CT-07:   External Pipe Corrosion

Throughout the history of most building properties and plant operations, some degree of corrosion at the external surface of the chill and cold water piping will occur.  Surface pipe corrosion can range from cosmetic and mild, to severe and system debilitating.  Corrosion under insulation, CUI as it is known, is generally a long term problem taking decades to develop.  This partially explains its low priority in most building and plant maintenance plans and procedures.

The presence of condensation at the outer pipe wall produces much the same corrosive effects typically associated with interior pipe corrosion, except that microbiological attack rarely occurs.  In extreme cases, condensation will build up to the point where the insulation becomes completely saturated with water.  Generalized corrosion, excessive surface pitting, as well as the deterioration and failure of the insulation itself, usually results. Although presumed to be a long term problem, under certain conditions, exterior corrosion can produce a noticeable impact within 10 years.

• Cold Insulation Of Secondary Importance

While fiberglass, foam, and other forms of insulation serve to provide important protection against unwanted heat transfer, their equal or more important function for cold water systems is to prevent the migration and condensation of moisture onto the outer pipe surface.  Such function is rarely considered in the selection of cold water piping insulation, however, with standard insulation tables providing recommended thickness values dependent mostly upon pipe diameter.

The thinner the insulation, the greater the transfer of both heat and moisture to the pipe surface, and therefore the greater the resulting corrosion problem.  Even installing 2 in. or thicker fiberglass insulation, although excellent at reducing heat transfer losses, is often not itself sufficient to prevent the infiltration of moisture and condensation at a cold pipe surface.

Moisture easily migrates to the surface of the smaller diameter piping – where multiple turns, connections, and fixtures such as valves, strainers, and gauges make an effective insulation job difficult, if not impossible.  Any crack or sealing failure then becomes an entry point for moisture to permeate under the insulation and travel along the pipe surface for significant distance.

In many examples, therefore, the larger diameter main piping will show little or no surface deterioration, whereas the smaller distribution and run-out lines will reached near the point of failure. Identifying small diameter pipe where corrosion has completely worn away the exterior threads is not at all unusual.

The previous use of hard formed insulation decades ago was extremely effective at isolating the cold pipe surface from outside moisture.  Commonly used fiberglass insulation of today, which offers a semi-permeable cloth or paper outer covering, provides no real moisture barrier, however, and requires additional steps such as coating or painting over the insulation in order to stop a corrosion problem.  Other preventative steps are also often necessary.

• A Combination Of Factors

For inside locations, exterior corrosion is dependent upon a combination of the thickness and condition of the insulation, the water temperature, and the relative humidity in the area.  Generally, corrosion will be much more severe at the supply side piping simply due to the presence of lower pipe surface temperatures – typically lower by a factor of 10º F.

Depending upon the humidity level present, it is not unusual to find a 55º F. chill water return line free of any surface corrosion while the 45º F. supply side pipe has been severely pitted and deteriorated.  Likewise, the same chill water supply line may show high pitting in a more humid mechanical room or steam room area, and none at all once it passes into an air conditioned tenant space having less humidity.

Brine and ammonia refrigeration systems, or those operating at 35º F. and below, are most susceptible to outer surface corrosion.  Heavy insulation to limit the travel of moisture, and a strong preventative coating at the pipe itself to serve as a moisture barrier, are necessary to counter the threat from such low temperatures.  The humidity level is often the most critical factor, and we have documented condensation and the complete destruction of condenser water pipe even at 75º F. where it has traveled through a heavily moisture saturated steam station area.  The higher the moisture content, the less cold is needed to create an exterior corrosion problem.

• Easily Avoidable

Unlike interior pipe corrosion which can never be stopped, and which exists due to a complex variety of physical and chemical causes, piping failures due to the extreme outer corrosion of chill and cold water systems are almost totally preventable.  Such failures are generally attributed to inadequate maintenance and neglect, and/or the failure to inspect for and recognize such corrosion problems.

Chill water pipes are not the only victims of such outer surface corrosion.  A common occurrence in steel domestic house tank evaluations is to find extreme outer corrosion at the bottom or underside surfaces.  Here, inaccessibility often prevents adequate coating or painting.  For the same reason that it is out of sight and therefore out of mind, normal maintenance to the side walls is rarely performed at the bottom surface.

The below photo gallery dramatically illustrates the ultimate consequence of inadequate pipe insulation and/or maintenance. Additional examples of this problem are provided in our Photo Galleries.

Different Issues Relating To Insulation Failure

	Slow Deterioration 25 years of high humidity conditions effectively penetrated the insulation to slowly cause a severe external corrosion condition. Where still sufficient wall thickness remained, extensive rust removal, painting, and re-insulation was required. Surprisingly, sufficient remaining wall thickness yielded an acceptable remaining service life of 25 more years at this 8 in. chill pipe at all except the most heavily deteriorated areas.
	Generalized Mild Corrosion External corrosion due to insulation failure actually produced a uniform layer of rust product of approximately 1/2 in. thick.  In this case, a general ultrasonic piping evaluation discovered this condition, as well as showed it was a uniform loss and not a high pitting condition, as shown above. With acceptable wall thickness remaining, the pipe was coated with a rust reverter, re-insulated, and returned to service.
	Moisture Condensation This shows the very early stages of a future external corrosion problem.  Here, the use of 1 in. of fiberglass insulation over a 12 in. chill supply pipe at 44º F. resulted in the insulation becoming water saturated after only 5 years of service. Random areas of mild corrosion were found, and re-insulation with a thicker fiberglass material was performed.
	Waterlogged Insulation A good view of water saturated fiberglass insulation in worse condition.  While it typically requires many years to become fully saturated, this recently installed chill water piping was found to have the beginning of a serious exterior rusting problem after only one year in service – this due to poorly installed 1 in. insulation. Found at its beginning stages, mostly in the more humid mechanical areas, coating with a rust reverter and re-insulation was the only option.
	Deep Pitting Corrosion from exterior sources can take the route of general corrosion to deep localized pitting – similar to interior pipe problems.  Ten years of sub-zero service at this Ammonia plant produced random and deep pits up to 0.150 in. along the top of this pipe.
	Small Fittings Small pipe fixtures such as pressure and temperature gauges and instrument transmitters are at special risk.  This is due to the difficulty of insulating them, their inherently thin pipe wall, the standard selection of schedule 40 pipe, and to losses suffered when threaded.  Few in the industry recognize that the smallest diameter fittings at any piping system always represent the greatest threat.
	Imminent Failure Such small pipe sections are always at the greatest threat. They are the most difficult to insulate, have gaps for air to easily infiltrate, and are usually insulated with very thin materials. In addition, more than 60% of the original pipe is usually cut away while threading, which is usually for the purpose of attaching a brass valve – a source of galvanic activity.
	Stratified Rust Like interior rust deposits, corrosion created by a water condition at the exterior of a pipe often results in a buildup of multiple layers of rust.  As new rust develops and expands, as shown above, it appears to grow the pipe in diameter until fracturing the outer rust band. Since rust is directly proportional to pipe wall loss, the visual condition of corroded pipe becomes greatly informative.  Where stratification exists, and especially at smaller diameter lines, little service life remains before its inevitable failure.  Also in this condition, UT testing cannot be safely performed.
	Wet Insulation An obvious indicator of a problem. cold water piping will attract area moisture through any paper or cloth covered insulation to condense on its surface.  Once saturating the insulation itself, the moisture will often produce droplets at its exterior.  This will also occur at pipe having an outer covering although it typically requires more time.  While such coverings are beneficial as a first defense against moisture migration, they quickly become detrimental by trapping and retaining more water against the pipe; potentially hiding a CUI issue for years or decades.
	Insulation Failure Under insulation corrosion is commonly due to a combination of insulation failure itself, area moisture levels and cold pipe temperatures.  It may also be due to insulation failure alone – which is an important distinction related to maintenance or the lack thereof. A greater threat always exists for cold water pipe since insulation damage is not necessary.
	Random Deep Pitting Deep pitting offers much greater threat since the corrosion activity is focused against specific areas like a drill bit – rather than a lesser and more uniform wall loss against the entire pipe surface.  Whereas a generalized exterior corrosion rate of 2-3 MPY can often be tolerated, a pitting rate of 10-15 MPY will certainly produce a piping failure if undetected or ignored. Deeper pitting is often found where air infiltration exists at damaged insulation, open seams, at pipe hangers, and wherever the insulation has not been properly installed and maintained.
	Soft Foam Insulation Severe corrosion activity is well known to occur under closed cell foam piping insulation.  Such is less often specified for larger diameter mains, and is commonly used to protect the smallest run-out piping and small fixtures such as drains which are difficult to insulate using fiberglass.  The lack of skin irritation and the neat smooth appearance of soft form is another reason for its increased use. The problem its accelerated deterioration where it then shrinks and cracks, allowing moisture to enter.  As soft foam insulation deteriorates, it also produces a weak acid that accelerates steel and copper pipe corrosion.
	Fiberglass Failure to Foam Failure Soft foam is often viewed as the superior alternative to fiberglass, although our UT investigations have shown it to produce even more damage when it fails.  Not to be confused with thicker hard shell “foamglass” insulation which performs well in even the most humid environments, the replacement of fiberglass insulation with soft foam insulation is likely to make matters even worse.
	Return Side – Lower Corrosion The left and below photographs offer a good side by side comparison of the much greater damage to supply side chill water piping which normally occurs when the insulation is inadequate, and surface temperatures are sufficiently cold. This set of supply and return pipes existed parallel to each other and are identically insulated, yet the return side pipe shows much less exterior corrosion.
	Supply Side – Higher Corrosion The supply side component of the same chill water loop shows severe pitting, with an estimated depth of penetration into the pipe of about 0.100 in. Given that this is 2 in. extra heavy threaded pipe having a measurable remaining wall thickness in other areas of under 0.200 in., and thread cut depth of 0.095 in., no possible service life remains. Ultrasonic testing cannot be performed due to its rough surface conditions.
	Maintenance Neglect A not uncommon problem often caused by the difficulty in insulating certain piping areas.  This 12 in. chill water main supply pipe, located in a steam station area, showed widespread deterioration due to a complete lack of insulation in many areas.
	Inside Vs. Outside These chill water pipe cutouts provide an ideal comparison of interior and exterior corrosion. At the left we can see the exterior surface having been pitted due to a failure of the fiberglass insulation. At the right shows the interior surface covered with a light coating of iron oxide. Overall wall thickness is acceptable, and a comparison of inside and outside surface conditions shows that the effect of exterior corrosion has been the major deteriorating factor shortening system life.
	External Leak A slow leak over years at the packing gland for a top mounted valve produced devastating losses at this domestic hot water tank. The insulation provided some concealment for the problem, but eventually deteriorated completely. A failure to address this corrosion problem resulted in significantly greater wall loss.

• Preventative Inspection

The first step toward protecting against outer pipe wall corrosion is to find out if it exists, and to what extent – a step requiring some investigative effort.  Recommended prime locations to remove samples of insulation for inspection are high humidity areas such as steam rooms, shaftways, mechanical rooms, open areas, and rooftop or other locations exposed to the elements.  In many examples, water will migrate through the insulation to produce a noticeable watermark, stain, or crystallization at the surface.

Supply side piping should be checked first.  Hard shell older style insulation, or insulation which is hand formed of plaster or similar materials is usually very effective at holding back any moisture from penetrating, and is especially effective for small diameter pipe.  Standard unpainted paper or cloth covered fiberglass, on the other hand, provides little barrier to moisture, and is always a high priority location to check.  When painted, fiberglass insulation can be effective at retarding moisture penetration – this depending upon the composition of the coating and its thickness.  A single coat of paint is generally not sufficient.

Old style cork insulation, still in use from years ago at the oldest building properties, actually attracts moisture, and has been shown to cause the most severe cases of outer pipe corrosion we have measured.

While soft foam type piping insulation is convenient to apply, it too has been shown to fail in preventing the migration of moisture to the pipe surface.  Painting foam insulation is generally not an option.  In addition, a recognized chemical reaction between the components of the foam and the pipe itself has been cited in the on this internet site as greatly accelerating pipe surface corrosion.

Broken, missing, crushed and other damaged sections of insulation will obviously lead to outer pipe corrosion problems, and should be repaired or replaced as soon as it is discovered.  Missing insulation can be frequently found at transitions through walls and floors, or in areas of heavy traffic.

More information on this subject is available in Technical Bulletin Pi-03.

• Greatest Threat At Smallest Diameter Pipe

A high amount of outer corrosion can be typically found at smaller diameter pipe simply due to the inherent difficulty of insulating its many elbows, tees, valves, strainers, and other small piping components. Unfortunately, it is also that smaller diameter piping which has a thinner initial wall, and which is substantially weakened by the amount of metal cut away during the threading process. Small diameter pipe requires little outer corrosion, therefore, to create a failure condition.

Our standard recommendation to ensure against moisture caused outer pipe corrosion is to install 2 in. or heavier fiberglass pipe insulation on all cold water pipe of 50 º F. and below.  In addition, a high solids paint, epoxy coating, or hard shell outer covering, sealed thoroughly at all seams, is critically important in order to resist moisture penetration.

Since the surface of the pipe which is insulated is never seen, it is almost never painted.  As a result, any moisture which may accumulate at the pipe surface is able to attack completely unprotected steel.  As a precaution against such possibility, CVI recommends painting all cold water piping with a strong rust preventative coating prior to being insulated.  This is a very simple yet worthwhile precaution to specify in any new piping construction if long term and trouble free operation is desired.

• General Insulation Recommendations

The following list offers some worthwhile recommendations for providing the maximum heat transfer efficiency and condensation protection of any insulated chill or cold water piping system:

• • • Install heavier insulation. Piping specifications generally require 1” thick insulation for 12 inch black pipe chill/cold water systems at 70% relative humidity. At 80% relative humidity, 2” thick insulation is specified. Consult your insulation supplier or contractor. Consider the humidity conditions of the area the pipe will travel through.
    • Maintain a good moisture barrier at the outer pipe surface. Require smooth seals and joints throughout the entire installation in order to prevent moisture penetration. Stapled insulation, without being sealed, is a prime cause of insulation failure.
    • Consider various outer tape type wraps to the pipe which provides an impenetrable barrier to moisture.
    • Install a secondary metal, PVC, or other hard vinyl outer jacketing over the existing insulation. Overlap and seal the adjoining sections using a bead of silicone or other waterproof adhesive.
    • Protect all insulation from physical damage. Provide steps and bridges over insulation in high traffic areas. Caution staff against standing on insulated pipe. Repair all cuts and rips in the insulation immediately.
    • Apply an isolating seal of mastic or other waterproofing material at regular intervals between sections of insulation. This is especially important for outdoor locations. Should a breakdown at a particular area of insulation occur for any reason, the resulting condensation and water will be prevented from migrating throughout the adjoining insulation and piping.
    • Apply a good quality rust preventative coating to the base piping as soon as it is in place, and prior to insulating. Paint around the entire pipe circumference.
    • Migrating Vapor Corrosion Inhibitors (VCI) may be applicable to retard corrosion at already insulated piping which is known to have a corrosion problem, and cannot be immediately addressed.
    • Paint the insulation immediately after it is installed as a moisture barrier. A initial heavy coating of paint will penetrate the semi-permeable cloth or paper covering of the insulation to further protect against moisture migration. Require that the entire circumference of all piping is covered in order to seal the entire surface.

Insulated piping located outside exists under even greater threat from insulation failure due to varying environmental conditions. This places even more importance upon the above recommendations.

• Latest Technology

Some latest nondestructive testing technology does exist that can scan through piping insulation quickly and effectively to identify any hidden corrosion problem. This is best to contract as a service, and is an extremely cost effective means to safeguard against the above problems. Other sensing devices can detect waterlogged insulation and rust deposits at both the inside and outside of the pipe.

CorrView International strongly recommends the above preventative measures as a means to avoid this potentially devastating but very avoidable operating problem.

© Copyright 2005 – 2025 – William P. Duncan, CorrView International, LLC