Corrosion of wire screens in sulphur recovery units

Autor: V. K. Pareek, J. D. Mumford, T.A. Ramanarayanan, Adnan Ozekcin
Rok vydání: 1995
Předmět:
Zdroj: Journal of Materials Science Letters. 14:173-175
ISSN: 1573-4811
0261-8028
DOI: 10.1007/bf00318246
Popis: The chrome-nickel austenitic stainless steels (SS) including the widely used 18Cr-8Ni (type 304) have performed well in sulphur containing environments encountered in the petrochemical industries. However, during a planned turnaround 304 steel components (support screens) used in a sulphur recovery unit showed corrosion and scaling ranging from 10 to 750 ,urn thick. The gaseous environment in the sulphur recovery unit contains species such as H2S, CS2, SO2 and water vapour, and during turnaround oxygen is injected into the unit through the screens. The oxygen injection results in a temperature rise, which was not recorded. The aim of this study was to provide an understanding of the mechanism of degradation of the 304 SS wire screens by characterization of scale and the underlying alloy. Sorrel and Hoyt [1] investigated the corrosion resistance of 304 SS to sulphur attack and found this material to be at least ten times as resistant as carbon steel. Backensto et al. [2] also studied the sulphidation of various grades of SS and stated that extra low carbon grade and the stabilized grades of SS sulphidized at the same rates as the plain 304 SS. Sorrel and Hoyt [1] questioned the validity of their statement and pointed out that the effect of sensitizing treatment was not investigated by Backensto et al. [2]. Natesan and Chopra [3] sulphidized 304 SS under low sulphur pressures and suggested that an alloy with equal amounts of Fe, Cr and Ni is suitable for mixed sulphidation/oxidation environments. Verma [4] has indicated that depletion of chromium content in austenitic 310 SS matrix by internal sulphidation lowers the resistance of the alloy to sulphur attack. In fact, it has been recommended [5] that steels for coal gassification units, operating in mixed oxidation/sulphidation environments should have a minimum of 25% chromium for sulphidation resistance. Thus, it is evident from the above different studies that the chromium content of an alloy is crucial to provide resistance against sulphidation. Consequently, the present investigation was directed at correlating the bad performance of the support screens with the chromium distribution in the scale as well as in the alloy. Four wire screen samples, designated A2, A3, B2 and C1, were investigated. These had colours ranging from brown to reddish brown to black and showed visual evidence of scaling. Sample C1 was hardly corroded while sample A2 showed maximum corrosion. In many regions of sample A2, the loss of wire thickness was as large as 50%. Detailed investigations were therefore carried out on sample A2. A cross-section of the wire in the most corroded region, shown in Fig. la, reveals a compact corrosion scale of about 750/~m. An expanded scanning electron microscope (SEM) image of the corrosion scale is shown in Fig. lb. A multilayer structure is evident in the corrosion scale, as shown in Fig. lb. The different layers of the scale, representing sulphides of different composition, are listed in Table I. The iron content in the overall scale decreases progressively from the outer layer to the inner layer whereas the chromium content increases. The subsurface region beneath the corrosion scale
Databáze: OpenAIRE
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