THE MATERIAL CORROSION TEST USING LOOP SYSTEM UNDER ACIDIC CONDITION AT GEOTHERMAL FIELD IN JAPAN

Abstract

We carried out the material corrosion in acidic fluid under two-phase conditions at Yanaizu-Nishiyama, Okuaizu geothermal power plant using small loop test system in February of 2017.
We carried out several corrosion tests, 1 day, 4 days and 15 days testing under about pH 3.8 and 165 degreeC condition which gave us low corrosion rates. And we adjusted pH using H2SO4 or HCl to pH 3 and carried out one day test to determine the effect of a lower pH.
The loop-test system was about 2 meter high and 3 meter wide. The test coupons used were about 30 mm long and 8mm outer diameter and 5.2 mm inner diameter tube shape and inserted over a central rod with 8 test coupons in the loop system. A carbon steel, a low alloy steel and a 13% Cr stainless steel were tested in the loop system.
The corrosion rate was calculated from weight change of the coupons. This was compared to an estimation system for corrosion rate using previous corrosion data developed by AIST from 1980 to 2003 based on temperature, pH and metal chemical composition.
The corrosion rate of the pH 3.8 condition of carbon steel was from 0.1 to 0.3 mm/year and was lower than estimated using previous data which predicted 1.5 to 1.8 mm/year. The low alloy steel and the 13% Cr stainless steel showed the same trend as carbon steel. Two of the reasons for the observed differences were that the two-phase tests had a lower liquid/vapor ratio in the two-phase line than tested previously and the second reason was especially relevant on carbon steel, namely the precipitation of uniform layers of As and Sb metal was found.
In the pH control tests at pH 3 adjustment done using HCl acid showed a different trend. The corrosion rate of carbon steel was about 27 mm/year, and about 10 times larger than the estimated rate. The addition of HCl was proposed to have accelerated the galvanic corrosion under the deposited heavy metal.