ABSTRACT
Galvanized steel pipe is a very common material used in domestic water systems, primarily for larger pipe diameters (DN 50 or 80 (NPS 2 or 3) and above) due to the expense of copper pipe. While galvanized pipe is allowed in most building codes, it has corrosion issues when used for hot, softened water service. The low mineral content in soft water is desirable for reducing scale build-up, but detrimental to corrosion resistance. When there is sufficient calcium hardness in the water, the calcium reacts with the galvanizing to form a protective layer on the internal pipe surface. Without that protective layer, the pipes are significantly more susceptible to corrosion. Presented in this paper are case studies of corrosion issues in galvanized water piping in a stadium, hotel, and county government building. Details of the corrosion mechanism and mitigation strategies are also discussed.
INTRODUCTION
Hotels, luxury residences or condos, stadiums, hospitals, municipal buildings, etc. often have specifications that require the builder to use copper for all potable water piping that is 50 mm (2 inches) in diameter or smaller (referred to as DN 50 (NPS 2)), but allow substitution of other suitable materials for larger piping. Large hot water systems in these types of complexes use pumps to circulate the hot water, such as with instant hot water supply systems. They have defaulted to using galvanized carbon steel piping for these larger diameter pipe sections. This is due to the lower cost of galvanized pipe compared to copper or other materials. Use of galvanized pipe in commercial buildings is in contrast to plumbing practices in homes and smaller residential units that typically do not allow galvanized pipe. The degradation of the galvanized piping in a soft water system can occur in a short period of time, and problems have been witnessed in as few as two years after installation (Figure 1). Instead of dealing with spots on their dishes, clients are often confronted with brown water and iron stains from the corroding pipe. This discoloration and particulate creates a major issue that often comes back to the builder as a law suit.