Corrosion of metals and alloys in sulfate melts at 750°C

The corrosion of Ni, Co, Ni-10Cr, Co-21Cr, and IN738 was studied at 750°C in the presence of molten sulfate mixtures (Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄) and in an atmosphere consisting of O₂+0.12% SO₂-SO₃. The corrosion was observed to be similar for both Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄ melts. The corrosion of Ni and Co tookplace by the formation of a mixed oxide plus sulfide scale, very similar to the corrosion in SO₂ or SO₃ alone. The initial stage for the corrosion of Ni-10Cr involved the formation of a thick NiO+Ni₃S₂ duplex scale, and Cr sulfide was formed during the later stages. A pitting type of morphology was observed for both Co-21 Cr and IN738. The pit was Cr sulfide at the beginning, and subsequently the sulfides oxidized to Cr₂O₃. A base-metal oxide layer was present above the pit, and this was observed to be formed very early in the corrosion process. A mechanism is proposed to explain this. In general, the formation of sulfides appears to be the primary mode of degradation in mixed sulfate melts.     

几种含铌镍基合金的热腐蚀研究

用硫酸钠沉积法研究了四种镍基合金-Ni-16Cr,Ni-16Cr-2Nb,IN738和537合金-在900～1000℃之间的热腐蚀行为。用热天平测量了热腐蚀动力学曲线,用金相、SEM、电子探针、X 射线衍射和~(3δ)S 示踪法观察和分析了腐蚀产物。研究发现,Ni-16Cr-2Nb 合金中的铌富集在表面氧化膜内,形成 NaNbO_3而降低熔盐中的 O~(2-)浓度,抑制 Cr_2O_3膜与熔盐的反应;但在复杂成份的合金中。铌的上述作用可能促进氧化膜的酸性熔融。鈪同时能促进硫在 Ni-16Cr-2Nb 变形合金晶界上的连续分布,加速内硫化过程。     

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.     

The effect of surface deformation on the hot corrosion behavior of some nickel-chromium alloys

The resistance of some NiCr alloys to Na₂SO₄-induced hot corrosion at 900°C has been found to be strongly affected by surface treatments such as grit-blasting and mechanical indentation. These cold-work procedures, when followed by recrystallization anneal, produced a large number of new grain boundaries in the deformed region. When deformed-and-annealed specimens of NiCr, Ni15 Cr and IN 738 were placed directly in contact with molten Na₂SO₄, the grain boundaries facilitated sulphur penetration and enhanced metal wastage. The deleterious effects of mechanical treatments could, in the case of Ni15 Cr, be offset by suitable thermal treatments such as pre-oxidation prior to Na₂SO₄ exposure.     

The influence of electrode potential on the corrosion of gas turbine alloys in sulfate melts

The influence of the electrode potential on the corrosion behavior of a series of nickel- and cobalt-base gas turbine alloys has been investigated in a (mole %) 53Na₂SO₄+7CaSO₄+40MgSO₄ melt at 1073 and 1173 K and in a 90Na₂SO₄+10K₂SO₄ melt at 1173 K. Only acidic fluxing is observed in the (Na ₂,Ca, Mg)SO₄ melt at positive potentials while a protective scale rich in MgO is formed on all alloys at negative potentials. This scale prevents basic fluxing because MgO is insoluble in neutral and basic melts. The breakthrough potential for acidic fluxing is a function of the material composition. Increasing chromium content of the alloys extends the potential range of protective film formation. Acidic and basic fluxing are observed in the (Na, K)₂SO₄ melt. Acidic fluxing occurs at positive and basic fluxing at negative potentials. A protective scale is formed in an intermediate (neutral) potential range on the high-chromium alloys IN-597 and IN-738 LC. Here, too, the breakthrough potentials for acidic and basic fluxing are influenced by the composition of the alloys.     

Inhibition mechanism of S-containing additives towards intergranular corrosion of a sensitized stainless steel

The comparison of the polarization curves recorded on both pure Fe, Cr, Ni, and Fe-18Cr-8Ni, Fe-8Cr-8Ni alloys, and Fe-10Ni, Fe-17Cr alloys in 1 N H₂SO₄ at 70°C clarifies the mechanism of inhibition of IG corrosion on sensitized AISI 304 SS by S-containing additives. These additives stimulate the anodic dissolution process of Fe-18Cr-8Ni alloy and inhibit this process on Fe-8Cr-8Ni alloy. The anodic behaviour of Fe-18Cr-8Ni is similar to that of pure chromium, while the behaviour of Fe-8Cr-8Ni is similar to that of pure iron.     

Cyclic oxidation behavior of some plasma-sprayed coatings in Na2SO4-60%V2O5 environment

Cyclic oxidation behavior of plasma-sprayed NiCrAlY, Ni-20Cr, Ni₃Al, and Stellite-6 coatings was investigated in an aggressive environment of Na₂SO₄-60%V₂O₅ by thermogravimetric techniques for 50 cycles. These coatings were deposited on a Ni-base superalloy, namely Superni 600; 10Fe-15.5Cr-0.5Mn-0.2C-Bal Ni (wt.%). X-ray diffraction, scanning electron microscopy/energy dispersive x-ray (SEM/EDX), and electron probe microanalyzer (EPMA) techniques were used to analyze the oxidation products. The uncoated superalloy suffered accelerated oxidation in the form of intense spallation of its oxide scale. After deposition of the NiCrAlY coating, the superalloy showed a minimum mass gain, whereas after application of the Stellite-6 coating, a maximum mass gain was observed among the coatings studied. All of the coatings were found to be useful in reducing the spallation of the substrate superalloy. Moreover, the coatings were successful in maintaining continuous surface contact with the base superalloy during the cyclic oxidation. The phases revealed for the oxidized coatings were mainly the oxides of chromium and/or aluminum and the spinels containing nickel-chromium/cobalt-chromium/nickel-aluminum mixed oxides, which are reported to be protective against high-temperature oxidation/hot corrosion.     

Elektrochemische Untersuchungen über die Stimulation der kathodischen Teilreaktion und der freien Korrosion von Silber und Chromstählen in Alkalisulfatschmelzen

Electrochemical investigations into the stimulation of the cathodic partial reaction and free corrosion of silver and chromium steels in alkali sulfate melts Information is presented concerning the influence of vanadium pentoxide, sodium metavanadate, sodium vanadate, molybdenum trioxide, tungsten trioxide and potassium chromate on the cathodic accumulated corrosion density potential curve on platinum and on the free corrosion of silver and ferritic chromium steels (Werkstoff- No. 1.4713 and 1.4742) in alkali sulfate melts at temperatures between 625 and 800 °C. All the compounds mentioned, except WO₃, produce a considerable stimulation of the cathodic partial reaction. Contents amounting to about 2 mole-% give rise to an increase of the accumulated current by appr. two orders of magnitude. In all the cases studied limiting diffusion currents are observed. V₂O₅ is reduced to V₂O₄ and potassium chromate to Cr₂O₃. The influence of WO₃ is relatively small because of its insolubility in alkali sulfate melts. Addition of acidic compounds (V₂O₅, NaVO₃, MoO₃) shifts the redox potential of the melt by several hundres millivolts toward more positive potentials, while the addition of salts (Na₃VO₄, K₂CrO₄) has but little influence on the position of the redox potential. The corrosion of silver is accelerated, in particular by acidic compounds (V₂O₅, MoO₃), which shift the free corrosion potential to much more positive values: in the system (K, Li)₂SO₄? V₂O₅ the corrosion rate of silver at 750 °C increases by six orders of magnitude. Below critical potential (rupture potential) the ferritic chromium steels form oxidic surface layers resembling passivation layers. This effect is connected with the formation of porous, sulfide containing layers only in those cases where the free corrosion potential is above the rupture potential. This effect is attained e.g. by addition of V₂O₅, and in the case of material No. 1.4713 also by addition of NaVO₃ to the sulfate melt. Higher contents of Na₃VO₄ in the melt, however, may give rise to a stimulation of the anodic partial reaction of metal dissolution.     

The hot corrosion of nickel-based ternary alloys and superalloys for gas turbine applications—I. Corrosion in SO2/O2 atmospheres

Hot corrosion studies of IN738 superalloy, Ni-22Cr control alloy as well as Ni-20Cr-X ternary alloys containing additions in w/w 3Al, 4Mo, 0.3Si and 5V in SO₂/O₂ equilibrated atmospheres were carried out. The investigations covered two SO₂/O₂ ratios of 2:1 and 1:4 at two temperatures of 700 and 900°C. In some cases irreproducible kinetic patterns pointed to the presence of Kirkendall voids opening between metal and scale and also to voids occurring as a result of sulphide instabilities and their subsequent conversion to oxides in the prevailing gas compositions and also to metal oxide formation in the dissociation pO₂ of NiO. The change in SO₂/O₂ ratio did not alter the corrosion rate, suggestive that the pSO₃ in the gas phase (necessary for the formation of NiSO₄) or pS₂ within the NiO layer is enough for Ni₃S₂ formation (the precursor of hot corrosion). Rates at 700°C were much greater than those at 900°C. Al, Si and V were beneficial at 700°C whereas at 900°C, V additions enhanced the corrosion rate of the control alloy due to the formation of a molten phase.     

Performance of high-velocity oxyfuel-sprayed coatings on an Fe-based superalloy in Na2SO4-60%V2O5 environment at 900 °C part II: Hot corrosion behavior of the coatings

NiCrBSi, Cr₃C₂-NiCr, Ni-20Cr, and Stellite-6 coatings were deposited on an Fe-based superalloy by the high-velocity oxyfuel (HVOF) thermal spray process. The hot corrosion behavior of the coatings in an aggressive environment of Na₂SO₄-60%V₂O₅ at 900 °C under cyclic conditions was studied. The thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction, scanning electron microscopy/energy-dispersive x-ray and electron probe microanalysis techniques were used to analyze the corrosion products. Hot corrosion resistances of all the coatings were found to be better than the uncoated superalloy. The Ni-20Cr coating was found to be the most protective, followed by Cr₃C₂-NiCr coatings. The Ni-20Cr coating had reduced the mass gain by 90% of that gained by the uncoated superalloy. The hot corrosion resistance shown by the Cr₃C₂-NiCr coating was slightly better compared with the NiCrBSi coating; however, both of the coatings performed better than the Stellite-6 coating. The Stellite-6 coating was the least effective among the coatings studied, but it was still successful in decreasing the mass gain to about one fourth compared with the uncoated superalloy. The formation of oxides and spinels of nickel, chromium, or cobalt may be contributing to the development of hot corrosion resistance in the coatings. This article focuses on the hot corrosion behavior of HVOF coatings. The characterization of these coatings has been presented in part I included in this issue.     

Cyclic and isothermal oxidation behavior on some Ni-Cr alloys

Additions of 3 wt.% Mn and 3 wt.% Si were made to Ni-20Cr. These alloys, along with Ni-20Cr and Ni-40Cr were oxidized for 100 1-hr cycles at 1100°C and 50 1-hr cycles at 1200° C. Oxidation behavior was judged by sample weight and thickness change, metallography, x-ray diffraction, and electron microprobe analysis. These tests showed that Ni-40Cr and Ni-20Cr-3Si were about the same and were the most oxidation-resistant alloys. Ni-20Cr-3Mn was not as oxidation resistant, especially at 1200° C. Ni-20Cr was far less oxidation resistant than any of the other alloys. The Ni-40Cr and Ni-20Cr-3Si relied on a protective layer of Cr₂O₃ for their oxidation resistance. A SiO₂ layer was noted beneath the Cr₂O₃ layer on the Ni-20Cr-3Si, but had apparently only a second-order effect. The source of improved protection of the Ni-20Cr-3Mn was apparently the formation of a relatively adherent MnCr₂O₄ layer at the metal-oxide interface.     

Hot-Corrosion Behavior of TiAl-Base Intermetallics in Molten Salts

The hot-corrosion behavior of Ti-50Al,Ti-48Al-2Cr-2Nb, and Ti-50Al10Cr alloys was investigatedin (Na, K)₂SO₄ andNa₂SO₄ + NaCl melts. TiAlintermetallics showed much better hot-corrosionresistance in (Na, K)₂SO₄ at900°C than the Ni-base superalloy K38G. Two types ofcorrosion products formed on Ti-50Al: some areas werecovered with a continuous Al2O3 scale, whereas otherareas formed a mixed Al₂O₃ +TiO₂ scale; TiS existed at the scale-alloyinterface. A mixed Al₂O₃ +TiO₂ scale formed on Ti-48Al-2Cr 2Nb, and nosulfide was found beneath the scale. An adherentAl₂O₃ scale, however, formed onTi-50Al-10Cr, which provided excellent hot-corrosionresistance. All three alloys suffered severe hotcorrosion in Na₂SO₄ + NaCl meltsat 850°C. A mixed Al₂O₃ +TiO₂ scale formed on all three alloys andmany voids and pits existed in the scale-alloy interface. Thehot-corrosion mechanisms are discussed.     

Type II Hot Corrosion Screening Tests of a Cr<Subscript>2</Subscript>AlC MAX Phase Compound

Low-temperature hot corrosion tests were performed on bulk Cr₂AlC MAX phase compounds for the first time. This material is a known alumina-former with good oxidation and Type I high-temperature hot corrosion resistance. Unlike traditional (Ni,Co)CrAl alumina formers, it contains no Ni or Co that may react with Na₂SO₄ salt deposits needed to form corrosive mixed (Ni,Co)SO₄–Na₂SO₄ eutectic salts active in Type II hot corrosion. Cr₂AlC samples coated with 20K₂SO₄–80Na₂SO₄ salt were exposed to 300 ppm SO₂ at 700 °C for times up to 500 h. Weight change, recession, and cross-sectional microstructures identified some reactivity, but much reduced (<?1/10) compared to a Ni(Co) superalloy baseline material. Layered Al₂O₃/Cr₂O₃ scales were indicated, either separated by or intermixed with some retained salt. However, there was no conclusive indication of salt melting. Accelerated oxidation was proposed to explain the results, and coarse Cr₇C₃ impurities appeared to play a negative role. In contrast, the superalloy exhibited outer Ni(Co) oxide and inner Cr₂O₃ scales, with Cr–S layers at the interfaces. Massive spallation of the corrosion layers occurred repeatedly for the superalloy, but not at all for Cr₂AlC. This indicates some potential for Cr₂AlC as LTHC-resistant coatings for superalloys.     

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.     

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。     

Effects of mode-I stresses on the oxidation and failure mechanisms of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two alloys, Ni-20Cr and Ni-15Cr-8Fe, as wire specimens, were exposed to sulfur dioxide between 325 and 800°C, with applied external stresses (mode I). Their mechanical properties have been investigated, and the variation of their radius has been precised by conductivity measurements. For the Ni-20Cr alloy, below 550°C, the failure process combines cracking and corrosion: Cr₂S₃ crystals formed at the tip of the cracks facilitate their propagation. Above 550°C, no barrier effect is observed, and intergranular corrosion takes place. For the Ni-15Cr-8Fe alloy, mode-I stresses bolster intergranular corrosion.     

Effect of a sputtered TiAlCr coating on hot corrosion resistance of gamma-TiAl

The effect of a sputtered Ti-50Al-10Cr coating on hot corrosion Behaviour of gamma-TiAl was investigated in (Na,K)₂SO₄ and Na₂SO₄+NaCl melts. The TiAlCr coating was effective in improving the hot corrosion resistance of TiAl in (Na,K)₂SO₄ melts due to the formation of a continuous Al₂O₃ scale. In Na₂SO₄+NaCl melts, however, the TiAlCr coating had little effect on the hot corrosion resistance of TiAl because it suffered from severe corrosion due to the formation of an Al₂O₃+TiO₂ mixed scale which spalled easily.     

Studies on the hot corrosion of a nickel-base superalloy,Udimet 700

The hot corrosion of a nickel-base superalloy, Udimet 700, has been studied in the temperature range 900–950°C. The effect of the amount of Na ₂SO₄ on the corrosion kinetics was determined. Large weight gains and severe corrosion were associated with two different modes of degradation: (1) formation of large, interconnected sulfides beneath the external scale, and (2) formation of a Na₂MoO₄-MoO₃ melt. The corrosion due to formation of the Na₂MoO₄-MoO₃ melt occurred for all the salt-coating thicknesses, whereas, large sulfides were formed only for the heavier coatings of Na₂SO₄. The formation of Na₂MoO₄-MoO₃ melt required an induction period, and the length of the induction period was observed to be a function of the amount of Na₂SO₄ and of temperature.Work funded under NASA Grant NCC 3-43.     

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%.     

The salt-induced corrosion behaviour of FeCr alloys at elevated temperatures—II. Alloys rich in chromium

The oxidation behaviour of two Na₂SO₄-coated, chromia-forming, iron-based alloys at 900°C has been studied thermogravimetrically, and the reaction products examined in detail by metallographic and E.P.M.A. techniques. Na₃SO₄ coatings markedly enhance the oxidation rates of both alloys and result in the formation of thick, compact, stratified scales. On the basis of subsequent experiments designed to characterize the singular roles of sodium oxide and sulphur, and in the absence of scale fluxing, it is postulated that the formation of sulphides in the alloy substrate and the mechanical failure of scale are responsible for the enhanced oxidation. Sodium chromate, a feature of the hot corrosion reactions of Na₂SO₄-coated chromia-forming, Ni- and Co-based binary alloys, is shown not to be a by-product of the corrosion reaction of equivalent iron-based alloys. Instead salt/scale reactions result in the formation of sodium-iron oxide, which is capable of assisting in the corrosion reaction, albeit in a minor way. The role of NaCl additions during Na₂SO₄ induced corrosion is also examined.