Research Papers

Adhesion of Oxides Grown in Supercritical Water on Selected Austenitic and Ferritic/Martensitic Alloys

[+] Author and Article Information
D. Artymowicz

Department of Chemical Engineering
and Applied Chemistry,
University of Toronto,
200 College Street,
Toronto, ON M5S 3E5, Canada
e-mail: dorota.artymowicz@utoronto.ca

C. Bradley, B. Xing, R. C. Newman

Department of Chemical Engineering
and Applied Chemistry,
University of Toronto,
200 College Street,
Toronto, ON M5S 3E5, Canada

Manuscript received May 30, 2015; final manuscript received October 28, 2016; published online March 1, 2017. Assoc. Editor: Thomas Schulenberg.

ASME J of Nuclear Rad Sci 3(2), 021006 (Mar 01, 2017) (8 pages) Paper No: NERS-15-1104; doi: 10.1115/1.4035331 History: Received May 30, 2015; Revised October 28, 2016

A series of austenitic alloys (800H, H214, I625, 310S, and 347) with different surface finishes were exposed to supercritical water (SCW) at 550 °C and 2.5 × 107 Pa for 120 h, 260 h, and 450 h in a static autoclave with an initial level of dissolved oxygen of 8 ppm. Indentation with a hardness indenter was used for assessment of oxide adhesion. This was compared with the results of a similar test on SCW-oxidized ferritic alloys. Delamination in all the tested ferritic alloys was insufficient for quantification of the results but allowed for qualitative comparison within this group. In the set of austenitic alloys, oxide on stainless steel (SS) 347 exfoliated during cooling from 550 °C, and from the remaining four alloys, only oxide on H214 delaminated, which made the qualitative comparison across the whole group impossible. Energy dispersive X-ray spectroscopy (EDX) revealed that under delaminated external Cr2O3 on H214 alloy, there was a submicron thick layer of Al-rich oxide. To investigate a possible oxide spallation on austenitic samples during exposure, mass loss obtained through descaling was compared with mass gain due to SCW exposure. The results indicated that the applied descaling procedure did not, in most cases, fully remove the scale. Apart from one case (SS 347 with alumina surface finish), there was no clear indication of oxide spallation.

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Fig. 1

Example of the extent of delamination determination, Fe–13Cr–1.5Si ferritic/martensitic alloy. The inner and outer circles on the micrograph (d) represent the plastic zone of radius R and the equivalent delamination zone of radius r, respectively.

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Fig. 2

Extent of delamination of ferritic/martensitic alloys (dashed line separates 9% Cr from 13% Cr alloys)

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Fig. 3

Delamination modes observed in (a) Fe–13Cr–1.5Si and (b) Fe–9Cr. The capital letters designate: A—the outer, Fe-rich oxide; B—outer/inner oxide interface (this is the original surface of the coupon); C—the inner mixed Cr-rich oxide; and D—inner oxide/metal interface. Fe-13Cr-1.5Si was the only F/M alloy with delamination mode depicted in Fig. 3(a). All remaining F/M alloys delaminated as shown in Fig. 3(b). The scale bar is 50 μm.

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Fig. 4

Indented 310S sample showing cracks without delamination

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Fig. 5

SEM micrograph of alumina finished coupon of SS 347 after 450 h exposure to SCW showing exfoliation

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Fig. 6

SEM micrograph of indented exposed H214 sample with alumina fine polished surface. The white line shows the delamination extent.

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Fig. 7

Partially delaminated oxide on indented H214 alumina polished coupon

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Fig. 8

SEM micrograph of a cross section of 600 grit finished H214 after 450 h SCW exposure. A segment of the interface between the Cr-depleted zone and the unoxidized metal is indicated with a set of black arrows.

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Fig. 9

EDX line scan taken on a cross section of the 600 grit finished H214 coupon exposed to SCW for 4150 h. The size bar is 3 μm.

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Fig. 10

A schematic representation of an oxidized surface of a metal. The mass of the oxide was separated into that of metallic elements (Moxm) and oxygen (Moxo). m0, m1, and m2 represent the mass of an unexposed, exposed, and descaled coupon, respectively. A section of the oxide marked with gray net pattern (Moxex) indicates the part of oxide that exfoliated during the exposure.

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Fig. 11

Mass gain to mass loss ratio for austenitic alloys exposed to SCW. Upper and lower graphs show data for coupons with as-received and alumina surface finish, respectively. The exposure numbers 1, 2, and 3 correspond to exposure times 120 h, 260 h, and 450 h, respectively, and align with the vertical grid lines: dotted for 120 h, dashed–dotted for 260 h, and dashed for 450 h. The two continuous gray horizontal lines indicate the upper (Cr2O3) and lower (NiO) bounds of α excluding Al2O3. The horizontal-dashed line indicates the value of α for Al2O3. Majority of measurements were performed on three samples, and error bars represent the standard deviation of the mean. In four cases, the measurements were performed on a single sample and no error could be associated with the reported results. These four cases are S347_AR_120 h, S347_AR_450 h, S347_ALU _450 h, and I625_ALU_240 h.




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