Corrosion Rates in Upper Mahiao Zero Effluent Disposal System

Abstract

Evaluation of the corrosion rates at Upper Mahiao Zero Effluent Disposal System (UM ZEDS) was conducted after the installation of new spray nozzles at the spray tower and after dosing the spray tower basin fluids with caustic soda to elevate the fluid pH. The chemistry of the return condensates from the spray tower has improved with a pH of 6.6 to 7.2 and a corrosion rate of 0.5 mm/yr compared to the pilot testing at 0.6 mm/yr (Villa, 2003). Raising the pH to around 8.0 by on-line caustic treatment at 28 li/hr of 50% NaOH solution significantly reduced the corrosion rate to an acceptable 0.15 mm/yr. However, this is slightly higher than the corrosion rate limit of 0.12 mm/yr for a corrosion allowance of 3.0 mm for 25 years plant operation. A more stable passive film of hematized magnetite layer developed with NaOH dosing and is more refined and adherent, preventing further corrosion in the carbon steel pipeline. The results also indicate that in the untreated section of the line, it will take a longer period for the material to passivate and the corrosion product formed is non-protective, which explains why the condensate was still corrosive even at slightly elevated pH.