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51318-10990-Localized Corrosion of Mild Steel under Iron Sulfide Layers in CO2/H2S Environment

Mild steel specimens (API 5L X65) were pretreated to form a pyrrhotite layer on the surface using high temperature sulfidation in oil, then exposed to a range of aqueous CO2 and H2S corrosion environments, leading to initiation of localized corrosion.

 

Product Number: 51318-10990-SG
Author: Saba Navabzadeh Esmaeely / Gheorghe Bota / Bruce Brown / Srdjan Nesic
Publication Date: 2018
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Due to the electrical conductivity of pyrrhotite it was hypothesized that its presence in the corrosion product layer on a steel surface could lead to localized corrosion. Mild steel specimens (API 5L X65) were pretreated to form a pyrrhotite layer on the surface using high temperature sulfidation in oil. The pretreated specimens were then exposed to a range of aqueous CO2 and H2S corrosion environments at 30 and 60C. X-ray diffraction data showed that the pyrrhotite layer changed during exposure; in an aqueous CO2 solution it underwent dissolution while in a mixed CO2/H2S solution it partially transformed to troilite, with some mackinawite formation. This led to initiation of localized corrosion in both cases.

Propagation of the localized attack was enhanced due to a galvanic coupling between the pyrrhotite layer and the steel surface. The intensity of the observed localized corrosion varied with solution conductivity (NaCl concentration); a more conductive solution resulted in higher localized corrosion rates consistent with the galvanic nature of the attack propagation.

Key words: Pyrrhotite, H2S corrosion, CO2, localized corrosion, troilite, XRD

Due to the electrical conductivity of pyrrhotite it was hypothesized that its presence in the corrosion product layer on a steel surface could lead to localized corrosion. Mild steel specimens (API 5L X65) were pretreated to form a pyrrhotite layer on the surface using high temperature sulfidation in oil. The pretreated specimens were then exposed to a range of aqueous CO2 and H2S corrosion environments at 30 and 60C. X-ray diffraction data showed that the pyrrhotite layer changed during exposure; in an aqueous CO2 solution it underwent dissolution while in a mixed CO2/H2S solution it partially transformed to troilite, with some mackinawite formation. This led to initiation of localized corrosion in both cases.

Propagation of the localized attack was enhanced due to a galvanic coupling between the pyrrhotite layer and the steel surface. The intensity of the observed localized corrosion varied with solution conductivity (NaCl concentration); a more conductive solution resulted in higher localized corrosion rates consistent with the galvanic nature of the attack propagation.

Key words: Pyrrhotite, H2S corrosion, CO2, localized corrosion, troilite, XRD

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