics measurements.³ Carbon diffusion coefficients were measured by the standard sectioning method in the samples oxidized in air. A. generalized equation for carbon diffusivity in Fe-Cr-C alloys is developed in terms of N_Cr[wt.%], N_C[wt.%], and T[K].     

Grain size effects on the oxidation of two ternary Cu-Ni-20wt.% Cr alloys at 700-800 °C in 1 atm O2

Two nanocrystalline two-phase Cu-Ni-Cr alloys, both prepared by mechanical alloying and containing about 20 at.% Cr but with different Ni contents (40 and 20 wt.%, respectively), have been oxidized in 1 atm O₂ at 700-800 °C. Their oxidation behavior has been compared with that of two cast alloys of the same composition, already studied previously, to examine the effects of a large reduction of the size of the individual phase grains and particles. The nanophase alloy with 40 wt.% Ni formed a flat external layer of chromia of regular thickness, while the corresponding cast alloy produced a very irregular chromia layer, often protruding deeply into the alloy, only after an initial stage of rather fast corrosion involving also copper and nickel, associated with some degree of internal oxidation. By oxidation at 700 °C the nanophase alloy with 20 wt.% Ni formed an irregular chromia layer associated with low corrosion rates. The corresponding cast alloy formed complex scales containing Cu, Ni and Cr oxides, extending into the alloy in the form of large pegs, even though a very irregular and discontinuous innermost chromia layer was still able to produce low corrosion rates. On the contrary, at 800 °C both alloys formed complex scales containing mixtures of the oxides of the three metal components. However, the scales grown on the cast alloy were much more irregular in thickness and formed large protrusions into the alloy. In spite of this, the corrosion kinetics of the nanophase 20 wt.% Ni alloy at 800 °C were more irregular and, except for an initial stage, less protective than that of the cast alloy with the same composition.     

Corrosion behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and their corrosion behavior was compared with that of 304SS in 5 wt.% HCl solution. As compared to 304SS, E_corr, E_pit and E_rp values had shifted to a more negative region in 304SS containing TiC, indicating faster corrosion rate by TiC addition. The addition of TiC particles to 304SS resulted in no rapid pit propagation but maintained a high corrosion rate in the whole immersion time investigated.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.     

Effect of Mn/Mo incorporated oxide layer on the corrosion behavior of AA 2024 alloy

The oxide layer formed over AA 2024 using 10 wt.% H₂SO₄ (plain oxide, PO) was modified by Mn/Mo oxyanions (permanganate/molybdate modified oxide, PMMO) as an alternative to Cr(VI) ions to enhance the corrosion resistance. The corrosion current density values obtained for PMMO was found to be 2.8% and 1.4% of hydrothermal treated oxide (HTO) and PO respectively after 168 h immersion in 3.5% NaCl solution. The electrochemical studies showed the higher barrier layer resistance for PMMO. The improved corrosion behavior of PMMO was observed based on the damage function calculated. Similar observations were confirmed by continuous salt spray test.     

Corrosion phenomena of alloys and electrode materials in Molten Carbonate Fuel Cells

The corrosion behavior of different alloys and the electrical conductivity of the growing corrosion scales was investigated under simulated and real molten carbonate fuel cell conditions. The corrosion of the usually used NiO cathode material was also investigated. In several exposure tests in oxidizing atmospheres, the FeCrMnNi steel 1.3965 showed a higher corrosion resistance to the aggressive carbonate media than the FeCrNi alloy 1.4404 (SS316L). This superior corrosion resistance is explained by the formation of a mixed (Fe,Ni,Mn)_xCr_3‐xO₄ spinel layer, which reduces the outward diffusion of iron ions more than the mixed (Fe,Ni)Cr₂O₄ spinel formed on austenitic FeCrNi steels. Oxide debris, which spalls off the current collectors, was investigated by XRD. The corrosion scales spalled off mainly at the curved area of the current collector and not at the cathode/current collector interface. The debris was strongly magnetic and consisted of several, in some cases lithiated iron oxides, whereby α‐Fe₂O₃ (hematite), γ‐Fe₂O₃ (maghemite) and Fe₃O₄ (magnetite) formed most of the debris. The investigations of the electrical conductivity of the corrosion scales have shown that the electrical conductivity is limited by the inner, Cr‐containing oxide of the multi‐layered corrosion scale. Cr‐rich alloys which contain more that 20 wt.% Cr showed extremely high ohmic resistance of the corrosion scale, much higher than that of alloys containing less than 20 wt.% Cr due to the formation of highly conductive mixed spinel layers. Small additions of Al in the alloy increased the ohmic resistance of the corrosion scale by many orders of magnitude. Corrosion tests in the fuel environment showed, that common uncoated stainless steels are not suitable for the use as anodic current collectors. The corrosion resistance in the anodic gas atmosphere is determined by the Cr and the Ni contents of the alloy. Only the model alloy NKK which contains 45 wt.% Ni and 30 wt.% Cr showed an acceptable corrosion resistance. The NiO dissolution and the Ni precipitation was investigated by single cell tests. These tests showed, that the replacement of the metallic Ni by an Al support (which is necessary to avoid cracks inside the ceramic) decreases the amount of metallic Ni in the ceramic matrix significantly. Therefore shorting of a fuel cell having a NiO cathode and a LiAlO₂ matrix with an Al support for the mechanical support is not expected in the target lifetime of 40 000 h. The double layer LiCoO₂‐NiO cathode also showed a significant reduction in Ni precipitation after testing. Due to the improvements and development in materials the MCFC‐lifetime has been trebled in the last few years.     

Hot corrosion of YSZ/Al2O3 dispersed NiCrAlY plasma-sprayed coatings in Na2SO4-10 wt.% NaCl melt

This study examines the effect of ytrria stabilized zirconia (YSZ) dispersion on hot corrosion behaviour of NiCrAlY bond coat. Hot corrosion studies were conducted on NiCrAlY and NiCrAlY containing 25, 50 and 75 wt.% YSZ coatings obtained through the air plasma spray technique, in Na₂SO₄ + 10 wt.% NaCl environment at 800 °C. The results show that YSZ dispersion lowers the overall hot corrosion tendency of the NiCrAlY, though it enhances the inherent hot corrosion tendency of its metallic constituent (NiCrAlY). Furthermore, there exists a threshold oxide level beyond which it adversely affects the hot corrosion of the coating.     

CHARACTERISTICS AND FORMATION MECHANISM OF CORROSION SCALES ON LOW--CHROMIUM X65 STEELS IN CO2 ENVIRONMENT

利用高温高压CO₂腐蚀模拟实验以及ESEM, EDS, XPS和SEM等分析技术, 研究了4种不同含Cr量的X65管线钢的腐蚀速率、腐蚀形态和腐蚀产物膜结构特征. 结果表明: 含Cr量高的钢平均腐蚀速率小, 无Cr和含1\%Cr的钢的腐蚀形态为局部腐蚀, 含3%和5%Cr的钢的腐蚀形态为全面腐蚀. 在高温高压CO₂腐蚀环境中, 含Cr钢的腐蚀产物膜为FeCO₃和Cr(OH)₃竞争沉积形成的多层结构, 其中1Cr-X65和3Cr-X65的腐蚀膜具有3层结构, 5Cr-X65的腐蚀膜是双层结构. Cr在腐蚀产物膜层中出现局部富集, 远高于基体中的Cr含量. 高含Cr量使腐蚀产物膜中的Cr(OH)₃含量高, 并提高了腐蚀膜的保护性能, 从而引起腐蚀形态发生转变, 腐蚀速率降低. FeCO₃和Cr(OH)₃共沉积层膜对低铬钢的抗CO₂腐蚀性能具有关键的影响.     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

Effect of carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.