三种β-NiAl相材料的短期热腐蚀行为比较研究

在Ni-30Al(mass%,下同)和Ni-20Cr-10Al铸态合金上进行粉末包埋渗铝,分别得到纯β-NiAl相涂层和β-(Ni,Cr)Al相涂层.在这两种渗铝试样和铸态Ni-20Cr-13Al合金(含β-NiAl、γ′-Ni3Al、α-Cr相)上涂覆了硫酸钠薄膜,进行900℃热腐蚀测试.经过10小时热腐蚀测试后,三种试样表面均生成Al2O3膜.腐蚀动力学分析表明,Cr对提高材料的抗热腐蚀性能是有益的;Ni-20Cr-10Al渗铝涂层试样的增重最低,仅为铸态Ni-20Cr-13Al合金试样增重的80%左右,Ni-30Al渗铝涂层试样增重的50%左右.     

Effect of alloy element contents on caustic stress corrosion cracking of several stainless steels

The stress corrosion cracking behavior in caustic solutions (200 g/l sodium hydroxide, 10 g/l sodium chloride) of three austenitic (18Cr-10Ni-2.5Mo, 20Cr-25Ni-4.5Mo, 27Cr-31Ni-3.5Mo) and three duplex (23Cr-4Ni, 22Cr-5Ni-3Mo, 25Cr-7Ni-4Mo-N) stainless steels was examined. U-bend and Slow Strain Rate (SSR) tests were performed at 200–250°C. The negative influence of nickel in the lower range content for the 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo has been shown; when the nickel content is significantly increased (>30%), as in the case of the steel 27Cr-31Ni-3.5Mo, an increase of SCC resistance has been detected. The negative effect of molybdenum, mainly on the behaviour of duplex stainless steels, has also been evidenced. The duplex stainless steels show better caustic SCC resistance than austenitic stainless steels type 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo. The best behaviour has been found for the less-alloyed steel 23Cr-4Ni.     

Effect of carbon and silicon on the structure and corrosion resistance of 18cr-8ni cast steel in concentrated nitric acid solutions

The solutions of nitric acid can be divided into two concentration ranges: range I from 0 to 60–70% and range II from 80 to 100%. The 18Cr-8Ni cast steel with about 4% Si and up to 0.12% C exhibits very good corrosion resistance in concentrated nitric acid solutions (up to 98%); however, it is slightly inferior to a corresponding 18 Cr-13 Ni grade. The investigations were concentrated on the effect of the carbon and silicon contents on structure and corrosion resistance of an 18 Cr-8 Ni cast steel in cold nitric acid solutions. The following materials were used in these studies: a cast stainless steel of type 18Cr-8Ni with 0.48–6.75% Si and 0.021–0.220% C. A detailed analysis of the main result shows that

• 1. in cold HNO₃ solutions of concentrations above 80% the silicon content in a cast steel must not be lower than 2.5-3.0%;
• 2. the 18Cr-8Ni cast steel containing more than 4% Si and up to 0.12-0.13% carbon is practically not susceptible to corrosion in cold HNO₃ over the whole range of concentrations.

     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Kinetik der Lochfraßkorrosion bei konstantem Potential

Kinetics of the pitting corrosion at constant potential Measurements of the depth and diameter of corrosion pits on nickel and 18Cr-12Ni-2Mo steel were performed. It was found that the exponent b in equation i ～ t^b depends upon the Cl^?/SO₄^2? ratio in the electrolyte. Kinetics of the pit growth at constant potential is discussed, and models for the development of Pits in shape of caps and cylinders are given. Experimental results fit in with those models.     

Characterization of plasma sprayed Fe-10Cr-10Mo-(C,B) amorphous coatings

Alloys of Fe-10Cr-10Mo containing a large amount of carbon and/or boron were plasma sprayed by low-pressure plasma spraying (LPPS) and high-energy plasma spraying (HPS). The as-sprayed coatings obtained by the LPPS process are composed of only an amorphous phase, while as-sprayed coatings obtained by the HPS process are a mixture of amorphous and crystalline phases. The amorphous phase in these coatings crystallizes on tempering at about 773 to 873 K, and the crystallization temperatures depend on the content of carbon and boron. Thermal stability of the amorphous phase containing boron is higher than those phases containing carbon. A very fine mixed structure of ferrite and carbide, borocarbide, or boride is formed by decomposition of the amorphous phase, bringing about a hardness of 1200 to 1400 DPN (Vickers hardness). The coatings containing carbon retain a hardness of more than 1000 DPN, even on tempering at temperatures of 1073 K or higher. The anodic polarization behavior of the coatings exhibits an activation-passivation transition in 1N H₂SO₄ solution. The active and passive current densities of the as-sprayed amorphous and tempered crystalline coatings containing carbon is lower than the coatings containing boron. The corrosion resistance of the as-sprayed and crystallized coatings containing carbon is superior to a SUS316L stainless steel coating.     

The characteristics of high temperature Gas Nitriding heat treatment and tempering in 0.8Mo added 17Cr-1Ni-0.5C

The effect of the High Temperature Gas Nitriding (HTGN) and tempering treatment of 17Cr-1Ni-0.5C-0.8Mo (CNMo) steel was experimentally investigated. The HTGN was carried out at 1050 °C for 1 h in a gaseous atmosphere containing 98.07 kPa of nitrogen. Chromium nitrides in the austenite and martensite phase appeared at the nitrogen-permeated surface layer after the HTGN treatment. The hardness of the outmost surface of the HTGN treated specimen measured 708 Hv. When it was tempered at 500 °C for 1 h, the hardness of the outmost surface was 763 Hv as a result of the precipitation of mostly micro Cr₂N, which was densely packed with a small amount of Cr₂₃C₆ and the secondary hardening effect. In addition, an improvement in the corrosion resistance was observed in the tempered specimen.     

New stainless steels for sea-water applications. Part II: Comparison of corrosion and mechanical properties of laboratory and commercial ELI ferritic and superaustenitic stainless steels

This paper represents a follow-up to the first part of the work on new stainless steels for sea-water service. Four laboratory ELI (Extra Low Interstitial) ferritic stainless steels (types 25 Cr-4 Ni-4 Mo), two commercial ELI ferritic stainless steels (types 25 Cr-4 Ni-4 Mo? Ti and 26 Cr-2.5 Ni-3 Mo? Ti) and two highly alloyed austenitic stainless steels (types 20 Cr-25 Ni-4.5 Mo? Cu and 20 Cr-18 Ni-6 Mo? N) have been investigated. With a view to establish the performance of these new alloys in chloride containing environments, systematic electrochemical and laboratory exposure tests have been carried out to define how various factors affect its susceptibility to intergranular, pitting, crevice and stress corrosion. Tension tests were also performed. From the comparison of the localized corrosion resistance and mechanical properties it has been concluded that the laboratory Ti, Ti + Nb or Nb stabilized ELI ferritic stainless steels and the commercial type 25 Cr-4 Ni-4 Mo? Ti of analogous composition could be a valuable alternative to the more expensive highly alloyed stainless steel type 20 Cr-25 Ni-4.5 Mo? Cu which has been especially developed and already used for industrial sea-water applications.     

Corrosion Behavior of 9Cr-1Mo Steel in Sulfur Dioxide Environment

Corrosion behavior of annealed 9Cr-1Mo steel was studied in SO₂ environment at 1173 K, at flow rates from 8.33 × 10^?7 to 33.33 × 10^?7 m³/s, and parabolic rate law was followed. The rate constants were found to be independent of flow rate, within the range of flow rate investigated. Corrosion at temperatures from 973 to 1173 K, at a constant flow rate of 16.66 × 10^?7 m³/s, at 1 atmospheric pressure, for 6 h also exhibited parabolic law, however, the rate constants were observed to increase significantly with rise in temperature. The outer layer of the scale formed at 973 K was essentially of iron oxide, with small amount of chromium oxide whereas the inner layer was predominantly of chromium sulphide and chromium oxide. The scale formed at 1173 K was multilayered, in contrast to double layered formed at 973 K and 1073 K. The outer thick layer of the scale formed at 1173 K, consisted of iron oxide followed by thin substrate of chromium sulphide, iron sulphide/iron oxide, and chromium sulphide/chromium oxide toward the substrate. A model is proposed for the process of corrosion of 9Cr-1Mo steel in SO₂ environment, based on the present investigation.     

Effect of yttrium on the sulfidation behavior of Ni-Cr-Al alloys at 700°C

The sulfidation of Ni-10Cr-5Al, Ni-20Cr-5Al, and Ni-50Cr-5Al, and of the same alloys containing 1% Y, was studied in 0.1 atm sulfur vapor at 700°C. The sulfidation process followed linear kinetics for all the alloys except Ni-50Cr-5Al-1Y, and possibly Ni-50Cr-5Al, which followed the parabolic law. The reaction rates decreased with increasing chromium content in alloys without yttrium, and the addition of yttrium reduced the rates by at least a factor of two for the alloys containing 10 and 20% Cr and by an order of magnitude for Ni-50Cr-5Al. Alloys containing 10 and 20% Cr (with and without yttrium) formed duplex scales consisting of an outer layer of NiS_1.03 and an inner lamellar layer of a very fine mixture of Cr₂S₃ and A1₂O₃ in a matrix of NiS_1.03. The two alloys containing 50% Cr formed only a compact layer of Cr₂S₃, which was brittle and spalled during cooling. The lamellae in the duplex scales were parallel to the specimen surface and bent around corners. The lamellae were thicker than those on Ni-Al binary alloys. The lamellae were also thicker in scales on the 20% Cr alloy than on the 10% Cr alloy. The presence of yttrium refined the lamellae and increased the lamellae density near the scale/metal interface in the 10% alloy, but in the 20% Cr alloy the lammellae were thicker and more closely spaced. Platinum markers were found in the inner portion of the exterior NiS_1.03 layer close to the lamellar zone. A counter-current diffusion mechanism is proposed involving outward cation diffusion and inward sulfur diffusion, although diffusion was not rate controlling for alloys containing 10 and 20% Cr. Auger analysis of scales formed on Ni-50Cr-1Y showed an even distribution of yttrium throughout the layer of Cr₂S₃, suggesting that some yttrium dissolved in the sulfide. The reduced sulfidation rate of samples containing yttrium is explained by the possible dissolution of yttrium as a donor. The presence of Y⁴⁺ would then decrease the concentration of interstitial chromium ions in the N-type layer of Cr₂S₃, which would decrease the reaction rate.     

Oxidation and hot corrosion of nickel-based alloys containing molybdenum

The oxidation of Ni-15% CrMo alloys has been studied at 900°C in flowing and static oxygen atmospheres. In flowing atmospheres, molybdenum has no effect: all the alloys oxidize in a protective manner. However, in static atmospheres the oxidation rate of alloys with > 3% Mo eventually accelerates, and catastrophic destruction of the alloy takes place. Under these circumstances a molybdenum-rich oxide layer is detected adjacent to the alloy.When specimens are coated with Na₂SO₄ prior to oxidation, alloys containing > 3% Mo again suffer catastrophic degradation, in either flowing or static atmospheres, and again a molybdenum-rich oxide layer is observed. This suggests that the principal role of the salt coating is to prevent the escape of MoO₃ to the atmosphere.The morphology of the attack in the rapid propagation region is very similar to that obtained in pre-sulphidation/oxidation experiments in the absence of salt and that particular aspect of the reaction is not greatly affected by molybdenum; the aluminium content is more important in determining the nature of the propagation.Attack similar to that exhibited by molybdenum-containing alloys can be obtained with Ni-15%Cr binary alloys in the presence of MoO₃ vapour in the atmosphere, and this might suggest that the MoO₃ reacted with the Na₂SO₄ to produce an acid (SO₃-rich) salt, leading to acidic fluxing. However, very similar types of attack were obtained when Na₂MoO₄ was added to the Na₂SO₄, and this should not have affected the acidity of the salt at all.These experiments suggest that acidic fluxing may not be important in the hot corrosion of alloys of this type (molybdenum-containing) and that when catastrophic corrosion is observed, its initiation is probably due to the formation of a molybdenum-rich oxide layer, molten during the reaction. There appears to be a threshold molybdenum content below which attack does not occur, and this seems insensitive to an increase in the chromium content from 15 to 25%.     

High-Temperature Corrosion Behavior of Different Regions of Weldment of 2.25Cr-1Mo Steel in SO<Subscript>2</Subscript> + O<Subscript>2</Subscript> Atmosphere

This paper investigates the corrosion behavior of different regions of weldment of 2.25Cr-1Mo steel exposed in mixed oxidation and sulfidation (SO₂ + O₂) environment up to 500 h at 773 K. Microstructural investigation and characterization of oxide scales are done using SEM, TEM, and XRD. The obtained results infer that heat-affected zone corrodes faster than both base and weld metal. The reaction kinetics follows a parabolic growth rate for all regions. The higher corrosion rate of heat-affected zone is attributed to the formation of Cr₂₃C₆ secondary precipitates leading to depletion of protective inner scale of the Cr-rich oxide during welding.     

Increased Lifetime for Biomass and Waste to Energy Power Plant Boilers with HVOF Coatings: High Temperature Corrosion Testing Under Chlorine-Containing Molten Salt

Heat exchanger surfaces of waste to energy and biomass power plant boilers experience often severe corrosion due to very aggressive components in the used fuels. High velocity oxy-fuel (HVOF) coatings offer excellent protection for boiler tubes against high temperature corrosion due to their high density and good adherence to the substrate material. Several thermal spray coatings with high chromium content were sprayed with HVOF technique. Their mechanical properties and high temperature corrosion resistance were tested and analyzed. The coating materials included NiCr, IN625, Ni-21Cr-10W-9Mo-4Cu, and iron-based partly amorphous alloy SHS9172 (Fe-25Cr-15W-12Nb-6Mo). High temperature corrosion testing was performed in NaCl-KCl-Na₂SO₄ salt with controlled H₂O atmosphere at 575 and 625 °C. The corrosion test results of the coatings were compared to corrosion resistance of tube materials (X20, Alloy 263 and Sanicro 25).     

Nickel-chromium plasma spray coatings: A way to enhance degradation resistance of boiler tube steels in boiler environment

Boiler tube steels, namely low carbon steel ASTM-SA-210-Grades A1 (GrA1), 1Cr-0.5Mo steel ASTM-SA213-T-11 (T11), and 2.25Cr-1 Mo steel ASTM-SA213-T-22(T22), were used as substrate steels. Ni-22Cr-10Al-1Y powder was sprayed as a bond coat 150 μm thick before a 200 μm final coating of Ni-20Cr was applied Coatings were characterized prior to testing in the environment of a coal fire boiler. The uncoated and coated steels were inserted in the platen superheater zone of a coal fired boiler at around 755°C for 10 cycles, each 100 h. Coated steels showed lower degradation (erosion-corrosion) rate than uncoated steels showed. The lowest rate was observed in the case of Ni-20Cr coated T11 steel. Among the uncoated steels, the observed rate of degradation was the lowest for the T22 steel.     

The hot corrosion of Ni-based ternary alloys and superalloys for application in gas turbines employing residual fuels

Hot corrosion was studied in melts of Na₂SO₄, NaVO₃ and mixtures thereof at temperatures ranging from 680 to 900°C. The specimens tested were Ni, Ni-10Cr, Ni-22Cr, Ni-30Cr, Ni-20Cr-3Al, Ni-21Cr-0.3Si, Ni-20Cr-5V and IN738 superalloy. The effect of adding Cr to Ni was found to be beneficial in the Na₂SO₄ melt; however, on increasing the VO₃⁻ concentration in the melt, this effect diminished, becoming harmful in pure NaVO₃ due to the formation of the non-protective CrVO₄. Al was found harmful in Na₂SO₄ + NaVO₃ melts. Cr depletion was found in rich VO₃⁻ melts but internal corrosion was more obvious in the SO₄²-rich melts. Corrosion in rich VO₃⁻ melts was aggressive due to the fluxing action of the salt which takes place along internally sulphidized areas. Higher temperatures in NaVO₃ melts increased the corrosion rate of the studied alloys. IN738 superalloy, in comparison with the NiCr-based alloys, suffered tremendous internal attack due to its γ′ precipitates which became sulphidation-prone areas, which in turn were fluxed by the VO₃⁻ melt. Cyclic polarization measurements of the last five mentioned alloys were carried out in Na₂SO₄/10 mole % NaVO₃ melt at 900°C. I_corr was calculated by regression analysis and the results correlated with the other two techniques.     

Ni基合金在900 ℃75%Na2SO4+25%NaCl盐膜下的热腐蚀

研究了纯Ni、Ni-10Cr和Ni-10Cr-5Al合金在900 ℃空气中涂敷Na2SO4+25%NaCl盐膜时的热腐蚀行为。结果表明,Ni-10Cr合金的腐蚀动力学曲线近似服从抛物线规律,而纯Ni和Ni-10Cr-5Al合金的腐蚀动力学曲线分段服从抛物线规律。Ni-10Cr的腐蚀增重最小,抗热腐蚀性能最好,Ni-10Cr-5Al次之,纯Ni的腐蚀增重最大。Ni-10Cr-5Al合金热腐蚀24 h后氧化产物分为3层,外层氧化产物主要是NiO,中间层氧化产物为Cr2O3和Al2O3,最内层有少量Cr2S3和Al2S3。Ni-10Cr合金热腐蚀24 h后氧化膜分为3层,外层氧化产物是NiO,中间层是不连续的Cr2O3,内腐蚀区有少量Cr2S3。由于熔盐中NaCl的存在,Ni-10Cr-5Al和Ni-10Cr合金的腐蚀产物会变得疏松多孔。     

Hochtemperaturkorrosion des 2.25Cr-1Mo-Stahls unter simulierten Müllverbrennungsbedingungen

High temperature corrosion of 2.25Cr-1Mo in simulated waste incineration environments The corrosion of the 2.25Cr-1Mo steel was investigated by thermogravimetry in He? O₂? HCl? SO₂ at 500°C with and without deposits, taken from a waste incinerator. Additions of 500 vppm HCl to He-O₂ without deposits lead to accelerated corrosion due to the formation of FeCl₂ at the metal/oxide interface, its evaporation and oxidation lead to Fe₂O₃ at the outer surface (“active oxidation”). SO₂ causes FeS₂ formation on the FeCl₂ which retards the corrosion process. Under deposits, accelerated corrosion was observed in He-O₂ due to the reaction of alkalichlorides of the deposits with the oxide scale to ferrate and chlorine, which causes active oxidation. In He? O₂? HCl occurs strongly accelerated corrosion, due to transformation of the sulfates in the deposits by reaction with HCl to alkalichlorides. Addition of SO₂ shifts this equilibrium to enhanced stability of the sulfates, thus the corrosion in He? O₂? HCl? SO₂ is reduced.     

Corrosion resistance of TiN and MoN coated grey cast iron in acid solutions

Grey cast iron containing 3.39% C, 2.76% Si, 0.37% Mn, 0.05 %, P and 0.005 % S, with different shape of graphite was covered with Mo and Ti nitrides by cathodic are plasma deposition method at 400°C. Incorporation of C from the substrate caused the formation of molybdenum carbide. Formation of diffused interphase layer due to incorporation of Mo into the substrate caused increase in the coating adhesion. Electrochemical measurements showed that H₃PO₄ and H₂SO₄ solutions penetrated to substrate through flaws in coatings, mainly occurred at the graphite particles. The local corrosion degradation of material was accompanied by hydrogen evolution and formation of microcracks. In the case of coated vermicular cast iron merging of microcracks led to falling off the pieces of coated material. The Mo containing coatings exhibited lower susceptibility to crack formation but higher susceptibility to environmental degradation than the Ti containing coatings. The highest protection in acid solutions provided the complex (Mo,Ti)N coating.     

Corrosion behavior of titanium-clad carbon steel in weakly alkaline solutions

The electrochemical polarization curves and corrosion potentials during long-term immersion of Ti, carbon steel, carbon steel tightened-Ti and Ti-clad carbon steel specimens were investigated in bentonite-contacting solution (mixture of sodium sulfate and sodium carbonate solutions), sodium sulfate solution and borate solution adjusted to pH=9.0-9.84. Ti and carbon steel were passivated during immersion in borate solution, while carbon steel was corroded in the solutions containing SO₄²⁻ ions. The immersion potentials of Ti-clad steel and steel-tightened Ti specimens were controlled by the corrosion potential of carbon steel (about 0 V vs. RHE at 298 K). The Ti side of the clad specimen was, therefore, polarized cathodically at this immersion potential, and this caused hydration of and/or hydrogen penetration into the oxide film, resulting in degradation of its barrier property. Furthermore, the cathodic current on the Ti side was partially coupled with the anodic current in the corrosion reaction on the carbon steel side, resulting in acceleration of the overall corrosion rate.     

Characterization and Performance of Magnetron-Sputtered Zirconium Coatings Deposited on 9Cr-1Mo Steel

Zirconium coatings of different thicknesses have been deposited at 773 K on 9Cr-1Mo steel substrate using pulsed DC magnetron sputtering. These coatings were heat treated in vacuum at two different temperatures (1173 and 1273 K) for one hour. X-ray diffraction (XRD) analysis of Zr-coated samples revealed the formation of α-phase (HCP structure) of Zr. XRD analysis of heat-treated samples show the presence of Zr₃Fe and Zr₂Fe intermetallics. The lattice parameter of these coatings was calculated, and it matches with the bulk values when the thickness reached 2µm. In order to understand this, crystallite size and strain values of these coatings were calculated from XRD plots employing Williamson-Hall method. In order to assess the performance of the coatings, systematic corrosion tests were carried out. The corrosion current density calculated from the polarization behavior showed that the corrosion current density of the uncoated 9Cr-1Mo steel was higher than the coated sample before and after the heat treatment. Studies using electrochemical impedance spectroscopy confirmed that the coated steel has higher impedance than the uncoated steel. The corrosion resistance of 9Cr1Mo steel had improved after Zr coating. However, the corrosion resistance of the coating after heat treatment decreased when compared to the as-deposited coating. The microstructure and composition of the surface oxide film influence the corrosion resistance of the Zr-coated 9Cr1Mo steel.