Materials at High Temperature Vol 18, Issue 1, 2001

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High temperature oxidation of the bicrystals of Incoloy alloy 909 and an Fe-36% Ni alloy in air under tensile stress†

Xiping Guoa and Kiyoshi Kusabiraki

Faculty of Engineering, Toyama University, Toyama 930-8555, Japan

aPresent address: State Key Laboratory of Solification Processing, Northwestern Polytechnical University, Xian 710072, China

The bicrystals of both an Fe-36% Ni alloy and Incoloy alloy 909 were oxidized at 1200 and 1250 K in air respectively with a tensile stress of 2.5, 4.9, or 14.7 MPa. The thickness of external scale, intergranular subscale and intragranular subscale all exhibited linear relationships with the square root of oxidation time, indicating that oxidation followed parabolic law. In oxidized Fe-36% Ni bicrystals, leading cellular oxide nodules formed in intragranular subscale and a tensile stress promoted intergranular oxidation and the formation of cavities at the grain boundary. In oxidized Incoloy alloy 909 bicrystals, intergranular subscale only slightly led the internal subscale. However, intergranular oxidation was obvious in equiaxed specimens of this alloy. The external scale was stratified in Fe-36% Ni alloy, but it exhibited cellular or pocket-like structure in Incoloy alloy 909. The latter alloy possessed a much better resistance to oxidation than the Fe-36% Ni alloy.

Keywords: Incoloy alloy 909, Fe-36% Ni alloy, bicrystal, high-temperature oxidation, scale formation, oxidation kinetics, tensile stress

Corrosion performance of types 310S and 410 stainless steels in forming gas containing 1% H2S or 1% H2S/1% HCl at 600°C†

T.P. Levi1, K.A Lichti1 and A.J.Tack2

1Materials Performance Technologies Limited, PO Box 31-310, Lower Hutt, New Zealand

2MPT-Matcor Pte Ltd., 16 Science Park Drive, #03-04A The Pasteur, 118227 Singapore

The corrosion of types 310S and 410 stainless steels in forming gas containing 1%H2S or 1%H2S/1%HCl has been studied at 600°C. In the conditions chosen, sulphide scales formed on both materials and significant differences in corrosion rate and depth of attack were observed. Thermogravimetric results showed that for both materials there was an initial transient period of rapid sulphidation after which linear or parabolic kinetics were observed. Type 310S and 410 both developed non-adherent, multi-layered sulphide corrosion products. Scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction were used to examine the structure and composition of these sulphide scales. The differences in corrosion morphology are in agreement with the variations in the thermogravimetric experiments. Mechanistic corrosion models are being developed.

Keywords: stainless steels, high temperature corrosion, hydrogen sulphide, hydrogen chloride

Scale formation on Haynes 230 alloy in a synthetic biomass ash between 400 and 800°C

C. Liu, J.A. Harvey and J.A. Little

Department of Materials Science and Metallurgy, Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK

Hot corrosion of Haynes 230 alloy by a synthetic biomass ash has been investigated under isothermal conditions (400, 600 and 800°C). The ash composition simulates typical ash deposits formed on fireside heat exchanger tubes under biomass co-combustion situations. The alloy shows oxidation and a little hot corrosion at 400°C for up to 100 h. Reaction at the medium temperature of 600°C causes hot corrosion enhanced oxidation and localised intergranular attack. The localised severe scale growth and intergranular attack are found to be associated with the alkali salts deposited on the alloy surface. Hot corrosion dominates at 800°C and causes significant material wastage. Possible mechanisms responsible for the synergistic effect between oxidation and hot corrosion are discussed.

Keywords: Haynes 230, synthetic ash, scale formation, oxidation, hot corrosion

Corrosion fatigue of alloy 625 weld claddings in combustion environments

K. Luer1, J. DuPont1,A. Marder1 and C. Skelonis2

1Energy Research Center, Lehigh University, Bethlehem, PA, USA; 2Pennsylvania Power & Light Allentown, PA, USA

Alloy 625 weld clad boiler tubes retrieved from two supercritical pulverized coal-fired utility boilers after less than 24 months service were extensively cracked. Most of the cracks tapered towards a single crack tip whereas some cracks branched at the crack front. Corrosion products in the cracks consisted of a primary phase and a secondary phase or “spine”. The chemistry of the corrosion products was complex. The primary phase was considered a Cr-based oxide or oxide–sulfide phase mixture whereas the composition of the secondary phase was considered a nickel-based sulfide. The cause of the cracking was attributed to corrosion-fatigue. Corrosion was attributed to traditional oxidation and sulfidation mechanisms commonly found in combustion environments. Sources of thermal fatigue stress and stress concentrators were identified. In the dendritic weld microstiucture, dendrite cores depleted in Mo and Nb were susceptible to preferential oxidation–sulfidation. Cracks initiated at these areas and propagated the farthest in the valley of weld ripples following the main axis of the columnar dendrite cores. To explain the corrosion morphology within the cracks, a mechanism was proposed based on fatigue cracking of layered multiphasic corrosion scales formed on a compositionally segregated weld microstructure.

Keywords: corrosion fatigue, alloy 625 weld clad boilers, combustion

Corrosion behavior of ferritic steels on the air sides of boiler tubes in a steam/air dual environment†

Kiyoakazu Nakagawa1,Yasuo Matsunaga1 and Takahiro Yanagisawa2

1Research Laboratory , Ishikawajima-Harima Heavy Industries Co., Ltd, Tokyo, Japan

2Boiler Basic Design Dep., Ishikawajima-Harima Heavy Industries Co., Ltd, Tokyo, Japan

The corrosion behavior of heat resistant ferritic steels for boiler tubes in a steam/air dual environment simulating the non heated areas at 823, 873 and 923 has been investigated. Corrosion rates of the ferritic steels on the air sides under a condition of steam/air dual environment are increased more significantly than those in simple air conditions because the permeation hydrogen from steam sides. Corrosion morphology on the air sides closely resembles that on the steam sides, namely, a two-layered oxide scale is formed. However, Fe2O3 is observed in the outermost layer of the scale. The inner scales on the air sides are found to grow parabolically with time for all steels tested. The hydrogen permeation current measurements show that the amount of permeated hydrogen from the steam sides decreases parabolically with time, and also decrease, with increasing Cr contents in the steels.

Keywords: corrosion behavior, ferritic steels, boiler tubes

Salt corrosion of a hot-pressed silicon nitride in combustion environments with different sulphur contents

F.A. Costa Oliveira1 and D.J. Baxter2

European Commission – DG Joint Research Centre, Institute for Advanced Materials, P.O. Box 2, NL-1755 ZG Petten, The Netherlands

1On leave from INETI, Instituto Nacional de Engenharia e Tecnologia Industrial, Estrada do Paço do Lumiar, 1649-038 Lisboa, Portugal.

*To whom correspondence should be addressed. E-mail: BAXTER@JRC.NL

Dense Si3N4 hot-pressed with the aid of 9 wt% Y2O3 was exposed to salt-containing combustion environments at 1,000°C and 1,200°C for up to 500 h in a low-velocity burner rig. Sodium was added as in the form of synthetic sea water. Tests were carried out above the dew point of Na2SO4 in order to prevent sodium sulphate deposition on exposed ceramic surfaces. The concentration of sulphur in the fuel used, was either 0.01 wt% (aviation kerosene) or 1 wt% (industrial or marine diesel fuel). Considerable corrosive degradation was observed for materials exposed in the low-sulphur fuel, as a result of the formation of a low viscosity Na2Ox(SiO2) corrosion product. The rate of corrosion in the high-sulphur fuel combustion environments was similar to that recorded in ‘dry’ air. A mechanistic model based on the experimental data available is presented.

Keywords: corrosion, oxidation, Si3N4, Na2SO4, burner rig

Test methods and data on the mechanical

properties of protective oxide scales†

M. Schütze1, S. Ito2,W. Przybilla3, H. Echsler1 and C. Bruns4

1Karl-Winnacker-Institut der DECHEMA e.V., D-60061 Frankfurt/Main, Germany

2Nippon Steel Corp., Chiba 293-8511, Japan

3ELF-Atochem Deutschland GmbH, D-53121 Bonn, Germany

4Witzenmann GmbH Metallschlauch-Fabrik, D-75175 Pforzheim, Germany

The present paper describes the key mechanical properties of oxide scales with regard to model equations and investigation methods. Modified test methods are introduced for the determination of values describing scale failure, growth stresses in oxides and creep of the scales. Furthermore, a summary of scale fracture data from the literature is given. As a conclusion it is discussed that failure strains of oxide scales can be estimated from the model equations and using basic mechanical property data like fracture toughness and geometrical scale parameters as physical defect size, interface roughness, scale thickness etc. without complicated mechanical tests. In the end a universal mechanical properties diagram is introduced, which describes fracture and creep behavior of the oxide scales.

Keywords: oxide scales, mechanical properties, critical strains, fracture toughness, scale defects