| Title | Surface Analysis of Carbon Steel and Corrosion-Resistant Alloys Exposed to Acid-SO4= Chloride Type Geothermal Fluid |
|---|---|
| Authors | J. Nogara and S.J. Zarrouk |
| Year | 2015 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | corrosion, geothermal fluid, CRA, surface analysis, Acid sulphate, low-pH, ESEM-EDS, XRD |
| Abstract | Carbon steel and corrosion resistant alloys (CRA) coupons were exposed to low pH acid-SO4=chloride corrosive geothermal fluid at high temperature and pressure condition. The objective is to gain better understanding of corrosion damage to standard low carbon steels normally used as casing material for geothermal well construction. Corrosion-resistant alloys were likewise evaluated for performance as these may be used as alternative materials in the future. Corrosion tests were conducted in a geothermal production well that has reported casing damage caused by flowing corrosive fluids. Nine 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 a two-phase acid-SO4= chloride type fluid with pH between 3.54 – 3.72 (measured in the laboratory). The corrosive fluid is at a temperature of 170 °C and at low-flow fluid conditions. Surface evaluation techniques are used to characterize the corrosion behavior 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 this fluid environment 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. Crevice corrosion is intense in carbon steel sections shielded by insulating spacers ("gasket-corrosion") in the acid-SO4= chloride fluid environment. Zinc sulfide scales did not form in the crevices. Corrosion product formed in the creviced areas of the carbon steel coupons consists mainly of iron oxide (magnetite) and iron oxy-hydroxides (goethite). |