Flow accelerated corrosion of the heat exchanger carbon steel tubing in air cooled condensers

Abstract

The water/steam mixture exhausted from the low - pressure turbine is directed to the A-frame steel heat exchange structure via lower/upper ducts in both fossil and combined cycle plants. FAC (flow accelerated corrosion) has been described as the degradation or dissolution of the surface layers of low carbon steel at a relatively low temperature and pressure. FAC can cause serious damage in thin-walled heat exchange tubing in air cooled condensers. The appearance of the protective layers on the interior of carbon steel equipment depends largely on the chemistry of the water and steam cycle. This research investigates the influence of operating variables conditions on FAC, these variables include temperature, pH, and fluid velocity. This investigation also studies the performance of a different type of inhibitors (ammonia, film forming amines (FFA), and neutralizing amines) and their effects on the growth and dissolution of the oxide layers on the heat exchanger steel tubes. This investigation is accomplished by performing a dynamic fluid electrochemical measurement using Electrochemical Impedance Spectroscopy (EIS) technique and Rotating Cylinder Electrode (RCE) set-up. Two temperatures were used; 25 and 50oC and two pH 9.0 and 9.50 values were tested. The rotating speeds were selected based on the operating conditions to confirm the turbulence flow conditions. pH values were controlled by auto feeding of the ammonia to the electrochemical corrosion cell which mainly contains three electrodes; a working electrode, a reference electrode and a counter electrode. EIS technique provided a quantitative information about the electrochemical reaction at the electrode/electrolyte interface as it explains how the system response to the AC current used at EIS such as the charge transfer resistance and phase angle. A good correlation of impingement test with the flow accelerated corrosion was achieved by testing the electrochemical behavior of the electrode surface using electrochemical impedance spectroscopy technique and liquid drop impingement (LDI) test.