Solubility study on protective oxide films in molten chlorides created by refuse incineration environment

Solubility measurements of Cr₂O₃, NiO, Co₃O₄, Fe₃O₄ and SiO₂ in molten NaCl‐KCl at 727 °C and NaCl‐KCl‐Na₂SO₄‐ K₂SO₄ at 550 °C were conducted in three different levels of basicity. The dissolution behavior of the oxides in the molten chlorides showed almost the same tendency as that shown in the result in molten Na₂SO₄ obtained by other researchers. In refuse incineration environment, protective Cr₂O₃ film is easily changed into CrO²⁻₄ by molten chlorides contained in deposits because the basicity of the molten chlorides is made more basic by the effect of H₂O in the combustion gas. In order to improve corrosion resistance of alloys, it would be better to add some elements such as Mo, V or W, which are expected to reduce the solubility of Cr₂O₃, and/or some elements such as Si or Ni, which are expected to produce protective oxide films having low solubility in a basic solution.     

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.     

Differences of the high-temperature corrosion of ferrite steel T92 caused by applied stresses and NaCl–Na2SO4 molten salts

The corrosion behavior of T92 in the environment characterized by the applied stress and NaCl–Na₂SO₄ molten salts was studied by scanning electron microscope and energy dispersion spectroscopy. The dissolution and precipitation of oxides in molten salts resulted in changes in the elemental distribution in corrosion scales. Moreover, the applied stress was found to accelerate the outward diffusion of Cr causing fast deformation of protective oxides because of the easy diffusion path available. On the contrary, applied stresses also compacted the corrosion layers and enlarge the gaps between layers. These two opposite effects indicate that there should be a threshold stress for corrosion relief.     

Dissolution behavior of synthetic Mg/Zn-ferrite corrosion products in EDTA and NTA based formulations

Dissolution behavior of Mg/Zn-ferrites in citric acid—EDTA/NTA formulations with or without ascorbic acid is reported. This study is relevant to the dissolution of such corrosion product oxides during the chemical decontamination of primary systems of nuclear reactors wherein Zn²⁺/Mg²⁺ ion passivation is practiced/proposed. Two sets of oxide systems namely (a) mixed phase system containing (Mg/Zn ferrites + MgO/ZnO + Fe₃O₄) and (b) single phase system of (MgFe₂O₄/ZnFe₂O₄) are investigated. In the absence of ascorbic acid, the dissolution of single phase oxides is hindered significantly unlike the mixed phase system. However, the single phase could be dissolved completely in presence of externally added Fe₃O₄. The dissolution rate coefficients are determined using two kinetic models. Activation energy for Mg-ferrite dissolution in mixed phase and single-phase is ∼35 kJ mol⁻¹ while in Zn-ferrite case it is ∼57 kJ mol⁻¹.     

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.     

Measurements of pitting corrosion currents of zinc in near neutral media

Using a simple model cell the susceptibility of the zinc electrode to pitting corrosion by SO₄²⁻, SO₃²⁻, S₂O₃²⁻ and S²⁻ anions were examined in naturally aerated carbonate solutions. It was found that, pitting started after an induction period, τ, which depended on the type and concentration of the aggressive and passivating anions. The pitting corrosion current increased with time until steady state values were attained. These values depended on both the type and the concentration of the passivating and pitting anions. For the same concentration of the passivating anions, the corrosion current varied with the concentration of the aggressive anion according to the relation: logi_pit.=a₁+b₁logC_agg. At a constant concentration of the aggressive anion, the corrosion current varied with the concentration of the passivating anions according to: logi_pit.=a₂−b₂logC_pass. The constants a₁ (a₂) and b₁ (b₂) were determined for all the systems studied. From the values of a₁ the corrosivity of the sulphur-containing anions is found to decrease in the order SO₄²⁻>SO₃²⁻>S₂O₃²⁻>S²⁻.     

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…     

Oxidation Behavior of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel-Coated Interconnects in Wet Air

Corrosion of boilers and heat exchangers is accelerated in the presence of vanadium, sodium, and sulfur from low-grade fuels. Several iron- and nickel-based alloys were immersed in 60 mol% V₂O₅–40Na₂SO₄ salt for 1000 h in order to investigate their degradation behavior at 600 °C in air. Materials performance was analyzed by means of substrate recession rate and metallographic characterization. Their corrosion mechanism is characterized by the formation of a sulfide/oxide layer adjacent to the metal, the dissolution of scale oxides in the molten deposit, and their precipitation near the outer surface of the deposit. High Ni- and Cr-containing alloys show the lowest metal loss rates. Al addition was detrimental due to low-melting eutectic AlVO₄–V₂O₅ formation. Fe–Cr-based alloys showed the highest metal loss rates. In such alloys, high Cr additions (above 20%) did not improve the performance due to the negative synergetic effect by simultaneous dissolution of Fe₂O₃ and Cr₂O₃. The predominant salt composition at the corrosion front varied from vanadate rich to sulfate rich during the exposure. This change in the attacking salt makes it difficult to find a protective material for mixed sulfate–vanadate-induced corrosion.     

Hot-Corrosion Resistance of a Sputtered K38G Nanocrystalline Coating in Molten Sulfate at 900°C

The hot-corrosion behavior of a nanocrystalline coating of K38G alloy, prepared by magnetron sputtering and cast K38G in molten 75 wt.% Na₂SO₄+25 wt.% K₂SO₄ at 900°C was studied. The coating eliminated internal sulfidation during the early stage of corrosion as a result of the formation of a continuous and compact Al₂O₃ scale. The nanocrystallization of K38G alloy prolonged the incubation of breakaway corrosion and improved the corrosion resistance of K38G. The relevant corrosion mechanism is discussed.     

Impedance investigation of the anodic iron dissolution in perchloric acid solution

The anodic iron dissolution in 0.5 mol dm⁻³ perchloric acid (HClO₄) was investigated by the electrochemical impedance spectroscopy, the potentiodynamic sweep and the scanning electron microscopy measurements. The anodic polarization behavior of iron in HClO₄ solution showed that the strong current oscillations occurred in a narrow potential region, particularly the pitting corrosion was observed in the active dissolution region. These characteristics were quite different from those of iron in the sulfuric acid (H₂SO₄). At the potentials 82 and 132 mV more positive than the corrosion potential (−482 mV vs. SCE), the impedance spectra for the iron in HClO₄ displayed two inductive arcs; however, by gradually increasing potential the lower frequency inductive arc disappeared at −300 mV at first, and then the higher frequency inductive arc changed into a capacitive arc at −250 mV. Based on the impedance display of iron at various potentials, a reaction model involving two adsorbed intermediate species was proposed, in terms of which the impedance behavior at different potentials were described. Occurrence of the pitting corrosion in active dissolution region was explained.     

Hot-Corrosion Studies of Separator Plates of AISI-310 Stainless Steels in Molten-Carbonate Fuel Cells

The corrosion behavior of a stainless steel 310S was investigated in amolten carbonate electrolyte at 650°C. The LiFeO₂ andLiCrO₂ oxides were formed in contact with the eutectic 62 mol%Li₂CO₃–38 mol% K₂CO₃mixture. Despite the low solubility of these oxides, the presence offissures in the LiFeO₂ layer does not provide good protection ofthe metallic material. The electrochemical-impedance spectroscopy (EIS)technique has shown low values of polarization resistance up to 80 hr and anincrease of the electrolyte resistance because of the dissolution of part ofthe scale after immersion. A mechanism for the corrosion of stainless steel310S in molten carbonates is proposed between 0 and 80 hr, taking intoaccount thermodynamic simulations, X-ray diffraction (XRD), scanningelectron microscopy (SEM) characterizations, and ac-impedance (EIS)results. This mechanism confirms a high corrosion rate and nonprotectivescale formation during the first hours of immersion. An external anddiscontinuous iron-rich oxide layer covered by a thin chromium-rich oxidelayer and an intermediate chromium-rich oxide and an internal spinel layerwere observed. Internal oxidation of the substrate also occurred to form(Fe, Cr)₂O₃ oxide. The dissolution in the melt of apart of the scale has been detected by EIS.     

Effects of solution pH and Cl on electrochemical behaviour of an Aermet100 ultra-high strength steel in acidic environments

In this work, the effects of solution pH and Cl⁻ on the electrochemical behaviour of an Aermet100 ultra-high strength steel in 0.5 M (Na₂SO₄ + H₂SO₄) solution were studied by polarization curve and electrochemical impedance spectroscopy (EIS) measurements, combined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterization. The results show that, when solution pH is below 4, the steel is in the active dissolution state, and corrosion current decreases with the increase of pH. There exists a critical pH value, above which the steel is passivated. Moreover, the oxides and hydroxides of Fe, Co, Ni and Cr are the primary components of the passive film. With addition of Cl⁻, pits are initiated on the steel electrode.     

The corrosion of Fe3Al alloy in liquid zinc

A systematic study of the isothermal corrosion testing and microscopic examination of Fe₃Al alloy in liquid zinc containing small amounts of aluminum (less than 0.2 wt.%) at 450 °C was carried out in this work. The results showed the corrosion of Fe₃Al alloy in molten zinc was controlled by the dissolution mechanism. The alloy exhibited a regular corrosion layer, constituted of small metallic particles (diameter: 2-5 μm) separated by channels filled with liquid zinc, which represented a porosity of about 29%. The XRD result of the corrosion layer formed at the interface confirmed the presence of Zn and FeZn_6.67. The corrosion rate of Fe₃Al alloy in molten zinc was calculated to be approximately 1.5 × 10⁻⁷ g cm⁻² s⁻¹. Three steps could occur in the whole process: the superficial dissolution of metallic Cr in the corrosion layer, the new phase formation of FeZn_6.67 and the diffusion of the dissolved species in the channels of the corrosion layer.     

Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 °C and examined in 0.1 M Na₂SO₄ without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.     

Hot corrosion of materials: a fluxing mechanism?

Hot corrosion is the accelerated oxidation of a material at elevated temperature induced by a thin film of fused salt deposit. Fused Na₂SO₄, which is the dominant salt involved in hot corrosion, is an ionic conductor, so that the corrosion mechanism is certainly electrochemical in nature. Further, the acid/base nature of this oxyanion salt offers the possibility for the dissolution (fluxing) of the normally protective oxide scale. Non-protective precipitated oxide particles are often observed in the corrosion products. In this paper, the status of knowledge for the solubilities of oxides in fused Na₂SO₄ is reviewed, and the effects of various influences on a fluxing mechanism are discussed. An evaluation of a “negative solubility gradient” as a criterion for continuing hot corrosion is made.     

The anodic dissolution of Mg in NaCl and Na2SO4 electrolytes by atomic emission spectroelectrochemistry

The dissolution of Mg has been investigated with atomic emission spectroelectrochemistry at open circuit and during potentiostatic control in chloride (NaCl) and sulfate (Na₂SO₄) electrolytes. A dissolution stoichiometry of approximately n = 2 is observed under all the conditions of these experiments with no evidence for the commonly invoked Mg⁺ intermediate. Nevertheless, in chloride electrolyte it is shown that anodic polarization results in a cathodic activation of the surface with increased hydrogen production during the polarization and an increased corrosion rate following the pulse. The formation of insoluble Mg oxides/hydroxides was also investigated in the sulfate electrolyte.     

Oxidation and Hot Corrosion Behavior of Plasma-Sprayed MCrAlY–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

The oxidation and hot corrosion behavior of two atmospheric plasma-sprayed NiCoCrAlY–Cr₂O₃ and CoNiCrAlY–Cr₂O₃ coatings, which are primarily designed for wear applications at high temperature, were investigated in this study. The two coatings were exposed to air and molten salt (75%Na₂SO₄–25%NaCl) environment at 800 °C under cyclic conditions. Oxidation and hot corrosion kinetic curves were obtained by thermogravimetric technique. X-ray diffraction analysis and scanning electron microscopy with energy-dispersive x-ray spectrometry were employed to characterize the coatings’ microstructure, surface oxides, and composition. The results showed that both coatings provided the necessary oxidation resistance with oxidation rates of about 1.03 × 10^?2 and 1.36 × 10^?2 mg/cm² h, respectively. The excellent oxidation behavior of these two coatings is attributed to formation of protective (Ni,Co)Cr₂O₄ spinel on the surface, while as-deposited Cr₂O₃ in the coatings also acted as a barrier to diffusion of oxidative and corrosive substances. The greater presence of Co in the CoNiCrAlY–Cr₂O₃ coating restrained internal diffusion of sulfur and slowed down the coating’s degradation. Thus, the CoNiCrAlY–Cr₂O₃ coating was found to be more protective than the NiCoCrAlY–Cr₂O₃ coating under hot corrosion condition.     

Oxidation of green rust suspensions containing different chromium ion species

The oxidation of hydrosulphate green rust (GR2(SO₄²⁻)) suspension containing different chromium ion species was investigated by X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy. The pH, oxidation-reduction potential and amount of dissolved oxygen in aqueous solutions were measured during the reactions. The results show that the addition of Cr(III)₂(SO₄)₃ solution suppresses the transformation of GR2(SO₄²⁻) into iron oxyhydroxides and oxides in aqueous solution, while the addition of Na₂Cr(VI)O₄ solution promotes the transformation of GR2(SO₄²⁻) in which Cr(VI) is reduced to Cr(III); α-FeOOH particles were refined by the addition of the chromium ions.