Corrosion of Thermally Insulated Galvanized Piping in a Low-Temperature Hot Water System

Abstract

This paper presents the case study of internal oxygen corrosion and corrosion under thermal insulation (CUI) that occurred in a low-temperature hot water (LTHW) system of a nine-storey building. High dissolved oxygen (DO) levels (>4 mg/L) resulted in severe tuberculation and through pitting of thin-walled carbon steel piping. Leachate from the phenolic foam insulation subsequently contacted the galvanized surface, resulting in high corrosion rates of approximately 1 mm/year. Multiple leaks were experienced from the horizontal ceiling pipe runs within 18 months of commissioning. The high DO levels were caused by incorrect pressurization of the system and possible under sizing of the vacuum degassing equipment. Inhibitor levels were also very low, resulting from historic under stocking. Within the building sector, galvanized steel pipe is still widely used under thermal insulation despite non-compliance with Section 4.3.5 of NACE Standard SP0198-2016. The CUI failure experienced in this case study illustrates the consequence of systemic non-compliance. Recommendations are proposed for mitigating the identified causes of failure, with important ramifications for LTHW system piping material selection, in the building industry.