Photo Gallery F7: Galvanized Steel / Domestic Hot Water Service
Galvanized steel pipe should never be used for any hot water service due to the reason that heat destroys the zinc protective finish. Although the higher temperatures found at domestic hot water systems are mostly involved, accelerated deterioration can even be measured at galvanized steel run-out waste pipe serving kitchens and bathrooms. Dishwasher discharges are especially damaging. Highest deterioration is typically closest to the outlet of the hot water furnace or water heater simply due to the increased temperature, and lower at the furthermost extremes.
For most older buildings were galvanized steel pipe has been installed for domestic cold water service, brass was installed for the hot water side due to this well known vulnerability. However, certain cities and geographical areas such as Chicago and elsewhere specified galvanized steel pipe for hot water service, which has now resulted in widespread hot water system failures. Our frequent visits to Chicago has suggested that most larger high rise apartments and condominiums have this problem, as well as much of the Chicago infrastructure. A very common finding from any ultrasonic testing investigation of galvanized steel hot water pipe is to show significant pitting – in itself defining that the galvanized protective finish no longer exists.
Once this occurs, the underlying bare carbon steel pipe corrodes at an accelerated rate due to different galvanic potentials; producing significant rust deposits then restricting flow and reducing line pressure. Iron oxide rust product generated as a result of such higher corrosion activity can reduce inside pipe diameter by 80% or more, as the below photographs show. Dramatic side by side examples of galvanized steel domestic cold water pipe showing no wall loss next to its adjacent and heavily deteriorated galvanized steel domestic hot water pipe dramatically demonstrate the threat involved.
Since such piping systems are typically threaded, a further threat exists at each connection point due to the loss of approximately 50% of the pipe wall during the threading process. As pipe diameter decreases, so does its wall thickness, resulting in lesser service. For any building having a galvanized steel domestic hot water system, the first failures are typically on the threads of the smallest and hotter supply side circulating lines. The problem is typically addressed by repairing the random failures although by typically installing copper pipe and brass isolation valves, a new galvanic corrosion threat is created to further accelerate corrosion activity at the older galvanized steel pipe. Spot repairs continue until ultimately a total thread separation occurs to produce catastrophic damage and full pipe replacement is considered.
An even greater threat exists at all mechanical swing type expansion joints given that in addition to its inherent physical weakness, expansion and contraction adds additional stress.
No options exist other than to replace the pipe with copper, as new galvanized steel pipe will fail even sooner. Once the problem is discovered, substantial wall loss will have occurred – thereby preventing any form of relining or internal pipe coating. Efforts to epoxy line pipe at the buildings we have been involved have been unsuccessful.
