ABSTRACT

Samples of alloy 800 H (UNS N08810) and alloy 617 (UNS N06617) were oxidized in o

simulated process gas atmosphere. The principal ele~nents of the gaseous environment were 50 %H20,

35?%0H2 and 5°A of C02, CO, and CH4, respectiw~ly and a total pressure of 140 IcPa. This gas in it

state of thermodynamic equilibrium establishes oxyg,m partial pressures ranging from 3.0 x 10-17 I@,

at 800 ?C to 3,2 x 10-14 kPa at 950 ?C. The expo ;ure time was from 600 to 5000 hours, Ailer th~;

exposure the alloy surfaces showed compact oxide ayers, On alloy N08810 a double oxide layer wal;

built. A first layer was identified as the spine] phas;, Mn( 1+x)Cr2( 1-x) Tix04 The second one is the

corundum type phase, Cr2-YTiyO~, On alloy N066 17, lack of manganese produces the growth of o

single layer, which was also Identified as the phase Cr2-yTiy03

INTRODUCTION

Gasification of coal requires temperatures M high as 800-1000 ?C. These temperatures are

produced conventionally. From the stand point of an optimization of the resources the heat generated by

a high temperature reactor has been taken into account through the German PNP-Project (Prototype

plant for Nuclear Process heat). In this project high temperature alloys such as alloy 617 (N06617) ani

alloy 800 H (N088 10) are possible candidates fo - the necessary heat transfer walls. In order to

understand the scale formation mechanism and simulate the oxidizing process and growth of the oxide

layers on these alloys, samples were oxidized in a silnulated PNP gas atmosphere. The thermodynamic

equilibrium of such an atmosphere establishes very low oxygen partial pressures, at the oxidation

temperatures of interest. This work presents qualit ~tive and quantitative characterization of the oxide

scales using techniques of electron probe microanaly:~is (EPMA) and X-ray diffraction data.

EXPERIMENTAL

Weighed samples of a Ni-based alloy (UNS N066 17) and an Fe-based (UNS N088 10) allo~,

Table 1, were oxidized in a simulated process gas atmosphere. The principal elements of the gaseous

environment were 50 %H20, 35%H2 and 5°/0of C02, CO, and CH4, respectively. The total pressure

was 140 kPa. The components of this atmosphere in equilibrium, establishl J2 an oxygen partial pressure

ranging from 3.0 x 10-] 7 to 3.2 x 10-?4 kPa and a carbon activity from 2 x 10-2 to 3.5 x 10-3, Table 2.

Oxidation temperatures and exposure times for the ir vestigated samples are given in Table 3.

After the oxidation experiments, the sampl>s were weighed, cross-sectioned, embedded in a

synthetic resin and examined metallographically. The elements of the oxide scales were determined

qualitatively by electron, X-ray images and linesc ans and quantitatively by stepscans, all of them

techniques of the Electron Probe Microanalysis (C 4MEBAX MBX 100). A linescan is a continuos

sweep of the electron beam, on an imaginary line otI the sample?s surface Contrary to this technique,

by the stepscan, the electron beam is placed on a sanip]e area (radius about 1pm) and each measurement

is taken one at a time. X-ray ditiraction measurements were also carried out.

Using the stepscan technique, quantitative plmctual analyses were done on three regions of the

chrome oxide layers, parallel to the surface. These -egions were arbitrary named, A (outside), B

(middle ) and C , ( inside). Later it will be shown that these nominations correspond to regions in the

chrome oxide at the interphase of a spinel phase, at the center, and at the interphase oxide/ substrate,

respectively. Concentration profiles were also measured in the depletion zone and integrated in order to

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