Assessment of Corrosion Mechanism Affecting Stainless Steel Tee in the Zero Liquid Discharge System

This study examines the root cause of premature, localized leakage in stainless steel piping at the reverse osmosis (RO) header of a Zero Liquid Discharge (ZLD) system within a combined-cycle fossil fuel power plant. Several failures occurred at multiple welds and in adjacent base metal locations.

A detailed failure analysis was performed to identify the underlying mechanism. Using optical microscopy, scanning electron microscopy (SEM), and energy‑dispersive X‑ray spectroscopy (EDS), the investigation confirmed that Microbiologically Influenced Corrosion (MIC) was the contribution for the degradation. Biological activity initiated localized depassivation, compromising the stainless steel’s protective oxide layer. This was followed by accelerated corrosion driven by galvanic interactions between the ferrite–austenite two-phase microstructure of the weld material and the base material.

As a corrective measure, the entire piping system was replaced. To prevent future occurrences, it is strongly recommended that the engineering design be reviewed to eliminate stagnant water and low‑flow conditions in this critical section of the ZLD system, thereby reducing the likelihood of biological growth and electrochemical corrosion.