Corrosion Testing of UNS N06625 in a Simulated High Temperature Geothermal Environment

Thorhallsson, Andri Isak (University of Iceland) | Stefansson, Andri (University of Iceland) | Karlsdottir, Sigrun Nanna (University of Iceland)

OnePetro 

ABSTRACT

A 3-week corrosion testing of UNS N06625 was conducted in supercritical fluid at 350°C and 10 bars. The aim of corrosion testing was to simulate high temperature geothermal environment i.e. IDDP-1 conditions where previous on-site corrosion testing of UNS N06625 and other alloys had been conducted. The simulated environment had lower concentration of H2S and CO2 in the steam comparing IDDP-1 environment. In addition, no silica scaling was precipitated on the samples nor HF was used in the simulated experiment. The corrosion rate was determined with weight loss comparison and the corrosion forms were analyzed with SEM, XEDS and light microscope. The result of the simulated experiment shows that some localized corrosion is occurring and the corrosion rate of UNS N06625 in simulated environment is similar to the corrosion rate observed in IDDP-1.

INTRODUCTION

In the first deep drilled well (IDDP-1) in the Iceland Deep Drilling Project in-situ weight loss experiment was conducted from an exhaust pipe from wellhead of the borehole for several corrosion resistant alloys and carbon steel types. Superheated geothermal steam was obtained from the well with 450°C and 140 bars at wellhead. The weight loss experiments were done in exhaust pipe with throttled steam from the wellhead where the temperature was 350°C and pressure 12-13 bars [1]. The geothermal fluid (see Table 1) from IDDP-1 contained corrosive components such as CO2 and H2S and additionally HCl and HF gases [2] Due to the pressure drop, large amount of silica precipitated in the exhaust pipeline and hence silica covered the corrosion samples during the testing. All the corrosion samples had very low corrosion rates i.e. the corrosion resistant alloys and generally low corrosion resistant carbon steel. The corrosion resistance of N06625 was measured to be less than 0.001 mm/year but localized cracks and pits were observed on sample after testing. From the corrosion testing, it was concluded that the silica scaling prevented general corrosion but promoted under deposit corrosion [1] of UNS N06625. Same was concluded from the results of the testing of other samples.