Corrosion in Concrete Structures of the Upper Mahiao ZEDS
Leyte Geothermal Production Field – Characterization and Implications

S.G. Ramos, E.H. Alcober, R. R. Villa, Jr., J. B. Rosell

PNOC Energy Development Corporation
Energy Center, Fort Bonifacio, Taguig City

Abstract

Corrosion was initially documented in the concrete spray tower posts of the Upper Mahiao Zero Effluent Disposal System (ZEDS) in Leyte Geothermal Production Field (LGPF) during the October 2004 annual Preventive Maintenance Service (PMS). Field observations showed deterioration of the cement matrix to a weak and powdery state leaving the rock aggregates protruding out of the concrete surface.

Detailed petrographic examination showed the aggregates to be mainly igneous rocks; while the vuggy cement matrix showed intense alteration to finely crystalline calcium carbonate or calcite. Phenolphthalein tests indicate a depth of carbonation ranging from 4 to 5 mm. SEM-EDX analysis of the cement matrix revealed the dominance of Ca, C, and O confirming the CaCO₃ composition of secondary calcite in the matrix.

The soft friable nature of the concrete sample, and the development of secondary calcite in the cement matrix indicate that the spray tower posts were attacked by carbonic acid formed by dissolved CO₂ in the power plant effluents. Apart from decomposing the cement paste, carbonation also lowers the pH of concrete hastening corrosion of reinforcing steel bars.

Mitigating measures were applied in October-November 2005 to protect the spray tower posts from further carbonic acid attack. These included using a special high-density cement mix blended with fly-ash. The spray tower posts were also jacketed up to fluid level, and the basin was lined with molded high-density polyethylene plastic that is both durable and resistant to corrosion attack.