Subscribe to our newsletters & stay updated
When I lived in Texas in the late 1970s, I worked as a plumbing design draftsman with a large mechanical, electrical and plumbing (MEP) engineering firm. At a plumbing industry association meeting in Texas, I met John H. Fitzgerald III, a corrosion engineer from Brooklyn, N.Y., who graduated from Yale University School of Engineering.
He was working with a large corrosion consulting firm on petrochemical, industrial and underground utility corrosion projects in Texas and across the United States. A few months later, John asked me to assist him with preparing some standard detail drawings and notes he wanted to use in his work.
In those days, many underground fuel storage tanks and piping were made of steel. They needed corrosion protection in the form of cathodic protection with magnesium anodes (connected with copper wires and thermit welds), which would allow the less-noble metal (magnesium) to corrode in order to protect the steel tanks.
John mentored me while we worked together. He taught me about corrosion cells, electrochemical reactions, the electromotive (galvanic) series of metals, electromotive force, anodes and cathodes. He also taught me about common forms of corrosion, such as: galvanic, pitting, uniform, concentrated cell, impingement, stress crack, selective attack and differential environmental soil condition
Galvanic corrosion is caused by dissimilar metals connected in a corrosion cell (Figure 1). John said it is important to specify and use dielectric unions or dielectric waterways at the connection points of dissimilar metal piping systems. Back then, there was not much talk of microbiologically induced corrosion (MIC). However, since then, much research has been conducted on MIC, especially in fire protection systems.
We discussed stray currents, which also cause corrosion, and protective currents, the latter of which are intentionally induced currents to control corrosion in underground structures. John was involved in a project in Detroit where stray electrical currents from streetcar electrified rails caused excessive corrosion in underground utility pipe, valves, tanks and other underground steel structures.
John taught me how Faraday’s law is used to compute corrosion/oxidation-reduction rates. Rates of corrosion over time can be used to determine the life expectancy of a metal structure based on original wall thickness, current wall thickness and material loss.
He also taught me about Ohm’s law, which states that the current flowing between two metals in a corrosion cell is directly proportional to the voltage and inversely proportional to the resistance. The resistance is dependent on several factors and increases or decreases the rate of corrosion, as factors such as electrolytes or conductive wet soils in the corrosion cell change.
I learned about pH levels of water, acidity, oxygen content and oxidation. I learned that using chemicals such as phosphates and sodium silicate in water mains to form a film on the interior walls will protect the cast iron or ductile iron pipe from the water. And we talked about how this also protects the water from metals (copper, iron, lead, zinc, tin, etc.) from dissolving into the water.
My Corrosion Education
Passivation is a process of adding chemicals to keep water in a pipe slightly above 7.0 on the pH scale to coat the interior of older cast-iron or ductile-iron water mains. Another method is coating pipe with epoxy or other corrosion-resistant materials.
Note that some water treatment consultants add sodium silicate to building water systems, which creates a coating on the interior of the water pipe for condenser water, heating water, chilled water systems and, in some cases, domestic water systems. Extreme caution should be used when adding chemicals to domestic water systems so as not to cause a system to fail because of the scale buildup that seizes up moving parts. Layers can build up on the moving parts of mixing valves, strainers, isolation valves or balancing valves, rendering them inoperable.
Layers of sodium silicate or phosphates also can build up on heating surfaces, causing a significant loss of heat transfer efficiency. The addition of sodium silicate or phosphates may trigger a need to comply with the licensing at the state level and periodic testing to meet Safe Drinking Water Act requirements.
We talked about the exterior pipe-coating systems still being developed. Much of the field-applied coatings of the day were coal-tar or asphalt coatings, and the industry was switching to field-applied enamel and epoxy, factory-applied coatings, and heat fusion-bonded epoxies or fusion-bonded powdered epoxies for patching scrapes of metal pipe coatings, scratches or holidays (pinholes) in coatings and joints.
The options beyond coatings were plastic pipe and other materials that were compatible with the application. John and I were both involved in projects involving corrosion caused by acidic soils. The soils at a major airport, where we both had worked, had soil conditions known to be corrosive, and any exposed underground metal corroded rapidly.
John would always tell me that in order to prevent corrosion, the first consideration should be to choose a material that will not corrode. However, if an application or design requires steel or some other metal pipe material (such as liquid fuel), then coatings should be selected that are compatible with the wet soil or environment around the pipe to minimize corrosion.
We talked about increased temperatures accelerating the effects of corrosion, but we never talked about water velocity and the effects of velocity and erosion. In the large ductile iron water mains, velocity and erosion were not a problem.
Case Study: Circulating Pump Erosion
In the 1990s, John and I met up again at a meeting of the Eastern Michigan Chapter of the American Society of Plumbing Engineers (ASPE); I was chapter president. I invited John to speak about corrosion at our local chapter meeting, and he called upon me to discuss a perplexing case in a high-rise apartment building in Chicago.
He showed me what appeared to be a corroded pump impeller and said the corrosion seemed to only occur in the circulating pumps. The pump impellers were perpetually changed; within a couple of weeks, the impeller was corroded away to almost nothing.
“It’s the strangest thing, but the pipe, valves and the pump body are fine,” he said, adding that the high-rise also had problems with its hot water circulating system. The maintenance person had been replacing the small circulating pumps with bigger and bigger pumps, with very little improvement in the hot water temperature at the fixtures.
When systems are not balanced properly, I often see maintenance personnel install larger pumps to try and get circulation. In many cases, they use large pumps with cast-iron construction intended for heating hot water systems. (Figure 2 illustrates a common hot water recirculation system where larger pumps are installed in basement mechanical rooms trying to solve air trapped at the top of the system.)
Looking at the impeller, I said, “I think I know what the problem is, but I want to ask a few more questions to make sure.”
I learned that the impeller was from a circulating pump in a mechanical room on the top floor of the 12-story building. The water heaters were in the penthouse mechanical room, and a hot water main went down to the first-floor ceiling, where the mains were distributed along hallways and then upward in vertical risers behind each bathroom and kitchen area in the building. The hot water lines terminated above the top-floor ceiling into a hot water return pipe running across the penthouse to the water heaters (Figure 3).
“The problem does not appear to be a corrosion problem; it is an air problem, causing cavitation in the pump and eroding the impeller in the circulating pump,” I said.
Erosion Vs. Corrosion
John and I then discussed the difference between corrosion and erosion. We talked about the Copper Development Association’s recommendation for a maximum velocity in copper pipe, and how velocity erosion of copper and copper alloys (such as the brass impeller in this pump) can occur.
I explained that air in the distribution piping would collect at high points in the system. The air will get caught in the eye of the impeller while it is spinning at thousands of revolutions per minute (rpm). The air bubbles collapse and expand rapidly (cavitate) the pump, destroying the pump impeller. This is why erosion only occurred in the pump impeller in this case.
Hot water circulation pumps are typically small, fractional horsepower pumps of bronze (mostly copper) and are susceptible to erosion, especially if there is cavitation. Depending on the pump selection, the pump will be spinning at about 1,750 rpm.
I explained that the outer edge of a typical small impeller with a small circumference would travel the distance of the circumference every rotation of the impeller. This means the velocity of the outer edge of a typical small impeller will be between 10 to 30 feet/second (fps), depending on the rpm of the pump selected and the size of the impeller.
However, larger pumps can have rpms up to 3,600. In this building, the pump had a 6-inch-diameter impeller with a circumference of about 18.8 inches. The impeller velocity of a larger pump in hot water at a high temperature would be about 45 to 90 fps. This velocity, combined with an air problem, would, more likely than not, cause cavitation and erosion of the impeller in a short period.
Domestic Hot Water Vs. Heating Hot Water
We talked about the difference between the domestic hot water (DHW) and the heating hot water systems. The DHW system is open, and the heating hot water system is closed with air eliminators.
DHW systems, when heated, include microscopic air bubbles that will coalesce into bigger bubbles in the distribution piping; the large bubbles rise to the top of the hot water pipe. Air collecting at the top of risers can cause a restriction to flow in some pipe layouts and risers. This is especially true in high-rise apartments and hotels, where the plumbing distribution pipes are often vertical and the flow must turn down at some point.
When a DHW return system has a high spot in the piping, air bubbles can combine, making a large air pocket in the hot water return pipe. The air pocket can resist flow and stop circulation when small circulators are located on lower floors. When the pump is at the top of the building, the air can get drawn into or enter the eye of the impeller on the circulating pump(s) and cause cavitation.
When the hot water return is collecting across the top floor, air will make its way to a high spot in the piping, typically before the hot water return pipe drops down to the circulating pump. Smaller pumps will see the flow drop off and a large air bubble can stop the flow. Large pumps may suck the air into the eye of the pump impeller and cause cavitation in the pump.
After my explanation, John stood back and asked, “That means it is not a corrosion problem?”
“It is an erosion problem, not a corrosion problem,” I replied.
“The student has become the teacher,” he said.
In our follow-up discussions with the maintenance person, we learned there were problems with hot water circulation and complaints of “too long to get hot water” or “no hot water,” which would happen after air collects and causes the circulating pump to stop pumping.
The maintenance person thought the solution was to install a bigger circulating pump, and when that pump didn’t work, he installed an even bigger, higher-rpm pump. Despite the bigger pump, within a couple of weeks, the brand-new bronze impeller was eroded away.
I gave the maintenance person some suggested solutions. When the hot water return pipe is at the top of a system, there should be an extension of the vertical riser to collect air, with the return branch taking off sideways. Any accumulated air bubbles should rise to an automatic air vent valve or a plumbing fixture off the top of the riser or high point to vent any accumulated air.
When the fixtures at the top are opened, the air bubbles are vented to the room. I suggested that he reinstall the original smaller pump and then do one of the following:
1. Install an air vent valve at the high point (Figure 4);
2. Route the hot water return line from a tee off the riser on the floor below the top floor. With this arrangement, air will continue upward to the plumbing fixtures above and would vent the air when used (Figure 5); or
3. Rise up off the top of the hot water return riser in the mechanical room. Route a branch to a fixture below, such as a kitchen sink or bathtub, that is used often and could vent air in the hot water return pipe before the air reaches the hot water return pump.
John later told me that one of my suggestions was followed, and it solved the problem.
John was employed by several well-known companies in the corrosion field, including Corrpro Cos., where he was employed as a corrosion engineer. Later, with its subsidiary PSG Corrosion Engineering in Detroit, he was a corrosion specialist. I understand he also ran his own corrosion consulting business.
He served as a technical editor at Materials Performance Magazine for 15 years and as the international president for the National Association of Corrosion Engineers (NACE).
NACE is an organization of about 35,000 members across 30 countries and is recognized globally as one of the authorities for corrosion control solutions. The organization offers technical training and certification programs, conferences, industry standards, reports, publications, technical journals and government relations activities. John was named a Fellow at NACE, and a NACE award is named after him.
John was a regular speaker, author and instructor for industry meetings and at NACE International seminars and conferences, and I understand that he taught some courses at Purdue University. He was well-known nationally as an expert on corrosion.
I am sure John will be dearly missed by all who knew him, including me. He was the corrosion guru to me. The field of corrosion engineering is much better today because of John’s contributions and passion, and his education of the people he met. His teachings live on in his writings, in his students’ and colleagues’ memories, and in the readers of this column.
Dedicated to the memory of John H. Fitzgerald III.