ABSTRACT

A new nickel-base high-temperature alloy, alloy 10 Al, which contains 30% iron, 10% aluminum and 8% chromium, has been developed. Alloy 10 Al has a lamellar eutectic two-phase microstructure with one phase being the intermetallic phase NiAl and the second phase being Ni(Fe,Cr) solid solution. The high-temperature corrosion behavior of the new alloy has been determined in both air and hot process gases containing methane, sulphur dioxide and hydrogen sulphide, respectively. It was found that the corrosion resistance against carburization, sulphidation and oxidation was excellent due to the formation of a dense protective alumina scale. The adherence of the alumina scale was increased by an addition of 0.1% haftium.

The concentration of chromium was found to have a remarkable impact on oxidation and high temperature corrosion resistance. Alloys without chromium showed an increased corrosion rate in both air and sulphur-containing gas atmospheres due to the initial formation of nickel oxides. In sulphidizing gases at least 4 % chromium are required to stabilize the formation of alumina and to prevent the formation of nickel/sulphur compounds.

INTRODUCTION

Hot corrosive gases containing high amounts of sulphur and/or carbon are encountered by metallic materials in a variety of processes in coal conversion, petrochemical industry and chemical industry. At moderate temperature iron or nickel base alloys containing 15-30 0/0chromium are widely used. Those materials are protected against corrosion attack by formation of a thin, protective chromia scale.

At high temperatures and/or low oxygen partial pressures, however, chromia scales become nonprotective. Compared to chromia-formers, materials forming alumina scales have several advantages. The rate constant of alumina formation is lower than that of chromia formation. Unlike chromia, alumina does not evaporate at temperatures above 1000 ?C and it is formed even under conditions with very low oxygen partial pressures due to its high thermodynamic stability. Consequently there are world-wide efforts to develop new alumina-forming high temperature materials both for structural applications and coatings. Special emphasis has been put on the development of nickel aluminizes NiAl and NisA1. Both materials are very resistant to high temperature corrosion because of their formation of a dense, protective alumina scale3>4.Their major drawback is their brittleness at low temperatures (NiAl) or high temperatures (NisAl).

It was our objective to develop a material which combines the excellent high temperature corrosion resistance of intermetallic phases on the basis of aluminizes with the fabricability of conventional nickel base wrought alloys. The present work focuses on the effects of aluminium and chromium on the high- temperature corrosion behavior of the newly developed nickel-iron-aluminium chromium alloy Alloy 1OAl.

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