ABSTRACT

Hydrazine is used as an oxygen scavenger to control corrosion in steam generating systems, despite being a genotoxic carcinogen. Alternative chemicals, nontoxic corrosion inhibitors or new oxygen scavenger-free water treatment technologies are preferred. A newly developed amine based vapor phase corrosion inhibitor was investigated. Electrochemical tests were conducted and showed a significantly lower corrosion rate in steam generating boilers and boiling water. Short term (720 hours) corrosion tests in boiling water showed a decreased corrosion rate from 5.3 mpy to 1.93 mpy for 50 mg/L VCI and 1.32 mpy (0.001 metal loss per year) for 100 mg/L VCI (Volatile Corrosion Inhibitor) addition. Long term (2,200 hour) corrosion tests in the hot steam generating closed loop system showed a decreased corrosion rate from 8.2-8.9 mpy for the control sample to 0.72-0.74 mpy when washed with 500 mg/L VCI solution. The closed loop system was subsequently maintained at 100 mg/L inhibitor for the test remainder. Inhibitor added at beginning of test resulted in corrosion rate of 1.09-1.24 mpy (with 100 mg/L VCI). XPS (X-Ray Photoelectron Spectroscopy) analysis showed that the amine based inhibitor promoted and stabilized a protective (Fe3O4) magnetite oxide on the pipe internals. This investigation confirmed the inhibitor can be an effective replacement for toxic hydrazine.

INTRODUCTION

The presence of dissolved oxygen in boiler feed water and steam generating systems can present serious problems in a steam generating plant by promoting corrosion and thick scale formation in the feed water system, the boiler and the steam condensate system. Therefore, it is important to remove oxygen from the feed water and also from the condensate where in-leakage can occur. The first step in the elimination of oxygen from the boiler feed water is mechanical deaeration. The second step involves chemical oxygen scavenging to remove the residual oxygen. For many years, sodium sulfite and hydrazine were the preferred chemical oxygen scavengers. However, sodium sulfite contributes solids to the boiler water and hydrazine was found to be extremely toxic.

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