Thermogravimetric Tests Research Articles

Effect of oxygen and oxidation on tensile properties of V5Cr5Ti alloy

Oxidation studies were conducted on V5Cr5Ti alloy specimens in an air environment to evaluate the oxygen uptake behavior of the alloy as a function of temperature and exposure time. The oxidation rates calculated from parabolic kinetic measurements of thermogravimetric testing and confirmed by microscopic analyses of cross sections of exposed specimens were 5, 17, and 27 μm per year after exposure at 300, 400, and 500°C, respectively. Uniaxial tensile tests were conducted at room temperature and at 500°C on preoxidized specimens of the alloy to examine the effects of oxidation and oxygen migration on tensile strength and ductility. Microstructural characteristics of several of the tested specimens were determined by electron optics techniques. Correlations were developed between tensile strength and ductility of the oxidized alloy and microstructural characteristics such as oxide thickness, depth of hardened layer, depth of intergranular fracture zone, and transverse crack length.

Technical noteRoles of silicon and aluminium in high temperature oxidation behaviour of low alloy steel ingots in a simulated combustion atmosphere

AbstractThe effects of (0·5-2%)Si and (0 or ∼0·4%)Al contents in a low alloy steel ingot on high temperature oxidation in a simulated combustion atmosphere (N2-6·55CO2-18·84H2O-3·75O2) at 1050°C have been investigated. The experimental methods include thermogravimetric tests and analyses of oxidation products. The results indicate that the major oxidation kinetics obeyed a parabolic rate law, and diffusion of Fe2+ in the FeO layer was the rate controlling step. Increase in Si content decreased oxidation, because the FeO + Fe2SiO4 layer between the scale and the substrate became more compact or thicker. The effect of Al depended upon the Si content. At 0·5%Si, alloying with Al reduced the oxidation due to the segregation of FeAl2O4 at the scale/base metal interface. At 1%Si, Al promoted oxidation due to the doping effect of p type semiconducting oxide by small amounts of Al3+. At 1·5 and 2%Si, Al did not affect oxidation.

Alumina-Metal (Fe, Cr, Fe0.8Cr0.2) Nanocomposites

Metal (Fe, Cr, Fe0.8Cr0.2)/Al2O3 composite powders containing from 2 to 20 wt% metallic phase were prepared by selective reduction of oxide solid solutions and studied using mostly TEM and magnetic measurements. The microstructure of the composites is strongly dependent on the morphological and microstructural characteristics of the starting oxide. Most of the metal particles are < 10 nm and are dispersed inside the matrix grains. Hydrogen reduction at temperatures < 1000°C results in the formation of an iron aluminate spinel phase at the Fe0/Al2O3 interface. Massive composites were prepared by hot-pressing and were found to contain both intragranular (≤30 nm) and intergranular (≤1 μm) metal particles. The former particles are responsible for the strong mechanical reinforcement observed at low metal content, while further enhancement is caused by the latter. Thermogravimetric tests showed that a high metal content and the presence of chromium are beneficial with respect to the resistance to oxidation in air, which is negligible below 1000°C.

Points to be considered in thermogravimetry

AbstractContinuous thermogravimetry is a common method to test materials in gaseous corrosive environments at high temperatures and to elucidate kinetics and mechanisms of high temperature corrosion. Recommendations how to conduct thermogravimetric tests are collected here, points to be considered including sample size and form, surface preparation, reaction chamber, starting procedure, sensitive measurements, gas supply and dosing. This study is meant as a starting foundation for establishing guidelines in high temperature corrosion research.

The thermal behaviour of the ethylenebisdithiocarbamates Maneb, Zineb and Mancozeb.: Part 1. The mechanism of the thermal decomposition

The thermal decomposition of Maneb, Zineb and Mancozeb has been investigated by means of thermal analysis techniques. IR, X-ray diffraction, gas chromatography, high pressure liquid chromatography, etc. were carried out on the samples, which were withdrawn at various stages during thermogravimetric testing or during thermal decomposition in vacuum at constant temperature. The mechanisms of the thermal decomposition and the molecular structures of Maneb, Zineb and Mancozeb are proposed.

High-temperature oxidation and thermal cycling of aluminum-electroplated stainless steels

An alumina coating, produced from the oxidation of an aluminum-electroplated deposit, improved the oxidation resistance in air of a ferritic, AISI-type 446 stainless steel, Fe-24Cr-1.2Al containing 0.15% of mischmetal, and an austenitic AISI 321 stainless steel containing 0.53% Ti, at least up to 1100°C. In thermal-cycling tests from 1000°C to room temperature, the alumina coating was adherent on the ferritic and austenitic steels, for at least 1000 and about 700 cycles, respectively. The addition of rare earths to the ferritic steels and titanium to the austenitic, provided good adhesion between the coating and substrate. The porous nature of the coating was found to be very beneficial by causing the coating to be more resistant to thermal and growth stresses. Oxidation mechanisms are discussed in the light of results obtained from the thermogravimetric tests and metallographic observations by SEM-ED analysis.

Developing matrices for hardmetals: T.J. Davies and A.A. Ogwu, (University of Manchester, UK)

Two ultra-high temperature HfB2–SiC ceramics were successfully consolidated by hot-pressing (HP) and spark plasma sintering (SPS). The powder mixture HfB2 + 30 vol% SiC was brought to full densification with the addition of 2 vol% TaSi2 as sintering aid, and applying the following conditions: 2100 °C for 3 min (SPS), or 1900 °C for 35 min (HP). The microstructure consisted of regular micrometric diboride grains and SiC particles homogeneously distributed. The major secondary phases were HfO2, and (Ta, Hf)-mixed or Hf carbides in the materials processed by SPS and HP, respectively. Both SiC and TaSi2 beneficially contributed to boost the sinterability of HfB2 at elevated temperatures.The mechanical properties showed interesting potential. Elastic moduli above 490 GPa were measured. Flexural strengths at room temperature and 1500 °C (in air) of the hot-pressed composite were 665 ± 75 and 480 ± 30 MPa, respectively. Machining-induced flaws rather than fabrication defects adversely affected the room temperature strength of the spark plasma sintered material, leading to premature failure. The steep cooling up to 1000 °C in about 2 min associated to SPS induced large unrelaxed thermal stresses, which enhanced the tendency to micro-cracking during machining. However, such a strained configuration had a beneficial effect on fracture toughness.In the temperature range of 1450–1650 °C both the as-fired materials tolerated acceptably the oxidation attack in air. Thermo-gravimetric tests at 1450 °C for 20 h had mass gains of 4.10 ± 0.02 and 3.30 ± 0.02 mg/cm2 for the materials processed by HP and SPS, respectively, and decelerating kinetics were recorded, although not conclusively parabolic.

Electronic transport in thermally grown Cr2O3

Electrical conductivity of thermally grown Cr2O3 has been measured as a function of temperature and over a range of oxygen partial pressures from that of air to that of the Cr/Cr2O3 equilibrium. The conductivity showed p-type behavior over the range of the present investigation. At temperatures above 1000°C, the conductivity values were independent of oxygen partial pressure and indicated intrinsic semiconductor behavior. The mobility of holes, determined by measuring conductivity at fixed compositions (i.e., fixed δ in Cr2-δO3), increased with temperature. This behavior can be attributed to hopping-type conduction. For δ ∼ 10−5, the activation energy for hole hopping was 0.248 eV, and the calculated hole mobilities were 5.4x10−2 and 2.4x10−1 V/cm2 · s at 500 and 1000°C, respectively. The oxidation kinetics of Cr were determined by measuring the electrical conductivity and electromotive force across the oxide layer at 875°C. The result agreed well with the oxidation data obtained in thermogravimetric tests.

The role of microstructure on oxidation of Ni-Cr-Al base alloys at 1023 and 1123 K in air

The oxidation behavior at 1023 and 1123 K in air has been followed for several Ni-Cr-Al ternary alloys and with small additions of refractory elements Nb, Mo, and Ta. Prior to testing, the alloys were characterized by microscopic and microanalytic methods. Microstructural observations have shown that the formation of oxide layers depends on the original microstructure. Discontinuous thermogravimetric tests were used to relate the behavior of different chromium, aluminum, and refractory-element additions, and eventually to service conditions in similar environments in which general oxidation takes place. The addition of refractory elements generally impaired the oxidation resistance.

Pozzolanic and cementitious reactions of fly ash in blended cement pastes

Results of thermogravimetric tests on hardened cement pastes containing fly ash are reported. Ashes derived from subbituminous and lignite coal were used at replacement levels of 30% and 50% by weight. Thermal analysis was performed on samples which were water cured for various ages from 3 days to one year. An analytical technique was used to split the non-evaporable water content of cement pastes into two components: water held by calcium hydroxide, and water held in other reaction products. The technique is used to identify and monitor the progress of reaction of the different types of fly ash. There are distinct differences in the manner in which the various ash/cement pastes gydrate. The subbituminous ash relies more upon the “pozzolanic reaction” between calcium silicate and calcium hydroxide to provide the strength-giving hydration product. Conversely, the lignite ash produces a substantial amount of hydrate by direct reaction between compounds of the ash and water. At early and intermediate ages the hydrate produced from the reaction of fly ash may combine substantially more water per unit weight than the hydrates normally produced through the reaction of cement.

Effect of coatings on oxidation of Ti-6Al-2Sn-4Zr-2Mo foil

The viability of using coated Ti-6Al-2Sn-4Zr-2Mo foils at 620°C in air was established through mechanical and thermogravimetric testing. Weight-grained and oxygen embrittlement were significantly reduced by the coatings. The residual tensile elongation of coated specimens was 2.5 times that of uncoated specimens. Comparison depth-profiling with X-ray diffraction verified the reduction of oxygen solid-solution in the α-phase for a selection of coated specimens.

Corrosion resistance of chromium carbide coatings in oxygen-sulfur environments

Oxidation of metallic materials at elevated temperatures in bioxidant environments is a potential problem in many systems in the chemical, petroleum and power-generating industries. The use of appropriate corrosion-resistant coatings on metallic components offers an avenue to minimize the material degradation and to extend the life of the component. This paper discusses results obtained when Type 310 stainless steel (SS) and chromium-carbide- and chromium-coated Type 310 SS were exposed to oxidizing and sulfidizing gas mixtures. The coatings were developed by an electro-spark deposition process in which short-duration, high-current electrical pulses were used to deposit the electrode material on the metallic substrate. Thermogravimetric test data, observations on the morphology of coating layers, and hardness depth profiles for coated and uncoated specimens after exposure to several gas mixtures at temperatures of 875 and 650 °C are presented.

Thermogravimetric Analysis and Pyrolysis Kinetics of Cotton Fabrics Finished with Pyrovatex CP

Unmodified cellulose and cellulose fabrics finished with different N/P mole ratios of Pyrovatex CP were subjected to thermogravimetric analysis and flame retardant tests. The relationship between kinetic data and flame retardancy are interpreted for the main cellulose pyrolysis stage; the mechanism of cotton fabrics finished with reactive flame retardants is also discussed.

ChemInform Abstract: CHEMICAL REACTIONS INVOLVED IN THE INITIATION OF HOT CORROSION OF IN‐738

AbstractDie NazsO4‐induzierte Heißkorrosion der präoxidierten Ni‐Legierung IN‐738 wird unter besonderer Berücksichtigung der langen lnduktionsdauer (ca.

Chemical Reactions Involved in the Initiation of Hot Corrosion of IN‐738

Sodium‐sulfate‐induced hot corrosion of preoxidized IN‐738 was studied at 975°C, with emphasis on processes occurring during the long induction period. Thermogravimetric tests were run for predetermined periods of time, after which one set of specimens was washed with water to give information about water‐soluble metal salts and residual sulfate. A second set of samples was cross‐sectioned dry and polished in a nonaqueous medium. Element distributions within the oxide scale were obtained from electron microprobe x‐ray micrographs. Evolution of was monitored throughout the thermogravimetric tests. Kinetic studies were performed for several pertinent processes, and rate constants were obtained for the following chemical reactions

ChemInform Abstract: CHEMICAL REACTIONS INVOLVED IN THE INITIATION OF HOT CORROSION OF B‐1900 AND NASA‐TRW VIA

AbstractDie Ni‐Basis‐Legierungen B‐1900 und NASA‐TRW VIA werden bei 900°C voroxidiert, mit Na2 S04 beschichtet und ebenfalls bei 900°Korrosionstests durchgeführt.

Corrosion testing of nickel base superalloys and coatings with molten sulphates at 900°C

AbstractThe work refers to the application of electrochemical testing procedures on the corrosion properties of different commercial nickel base superalloys. This included the following alloys: IN 738 LC, IN 939, Nimonic 81, Nimonic 90, IN 713 LC, IN 792 5 A, IN 100, IN 853, IN 617, Nimonic 105, B 1925, N 33 Hastelloy X, Mar M 246, Mar M 421, Mar M 432, IN 671, Nicrofer 7520, IN 800.Beside a short galvanostatic test with subsequent metallographic measurement of the resulting corrosion damage, potential/current density curves and potential/time curves were taken from selected alloys. In all cases the corrodent was a melt of (Na0.9, K0.1)2SO4 at 900°C. Thermogravimetric tests were run for comparison of three alloys (IN 738 LC, IN 939, IN 713 LC). The existence of ranges of passivation and induction periods could be established. The galvanostatic corrosion test was also applied to the testing of two different coatings (S 57 and LDC 2) on IN 738 LC and Mar M 246 or IN 100. In all cases scale compounds and intermetallics in coatings were analyzed by XRD. The electrochemical test for nickel base superalloys is a useful tool for a quick ranking of alloys. For the assessment of coatings more work will be necessary.

Chemical Reactions Involved in the Initiation of Hot Corrosion of B‐1900 and NASA‐TRW VIA

Sodium sulfate‐induced hot corrosion of preoxidized B‐1900 and NASA‐TRW VIA has been studied at 900°C with special emphasis placed on the chemical reactions occurring during and immediately after the induction period. Thermogravimetric tests were run for predetermined periods of time after which one set of samples was washed with water. Chemical analysis of the wash solutions yielded information about water‐soluble metal salts and residual sulfate. A second set of samples was run, cross‐sectioned dry, and polished in a nonaqueous medium. Element distributions within the oxide layer were obtained from electron microprobe x‐ray micrographs. A third set of samples was subjected to surface analysis by ESCA. Evolution of was monitored throughout many of the thermogravimetric tests. Results are interpreted in terms of acid‐base fluxing mechanisms.

Sulfidation-oxidation of advanced metallic materials in simulated low-Btu coal-gasifier environments

The corrosion behavior of structural alloys in complex multicomponent gas environments is of considerable interest for their effective utilization in coal conversion schemes. Little understanding of the degradation mechanisms of advanced high-temperature alloys in conditions typical of low-Btu coal gasifiers presently exists. Analysis of scale and subscale characteristics of several alloy types after exposure to the aggressive simulated low-Btu gasifier environments yielded a reaction model for these sulfidation-oxidation conditions. Initial competition between reactive metal oxide and base metal sulfide nuclei is followed by base metal sulfide overgrowth, chromium sulfide formation at the scale-metal interface, dissociation near voids in the subscale, and internal chromium sulfide precipitation. Additions of aluminum, titanium, and an oxide dispersion improve the sulfidation resistance by increasing the number of oxide nucleation sites and their growth kinetics on the surface in the crucial competition stage. Thermogravimetric tests carried out in three mechanistic regimes agreed with these hypotheses.

Reactions between methanol and portland cement paste

The paper concerns the suitability of methanol replacement as a method for drying cement paste specimens prior to microstructural examination. Thermogravimetric tests on both cement paste and calcium hydroxide which were soaked in methanol for various periods show that methanol alters sample composition by reacting to form a carbonate-like product. Also, some of the methanol is found to remain with the solid at temperatures above 300°C. It is concluded that until more is known about the interaction between methanol and the cement paste constituents, the correct interpretation of test results of methanol treated specimens is likely to be very difficult.