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.     

Corrosion processes of Mg–Y–Nd–Zr alloys in Na2SO4 electrolyte

The corrosion behavior of Mg–Y–Nd–Zr (WE43 commercial alloy) was investigated in Na₂SO₄ electrolyte using potentiodynamic polarization curves, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiles, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) analyzes. SEM and EDS data show that Nd-rich precipitates are mainly located at the grains boundaries. Zr/Y-rich zones are distributed inside the most of the grains. XPS study indicates a depletion of Mg on surface that could be attributed to Mg dissolution and an enrichment of the addition element oxides. XPS and ToF-SIMS analyzes demonstrate that the corrosion films are made up of a magnesium hydroxide (Mg(OH)₂) outer layer and an inner layer containing magnesium oxide (MgO), yttrium oxide (Y₂O₃) and hydroxide (Y(OH)₃), mixed with a small amount of MgH₂, zirconium oxide (ZrO₂) and neodymium oxide (Nd₂O₃). The Y₂O₃ and Y(OH)₃ signals increase slightly in the inner layer towards the corrosion film/alloy interface. Unlike these compounds, ZrO₂ and Nd₂O₃ compound signals are constant inside the inner layer. It is concluded that: (i) neodymium, zirconium and yttrium play a key role in the slightly improved corrosion resistance of the alloy and (ii) the cathodic reaction is slower on WE43 than on pure Mg and AZ91.     

A Comparative Study of High Temperature Corrosion of Al2O3, SiO2 and Al2O3–SiO2 Forming Alloys in a Na2SO4–NaCl Atmosphere

The present paper is designed to provide a summary of our study on the high temperature corrosion of Al₂O₃, SiO₂ and Al₂O₃–SiO₂ forming alloys in the gas phase and liquid phase of Na₂SO₄–NaCl system by comparing their corrosion resistance at 1000 °C. The obtained results show that the alumina-forming alloy experiences severe internal corrosion in the gas phase compared to in the liquid phase due to oxide cracking. This results in an increase in the inward diffusion and/or penetration of constituents of the salts and oxygen to form internal Al-oxide and Cr-sulfides. In the liquid phase, however, the formation of yttrium sulfide beneath a continuous double oxides layer of Al₅Y₃O₁₂ and Al₂O₃ may be related to the high affinity of yttrium for sulfur. On the other hand, it is apparent from the cross-sectional observations that a SiO₂ and Al₂O₃–SiO₂ forming alloys form a continuous and dense oxides layer, and demonstrate a high resistance against internal oxidation and corrosion in both corrosive environments.     

Effect of Co on oxidation and hot corrosion behavior of two nickel-based superalloys under Na2SO4–NaCl at 900 °C

Nickel-based superalloys with and without Co by partial replacement of W were prepared using double vacuum melting. A comparison of the oxidation in air and hot corrosion behaviors under molten 75wt.%Na₂SO₄+ 25wt.%NaCl at 900 °C were systematically investigated. The results showed that partial replacement of W with Co promoted the formation of chromia scale and consequently decreased the oxidation rate. Besides, the addition of Co also retarded the internal oxidation/nitridation of Al and consequently promoted the growth of Al₂O₃ scale, which further decreased the scaling rate and improved the adhesion of scale. Moreover, the addition of Co also further improved the hot corrosion resistance under molten Na₂SO₄–NaCl salts.     

Corrosion behaviours of typical metals in molten hydrate salt of Na2HPO4·12H2O – Na2SO4·10H2O for thermal energy storage

ABSTRACT

The corrosion behaviours of stainless steel 316 (SS-316), copper (Cu) and aluminium 1060 (Al-1060) in composite molten hydrate salt phase change materials (PCM) composed of Na₂HPO₄·12H₂O – Na₂SO₄·10H₂O have been studied by electrochemical measurements in conjunction with SEM and XRD. The corrosion resistance of the metals in the composite molten PCM follows SS-316?>?Cu > Al-1060. The role of the composite molten PCM affecting the corrosion resistance and its mechanisms of the metals are as following: the experiments reveal that SS-316 readily gets passivated in the composite molten PCM medium with no considerable corrosion; Cu suffers an initial rapid anodic dissolution which is further impeded by the formation of spot-like salt precipitates on the substrate, mainly consisting of CuHPO₄·nH₂O, CuSO₄·nH₂O and Cu₂O; Al-1060 shows a continuous large dissolution in the studied PCM with a porous corroded surface morphology due to the complete destruction of passive film under the attack of OH^- ions.     

Corrosion properties of nickel free austenitic stainless steels in 0·5M H2SO4 and 0·5M H2SO4 plus 0·4M NaCl

Abstract

The corrosion behaviour of four nickel free austenitic stainless steels were investigated in 0·5M H₂SO₄ and 0·5M H₂SO₄ plus 0·4M NaCl solutions by means of potentiodynamic and potentiostatic anodic polarisation testing. The performances of the nickel free alloys are compared to those of an experimental intermediate nickel alloy (4%Ni) and a standard AISI 304 steel grade. Once passivity was reached all alloys displayed similar current densities i _p in 0·5M H₂SO₄, independent from alloying. Mo and Cu were shown to be beneficial in decreasing the active dissolution currents and i _crit values. The commercial AISI 304 steel displayed superior resistance to pit initiation during potentiodynamic testing, and AISI 304 steel displayed the highest E _pit value of all alloys tested. When tested potentiostatically the N and Mo alloyed nickel free alloys showed excellent resistance to pit initiation and growth. The dominant effect of N was associated with repassivation of incipient pits, while Mo appeared to act at an earlier stage, suppressing initiation.     

Corrosion and Electrochemical Behavior of Ni–P Coatings in 0.5 M H2SO4

An increase in the phosphorus content in Ni–P coatings (from 6.6 to 13.4%) is shown to be accompanied by amorphization of their structure and a decrease in the anodic dissolution rate (at low overvoltages) by two–three orders of magnitude, whereas the corrosion rate is decreased to a lesser degree. Polarization curves of the coatings measured in 0.5 M H₂SO₄ comprise two linear segments and no passive range. In the first segment, the b _a coefficient is shown to be anomalously high (0.4–0.5 V for amorphous alloys), and the dissolution is controlled by transient volume diffusion of nickel and accompanied by surface accumulation of phosphorus. Being added as a stabilizer, thiourea reduces the corrosion resistance of the coatings and increases the dissolution currents, which can be to a considerable degree eliminated by increasing the phosphorus content in the coatings. The adverse effect of Pb incorporation into the coatings manifests itself as a substantial acceleration of their anodic dissolution.     

Synergic Effect of Shot Blasting and Deposit RE on Corrosion Behavior of TP304H steel in SO2-O2 at 923K

The corrosion behavior of TP304H steel with combined treatment of shot blasting and electrophoresis deposited RE coating in 1%SO2 14%O2 85%Ar mixture gaseous has been studied at 923K for 150 hours. The results show that both RE coating and shot blasting can improve the corrosion resistance of TP304H steel, and combined treatment has the best effect. Combined treatment reduces 50% mass gain, enhances the continuous and compactness of scale and changes the scale phase forms from Fe2O3/Fe1-xS/Ni1-x/S to Cr2O3/FeCr2Sn/Ni3-xS2. The synergistic effects of combined treatment on corrosion resistance and the effect of shot blasting on corrosion kinetics are discussed. Shot blasting increases the outward diffusion and surface concentration of Cr, Ce coating promotes the selective oxidation of Cr.     

Electrochemical-Impedance-Spectroscopy (EIS) Study of Corrosion of Steels 12CrMoV and SS304 Beneath a Molten ZnCl2–KCl Film at 400 °C in Air

The corrosion resistance of 12CrMoV and SS304 steels in contact with a molten mixture of (55–45) mol.% ZnCl₂–KCl, similar to that found in waste-incineration plants, has been examined in air at 400 °C by the electrochemical-impedance-spectroscopy (EIS) technique. The initial Nyquist plots of 12CrMoV showed a semicircle at high frequency and a line at low frequency indicating a diffusion-controlled reaction. At a later stage, the Nyquist plots are composed of a small semi-circle at high frequency, a line at medium frequency and a large semi-circle at low frequency, similar to that shown by SS304 during the whole experimental test. The larger impedance of SS304 as compared to 12CrMoV may be attributed to the presence of Ni and to the higher Cr content of SS304. Equivalent circuits representing the features of the corrosion of 12CrMoV and SS304 are proposed to fit the corresponding impedance spectra, and the electrochemical parameters in the equivalent circuits are calculated.     

An investigation on hot corrosion behavior of YSZ-Ta2O5 in Na2SO4 + V2O5 salt at 1100 °C

This study compares the hot corrosion performance of yttria stabilized zirconia (YSZ), and YSZ-Ta₂O₅ (TaYSZ) composite samples in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1100 °C. For YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For TaYSZ, minor amounts of NaTaO₃, TaVO₅ and Ta₉VO₂₅ are formed as the hot corrosion products with only traceable amounts of YVO₄. Due to the synergic effect of doping of zirconia with both Y₂O₃ and Ta₂O₅, the TaYSZ sample has a much better hot corrosion resistance than YSZ.     

Hot corrosion behavior and mechanism of FGH96 P/M superalloy in molten NaCl–Na_2SO_4 salts

Hot corrosion behavior of FGH96 powder metallurgy(P/M) superalloy in 25 % NaCl + 75 % Na_2SO_4 molten salts at 650, 700, and 750 ℃ was investigated in this paper. The methods of mass loss measurement, X-ray diffraction(XRD), scanning electron microscopy(SEM),and energy-dispersive spectroscopy(EDS) were used here.The experimental results show that hot corrosion kinetics follows a square power law at 650 ℃, while a linear one at 700 and 750 ℃. The corrosion layer is detected to be composed of Cr_2O_3, NiO, and Ni2S_3 at each temperature.The cross-sectional morphologies and corresponding elemental maps indicate that the corrosion layer is a stratified structure of oxide and sulfide. The results and analyses confirm that the hot corrosion mechanism of FGH96 P/M superalloy is a cooperate process of oxidation and sulfidation. Furthermore, the relatively higher concentrations of Cr, Co, and Ti provide better corrosion resistance to the attack of S and Cl~-.     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

Effects of NH4+, Na+, and Mg2+ ions on the corrosion behavior of galvanized steel in wet–dry cyclic conditions

The effect of cations on the corrosion of galvanized steel (GS) is scarcely reported. In this study, a wet–dry cyclic test was conducted to study NH₄ ⁺, Na⁺, and Mg²⁺ cation effect on the corrosion behavior of GS available in Nepal. Fourteen wet–dry cycles (18 h wet and 6 h dry) were performed by exposing samples at 298 K with a relative humidity of 90% in a wet cycle and 50% in a dry cycle for 14 days. The cations strongly affect the corrosion rate of the GS sample estimated by weight loss and potentiodynamic polarization. The potentiodynamic polarization curves showed the inhibition of cathodic and anodic reactions by Mg²⁺ ion, while the NH₄ ⁺ ion only changed the cathodic reaction. Mg²⁺ ion was found to shift the corrosion potential to noble values compared with NH₄ ⁺ and Na⁺ ions. A compact and thin corrosion products layer was developed in Mg²⁺ salt solution in contrast to a thick and porous corrosion products layer in NH₄ ⁺ and Na⁺ salt solutions. Red rust due to corrosion of underlying steel appeared in the presence of NH₄ ⁺ and Na⁺ salt solutions. Finally, the weight loss data revealed that the corrosivity of cations for GS decreased in the order Na⁺ > NH₄ ⁺ > Mg²⁺.     

Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg–RE alloys: RE = Ce,La, Nd,Y, Gd

Corrosion behaviour was characterised in salt spray and in 3.5% NaCl solution saturated with Mg(OH)₂ of as-cast and solution heat-treated binary Mg–RE alloys. The corrosion rate in the immersion test for the solution heat-treated Mg–RE alloys was substantial, and was greater than that of high-purity Mg. These corrosion rates were probably caused by the particles in the microstructure and/or by Fe rich particles precipitated during the solution heat-treatment. The corrosion rate in the immersion tests for each as-cast Mg–RE alloy was greater than that of high-purity Mg, attributed to micro-galvanic acceleration caused by the second phase. Corrosion rates in salt spray had a general correlation with corrosion rates in the immersion tests, but there were differences. The values of apparent valence were always less than 2 consistent with Mg corrosion being only partly under electrochemical control.     

Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg–X alloys: X = Mn,Sn, Ca,Zn, Al,Zr, Si,Sr

Corrosion of binary Mg–X alloys (as-cast and solution heat-treated) was characterised by immersion tests in 3.5% NaCl solution saturated with Mg(OH)₂, and by salt spray. Alloys with high corrosion rates in immersion tests also had high corrosion rates in salt spray. Corrosion rates of the solution heat-treated alloys did not meet the expectation that they should be equal to or lower than those of high-purity Mg. There was circumstantial evidence that the higher corrosion rates were caused by the particles in the microstructure; the second phases had been dissolved. The corrosion rate of all alloys was faster than that of high-purity Mg.     

Effects of Cu,Mo and C on the corrosion of deformed 18Cr8Ni stainless steels in H2SO4/NaCl solutions

The effects of Mo, Cu and C on the rate and type of corrosion of 18Cr8Ni stainless steels after deformation into U-shaped specimens have been investigated in 5N H₂SO₄ solutions containing up to 0·4M NaCl at 40°C. General corrosion, intergranular corrosion, and/or transgranular stress-corrosion cracking were found to occur, the type of attack depending on the contents of Mo, Cu and C in the steel, the concentration of NaCl in the solution, and the degree of deformation. In particular, it was observed that carbon has an effect on intergranular corrosion in these solutions, which is quite different from that observed in Strauss and Huey tests.     

Acid-base behavior of cryptand 1, 4, 7, 10, 13, 16, 21, 24-octaaza-bicyclo [8, 8, 8] hexacosan-3, 8, 12, 17, 20, 25-hexone and complex formation

The bicyclic cryptand I, 4, 7, 10, 13, 16, 21,24-octaaza-bigcyclo [8, 8, 8] hexacosan-3, 8, 12, 17, 20, 25-hex-one (COBH) bearing diaminoethane groups along the eight-atom bridges was synthesized. The structure consists of discrete neutral macrobicyclic units; the two cycles share the two tertiary amine nitrogen atoms, which exhibit an endo-endo conformation. Three identical branches formed by I, 2-diaminoethane link the two tertiary amine groups. The protonation reactions ofcryptand (COBH) and its complex formation with copper (II) were investigated by potentiometry in water and in a DMSO/water (80: 20 in mass ratio) mixture as solvents. The cryptand acts as a bis-base through its two Nbridgehead and exhibits a strong cooperativity that favors the first protonation and makes the second one difficult (ΔpK≈5.0 ). An inward rotation of the amide groups to form hydrogen bonds accounts for this cooperativity. The interaction of COBH with copper (II) leads to several binuclear complex proton contents.     

Oxidation behavior of sulfidation processed TiAl–2 at.%X (X=Si,Mn, Ni,Ge, Y,Zr, La,and Ta) alloys at 1173 K in air

The oxidation behavior of sulfidation processed TiAl–2 at.%X (X=Si, Mn, Ni, Ge, Y, Zr, La, and Ta) alloys was investigated at 1173 K in air for up to 630 ks under a heat-cycle condition between 1173 K and room temperature. During the sulfidation processing the TiAl–2 at.%Ta alloy formed Ta-aluminides on the TiAl₃ layer, while the alloys containing Mn, Ni, Y, and Zr formed a TiAl₃ (TiAl₂ included) layer including a small amount of the third element, like the TiAl binary alloy. The cross-sectional microstructure of the TiAl–2 at.%Ta alloy shows the sequence: oxide scale/TiAlTa/TiAl₂/alloy substrate; and the cross sections of the alloys containing Mn, Ni, Y, and Zr are: oxide scale/Ti₃Al/alloy substrate. The TiAl–2 at.%Ta alloy showed some scale exfoliation at the initial stage of oxidation, but very little exfoliation after long oxidation times, whereas alloys containing other third elements such as Si and Ge showed little exfoliation at the first several cycles and then tended to exfoliate significantly, resulting in very rapid oxidation. The TiAlTa/TiAl₂ layers formed by the reaction between the Ta-aluminide and TiAl₃ improve the oxidation properties of the TiAl–2 at.%Ta alloy.