Corrosion of 9Cr Steel in CO2 at Intermediate Temperature I: Mechanism of Void-Induced Duplex Oxide Formation

Abstract

Under CO 2 exposure at an intermediate temperature, typically 550°C, 9Cr-1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe-Cr rich spinel oxide layer. It is proposed that the inner Fe-Cr spinel layer grows according to a mechanism involving void formation at the oxide/ metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO 2 , which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature II: Mechanism of Carburization

“…Since CO 2 is composed of oxygen and carbon, a more specific study on carbon localisation and its becoming in the scale and matrix has been carried out and is exposed in this paper. Its traceability through the steel surface could indicate if it is in good agreement with the proposed void-induced oxidation mechanism [1]. Moreover, it has been shown in the past, that carburization could be at the origin of a catastrophic phenomenon occurring on mild steel and 9-12 Cr steel called ''breakaway oxidation'' [2][3][4][5].…”

“…These observations could suggest a very low diffusion of carbon from the oxide/gas interface. • The Fe-Cr rich spinel oxide layer growing inside T91 steel [1] is slightly depleted in carbon compared to the initial bulk carbon content. Nevertheless, an accumulation bump of carbon still appears at the magnetite/Fe-Cr spinel oxide interface and its magnitude increases with time (between 110 and 1,000 h).…”

“…It has been shown in a previous paper [1] that 9%Cr-1%Mo steel forms under CO 2 at 550°C a duplex oxide scale made of an outer magnetite layer and an inner spinel oxide layer Fe 2.3 Cr 0.7 O 4 . This scale grows according to a void induced mechanism: the inner scale forms inside voids created by the condensation of iron vacancies at the oxide/metal interface.…”

“…Figures 3, 4 and 5 show the carbon profiles through T91 surface after exposition in CO 2 for 1, 322 and 1,000 h, respectively. The profile of Ni is added in these figures since it has been noted that its enrichment zone below the inner spinel oxide Fe 3-x Cr x O 4 corresponds exactly to the internal oxidation zone [1]. Several important features of the carbon distribution through the scale and the sub-scale can be observed:…”

“…Detail of the experimental procedures are given in [1]. After the corrosion tests, the samples are weighed and the corrosion layer characterized by different analytical techniques.…”