PD-10:   Galvanized Steel For Hot Water Service

Galvanized steel is commonly used for many building related piping systems including domestic cold water, sanitary waste, compressed air, storm drain, vent, and fire protection.  Its zinc protective coating has far greater resistance to corrosion than steel and essentially serves as a sacrificial anode to cathodically protect the underlying steel surface as long as the zinc remains.  Once the zinc protective coating fails, a more intense and focused deterioration of that affected area typically occurs.

In its earliest stage of failure, zinc deterioration may impact one or a few areas and penetrate through with the aggressiveness of a drill bit.  Where localized penetration is less severe and the corrosion process continues over a more widespread surface area, substantial rust product is produced as a result of a corrosion process which oxidizes the steel to approximately 12 times greater volume of less dense iron oxide.

Although not recommended, and with copper now the preferred material for domestic water service, galvanized steel pipe can be found at domestic hot water systems and is still specified by design engineers.  While a pipe failure or flood is the most common signal to a corrosion problem at galvanized hot water systems, complaints due to the loss of water flow and reduced pressure are also common indications.  Objectionable taste, poor water quality, or clogged water strainers and bathroom flushometer valves are also common signals to this much larger problem.

Different Corrosion Scenarios

Localized Pitting	Generalized Corrosion

Most galvanized pipe today is manufactured using the same “hot dipped” method of submerging the steel pipe into relatively pure molten zinc and applying a thick coating of zinc to varying density defined by its specific ASTM specification.  Typical coating thickness on most general galvanized products are 80-300 microns, and is specified in terms of weight of zinc per surface area, such as 250 grams zinc per square meter of pipe surface area.  The process is essentially unchanged from over 100 years ago, although the far greater number of failures of new galvanized steel pipe would suggest that some difference in today’s galvanized pipe products exists.

• Dramatically Varying Service

The estimated service life of galvanized steel pipe varies greatly depending upon the service and location it is employed, orientation, the local water quality / water chemistry, and operating temperatures.  For vent and storm drain applications having limited water contact, service life can range up to 100 years and greater.  Our investigation of a galvanized steel fire sprinkler system in San Francisco, installed in 1907 immediately following the great earthquake and fire, showed it in essentially new condition and still at schedule 40 ASTM specifications an amazing 103 years later.  Typically, however, service life is far less.

For domestic cold water, 50-60 years is a reasonable service life expectation for older pipe products, and in fact we routinely discover galvanized cold water pipe from the 1950’s still providing reliable service.  Conversely, run-out waste piping between bathroom fixtures to the cast iron risers typically provides less service than the exact same galvanized pipe carrying the water to the same location.

By far, the lowest service life for galvanized pipe is found at not so “dry” fire sprinkler systems where significant water always remains in the presence of abundant oxygen.  Although the anti-corrosive function of galvanized pipe to provide long service life should remain under such conditions, it typically fails at an advanced rate to produce deep and severe pitting.  This problem is further amplified at fire protection systems through the use of thin wall schedule 10 and schedule 7 materials; leaving relatively little pipe wall to corrode before producing a failure.

• Heat The Greatest Limiting Factor

While there are multiple factors which influence the service life of galvanized steel pipe, temperature is by far one of the most significant.  As water temperature increases, so does the deterioration of the zinc protective finish through a series of chemical / molecular level events.  The most aggressive deterioration of galvanized steel pipe is at higher water temperatures to near 200° F., although it will also occur at lower temperatures to near 90° F, such as commonly found at the run-out pipe exiting bathroom and kitchen facilities.  Galvanized pipe deterioration is directly proportional to temperature where all other factors are the same.

Galvanized steel pipe has traditionally been excluded from steam and steam condensate service due to its recognized high temperature limitation, although a belief or expectation that lower temperatures were not a concern is likely the reason for its application elsewhere.  In fact, our ultrasonic investigations have shown that any galvanized steel pipe providing drain service where water temperatures may be higher than 80° F, such as exiting a laundry room, cafeteria, laboratory sterilization, or gym locker room, etc., should expect to see an increase in corrosion activity and therefore lower than expected service life.

Such higher temperature limitations are not recognized or shared by those in the galvanizing industry, and in fact the opinion and claim of the galvanizing industry is that, “In long-term, continuous exposure, the recommended maximum temperature is 392° F.”  Unquestionably, however, higher temperatures at water filled pipe are clearly a threat.

• Real World Proof

Our ultrasonic investigation of dozens of galvanized steel domestic hot water piping systems have overwhelmingly confirmed this limitation, with premature failures reported as soon as 2 years after installation.  For older domestic hot water systems, the higher quality of galvanized steel pipe from before 1980 is only now showing heat related failure while more recently produced galvanized steel pipe may have already failed and been replaced.

For any larger domestic hot water system where a return recirculation loop exists at a typically lower temperature, by 10-15° F is still very noticeable ultrasonically in terms of less corrosion loss; further confirming this close and proportional relationship between water temperature and failure of the zinc protective finish.  Ultimate proof, of course, is often in a photograph, as we illustrate below.

Taken at the same 28 year old Chicago building property, ultrasonic testing identified the domestic cold water galvanized steel pipe at new or near new schedule 40 ASTM specifications and in near pristine condition.  Ultrasound could not identify the thin internal layer of “white rust” present, but the later removal of side by side hot and cold supply risers quickly confirmed its interior condition, shown bottom right.

In contrast, ultrasonic testing of the hot water side produced low wall thickness values to near 0.048 in. in many examples; defining the pipe as not only below recommended minimum thickness limits, but to have already reached the thread cut limit itself, and therefore risking failure.  Areas of deepest pitting represented the greatest threat of a leak or failure, a problem primarily located at the threads, although the eventual widespread deterioration of the entire pipe surface generated sufficient iron oxide rust deposits to severely restrict hot water flow.  Multiple failures of the hot water side had occurred for this specific property, with our investigation clearly defining the need for pipe replacement.

As the below side by side photographs of the same galvanized steel pipe, of the same age, under the influence of the same water conditions / chemistry show – temperature has a devastating impact against galvanized steel pipe.

Hot vs. Cold Water Conditions – 28 Year Old Pipe

Domestic Hot Water	Domestic Cold Water

We provide some additional examples of this condition in the photographs below taken from prior case history projects, and also offer a larger photo gallery of problems related to domestic hot water and domestic cold water piping in our photo galleries.  All below examples of pipe were from domestic hot water piping systems in the Chicago area, and illustrate various aspects of the problem.  Although Chicago has been heavily impacted by this issue due to the widespread use of galvanized steel in their hot water service specifications, such deterioration is common throughout the world.

Galvanized Steel Domestic Hot Water Pipe

Expansion Joint One of the first locations likely to fail at a domestic hot water system.  Twisting movement of the mechanical joint, as designed and intended, places additional stress on pipe already weakened by corrosion.	Internal Conditions Threat of failure is not the only concern raised.  Rust deposits are the end result of this corrosion condition, with the deposit itself the remnant of the original pipe wall.  Flow restriction can be up to 90% or greater.

Misinterpretation Galvanized pipe failure is often blamed upon some form of external influence, such as the nearby storage of chemicals, or cleaners.  In fact, dissolved rust and other elements leak through the failure point to evaporate on the surface; often producing this error in interpretation.	Early Indication The first stage of galvanized pipe corrosion problem whereby deep pitting will focus at one area to produce through wall penetration.  Small leaks will often evaporate away the hotter water and escape unnoticed until a larger failure occurs.  Conditions capable of producing the above leak have far greater impact against its threaded ends.

Flow Restriction Most pipe corrosion problems are discovered due to a failure and flood.  A less frequent indication, however, has to do with restricted flow and pressures due to the high volume of less dense iron oxide deposits created as galvanized steel pipe deteriorates.	Run-Out Threat A very common point of failure exists at the smallest threaded piping immediately closest to the riser.  This is due to significantly less wall thickness of smaller diameter pipe which is subject to similar deterioration caused by higher water temperatures.

• Scientific Investigation

This technical bulletin, and in fact almost all the information available on this website, is based upon real world hands on investigations of corrosion problems accumulated by CorrView International, LLC over the past 30+ years.  Although our background is firmly rooted in pure scientific research, we simply do not have the time, opportunity, or facilities to more scientifically document the explanation behind the many corrosion problems we encounter.

An excellent scientific paper appeared in the August 2013 issue of the International Journal of Material and Mechanical Engineering (www.ijm-me.com).  Authored by B. Pawlowski, J. Krawczyk, P. Bala of AGH University of Science and Technology, Krakow, Poland, this excellent article provides the underlying scientific explanation for a problem which is worldwide in nature.

This paper delves more deeply into the subject to reveal the even greater threat to galvanized steel hot water pipe having a longitudinal weld seam.  Water quality, a critical factor to most corrosion conditions, is also addressed in detail.

• A Growing Problem

This problem is escalating due to many older properties now having reached the end of their 25-40 years of useful service life.  Although knowledge regarding the above threat of galvanized steel pipe would be expected to have reached those in the engineering fields responsible for designing domestic hot water and other higher temperature piping systems, our involvement with properties exhibiting this problem after as little as 2-3 years of operation defines that it has not.

In some cases, originally designed domestic water copper piping systems were “value engineered” to galvanized steel pipe as simply a cost saving measure.  In others, continued reliance upon arguments made by those in the galvanized piping industry have been followed even against overwhelming evidence to the contrary.

© Copyright 2005 – 2026 – William P. Duncan, CorrView International, LLC