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Top 10 Corrosion Threats

By Corrview .com
Published: August 27, 2018
Key Takeaways

Many corrosion problems are self-inflicted due to a lack of awareness of the problem or maintenance neglect.

Source: Enterlinedesign/Dreamstime.com

There are many different corrosion problems that can significantly impact a facility. Some problems are very specific to properties of older age, such as the dezincification to older brass domestic water piping systems; or to certain types of piping systems, such as domestic galvanized steel pipe or dry fire pipe sprinklers. Corrosion issues are very much age-related, with very old 100-year properties simply running out of time, and newer buildings no longer providing the 50 years of trouble-free service that once could be relied upon. (For more insight on this topic, read Decline in Quality of Piping Making Corrosion Inevitable.)


Many corrosion problems are common, such as thread leaks, condenser water failures and fire sprinkler issues. Of the very many different types of corrosion events that can occur, the 10 corrosion issues below are most commonly brought to our attention. Some are very obvious by the presence, such as thread leaks and pinhole failures; while others, such as corrosion under insulation and dry fire internal rust deposits, require more proactive investigation.

Many, unfortunately, are self-inflicted due to a lack of awareness of the problem or maintenance neglect. Some were designed into the property when first constructed. Virtually all the corrosion conditions below will show themselves with age. Here are the common categories:


1. Dead End Condenser Water Lines

Dead End Condenser Water Lines

Any open cooling tower is essentially a large air washer that removes a wide range of airborne contaminants and then dumps them into your cooling water system. In addition to microorganisms, dirt, particulates and other foreign material, warm and fully oxygenated water raises corrosion activity dramatically.

Chemical treatment is always maintained at lower levels for open systems due to high chemical costs, resulting in greater corrosion losses. Water filtration, if it exists, is typically inadequate, which then allows any foreign particle entering the system to remain there permanently.

Added together, a substantial volume of iron oxide and other particulates accumulate at most open condenser water systems to produce many secondary corrosion problems.


A high priority location for high secondary under deposit corrosion is at all dead end areas of any condenser water system. This may be 2" or 3" futures at the risers, futures in an MER for additional pumps and refrigeration chillers, or dead ends following the last take-off tee. Dead end drain lines, often unnecessary and redundant, are high priority rust and dirt capture locations.

Small diameter cross connections to provide water flow, although very well intended, may circulate treatment chemicals but do nothing to the rust and dirt particulates that will settle. Side stream sand filtration, often installed incorrectly and always too far undersized, has little impact on removing the sediment capable of settling in such areas. This establishes localized areas of excessive corrosion activity often 10 or more times than that of the majority of the piping system.

With the control of rust and dirt exceptionally difficult for any open condenser water system, removing such dirt traps is the only reasonable course of action. In the above examples, 12" schedule 40 futures had lost an astounding 0.121" of pipe wall in only a few years due to dirt accumulation. In addition to their bottom-facing direction, the placement of dead ended futures as the first connection point to a condenser water downward supply riser absolutely guaranteed a surprise future failure—had ultrasonic testing not identified it first.

2. Dual Temperature Service

Dual temperature service.

A very common heating and cooling design originating back to the 1950s for apartments, condominiums and some office buildings, dual temperature piping systems are now reaching the end of their useful service life nationwide.

This elegantly simple design for heating and cooling placed generally thin wall and small diameter threaded schedule 40 carbon steel pipe at the perimeter column supports, whereby run-out lines fed window fan units either hot or cold water.

Aside from the issue of what the original design lifespan was of such piping systems, and whether or not they were actually intended to provide 60+ years of operation, various factors have significantly impacted their useful service.

First is the placement of mild carbon steel standard wall schedule 40 pipe, operating at temperatures of 7°C (45°F) or lower during summer months, at the building envelope where it is easily subject to high moisture infiltration.

Insulation, which was typically thin wall 1" fiberglass, was entirely inadequate for the application, easily permeable to moisture, and always poorly installed into an area—never again expected to see the light of day. The steel pipe itself was never painted, coated or protected from corrosion, which then allowed moisture to easily penetrate the insulation to aggressively destroy the pipe from the outside in.

Over many decades, significant and often devastating outer surface corrosion due to moisture condensation will destroy the entire piping system and progress to the point of failure while entirely hidden from view. Add to that threat normal 1/2 mil per year or greater corrosion losses to the inside of the pipe, a 50% wall loss at the thread cut and a galvanic interaction between the steel pipe and typically copper run-out piping to the units, and failure is virtually guaranteed.

Most dual temperature piping systems are found almost entirely destroyed by the time the problem is discovered decades after construction—thereby leaving total pipe replacement as the only recourse.

3. Fire Sprinkler Inlet Lines

Fire sprinkler pipe.

For all fire protection systems, the introduction of freshwater is the primary cause of their destruction. For older piping systems dating back to the 1920s and earlier, and where the fire pipe was almost never drained for testing or any other purpose, ultrasonic testing has often found the pipe in still near-new condition.

A high-priority area for corrosion at all fire protection systems is at the very beginning of the system at the water source. Here, a naturally higher movement of fresh city water produces higher corrosion losses, often in contrast to the remainder of the fire protection system.

Such a problem is amplified whenever a fire booster pump is present, and where required leaks at the packing seals then flow a greater volume of water up to the pump. Even a small constant drip, flowing 24/7, adds up to a substantial volume of freshwater that then accelerates corrosion activity at all pipe preceding the pump. The check valve after the pump prevents further water migration downstream—thereby limiting pipe damage to only before the pump.

Leaking over pressure relief valves, or drain valves, can produce the same result.

Fire inlet piping—which is cold to the touch or even sweating due to a lower than ambient inlet water temperature—immediately defines a movement of freshwater through the line, and should be investigated. Once identified, corrective actions are limited to correcting any leaks and reducing water flow through the pump seals.

Failures at fire inlet piping generally suggest a much more localized corrosion condition and not a major vulnerability for the entire fire protection system.

4. Galvanized Steel to Brass Valve

Corrosion of galvanized steel pipe and brass valve.

An almost guaranteed source of failure at any piping system is where galvanized steel pipe has been threaded directly to a brass valve. The only exception to this rule is where galvanized steel is sandwiched between two brass valves, in which case the destructive effect is greatly amplified.

Where galvanized pipe meets brass or copper, a strong galvanic potential exists between the dissimilar metals to quickly destroy the zinc finish. In effect, a small electrical current flows between both metals similar to a zinc-based battery. (A further examination of this topic can be found in the article Why Do Two Dissimilar Metals Cause Corrosion?)

As a result, pitting is severe in the immediate area of the connection, typically impacting the already weakened threads to produce a leak or failure.

This form of corrosion is common at domestic water systems and most pronounced at condenser water systems already under higher corrosion threats. In many cases, a prior failure due to galvanic activity between black carbon steel and a brass valve will be corrected using galvanized steel with the expectation that the galvanized steel will provide greater service.

In fact, however, the service life of the new galvanized pipe will likely be half of that of the steel pipe or less. With the far lower quality of today's galvanized steel pipe, we have documented such failures to occur in under three years.

The presence of any galvanized steel to brass pipe should always be noted for replacement prior to the inevitable failure.

5. Condenser Water By-Pass Configurations

Corrosion of a condenser water bypass.

A very common feature of most HVAC piping systems is by-pass connection used to temper water temperature. This may be across the supply and return lines of a condenser water system, package A/C unit or any other area where greater temperature and/or flow control is desired.

For open condenser water piping systems especially, any rust product, dirt or captured airborne particulate debris will be attracted to settle wherever water flow slows down or stops.

Water filtration, when provided, rarely offers full flow capacity, which in turn defines that any particulates not captured on the first pass will likely settle. With most condenser water filtration systems offering exceptionally low micron particle capture, but from a low rate of flow, any larger particle that can settle will settle to produce such problems.

Since most by-pass or crossover piping configurations are controlled by a valve at only one side, the entire remainder of the piping length becomes a large settling basin. Most such configurations we have investigated are rarely if ever used, and serve only as a source of future leaks and failure.

The vulnerability of such piping configurations is greatly dependent upon its physical layout. Bottom take-offs to a main condenser water supply attract particulates to fall into them and settle. Valves placed midpoint or at the furthermost extent of an open line turns all pipe prior to the valve into a settling basin. Long horizontal lines are more attractive to the problem than short lines where water movement and turbulence is constant. A dual by-pass configuration, of both manual and automatic valves, defines two likely problem areas rather than just one.

Periodically opening the by-pass loop for circulation accomplishes little to nothing to remove the deposits that have settled and hardened into place. Chemical dispersing agents may help in removing the lightest surface particulates but do nothing for the heavier deposits. Once established, hardened deposits prevent chemical and microbiological inhibitors from reaching the underlying bare steel—which then inevitably escalates wall loss significantly.

For a condenser water piping system, where wall loss throughout the system is 2–4 MPY, it is not unusual to measure severe under deposit pitting at a dead ended by-pass loop at 25 MPY.

Open condenser water piping systems are the most vulnerable, with any area of dead end or stagnant pipe providing a potential long-term threat.

6. Galvanized Pipe for Domestic Hot Water Systems

A galvanized pipe with corrosion.

Certain rules govern the use of materials for piping systems depending on material properties and often the physical limitations of those materials. Highly acidic waste requires certain forms of stainless steel, plastic or cast iron. Steam service is typically carried by steel pipe. For sanitary waste it is typically cast iron or ductile iron.

A limitation for certain piping materials also exists. Softer copper pipe is typically not used for high-pressure applications. Galvanized steel can be used for certain piping systems, such as a storm drain, but would not be considered for steam or steam condensate service due to its temperature limitations.

Cost also influences choice for certain piping systems. Whereby stainless steel would be ideal for domestic cold water service, and may be specified for a hospital, it is not typically used for commercial or residential properties due to its much higher cost. "Value engineering", a more formal term for lowering the project cost, is now a major aspect to any building design.

One example where the piping material itself is inappropriate for use is at domestic hot water service due to the fact that heat accelerates the deterioration of the zinc protective finish resulting in its advanced failure. Although galvanized steel pipe has been widely used for domestic cold water service for over 100 years, it has typically not been used for domestic hot water service due to this well-recognized physical limitation.

For older building properties located in cities like New York, Boston or Washington D.C., galvanized steel is found commonly for domestic cold water service, but then with brass pipe at the domestic hot water side. Where galvanized steel has been installed for domestic hot water service, system-wide advanced failure will occur, and has been documented in some more recent examples in as little as 1-1/2 years.

This is a widespread problem for the Chicago area due to the common use of galvanized steel for both domestic cold and hot water service. Although highly inappropriate for such service, this plumbing design specification was widely copied, evidently unaware of the difficulty that would appear decades later. ()

The failure of such piping systems is only related to water/pipe temperature and nothing else, with the pipe closest to the boiler or hot water source typically failing first. Smaller diameter pipe having inherently less wall thickness is always at greater risk, as are all threaded joints. (Related reading: Human Error as a Factor in Corrosion Failure.)

Once recognized, failure of the entire domestic hot water piping system is inevitable. Given that this is a chemical-based physical limitation of the pipe caused by elevated operating temperatures, there are no mitigating steps possible.

7. Internal Rust Deposits

Rust deposits inside pipes.

Rust deposits accumulate from various sources to produce a similar result of higher pitting activity and an advanced piping failure. Whether originating due to low flow conditions, an excessive corrosion rate, tuberculation deposits, microbiologically influenced corrosion (MIC) or any other source, internal rust deposits all lead to an advanced piping failure.

Part of the problems associated with such higher under deposit corrosion is the fact that it is generally unrecognized, always under reported and often discounted.

Corrosion coupons only report the relative corrosivity of the water itself, and not the wall loss occurring to the pipe. Favorable and welcomed corrosion coupon reports showing a 0.4 MPY corrosion rate, offer no possible indication to the 25 MPY pitting condition actually occurring at the bottom of a long horizontal by-pass line.

Chemical treatment providers all claim their products to have a special and unique ability to penetrate thick rust deposits and protect the underlying steel, when in fact 23 years of hard documentation through ultrasonic investigation has proven otherwise. Installing a 0.6 micron retention sand filter will clean the lightest particles suspended from the water and reduce its turbidity to everyone's applause, while doing nothing to remove the potentially thousands of pounds of rust and other debris attached firmly to its inside walls.

At a theoretical corrosion rate for a 12" condenser water system of 5 MPY, 65 lbs. of steel are corroded away for every 100 ft. of length per year. From this wall loss, over 2.8 cu. ft. of iron oxide rust product is created. Do the math.

Once established, internal rust deposits define a downward spiral for all piping systems, which inevitably leads to an advanced failure in those areas most vulnerable to such impact. Therefore, any corrective actions taken to reduce a high corrosion problem for any piping system, without first removing the rust build-up responsible for that same problem, becomes an unfortunate exercise in futility.

Rust deposits can only be removed through aggressive chemical cleaning and/or through physical means such as a high-pressure water jet. Both options have limitations to their application and success, and are often precluded from use due to other physical weaknesses within the system. With corrosion typically impacting all areas of a piping system to some degree, it is common to be faced with the difficult choice that correcting one corrosion issue may produce a piping failure elsewhere.

8. Schedule 40 Threaded Pipe

Threaded pipe with corrosion (rust) deposits.

Most threaded pipe found in older piping systems is schedule 80 or extra heavy material, which explains why many very old piping systems are still in service.

Depending on pipe diameter, threading removes approximately 50% of the pipe wall—meaning that the 0.154" wall thickness of 2" schedule 40 pipe is actually only 0.082" at the threads themselves.

Installed into any piping system of inherently higher corrosion threat, such as condenser water, threaded pipe is always the first component to fail. At a 5 MPY corrosion rate not uncommon for open condenser water systems, the entire wall thickness of 0.082" will be compromised in only 16 years. Actual failure is generally years earlier.

Based on the Barlow universal piping formula—which takes into account material strength, piping assembly, pressure, diameter and anticipated corrosion conditions—schedule 40 pipe does not meet minimum standards for use in open condenser water systems (although today it is universally specified).

For fire protection systems, this threat is even further amplified due to the approved use of threaded thin wall schedule 10 and schedule 7 pipe. In contrast to threading 2" schedule 40 pipe, which leaves a wall thickness of 0.083", schedule 7 pipe begins with only 0.084" of wall thickness. Removing another 0.071" for threading then reduces remaining wall thickness at its threads to approximately 0.012"—less than 1/3rd the thickness of a typical credit card!

For any HVAC application, the replacement of all small diameter threaded pipe will be likely required after 40 years of service and often before. For condenser water systems, threaded pipe will typically not provide more than 25 years of service, with most such systems requiring some pipe replacement after about 15–20 years of operation.

At only a small material cost increase, the use of heavier schedule 80 pipe in threaded applications offers very low-cost insurance to random failure years later.

9. Dry Fire Sprinkler

Dry fire sprinkler pipe with internal corrosion.

The number one question raised by everyone surprised at the failure of their dry fire sprinkler system is how dry pipe could have corroded.

Quite simply, the answer is that their "dry" fire sprinkler system is not dry and in fact never was dry. It can be labeled a dry system, be referred to as a dry system, and even argued a dry fire system, but it is never even close to dry once water is flowed into it the first time.

Pressure tested with water originally, and thereafter flow tested quarterly or more frequently, a significant volume of that water remains inside the pipe to create excessive corrosion conditions. Water does not fully drain from the pipe, as incorrectly believed, which then allows an abundance of air and oxygen above the water to produce corrosion rates of near 10 MPY and above.

The almost standard use of thin wall schedule 10 pipe and now schedule 7 further guarantees an early retirement of most dry fire systems. Under varying conditions, and heavily related to how much testing is performed, a dry fire system is not likely to provide beyond 20 years of service.

Galvanized steel pipe, commonly installed at dry and pre-action fire systems as a means to prevent such degradation, only produces a different form of more localized and accelerated pinhole corrosion. With lower quality galvanized steel pipe from sources worldwide, it is not uncommon to document their failure in less than five years.

The only benefit to galvanized steel pipe over carbon steel in a dry fire system is the far lesser volume of rust deposits created for galvanized steel pipe. A failure of carbon steel pipe is often linked to massive internal rust deposits capable of clogging the sprinkler heads themselves, while at a galvanized steel piping system surface, rust is only in the form of widely isolated tuberculation deposits under which severe pitting has occurred.

Both forms of failure, while representing significantly different outcomes where the piping is called into service during a true fire emergency, inevitably require pipe replacement. As one representative to a piping manufacturer once argued to us, "The real benefit of thin wall schedule 10 galvanized steel pipe over carbon steel pipe is that it will fail long before internal corrosion products can build up enough to clog it up."

We agree in terms of the threat, although still view the advanced failure of lower quality galvanized steel fire sprinkler pipe as totally unacceptable.

10. Insulated Outdoor Pipe

Pipe with thermal insulation.

For most locations, insulating outdoor condenser water pipe is totally unnecessary. Any 12" or larger diameter condenser water pipe under constantly flowing conditions and at water temperatures typically above 10°C (50°F) simply cannot freeze under any conditions, and therefore there is no necessity for its insulation.

Nevertheless, insulation is commonly installed, often for cosmetic reasons, or because efforts to paint the pipe itself may have failed. This in turn allows the hidden accumulation of water to destroy the piping from the outside in. Formally it is called CUI for corrosion under insulation, although we consider it more accurately termed CUPI, for corrosion under poor insulation.

Outdoor insulation is typically fiberglass covered by a metal or vinyl outer hard covering. Unfortunately, it is rarely installed and sealed to the degree necessary to prevent moisture and water infiltration. Cosmetically the insulation seems effective, but at the same time, often conceals a very aggressive corrosion condition. (This is discussed in more detail in CUI Myth: There’s no Corrosion under Insulation if the Jacketing isn’t Damaged.)

Similar to chill water and dual temperature piping systems, steel condenser water pipe is almost never painted or protected by a coating prior to insulation. If uninsulated, paint or some other protective coating would definitely be applied, but when insulated, it is not. For uninsulated pipe, observing a failure of the coating is typically addressed by removing any rust and applying new paint or coating.

When insulated, rust occurring to the pipe surface is completely hidden from view, and can proceed and accelerate unhindered. Since the insulation is not sealed between sections as required, any moisture entering one area of weakness can then travel laterally to impact a far greater area of pipe.

As water and moisture gradually infiltrates the outer covering, or migrates, it waterlogs the fiberglass. This reduces its insulation R value and intended benefit to near zero, but at the same time initiates a much greater threat to the pipe itself. Deterioration can easily reach 15 MPY or greater to destroy the roof-level pipe far sooner than any internal corrosion condition.

In addition to the hidden threat created by insulating outdoor condenser water piping, there is also the issue that once insulated, inspection becomes more difficult to perform.

Insulated pipe is always assumed to have performed the function intended and very rarely inspected as part of any preventative maintenance program. By the time most CUI problems are discovered, severe enough deterioration has often occurred to demand total pipe replacement.


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Written by Corrview .com

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CorrView International, LLC has been involved in the field of corrosion control and ultrasonic testing since 1981. Since 1981, CorrView has conducted 821 formal ultrasonic investigations involving 75 different categories of piping systems from condenser and chill water, to domestic water, to fire protection.

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