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CO2 and H2S corrosion of api 5l-b and 5l-x52 grade steels

Abstract:

Corrosion behavior of American Petroleum Institute (API) 5L grade B and X52 steels in 3% sodium chloride in distilled water was studied under CO2 and H2S gas mixtures (varying from 7(483) to 72(496) psi (kPa), and from 0.0017 (12) to 0.0060 (41) psi (Pa), respectively) at 100 °F (38 °C). Corrosion rates were determined through a coupon weight loss technique for both steels. In general, as CO2 partial pressure was augmented, corrosion rates increased for a given H2S partial pressure regardless of the type of steel. Nonetheless, as H2S partial pressure was increased (for a given CO2 partial pressure), corrosion rates varied differently according to the type of steel tested. For API 5L grade B, with incremental H2S levels, corrosion rates reached a maximum to then decrease, whereas for API 5L grade X52, corrosions rates kept increasing with H2S partial pressure. In many cases, corrosion rates for both steels were similar under the same conditions. However, in some instances (at 0.0017 psi (12 Pa) of H2S) API 5LX52 steel suffered from blistering and crack formation as that observed in Hydrogen Induced Cracking (HIC) of heavily deformed steel as revealed by surface and metallographic examination of the samples after testing. In such cases, where rates differed between the two steels, API 5LX52 exhibited larger corrosion rates (above 8 mpy (0.20 mm/y)) with no blistering for a given set of conditions. In addition to the mechanical and metallurgical (inclusions, plastic deformation, etc.) differences of a heavily deformed steel, the only effect attributable to variation in the presence or absence of blisters for API 5LX52 (when compared to 5LB steel) was the compaction and the competitive effects at which scales composed of Fe-S and Fe-C-O formed during corrosion,.