ABSTRACT

In CO-rich coal gasification atmospheres, sulfidation attack of metallic components can cause severe wastage problems. In these highly reducing environments, Si is one of the elements with a high enough affinity for oxygen to enable a silica-rich scale to be formed which may provide reasonable protection of the alloy substrate, In this study, the corrosion behavior of a commercial 12%Cr ferritic alloy is compared with the resistance of a series of specially cast 12%Cr and 12Y0Cr-12%Nl “model” alloys having Si levels ranging from 1.3 to 4 %. Two higher Cr-Ni austenitic alloys (25%Cr-20%Ni and 20%Cr-32%Ni) have also been tested. Coupons from each material have been exposed to a non-equilibrated CO-based gas mixture containing 0.8% H2S for up to 2000 hours. Weight changes and surface and cross-sectional examinations have been used to measure the kinetics and establish the morphology, type and extent of corrosive degradation.

INTRODUCTION

Metallic heat exchanger tubes in coal gasification plants can suffer high losses in section during exposure to the low oxygen, H2S-containing process gases generated during the high temperature combustion of cord. In these types of atmospheres, lower-alloy steels corrode at unacceptable rates at temperatures higher than about 250W-300ºC and simple 12%Cr steels are also inadequate. For higher temperatures, therefore, the use of austenitic stainless steels or the application of coatings is necessary but this, of course, increases component costs. Therefore, this study explores ways of improving the corrosion resistance of less expensive materials in the hope of extending their industrial use. One way of doing this is to add a strong oxide-forming element to the alloy which is capable of forming a protective scale at the operating temperature, even under the severely reducing process conditions.

The principal elements which have a high affinity for oxygen are Cr, Al and Si. However, Cr is a strong sulfide-former and can form a number of sulfides in addition to Cr2O3. With regard to the use of Al, A12O3 forms only slowly at low temperatures and also rare-earth additions are generally needed to improve the adhesion of Al-rich scales. The use of Si as an addition has, to a certain extent, been under-valued, mainly because of several possible drawbacks which may be introduced by the use of this element, not least of which is its adverse effect upon fabricability and weldability. In considering this element as an oxide-forming additive, the maximum allowable level in the alloy must therefore take account of fabrication and welding aspects as well as improvements in corrosion resistance.

The results described in this paper form part of a project funded by EPRI and the Commission of the European Communities’ Institute for Advanced Materials, which studied the corrosion behavior of a series of 6 specially cast 12%Cr and 12%Cr- 12%Ni alloys having Si contents ranging from 1.3 to 4.0 %. Samples of each alloy have been exposed in a laboratory autoclave to a CO-based gas mixture containing 0.8 % H2S, similar to the type of atmosphere found in a dry-feed entrained slagging gasifierl

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