Corrosion of titanium in phosphoric acid at 250℃

Corrosion studies of a commercially pure titanium in phosphoric acid solutions at 250 ℃ were carried out by immersion test in an autoclave. At lower phosphoric acid concentration (0.1 mol/L), the corrosion was mild. At higher phosphoric concentration (1.0 mol/L) corrosion, a 25 μm-thick white corrosion products layer was formed on the samples after 24 h immersion. XRD analysis shows that the white layer consists mainly of titanium oxide phosphate hydrate (π-Ti2O(PO4)2·2H2O). The corrosion product shows the morphology of fiber bundles. A thermodynamic analysis of the formation of the corrosion product is presented.     

Corrosion behaviour of IN625 in superheated steam at 800°C

Concerns with greenhouse gas emissions and the uncertainty of long-term supply of fossil fuels have resulted in renewed interest in nuclear energy as an essential part of the energy mix for the future. Canada is currently undertaking the design of Generation IV supercritical water-cooled reactor (SCWR) with improved higher thermodynamic efficiency and considerable plant simplification. The identification of appropriate materials for in-core and out-of-core components to contain the SCWR coolant is one of the major challenges for the design of Canadian SCWR. This study is carried out to evaluate the oxidation/corrosion behaviours of Inconel 625 (IN625) under superheated steam (SHS) at a temperature of 800°C (0.1?MPa) for up to 3000?hours. The results show that chromium-based oxide forms on the surface after exposure in SHS for 800, 2000 and 3000?hours. The oxide formation leads to initial weight increase followed by weight loss after extended exposure, likely due to chromium oxide dissolution. No localised oxide spallation was observed on all samples examined. Despite the slight weight reduction, a dense and adhered oxide layer, consisting primarily of Cr₂O₃ with some spinel (Ni(Cr,Fe)₂O₄), remained on IN625 after 3000?hours in SHS.     

The Corrosion Behavior of Carbon Steel in Sulfide Aqueous Media at 30°C

In this paper, we studied the effect of sulfide ions on the corrosion behavior of carbon steel to simulate the geological disposal of high-level radioactive waste. In geological storage conditions, sulfidogenic environment was sustained by sulfate-reducing bacteria. Corrosion tests were conducted in systems in a controlled atmosphere of 5% H₂/N₂. Batch experiments were conducted at 30°C for 1 month with steel coupons immersed in Na₂S solutions. The structural characterization of the corrosion products was investigated by scanning electron microscope/energy dispersive x-ray spectroscopy, confocal micro-Raman spectrometry, and x-ray diffraction. In the absence of sulfide ion, a magnetite (Fe₃O₄) corrosion product layer was formed on steel surface while in the presence of sulfide ions we observed the formation of a poorly crystallized irons sulfide at low-sulfide concentration (1 mg/L) and a solid adherent pyrrhotite layer at higher sulfide concentration (5-15 mg/L). The strong drop in steel corrosion rate with sulfide concentration was revealed and related to the formation of well-crystallized pyrrhotite.     

Hot Corrosion of an EB-PVD Thermal-Barrier Coating System at 950°C

Model thermal-barrier coating (TBC) systems consisting of cast NiCrAlYsubstrates and electron-beam, physically vapor-deposited (EB–PVD)partially yttria-stabilized zirconia (PYSZ) coatings with three differentmicrostructures were tested in 1-hr cycles at 950°C under hot-corrosionconditions (sodium and sodium/potassium sulfates) likely to occur inbiomass-derived fuel-fired gas turbines. In contrast to conditionsinitiating Type I hot-corrosion attack, a modified test procedure wasused in this study where periodically salt-coated specimens were subjectedto an oxygen atmosphere while SO_x was omitted, thus taking into account theessentially sulfur-free biomass fuel-combustion atmosphere. For comparison,similar tests were conducted on bare MCrAlY-type alloys. TBC failure byspallation of the PYSZ coating was observed between 300 and 500 1-hrcycles. Irrespective of PYSZ microstructure and deposit chemistry, failurewas primarily induced by crack formation and propagation within thevoluminous oxide scale formed as a result of hot-corrosion attack of themetal, rather than degradation of the ceramic layer. Since the major attackmode of the TBC seemed to be hot corrosion of the bond coat, this paperhighlights degradation mechanisms and microstructures of uncoated bond-coatcompositions. On the basis of the present results, implications of thefailure mode of EB–PVD PYSZ on in-service components are discussed.     

Corrosion of titanium in supercritical water oxidation environments

1　ＩＮＴＲＯＤＵＣＴＩＯＮＡｔｐｒｅｓｅｎｔ,ｔｒｅｍｅｎｄｏｕｓａｍｏｕｎｔｏｆｏｒｇａｎｉｃｗａｓｔｅｓｈａｓｂｅｅｎｐｒｏｄｕｃｅｄｉｎｍａｎｙｂｒａｎｃｈｅｓｏｆｉｎｄｕｓｔｒｙｅａｃｈｙｅａｒ.Ｍａｎｙｏｆｔｈｅｍｈａｖｅｈａｚａｒｄｏｕｓｅｆｆ     

The Corrosion of Co-15 wt.% Y at 600-800°C in Sulfidizing-Oxidizing Atmospheres

The corrosion of Co-15 wt.% Y has been studiedat 600-800°C inH₂-H₂S-CO₂ mixturesproviding a sulfur pressure of 10^-8 atm at600-800°C and of 10^-7 atm at 800°Cand an oxygen pressure of 10^-24 atm at 600°C and of10^-20 atm at 700-800°C. The corrosionrates in such sulfidizing-oxidizing atmospheres werecompared with those of pure cobalt and yttrium. Theaddition of yttrium to cobalt is only slightly beneficial, sincefor a yttrium content of 15 wt.% the corrosion rate isreduced quite significantly with respect to pure cobaltat 800°C under 10^-7 atm S₂,only to a limited extent at 600°C, and even slightlyincreased at 700°C. Moreover, the alloy corrodesconsiderably more rapidly than pure yttrium at800°C, when the latter behaves protectively. At 600 and 700°C, yttrium exhibitedbreakaway behavior, while the alloy corroded morerapidly than yttrium at short times, but more slowly atlong times. Under all conditions, except at 800°Cunder 10^-8 atm S₂, the alloy formsan external layer of cobalt sulfide overlying anintermediate region of very complex compositioncontaining a mixture of the compounds of the two metalsand an innermost region of internal attack containing compoundsof yttrium with both oxygen and sulfur. Thus, cobalt canstill diffuse through the intermediate region to formthe outer cobalt-sulfide layer at nonnegligible rates. The scaling behavior of the Co-15% Yalloy is discussed by taking into account the limitedsolubility of yttrium in cobalt as well as the presenceof an intermetallic Co-Y compound in thealloy.     

Corrosion resistance of electrodeposited RE-Ni-W-P-SiC-PTFE composite coating in phosphoric and ferric chloride

1 Introduction During process, store and use in various kinds of environment, corrosion, wear, fatigue and rupture may happen in all materials and products. These kinds of failure and damages widely occur in industries, causing enormous losses to nationa…     

Corrosion behaviour of X52 pipeline steel in high H2S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract

High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H₂S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H₂S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H₂S concentration at a temperature of 90°C. When the temperature and H₂S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe²⁺ ions and the inward diffusion of H₂S and HS^? species.     

Solubility of hemimorphite in ammonium sulfate solution at 25 °C

The solubility of natural hemimorphite in ammonium sulfate solution was measured by isothermal solution method at 25 °C and the dissolved residue of hemimorphite was investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) methods. The results show that zinc and silica in hemimorphite simultaneously dissolve in ammonium sulfate solution. The solubility of zinc in solution increases rapidly from 4.5381 mmol/kg in 0.5469 mol/kg ammonium sulfate solution to 11.5083 mmol/kg in 3.7038 mol/kg ammonium sulfate solution. The solubility of silica in solution increases slowly from 2.5509 mmol/kg in 0.5469 mol/kg ammonium sulfate solution to 7.2891 mmol/kg in 3.7038 mol/kg ammonium sulfate solution. The dissolved residue is the characteristic of hemimorphite Zn₄Si₂O₇(OH)₂·H₂O based on the results of the XRD, SEM and FTIR. Thus, no phase transition occurs in the dissolution process of hemimorphite in ammonium sulfate solution.     

Effects of rare earth elements on microstructure and tensile properties of Al-Si-Cu alloy at 250 °C

The effects of rare earth elements (La, Sm) on the high-temperature (250 °C) microstructure and mechanical properties of Al-Si-Cu alloys were analyzed. The experimental results show that with the addition of La and Sm, the α-Al was significantly refined, and the eutectic Si changed from acicular to rod-like and granular. XRD and SEM analysis shows that the rare earth phases in the alloy were mainly AlSiRe and AlRe. Fracture morphology observations show the fracture mode of the alloy changes from brittle and ductile fracture to ductile fracture. With the increase of La or Sm contents, the mechanical properties of the alloys at 250 °C increase at first, and then decrease. When the contents of La and Sm are 0.4wt.% and 0.2wt.%, the tensile strength of the alloy reaches maximum of 143.91 MPa and 201.48 MPa, respectively.

     

Metallography of fatigue crack initiation in coarse-grained Astroloy at 20°C

Fatigue crack initiation in coarse-grained Astroloy was studied by initiating cracks under bending loads. The specimen was not replicated in order to preserve any debris or extrusions associated with the cracks. Cracks were found to have initiated at pores and along slip lines. Debris and tonguelike extrusions were found coming from some cracks, but not all of them. Illustrations are given of the five different crack extrusion-related phenomena that were observed. It is concluded that the extrusion of material in the form of tonguelike or other debris may accompany the initiation of fatigue cracks, but that it is not a requisite part of the processes causing the crack.     

Corrosion Testing of some Austenitic Stainless Steels and of Inconel Alloy at 25°c in Acidic Chloride Solutions

Abstract

The corrosion of austenitic stainless steels types AISI 304, 310 and 316, and of Inconel alloy, was studied at 25°c, in 5% NaCl solution at an initial pH value of 2·5, and in 5% FeCl³ at pH 1·2. The resistance of the alloys in both corrosive environments was in the order: 310 > 316 > 304 > Inconel. Pre-treatment of the specimens with bubbling chlorine gas increased the subsequent corrosion rates of the alloys. Intermittent bubbling of gas mixtures such as Cl₂, N₂, and/or H₂S, increased the corrosion rate of Inconel alloy when Cl₂ was present, but decreased the corrosion rate when H₂ was present. Heat treatment of austenitic stainless steels increased the subsequent corrosion rates, whereas 16% pre-straining of annealed specimens slightly reduced the rates. Addition of trisodium phosphate to the corrosive solution reduced the corrosion rates and pitting tendency for all three types of austenitic stainless steel.     

Solubility of sulfur in pure Cr2O3 at 1000°C

Sulfur solubility in pure Cr₂O₃ was measured over a large range of oxygen (10^–6–10^–12 atm) and sulfur (10^–2 -10^–16 atm) partial pressures at 1273 K. Different methods of analysis were used; it was found that the neutronactivation technique produced the most reliable and reproducible results. The results obtained showed that the limiting solubility of sulfur in Cr₂O₃ varied between 16–93 ppm for the range of oxygen and sulfur pressures used in this study. The solubility was found to vary, depending on the combined effect of and . For a given , the amount dissolved increases with an increase of . An empirical equation, was found to best fit the experimental results and indicates a stronger influence of than of . A specific dissolution mechanism connot be formulated from this equation. However, a number of possibilities have been proposed, but none of these specific mechanisms seems to fit the experimental data.     

Characteristics of oxide pegs in Ti-and Y-doped CoNiCrAl alloys at 1150 °C

Oxide pegging is a widely accepted mechanistic model explaining the reactive element effect on the improved adherence of scale. However, previous models for the oxide peg formation process have not considered the effects of more than one active element added into the alloy during the peg formation. This study proposes a new model of oxide peg formation and growth for the doping of two reactive elements in an alloy(the precipitated Y and solid solute Ti). Different amounts of Ti and Y were added to a Co Ni Cr Al alloy, and the characteristics of the resulting oxide pegs, such as their linear density, size, and forming process, are obtained by examining alloy samples subjected to an isothermal oxidation operation at a temperature of 1150 °C. It is found that the amount of Y determines the density of the oxide pegs, and Ti does not form a Ti-rich oxide core if a Y-rich oxide exists in the sample. In samples with the same Y content, the oxide pegs primarily grow in length, and with increased Ti content, they grow along the b-phase boundary and into the alloy. Based on these results, a three-step model for oxide peg formation and growth is conceived.     

Investigation of the Fluorine Microalloying Effect in the Oxidation of TiAl at 900°C in Air

High-temperature oxidation resistance of gamma titanium aluminides can be achieved by the formation of a continuous scale of slowly growing Al₂O₃. The formation of such a scale was favored by the addition of small amounts of fluorine. It is shown that fluorine can have a beneficial effect on oxidation resistance in a certain F-range which is quantified by thermodynamic calculations and by experimental investigations. It is assumed that the F-effect offers a significant potential for improvement of the oxidation resistance of technological TiAl alloys.     

Long-time oxidation of iron at 1100°C in air as a function of time

Long-time oxidation experiments, 180 hr, 360 hr, and 660 hr, were carried out on AREMA iron in ambient air at 1100°C. Fe samples, 3 mm in diameter and 39 mm in length, were completely oxidized. Weight-gain measurements, electron microanalysis, and X-ray diffraction (XRD) analysis were used as experimental techniques. The completely oxidized samples remained cylindrical, compact, and solid. The content of hematite in the scale mass increased exponentially from 88.1% (180 hr) to 88.4% (660 hr), with the limit value 100% at t . No wustite and no pure Fe traces have been found in completely oxidized samples.     

Thermal oxidation of single-crystal aluminum at 550°C

Oxygen transport during oxide growth on (110), (111), and (100) Al crystals at 550°C is investigated by¹⁸O/SIMS combined with kinetic measurements and SEM and TEM observations. Starting with an electropolished surface, the experimental evidence suggests oxide growth by oxygen anion transport via local pathways through an outer amorphous Al₂O₃ layer and oxygen incorporation at the periphery of the underlying laterally growing -Al₂O₃ islands. The kinetics, island morphology and epitaxy are sensitive to substrate orientation. This oxide growth behavior is compared with oxide formation on a sputter-cleaned and annealed (111) Al surface.