Initiation and growth of iron metal dusting in CO-H2-H2O gas mixtures

The initiation and growth of iron metal dusting in CO-H₂-H₂O gas mixtures at 700 °C were investigated by surface observations of very early stages of the reaction. At first, iron was supersaturated with dissolved carbon and its surface became facetted. The nucleation of graphite and cementite depended on the surface crystallographic orientation. A fine grain structure at ground surfaces and a high carbon activity accelerated cementite nucleation. Further carburisation resulted in the formation of particulate areas mixed with deposited graphite, which accelerated the spallation of cementite and the protrusion of round particles. In some areas, large graphite mounds and bulk graphite were formed on the surface. Filamentous carbon was found in particulate areas and surrounding the graphite mounds. Based on these observations, a possible process of iron metal dusting was discussed.     

Effect of copper on metal dusting of austenitic stainless steels

Three steels, 304SS, 310SS and 800H, were alloyed with 5%, 10%, and 20% (by weight) copper, and then exposed to 68%CO-31%H₂-1%H₂O gas at 680 °C (a_C = 19 and p_O2=5.4×10^-25 atm) under thermal cycling conditions. Kinetic measurements showed that copper-free alloys all dusted, with 304SS experiencing the greatest metal wastage. Copper additions did not have any effect on metal wastage of 304SS, but reduced the attack on 310SS and 800H markedly at levels of 5% and 10%. However, increasing the copper content to 20% produced large copper-rich precipitates which accelerated dusting by promoting internal graphitisation.Dusting was associated with surface coking. When pitting occurred, on copper-free alloys and on copper containing 304SS, large coke structures grew above the pits. Internal grain boundary carburisation always took place, and intragranular carbides also precipitated when dusting occurred. A lamellar surface layer of internally precipitated spinel and austenite also developed in association with dusting. The copper effect is discussed in terms of its alloy solubility and its known beneficial effect in Ni-Cu binaries.     

High temperature corrosion of cast heat resisting steels in CO + CO2 gas mixtures

Two commercial variants of the cast heat resistant grade HP40Nb (Fe-25Cr-35Ni, Nb modified) were exposed to CO/CO₂ gases at 982 and 1080 °C in order to simulate exposure to the carbon and oxygen potentials realised in steam reformers under normal and overheated conditions. Both alloys developed external chromium-rich oxide scales, intradendritic silica precipitates and interdendritic oxide protrusions where primary, interdendritic carbides were oxidised in situ. Surprisingly, the lower silicon content alloy developed a more continuous internal silica layer, thereby slowing external scaling. Intradendritic oxidation was fast in both alloys, and is attributed to interfacial oxygen diffusion. Both alloys underwent rapid internal carburisation, indicating that their oxide scales failed to prevent carbon access to the underlying alloys under these reaction conditions.     

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.     

A correlation between phosphorous impurity in stainless steel and a second anodic current maximum in H2SO4

Phosphorous as a minor element (0.03%) in AISI 304 austenitic stainless steel greatly affects the polarization and corrosion behavior in sulfuric acid solution. The presence of P in stainless steel created a second current maximum in the anodic polarization curve and the current increased with increasing aging in the solution. An adhesive corrosion surface layer, rich with phosphate, formed on the surface of a P-containing steel during active dissolution. The layer lowered the cathodic Tafel slope at low current densities, and is likely, due to a change in hydrogen evolution mechanism. Phosphorous increases the H-adsorbed (and/or absorbed) atoms on the surface, leading to the appearance of a second anodic current peak that is interpreted as re-oxidation of hydrogen atoms. Also, P shifted the corrosion potential to the noble side, decreased effectively the active anodic dissolution, and lowered the corrosion rate.     

Tribocorrosion of 316L stainless steel and TA6V4 alloy in H2SO4 media

Tribocorrosion of stainless steel 316L and titanium alloy TA6V4 has been conducted in a sulphuric acid solution using an apparatus designed and built for evaluating the joint action of corrosion and wear. The material electrochemical and wear behaviours have been investigated during friction tests under electrochemical potential control. The specimens have been submitted to friction against an alumina ball under cathodic, free and anodic potentials. The friction coefficient, the wear rate and the current density were measured and the obtained results were discussed in terms of passivating film stability and repassivation kinetics.     

Oxidation of 310 steel in H2O/O2 mixtures at 600 °C: the effect of water-vapour-enhanced chromium evaporation

The oxidation of type 310 stainless steel was investigated at 600 °C in the presence of O₂ and O₂+10% and 40% H₂O. The effect of gas velocity was studied. The oxidized samples were investigated by grazing angle X-ray diffraction, SEM/EDX and SAM. The addition of H₂O to O₂ resulted in a change of oxidation behaviour. A strong dependence on flow rate was observed in O₂/H₂O mixtures. At low flow rates a thin (30-50 nm) protective α-(Cr,Fe)₂O₃ formed, the outer part being depleted in chromium. When the flow rate was increased beyond a critical value the protective oxide failed. Under these conditions ?5 μm thick α-Fe₂O₃/(Cr,Fe)₃O₄, oxide islands formed on the part of the surface corresponding to the centre of the alloy grains. The effect of water vapour is attributed to the water-vapour-assisted evaporation of chromium from the oxide, in the form of a chromium oxide hydroxide, probably CrO₂(OH)₂. The oxidation behaviour is rationalized using a qualitative mechanism proposed previously and parallels that of the 304L alloy.     

Oxidation behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and exhibited the uniform distribution of TiC particles in the matrix. The oxidation behavior of 304SS-2TiC and 304SS-6TiC (all in weight percentage) was compared with that of 304SS at 850 °C in air for 96 h using thermogravimetry analysis. For 304SS, the rate of weight gain was very slow initially, but accelerated suddenly to a very high level, forming breakaway oxidation. The addition of TiC particles to 304SS resulted in no breakaway oxidation and maintained a low oxidation rate in the whole reaction time investigated. Examination of oxide scale morphology and cross-section analysis by scanning electron microscopy and optical microscopy showed a significant scale spallation and a deep oxide penetration in the case of 304SS, but a rather continuous, dense and adherent oxide layer formed on the surface of TiC particle-reinforced alloys. XRD analysis revealed the presence of Cr₂O₃ together with spinel-type oxides in the oxide scale. For TiC-containing alloys, fine TiO₂ was also found on the surface and the amount of this oxide increased with TiC addition. The TiC addition developed finer matrix structure before oxidation, which accelerates chromium diffusion. As a result, scale adherence was improved and oxidation resistance was increased.     

Impact of oxyfuel atmospheres H2O/CO2/O2 and H2O/CO2 on the oxidation of ferritic–martensitic and austenitic steels

In future power plant technologies, oxyfuel, steels are subjected to steam rich and carbon dioxide rich combustion gases. The effect of simulated combustion gases H₂O/CO₂/O₂ (30/69/1 mol%) and H₂O/CO₂ (30/70 mol%) on the corrosion behavior of low alloyed steels, 9–12% chromium steels and an austenitic steel were studied. It was discovered that the formation of protective chromium rich oxides is hampered due to the carburization of the base material and the formation of chromium rich carbides. The kinetics of corrosion and carburization are quantified. The effect of temperature and the effect of gas pressure are analyzed statistically.     

Application of polyaniline to galvanic anodic protection on stainless steel in H2SO4 solutions

A stand-alone polyaniline (PANI) film electrode was made and then applied to protect 1Cr13 stainless steel (a type of stainless steel typically used in China, 0–15% C, 13% Cr) from corrosion in highly acidic solution, up to 5 M H₂SO₄. The stand-alone PANI electrode and 1Cr13 were coupled to study their galvanic interactions. PANI is a cathode while 1Cr13 is an anodic. The results indicate that PANI with a certain area can quickly passivate the stainless steel and effectively maintain the steady passive state for a long period of time. This specific method of PANI preventing 1Cr13 from corrosion is called galvanic anodic protection (GAP). Moreover, the efficiency, mechanism, and security of GAP were discussed. The results demonstrate that the stand-alone PANI electrode may have a potential application to galvanic anodic protection on stainless steel in highly corrosive H₂SO₄ solutions.     

Oxidation of stainless steel 304L in carbon dioxide

Oxidation of 304L stainless steel in a carbon dioxide atmosphere at 10⁵ Pa has been studied. Between 1193 and 1293 K the oxidation kinetics exhibit first a rapid increase, then a parabolic behaviour with apparent activation energy of (209 ± 8) kJ mol⁻¹ and obeys a Langmuir pressure law. After 1.15 mg cm⁻², the kinetics become almost linear.The reaction products are chromia at the grain boundaries, wüstite (Fe_1−xO) on the surface for weight gains greater than 0.30 mg cm⁻² and chromite. The very complex reaction mechanism takes into account random buckling for weight gains >1.15 mg cm⁻².     

Whisker growth morphology of high temperature oxides grown on 304 stainless steel

During the high temperature oxidation of as-cast Type 304 stainless steel there is a complex growth pattern of the oxide layers which is shown to be dependent on; alloy composition and local segregation, surface finish, temperature and atmosphere. Of interest is the appearance of whisker growth morphologies which grow in excess of 10 μm in length and at random orientations to the sample surface. This paper illustrates these features and discusses the formation mechanism in terms of the oxide microstructure.     

Notched tensile tests of cold-rolled 304L stainless steel in 40 wt.% 80 °C MgCl2 solution

The effects of cold-rolling (20% thickness reduction) and sensitization treatment (600 °C/10 h) on the microstructure, tensile properties and susceptibility to stress corrosion cracking of 304 stainless steel in 80 °C MgCl₂ (40 wt.%) solution were investigated. The increase in hydrogen traps, which retarded hydrogen diffusion to the strained region, accounted for the low loss in notched tensile strength (NTS) of such a cold-rolled specimen, as compared to the solution-treated specimen in the corrosive environment. By contrast, the high NTS loss of sensitized specimens in MgCl₂ solution was attributed mainly to the formation of stress-induced martensite near grain boundary regions.     

Oxidation of 316 stainless steel in supercritical water

The oxide scales of 316 stainless steel (316 SS) have been examined after exposure to supercritical water (SCW) with 2.0% H₂O₂ for up to 250 h. The exposed samples were analyzed using weight measurement, scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). It was found that mass gain of all samples increased with increasing temperature and exposure time. Higher temperature SCW resulted in rougher surfaces and thicker oxide scales. Duplex layer oxide structures with Ni-enrichment at the oxide/metal interface developed on all samples exposed to SCW, which were identified as Fe₂O₃/Fe₃O₄ + spinel/Cr₂O₃/Ni-enrichment/316 SS from the outer to inner layer. The possible oxidation mechanisms are also discussed.     

Susceptibility of stainless steel alloys to crevice corrosion in ClO2 bleach plants

Stainless steels, including duplex stainless steels, are extensively used for equipment in pulp bleaching plants. One serious corrosion problem in chlorine dioxide bleach plants is crevice corrosion of stainless steels, which is frequently the factor that limits their use in bleach plants. Crevice corrosion susceptibility of alloys depends on various environmental factors including temperature, chemical composition of environment and resulting oxidation potential of system. Upsets in the bleaching process can dramatically change the corrosivity of the bleaching solutions leading to temperatures and chemical concentrations higher than those normally observed in the bleach process. When the environmental limits are exceeded the process equipment made of stainless steel can be severely affected. Environmental limits for crevice corrosion susceptibility of eight stainless steel alloys with PRE numbers ranging from 27 to 55 were determined in chlorine dioxide environments. Alloys used in this study included austenitic, ferritic-austenitic (duplex), and superaustenitic stainless steels. The performance of the different stainless steel alloys mostly followed the PRE numbers for the respective alloys. The 654SMO alloy with the highest PRE number of 55 showed the highest resistance to crevice corrosion in this environment. Under the most aggressive chlorine dioxide bleach plant conditions tested, even alloys Nicr3127 and 654SMO with PRE numbers 51 and 55 respectively were susceptible to crevice corrosion attack. The two factors that seem to contribute the most to crevice corrosion and pitting in the investigated environments are temperature and potential.     

Corrosion behaviour of micro-plasma arc welded stainless steels in H3PO4 under flowing conditions at different temperatures

This paper studies the general corrosion behaviour of the micro-plasma arc welded AISI 316L stainless steel in phosphoric acid at different temperatures (25–60 °C) and at a Reynolds number of 1456. Galvanic corrosion has been studied using zero-resistance ammeter (ZRA) measurements and polarization curves (by the mixed potential theory). Results show that the microstructure of the stainless steel is modified due to the micro-plasma arc welding procedure. Coupled current density values obtained from polarization curves increase with temperature. ZRA tests present the highest i_G values at 60 °C; however, the values are very close to zero for all the temperatures studied. This is in agreement with the low value of the compatibility limit and of the parameter which evaluates the importance of the galvanic phenomenon. Both techniques present the most positive potentials at the highest temperature. This study reveals that micro-plasma arc welded AISI 316L stainless steels are appropriated working in the studied H₃PO₄ media from a corrosion point of view for all the temperatures analysed.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

Tribocorrosion behaviour of low temperature plasma carburised 316L stainless steel in 0.5 M NaCl solution

Experiments have been carried out to study the tribocorrosion behaviour of low temperature plasma carburised 316L stainless steel under unidirectional sliding in 0.5 M NaCl solution, using a pin-on-disk tribometer integrated with a potentiostat for electrochemical control. It is found that the carburised layer exhibits much better resistance to material removal than the untreated specimen, particularly at anodic potentials. No corrosion pits are observed inside the wear track on the carburised specimen at anodic potentials as high as 750 mV (SCE). The results are discussed in terms of the relative contribution of wear and corrosion to overall material removal by tribocorrosion.     

Acoustic emission monitoring of slow strain rate tensile tests of 304L stainless steel in supercritical water environment

An experimental set-up has been developed to perform slow strain rate tensile (SSRT) tests on tubular 304L stainless steel (SS) specimens in supercritical water (SCW) environment (550 °C, 250 bar). The supercritical water is circulated inside an internally pressurized tubular specimen mounted into a universal mechanical test rig and heated by a single loop resistance heating furnace. The set-up enables in situ monitoring of acoustic emission and electrochemical potential during the SSRT test. The SCW environment is found to significantly influence the mechanical performance of the material as a result of corrosion processes. A correlation between acoustic emission response and change of electrochemical potential is revealed. The findings are compared with preliminary results of tests performed on bulk SS specimens under the same condition in a commercial autoclave. The advantages and potentialities of both set-ups are discussed.