| Abstract |
Internal corrosion of carbon steel pipes exposed to acidic geothermal fluids is a major problem in the geothermal industry that may be controlled through the use of suitable corrosion resistant materials and barriers. In this paper, the abilities of three (3) types of metal alloys (i.e. K55, A53 Gr. B and SS316) and seven (7) types of protective coatings (Brands A - G) to resist corrosion in low-pH geothermal brine (pH25C < 4.0) were assessed using different analytical techniques in the laboratory. The sample coatings and alloys were subjected to immersion tests in low-pH geothermal brine at various temperatures and pressures (i.e. Rm. T-atm P, 70 0C-atm P and HT-HP) to simulate different operating conditions. Data on surface quality, corrosion rates and failure/corrosion mechanism of the test specimens after exposure were obtained using Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization Scans (PPS). Thermogravimetric Analysis (TGA) and X-ray Photoelectron Spectroscopy (XPS) were applied to characterize resulting precipitates while Scanning Electron Microscopy (SEM) was used to determine surface morphology. Results showed that EIS, PPS, XPS and SEM are good tools to use as standard analytical techniques for evaluation of coating and metal samples in the laboratory prior to field application. Corrosion rates for the metal alloy specimens increased with temperature. SS316 alloy generated the highest corrosion rate after 5 hour immersion in low-pH brine both at Rm. T and at 250oC. Precipitates found in solution and adherent to alloy surface were mostly composed of O, Si, Cl, C and trace amounts of metals. These appeared to provide inhibition in A53 Gr. B and K55 alloys but not in SS316 metal. PPS revealed that corrosion inhibition in A53 Gr. B and K55 alloys is controlled by the anodic reaction where iron component of metal is dissolved in the test media. All types of coatings that were tested exhibited protective properties in acidic environment but show decreasing trend with temperature. At higher temperatures, protection efficiencies of the coatings declined by ~20 – 50% while measured corrosion rates and visible coating defects (i.e. wrinkling and pinholing) increased. Utilization of these coatings may also induce deposition of precipitates composed of silica, oxides and inorganic salts inside the pipes. |