Surface Analysis of Carbon Steel and Corrosion-Resistant Alloys Exposed to Corrosive Geothermal Fluids

Abstract

This study pertains to an experimental research on the corrosion behaviour of carbon steel and corrosion resistant alloys (CRA) subjected to corrosivegeothermal fluids at high temperature and pressure conditions. The objective is to gain better understanding of corrosion damage to standard carbon steels normally used as casing material for geothermal well construction. Corrosion-resistant alloys are likewise evaluated for performance as these may be used as alternative materials in the future. Corrosion tests were conducted in wells from two geothermal fields that have reported casing damage caused by flowing corrosive fluids. Nine (9) types of metals are used for the tests consisting of carbons steels, stainless steels and nickel-base alloys. The corrosion-resistant alloys were selected based on an extensive review of the results of previous geothermal corrosion tests. These metals were exposed to the following low-flow fluid conditions: (a) two- phase acid-SO4= chloride type fluid with ambient pH between 3.54 – 3.72 at a temperature of 170 °C, and (b) high enthalpy geothermal steam with 13.8% (weight) non-condensable gas content (CO2 = 94.6%, H2S = 3.6%, plus minor gases) at 240°C. Surface evaluation techniques are used to characterize corrosion the behaviour of the metals, including coupled environmental scanning electron microscopy – energy dispersive spectroscopy (ESEM-EDS), X-Ray diffraction (XRD), light microscopy and metallographic sample preparation techniques. Results from the tests show that damage to the API standard carbon steel (> 30 days exposure) is significant in both environments in terms of surface penetration either as pitting or uniform corrosion. Generally, the CRA’s were resistant to corrosion attack, but a few of the CRA’s experienced localized corrosion in the form of pitting. Mineral scales and corrosion products formed on the surface of all the metals tested with varying thicknesses. The scale consists mainly of crystalline zinc sulphide (sphalerite) and minor iron sulfides which deposited from the acid-SO4= chloride fluids. Iron sulfides (pyrrhotite and troilite) and minor magnetite formed from the high NCG steam. Crevice corrosion is intense in carbon steel sections shielded by insulating spacers in the acid-SO4= chloride fluid environment. Zinc sulfide scales did not form in the crevices. A follow-up study on susceptibility of the CRA’s to both stress corrosion cracking and sulfide stress cracking to the same environment is recommended.