Effects of SO2 and SO3 on the Na2SO4 induced corrosion of nickel

The effects of SO₂ and SO₃ in the environment on the hot corrosion behavior of Ni in the temperature range 750–950°C has been studied. Below the melting point of Na₂SO₄ (884°C), rapid corrosion takes place by formation of a Na₂SO₄-NiSO₄ melt, which can penetrate the porous oxide scale and give rise to sulfide information by coming in contact with the metal. The distribution of the sulfides depends on the SO₂ level in the ambient gas. Continued corrosion occurs by a sulfidation-oxidation mechanism. At temperatures above the melting point of Na₂SO₄, accelerated degradation occurs via dissolution of the surface scale, followed by reprecipitation of the oxide in a nonprotective form.Deceased     

The effect of K2SO4 additive in Na2SO4 deposits on low temperature hot corrosion of iron-aluminum alloys

To understand the effect of K₂SO₄ additive in an Na₂SO₄ deposit on low temperature hot corrosion, the corrosion behavior of Fe-Al alloys induced by Na₂SO₄+K₂SO₄ was compared to that by Na₂SO₄ alone, and sulfation of Fe₂O₃ in the presence of either Na₂SO₄ or Na₂SO₄+K₂SO₄ was studied. It was found that K₂SO₄ additive promoted the low temperature hot corrosion, but did not change the corrosion-mechanism. Experimental results refuted the prior suggestions that the accelerated hot corrosion resulted either from the formation of K₃Fe(SO₄)₃ or from the stimulation of sulfation of Fe₃O₃. The earlier formation of the eutectic melt caused the accelerated hot corrosion, or in other words, the K₂SO₄ additive shortened the induction stage of hot corrosion.     

Electrochemical Impedance Study of Hot Corrosion of the Mo-Rich Ni3Al-Base Alloy IC6 at 750?C800???C Beneath Solid Na2SO4 Deposits

A two-electrode probe was used for electrochemical impedance studies of hot corrosion kinetics of molybdenum-containing Ni₃Al-base alloy IC6 covered with a solid Na₂SO₄ film at 750 and 800???C in air. The alloy was subject to catastrophic corrosion at both temperatures, forming a thick porous oxide layer, as a result of the formation of Na₂MoO₄?CMoO₃?CNa₂SO₄ melt. The experimental temperature affected the formation of the melt, the ionic conduction of the corrosion layer, and thus the impedance characteristics. For the corrosion at 800???C the Nyquist plots were composed of two capacitive loops at high-mid frequency and a line at low frequency indicating a diffusion-controlled reaction. At 750???C, however, the plots consisted of a single capacitive loop in the initial stage, followed by the same impedance features as at 800???C. Two equivalent circuits were proposed to fit the impedance spectra at the two temperatures. Based on the precise measurements of diffusion impedance, the diffusion flux of oxygen through the salt layer was calculated, and the main reduction reaction was also discussed.     

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.     

Hot Corrosion of a Single Crystal Ni-Base Superalloy by Na-Salts at 900°C

The hot corrosion behaviors of a single crystal (SC) Ni-base superalloy coated with Na₂SO₄ and 75 wt.% Na₂SO₄-25 wt.% NaCl mixture were studied in air at 900°C. The results showed that the corrosion productions were laminar structure, porous and easily spalled. And sulfides formed quickly in the deep SC superalloy under the corrosion production. The addition of NaCl into Na₂SO₄ considerably accelerated the corrosion of the SC superalloy, and the corrosion scale became more porous. The hot corrosion process was explained based on sulfide formation and its subsequent oxidation.     

Accelerated oxidation of iron induced by Na2SO4 deposits in oxygen at 750°C—A new type low-temperature hot corrosion

Na ₂ SO ₄ -induced accelerated corrosion of iron in oxygen at 750°C was observed. EDX, XRD, SEM, EPMA and some chemical examinations were carried out to understand the corrosion mechanism. The accelerated oxidation was attributed to the formation of abundant sulfide which has a highly defected lattice and allows rapid diffusion of iron ions. The sulfide resulted in turn from the formation of a liquid phase which was a eutectic melt of Na ₂ SO ₄ and Na ₂ O. The formation of and other possible effects of the melt were discussed. The accelerated oxidation was compared with the usual low-temperature hot corrosion, showing that it has most of the characteristics of low-temperature hot corrosion except that it occurred under basic conditions developed by the removal of sulfur from the sulfate deposits instead of the usual acidic conditions established by the SO ₃ in the atmosphere.     

Hot Corrosion Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and pre-oxidized in a vacuum environment, and its hot corrosion behavior in pure Na₂SO₄ and 75 wt.% Na₂SO₄ + 25 wt.% NaCl salts was investigated. The pre-oxidation treatment resulted in the formation of a dense and continuous α-Al₂O₃ scale on the coating surface. After being corroded for 150 h at 900 °C, the pre-oxidized coating exhibited better corrosion resistance to both salts than the as-sprayed coating. The presence of preformed Al₂O₃ scale reduced the consumption rate of aluminum, by delaying the formation of internal oxides and sulfides and promoting the formation of a denser and more adherent Al₂O₃ scale. Moreover, we investigated the corrosion mechanism of cold-sprayed CoNiCrAlY coatings in the two salts and discussed the effect of the pre-oxidation treatment.     

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.     

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%.     

Sulfate-induced hot corrosion of nickel

Nickel specimens with layers of Na₂SO₄ deposited on the metal surface have been reacted in O₂+4% SO₂ in the temperature range 660–900°C. At temperatures from 671°C (the eutectic temperature of Na₂SO₄+NiSO₄ liquid solutions) to 884°C (the melting point of Na₂SO₄), molten Na₂SO₄+NiSO₄ is formed in the scales above critical pressures of SO₃, and the molten sulfate causes accelerated hot corrosion of nickel. The rapid hot corrosion is preceded by an incubation period during which Na₂SO₄+NiSO₄ solid solutions and eventually molten sulfate are formed. The critical SO₃ pressures for formation of molten sulfate as a function of temperature have been delineated through experimental observations, and these are in agreement with theoretical estimates. When only solid solutions of Na₂SO₄+NiSO₄ can be formed, the reactions are slower than specimens with no Na₂SO₄ layer. The reaction mechanism is concluded to involve inward transport of SO₃/NiSO₄ and of oxygen through the molten sulfate distributed as a network in the NiO layer of the outer part of the scale. Beneath the NiO/molten sulfate layer, the scale consists of NiO with a network of Ni₃S₂. Sulfur, present as (Ni-S)_liq, is enriched at the metal/scale interface. Nickel diffuses outward through the Ni₃S₂ network in the inner layer to the boundary of the NiO/molten sulfate layer, where it reacts with the inwardly diffusing oxygen and SO₃/NiSO₄. The enrichment of sulfur next to the metal is concluded to be due to inward sulfur transport in the NiO+Ni-sulfide layer.     

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.     

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.     

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.     

Korrosion von silber in neutralen alkalisulfatschmelzen

The corrosion of silver in the ternary eutectic of (mole %) 78 Li₂SO₄, 13·5 K₂SO₄ and 8·5 Na₂SO₄ at 625°C is proportional to the oxygen pressure above the melt. The corrosion current density of completely immersed specimens is 1·4 × 16^?6 A cm^?2 in the unstirred melt under a 100% oxygen atmosphere. The corrosion rate is increased by stirring.Specimens incompletely immersed in the melt are corroded at a much higher rate than fully immersed specimens. The specimen surface protruding from the melt is covered by a thin film of the molten material. Local cells are formed where O₂ is preferentially reduced on the surface above the melt, and silver is preferentially dissolved from the immersed portion of the surface. The corrosion current density is controlled by the ratio of areas inside and outside the melt.     

Influence of gas phase composition on the Hot Corrosion of steels and nickel‐based alloys beneath a (Ca‐Na‐K)‐sulfate mixture containing PbSO4 and ZnSO4

The corrosion of steels and of nickel‐based alloys was studied in exposure experiments at 600 °C beneath a molten CaSO₄‐K₂SO₄‐Na₂SO₄‐PbSO₄‐ZnSO₄ sulphate mixture in N₂‐5 vol.% O₂ with and without additions of 1000 vppm HCl, 1000 vppm SO₂ and 1000 vppm HCl in combination with 250 vppm SO₂. In the N₂‐5 vol.% O₂ atmosphere, the corrosion products are iron‐ and nickel‐rich but chromium‐free precipitates of oxides in the solidified melt. Additionally, pits filled with layered corrosion products are formed, growing into the metal substrate. These layers consists of less soluble chromium‐rich oxides, containing varying amounts of zinc, (ZnCr₂O₄) alternating with potassium‐rich sulfates, most probably K₂S₂O₇. The addition of 1000 vppm SO₂ leads to a seperation of the melt in a K₂S₂O₇ part close to the metal surface and a Ca‐rich part on top in contact with the gas atmosphere. Compared to the N₂‐5 vol. % O₂ atmosphere accelerated corrosion was observed. In the K₂S₂O₇ part of the melt dissolved iron and nickel are identified, whereas in the Ca‐rich part iron‐ and nickel‐oxide precipitates are formed. Underneath the solidified salt, thin layers of sulfides are detected. In the N₂‐5 vol.% O₂‐1000 vppm HCl containing gas, the corrosive attack is also accelerated compared to the N₂‐5 vol.% O₂‐atmosphere. Much more oxide precipitates are found in the melt on every sample and the inward growth of the zinc‐free chromium‐rich oxides is significantly enhanced. Underneath the inward growing oxide small amounts of metal‐chlorides are detected. Compared to the SO₂ containing gas, the corrosive attack is enhanced for the iron‐based materials, but retarded for the nickel‐based alloys. In the 1000 vppm HCl‐250 vppm SO₂ containing gas, the corrosive attack is similar to the atmosphere containing only 1000 vppm HCl. In addition, sulfides are formed next to chlorides at the metal/scale interface.     

Effect of enamel coating on oxidation and hot corrosion behaviors of Ti-24Al-14Nb-3V alloy

The effect of enamel coating on the isothermal and cyclic oxidation at 900 °C in air and on the hot corrosion resistance of Ti-24Al-14Nb-3V in both 85% Na₂SO₄+15%K₂SO₄ and 15%NaCl+85% Na₂SO₄ molten mixed salts at 850 °C was investigated. The results indicated that Ti-24Al-14Nb-3V alloy exhibited poor oxidation resistance due to the formation of nonprotective Al₂O₃+TiO₂+AlNbO₄ scales and poor hot corrosion resistance due to the spallation of scales formed in molten Na₂SO₄+K₂SO₄ and NaCl+Na₂SO₄. Enamel coating suppressed the migration of oxygen and corrosive ions into the substrate to improve the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy. However, the dissolution of oxides components of the coating into the molten salts degraded enamel coating and the degradation of the coating involved a process by which Cl⁻ anion penetrated into the substrate through voids in the coating to accelerate corrosion of Ti-24Al-14Nb-3V alloy.     

高温防护涂层的熔盐热腐蚀研究进展

高温热腐蚀是热元件主要失效形式之一,Na₂SO₄和NaCl熔盐会加速高温下的热腐蚀,甚至导致灾难性事故发生。本文就Na₂SO₄和/或NaCl熔盐引起的热腐蚀进行了讨论,其中Na₂SO₄是主要的腐蚀反应物,详细介绍了2种典型的热腐蚀行为和性能特点。重点介绍了几种热腐蚀模型和机理,以及Na₂SO₄、NaCl、Na₂SO₄+NaCl熔盐的反应公式和腐蚀机理。根据目前的研究状况来看,制备防护涂层是缓解热腐蚀的最佳途径,总结了近年来MCrAlY涂层、NiAl涂层、热障涂层和新型涂层的发展情况,并探讨了进一步提高涂层耐腐蚀性能的方法。最后,展望了防护涂层的未来发展方向。     

Corrosion behaviour of Nimonic–75 and Ni–19.4 Cr–1.25 Al in NaCl and Na2SO4

The corrosion behaviour of Nimonic-75 and Ni-Cr-Al alloys has been investigated in NaCl and Na₂SO₄ by employing the half-immersion crucible test. Weight loss results obtained as a function of temperature (700–800°C) and concentration of NaCl in the mixture (1–30 wt%) show that the highest attack occurs in Na₂SO₄-3% NaCl mixture around 750°C, for Nimonic-75 and 800°C for Ni-Cr-Al, resulting in blister formation for both the alloys. The results also show that the addition of Al to Ni-Cr alloy drastically enhances corrosion. Pre-heating the Ni-Cr-Al alloy to recrystallization temperature for 2 to 4 h fails to indicate any inhibiting effect on catastrophic corrosion. The results strongly suggest that it is not only the concentration of NaCl in the mixture and the temperature of the salt mixture but also the volume fraction of liquid which plays a predominant role in the enhancement of corrosion attack.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

Effect of nanocrystallization on hot corrosion resistance of Ti-48Al-8Cr-2Ag alloy in molten salts

1 Introduction There was considerable interest in the use of γ-TiAl alloys within advanced gas turbines where they offered greater high temperature capability over conventional titanium at reduced mass. These factors would enable engines with greater th…