Corrosion behavior of various steels in the simulated fluidized bed boiler environment

The corrosion behavior of various austenitic stainless steels and high-alloy steels has been studied in simulated fluidized bed boiler environment to develop a new corrosion resistant austenitic stainless steel for the superheater tube. The superheater is usually not installed within the bed position, which is different from the evaporator installed within the bed position. Therefore, the superheater tubes are exposed to an oxidizing environment; but it is also necessary to estimate the corrosion resistance of the steels in a reducing environment. It is already known that the high temperature corrosion behavior in conditions where CaSO₄ is coated on the steels is more important than the erosion of the superheater tubes. The main results in this present study are as follows: The Nb bearing steels and low C steels showed good resistance to high-temperature corrosion in CaSO₄/CaO, e.g. 347, 304L and HR3C. The corrosion rate of all steels used increased with increase in temperature, particularly at temperatures higher than 650°C. Internal penetration was not detected at temperatures lower than 550°C, but it was detected at temperatures higher than 600°C, in particular, higher than 650°C. The corrosion thickness loss was almost the same as the internal penetration depth at 700 and 750°C in the 300 series steels placed in CaSO₄/CaO, including the fine grained 347 steel, while the internal penetration depth was larger than the corrosion thickness loss in high-alloyed materials such as Alloy 800 and 310 steels. At temperatures higher than 800°C, the same result was also obtained for the fine grained 347 steel. The corrosion during exposure to oxidizing or reducing gases without CaSO₄/CaO or CaS was slight, but when the test specimens were placed in CaSO₄/CaO or CaS, the corrosion rate sharply increased, regardless of the atmospheric gas composition. Cr, Si, Mn (less than 5 %), Mo and Nb are beneficial elements while C, Cu and Al are harmful elements. From the above results, the following steel was developed for high temperature corrosion resistance in CaSO₄/CaO: low C-22/25Cr-17/25Ni-3/5Mn-(2Mo)-Nb-0.08/0.2N-Al-(B).     

Corrosion of nickel-base and iron-base alloys in a simulated fluidized-bed coal-combustion environment

The modes of initiation and propagation of corrosion attack on a series of high-temperature alloys were studied in synthetic gas mixtures at 900°C. The gas mixtures were intended to simulate the oxygen and sulfur partial pressures experienced in reducing zones in a coal-fired fluidized-bed combustor and comprised mixtures of CO, CO₂, and SO₂. The alloys studied were candidates for in-bed heat exchanger tubing for an air-heater cycle operating at 843°C and 300–500 psig and so ranged from type 300-series stainless steels to nickel-base alloys. With the exception of two FeCrAlY alloys and types 304 and 347 stainless steels, it was found that sulfidation corrosion could be initiated on all the alloys within 0.25 hr; the rate of propagation of the corrosive attack depended on the flux of SO₂ in the environment and on the nickel content of the alloys. The presence of iron in the alloys appeared to slow the initiation of sulfidation, by forming a continuous iron oxide layer. The effects of various alloying additions are discussed, and a schematic model for the initiation of sulfidation is proposed.     

Metal dusting behaviour of Cr-Ni steels and Ni-base alloys in a simulated syngas mixture

Long-term laboratory exposure tests for various Cr and Ni content steels and Ni-base alloys were conducted at 650 °C in a 60vol.%CO-26%H₂-11.5%CO₂-2.5%H₂O gas mixture simulating syngas environments. Upon isothermal heating, alloys with 15% and 20% Cr had many pits on the surface after a brief exposure, while no pit was found on alloys containing of 60% Ni and more than 23% Cr exposed for up to 5000 h. The thermal cycling accelerated pit initiation drastically, resulting that all test specimens except 30%Cr-60%Ni alloy suffered from metal dusting. From a measurement of pit depths, Ni proved to be an effective alloying element to retard the pit growth: growth rate for 75% Ni alloy has achieved double-digit decrease compared to that for 20% Ni. Microscopic observations has revealed that platelet graphite aligned perpendicular at the boundary of gas/metal of pits. The length of the platelet graphite for high Ni alloys was appreciably longer than that for low Ni steels. This can be interpreted from the difference of super saturation of carbon.     

Corrosion of stainless steels in simulated diesel exhaust environment with urea

Laboratory exposures have been performed simulating a selective catalytic reduction (SCR) system with urea injection for NO_x reduction in diesel exhaust after‐treatment. The corrosion behaviour of three ferritic and one austenitic stainless steel was examined using X‐ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Continuous exposure to condensate did not cause any corrosion. Results show that cyclic interaction between high temperature and condensation aggravates the corrosion compared to isothermal exposure at 450 °C. All ferritic alloys exhibited more or less the same behaviour, while the austenitic steel performed better. In fact, the presence of urea decreased the corrosion compared to the environment without urea. The cyclic samples exposed with urea displayed iron sulphate on the surface. The sulphate appeared to decrease the oxide thickness. A sulphur enrichment in the form of sulphide also occurred in the inner chromium‐rich oxides of all cyclically exposed samples, both with and without urea. Thus, sulphidation is presumed to be involved in the corrosion process.     

Corrosion behavior of various steels in black liquor recovery boiler environment

It is important to examine characteristics of ash in order to understand the high temperature corrosion behavior of steels in the black liquor recovery boiler (BLRB) environment. The ash collected from the corroded superheater tubes was mainly composed of alkali-sulfates, alkali-chlorides and alkali-carbonates. The mole ratio of Na/K in the ash from BLRB ranged from 3 to 17. The concentration of sulfates varied over a range from 50 to 90 wt%, and chlorides from 0.1 to 10 wt%. On the base of these results, the corrosion behavior of various steels and the effect of alloying elements on the corrosion behavior of stainless steels in the BLRB environment were studied. From the results, Cr, Mn and Mo were significantly beneficial elements. N and Ni were also beneficial above 600 °C while C was an extremely harmful element. Subsequently, a new corrosion resistant stainless steel was selected as the material for the superheater in the BLRB.     

Corrosion behaviour of corrugated lean duplex stainless steels in simulated concrete pore solutions

This work studies the corrosion behaviour of two corrugated lean duplex stainless steels (SAF 2001 and 2304 grades) in eight alkaline solutions (carbonated and non-carbonated, saturated Ca(OH)₂ solutions with different chloride contents). 2001 stainless steel is a new grade in market because of its composition. 2304 is a grade previously studied under different conditions. However, its use as reinforcement in concrete is new. Studies are carried out by polarization curves following scanning electronic microscopy (SEM) and optical observations. Results are compared to those of carbon steel and austenitic AISI 304 and duplex SAF 2205 under similar conditions. After corrosion tests in alkaline media with chloride, ferrite tends to corrode selectively in 2304 duplex, while austenite corrodes selectively in 2001 under the same conditions. The influence of the duplex microstructure on attack development and morphology is analyzed. The electrochemical parameters obtained from the polarization curves suggest 2001 could replace 304 keeping the structure its corrosion performance (and with clear economical advantages). 2304 shows better corrosion behaviour than the more expensive 304, but somewhat lower than the excellent behaviour shown by 2205.     

Localised corrosion behaviour of alloys 33 and 24 in simulated flue gas desulphurisation environment

Abstract

Potentiodynamic anodic polarisation measurements have been carried out on type 316L stainless steel (as a reference material) and on alloys 33 and 24 in a simulated flue gas desulphurisation environment in order to assess the localised corrosion resistance. The results showed that the pitting corrosion resistance was higher in the case of alloys 33 and 24 than in the reference material owing to the higher contents of nitrogen, chromium, and molybdenum. An accelerated leaching study conducted on the alloys 33 and 24 showed only minor tendencies for the leaching of metal ions at various impressed potentials. Observations by SEM confirmed the lower tendency towards pitting of the alloys 33 and 24.     

Corrosion behaviour investigation of 460 low alloy steels exposed in the natural deep-sea environment

ABSTRACT

The corrosion behaviours of 460 low alloy steels were investigated by the field exposure corrosion experiments with various long periods and 1200–3000?m depths in the natural deep-sea environment. XRD analysis results indicate that corrosion products of β-FeOOH, γ-FeOOH and Fe₃O₄ increase and the corrosion product of α-FeOOH decreases with the exposure sea depth extending. Electrochemical experiment results reveal the corrosion status is aggravated with the exposure sea depth extending: corrosion potentials shift negatively to a minimum self-corrosion potential of ?0.590?V (vs. SCE) and charge transfer resistances decrease to a minimum of 278?Ω?cm². Corrosion behaviours analysis shows that the average corrosion rate decreases with the time increasing and reaches a maximum value of 101?μm?a^?1. The maximum pitting depth varies between 63.0?μm and 138?μm. Gray relational analysis shows that the corrosion rate of 460 steel is most influenced by the pressure and dissolved oxygen concentration of deep sea, exhibiting a positive correlation relationship.     

Corrosion and tribological properties and impact fatigue behaviors of TiN- and DLC-coated stainless steels in a simulated body fluid environment

In this study, the corrosion and tribological properties of TiN and DLC coatings were investigated in a simulated body fluid (SBF) environment. The ball-on-plate impact tests were conducted on the coatings under a combined force of a 700 N static load and a 700 N dynamic impact load for 10,000 impacting cycles. The results indicated that the TiN and DLC coatings could achieve a higher corrosion polarization resistance and a more stable corrosion potential in the SBF environment than the uncoated stainless steel substrate SS316L. The good corrosion protection performance of TiN could be due to the formation of a Ti-O passive layer on the coating surface, which protected the coating from further corrosion. The superior corrosion property of the DLC coating was likely attributed to its chemical inertness under the SBF condition. The TiN and DLC coatings also exhibited an excellent wear resistance and chemical stability during the sliding tests against a high density polyethylene (HDPE) biomaterial. Compared to the DLC coating, the TiN coating has a better compatibility with the HDPE. However, the impact tests showed that the fatigue cracks and the coating chipping occurred on the TiN coating but not on the DLC coating.     

Corrosion behaviour of tool steels in tannic acids

It is well known that cutting knifes in the wood industry often suffer from corrosion. Investigations showed that the corrosiveness of different wood types is responsible for a major part of the damage, and that different woods have a different corrosive impact. It is revealed that tannin, a water-soluble acid, which can be found in all woods in different concentrations, is the most aggressive acid contained in the wood, and so it is responsible for the main part of the corrosive attack. In view of the above, the ability of different cold-work steels to resist corrosion caused by tannic acid has been investigated. Often corrosion is measured by the mass loss of the sample divided by the surface of the sample under certain conditions during a fixed period of time. One problem with this method is that there is also a weight gain caused by oxidation or by other side reactions at the samples surface. So in some cases the weight of the sample even increases during the testing time, which makes the quantification of the corrosion impossible with this method. Due to the reasons already mentioned, another method was used to quantify the test results. This method is based on inductive coupled plasma–optical emission spectral analyses (ICP-OES), where the released Fe is quantified. The results of the corrosion resistant tests get correlated to the microstructure and the elemental composition of the analysed cold-work steels. The present investigations improve the understanding of important material parameters to enhance the corrosion resistance against tannin. It is revealed that the higher the primary carbide concentration is the more Cr or other passivating elements are necessary in the matrix to show a good protection against corrosion by tannin. Three materials with different elemental compositions have been investigated, and for one of those materials the influence of different heat treatments was also analysed. The investigations could show that annealing parameters and freeze-cycle processing (FCP) have less impact on the corrosion resistance to tannin than elemental composition and austenitising temperature.     

Corrosion behaviour of different hot rolled steels

The oxidation-corrosion behaviour of hot rolled alloys was examined by electrochemical impedance spectroscopy. The corrosion behaviour of the non-oxidised alloys was first determined in order to have a reference behaviour. Then, each alloy was oxidised for 1 and 3 days at 650 °C in air and its corrosion behaviour was also determined. For all the alloys, Fe₂O₃ was formed at the scale-gas interface. However, the Fe₂O₃ crystallographic structures varied as a function of the alloy composition. Differences in the corrosion behaviour are due to the thickness, the microstructure and the porosity of the scale. The new graphite chromium iron alloy (Hi-Cr + C) have an oxidation-corrosion behaviour close to the indefinite chill double paired (ICDP) one and is therefore thought to be a good candidate to replace the ICPD alloy. In the case of the high speed steel (HSS) alloy, the oxidation-corrosion kinetics are too slow to prevent sticking problems. The Co addition decreases the corrosion-oxidation rates for the non-oxidised and oxidised samples but this effect is limited in time.     

Corrosion behaviour of stressed magnesium alloys

Potentiodynamic polarisation and impedance measurements are used to examine the corrosion aspects of some Mg-based alloys, which were previously stressed in order to established the effect of mechanical deformation on surface electrochemical reactions. A first approach was made for the unstressed alloys. The electrochemical tests were carried out in a sodium borate buffer solution.     

Corrosion behaviour of Softmagnetic iron-nickel alloys

The corrosion behaviour of different FeNi alloys containing between 35 and 82 wt% nickel has been investigated in a humidity test between room temperature and 80°C. It has been found that higher nickel contents improve the corrosion behaviour. The poor corrosion behaviour of the low nickel containing alloys can be improved by grinding and cleaning the surface. With approximately 40 wt% nickel and 8 wt% chromium a softmagnetic alloy has been found with a corrosion behaviour similar to a high nickel alloy with approximately 80 wt% nickel. This FeNiCr alloy achieves also approximately the same saturation induction of 0.75 T as the 80 wt% nickel containing Fe-Ni-Mo alloy. The sufficient low coercive force of less than 2.5 A/m after an appropriate heat treatment and the adequate corrosion behaviour make this alloy suitable as a material for stamped parts in electromagnetic relays.     

Corrosion simulation of carbon steels in atmospheric environment

Atmospheric corrosion simulation of carbon steels has been performed in order to clarify the atmospheric corrosion mechanism of carbon steel. The corrosion simulation was conducted in a chamber by controlling the environmental factors such as temperature, relative humidity and temperature of carbon steels. When the corrosion simulation was conducted using only the data of temperature and relative humidity, although the carbon steel in the corrosion simulation showed almost the same corrosion behavior as that in outdoor environment, the corroded area of the former was smaller than that of the latter. In order to simulate dew condensation, the temperature of carbon steel was controlled by a cooling-system. As a result of the corrosion simulation, both the corrosion behavior and the corroded area were almost the same as those in outdoor environment. It was concluded that the initial atmospheric corrosion of carbon steel in the shielded environment could be simulated in the chamber by controlling the temperature, the relative humidity and the temperature of materials.     

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.     

Corrosion behaviour of Nickel-aluminium alloys in molten carbonate

The corrosion behaviour of nickel-aluminium alloys with aluminium contents of 2-50%, in molten carbonate has been investigated with electrochemical techniques in combination with post-test analysis of quenched specimens. For the 2 to 10% aluminium alloys a type of aluminium oxide is formed along the grain boundaries of the base metal at potentials of -1100 and -900 mV. At potentials of -700 nmV and more anodic an outer oxide scale is formed and also aluminium oxide formation takes place along the grain boundaries. The oxide scale contained nickel, aluminium and oxygen. The only corrosion product that could he detected by X-ray diffraction on specimens quenched after polarisation at -700 mV or more anodic is NiO; no type of aluminium oxide could be detected, probably due to the small amount of aluminium oxide formed. For the 20% aluminium alloy a type of aluminium oxide is formed along the grain boundaries at -1100 and -900 mV. After longer polarisation times also the aluminium in the interior of the grains is oxidised, which results in a two-layer microstructure: an outer layer of pure nickel, and an inner layer that is a honeycomb network of nickel filled with aluminium, oxygen and nickel, while high amounts of aluminium and oxygen could be detected along the grain boundaries. The amount of corrosion product was too small to be detected by X-ray diffraction. At potentials of -700 mV and more anodic, an oxide scale of irregular thickness is formed, but no oxide could be observed along the grain boundaries. The only corrosion product detected by X-ray diffraction on specimens quenched after polarisation at potentials in this range is nickel oxide. On the 50% aluminium alloy a continuous oxide layer is formed at all potentials. The only corrosion product formed at all potentials is α-LiAlO₂; the stable form of LiAlO₂ is γ-LiAlO₂. α-LiAlO₂ is probably an intermediate product that is slowly transformed to γ-LiAlO₂. The oxide layer provides good protection against further corrosion of the base material. The quasi-stationary polarisation curves are very similar to those of pure nickel, because no protective oxide scales are formed on the 2 to 20% aluminium alloys during preconditioning at -1100 mV.     

Corrosion behaviour of Nickel-chromium alloys in molten carbonate

The corrosion behaviour of nickel-chromium alloys with chromium contents ranging from 10 to 25% in molten carbonate under reducing gas atmospheres was investigated with electrochemical methods (cyclic voltammetry). The oxide scale was investigated, both after quenching from stationary conditions at fixed potentials and shortly after interrupting a cyclic voltammogram. The composition and the properties of the corrosion products, present as oxide layers, are potential-dependent. At potentials in the range of -1500 to -900 mV the layer consists of lithium chromite (LiCrO₂). For 18- and 25%-chromium alloys a continuous layer is formed. The lithium chromite formed on the 10%-chromium alloy at potentials in the same range does not form a continous oxide layer. At potentials in the range of -700 to +100 mV a continuous oxide layer is formed on all alloys. The corrosion product is a cubic solid solution of nickel oxide (NiO) and lithium chromite. In cyclic voltammetry experiments a hexagonal solid solution of lithium chromite and nickel oxide is formed at -700 mV. The hexagonal solid solution is an intermediate corrosion product between lithium chromite, which is stable at more cathodic potentials and a cubic solid solution of nickel oxide and lithium chromite which is stable at more anodic potentials. On the 10% chromium alloy with a non-continuous oxide scale, a cubic solid solution is formed at -500 mV. At about -300 mV the chromate formation and dissolution starts. For the 18- and 2.5%-chromium alloys, on which a continuous lithium chromite-scale is formed at the open circuit potential, a hexagonal solid solution of lithium chromite and nickel oxide is formed during the anodic scan at -700 mV. When the chromate formation and dissolution starts at about -300 mV, the chromium content of the scale decreases, the nickel content increases and the hexagonal solid solution becomes a cubic solid solution. The cathodic scan is very similar to the cathodic scan of pure nickel. During the cathodic scan the oxide scale mainly consists of a nickel-rich cubic solid solution of nickel oxide and lithium chromite because chromium has dissolved as chromate ions during the preceding anodic scan. The reactions are essentially the same as those on pure nickel [1, 2, 3].     

Corrosion behaviour of aluminium-magnesium alloys in molten sodium

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Corrosion fatigue crack growth behaviour of naval steels

Fatigue crack growth behaviour of two varieties of HSLA steels used in naval structural applications have been evaluated in air and 3.5% NaCl solution. In air both the HSLA steels showed similar resistance to fatigue crack growth. However, in 3.5% NaCl, the fatigue crack growth resistance of HSLA-80 steel was superior to that of HSLA-100. The apparent inferiority of HSLA-100 to corrosion fatigue crack growth resistance is attributed to rapid film formation and rupture, and occurrence of planar modes of failure. Effect of R-ratio on air fatigue and corrosion fatigue crack growth behaviour is rationalised by the concept of crack closure. Effect of cyclic frequency on corrosion fatigue behaviour is examined. It is noted that the mechanism of corrosion fatigue crack growth for the two HSLA steels changes with attendant change in the Paris slope. This leads to increase or decrease of crack growth rates, depending up on the ΔK range of interest.     

Mg-Al系和Mg-RE系合金在NaCl溶液中的腐蚀电化学行为

采用盐水浸泡法和电化学方法研究AZ91D镁合金和稀土镁合金Mg-3.0Nd-0.2Zn-0.4Zr(NZK)在5%(质量分数)NaCl溶液中的腐蚀行为.结果表明:NZK的腐蚀速率仅为AZ91D镁合金的1/2,这主要是由于NZK中阴阳极之间的电位差和阴阳极面积比小于AZ91D的所致;AZ91D的腐蚀主要集中在局部区域,形成较深的腐蚀坑,而NZK的腐蚀沿合金表面进行,形成比较均匀的浅腐蚀区域;NZK的腐蚀电位低于AZ91D的腐蚀电位; NZK稀土镁合金的耐点蚀能力高于AZ91D的.