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Roof Level Corrosion

Most building operators see the first indication that a corrosion problem may exist through evidence shown at the roof level.  Some problems may be as minor as a pinhole leak or a failed thread – while others may cause the loss of a major riser.  Problems may be hidden at the pipe’s interior, concealed within the insulation, or as obvious as a trail of rust flowing toward the roof drain.

In many instances, suspected wall loss finally becomes obvious by the presence of iron oxide deposits and chip scale in the cooling tower sumps and overhead distribution pans.  Rust and corrosion at the pipe exterior may also be noticed in areas directly exposed to the elements, or more frequently, may exist hidden under the pipe insulation and hard outer metal covering.

• Dual Corrosion Attack

Roof level pipe typically suffers a two front corrosion attack.  If it is drained down partially for winterization in sub freezing climates, higher corrosion rates by as much as 6 times or more above those existing at other water filled areas may be realized.  This often creates unexpected piping failures which are many decades earlier than would normally occur at the lower floors.  Totally drained pipe will deteriorate quickly throughout – although at a higher rate at the cooling tower area, where a greater infiltration of air exists in proximity to the open pipe.  See Technical Bulletin CT-03 about increased roof level corrosion activity during winter or temporary drain down.

Typically lacking or ineffective paint or coating protection at any steel pipe that is exposed to the outdoor elements initiates a second corrosion mechanism which can quickly produce severe wall loss and pitting.  A simple lack of regular maintenance is often the obvious cause of such unnecessary wall loss – which can easily exceed the normal 3-5 mil per year (MPY) corrosion losses taking place at the pipe’s interior.

• Common Problems

The below photo gallery, taken from CorrView International archives of previous corrosion and ultrasonic testing investigations, provides illustration of some very common exterior condenser water pipe conditions.  Further examples are provided in our Photo Gallery.

While an internal corrosion problem may elude detection for a few years, external corrosion problems are usually obvious, and if quickly and effectively addressed, rarely worsen to the point of producing a piping failure.  Given sufficient neglect and time, however, the pipe will eventually fail and replacement will be required at significant expense.

Common Failures

• Insulation Also Covers Up Problems

Some of the most severe examples of roof level corrosion are typically found under hard shell exterior insulation following many years of service.  This is due to the easy penetration of overspray, rain, and other moisture through the seams and joints of most insulation jobs, or to physical damage caused during maintenance work and everyday traffic.  In many examples, the problem is simply due to poor insulation installation in the first place.

Where insulation is lacking water will eventually saturate the fiberglass material to provide an untreated water condition at the exterior pipe surface – leaving the degree of pipe deterioration solely dependent upon the natural corrosion resistance of the pipe.  The difficulty in removing the metal outer covering usually deters any regular preventative inspection of the pipe.

Hidden from view over many years or decades, such exterior corrosion activity is often very advanced in its surface attack by the time it is detected.  The degree of severity can usually be judged by the volume of rust product between the fiberglass and pipe, as well as the depth of pitting at the surface.  In the most severe cases, measurable pit penetrations exceeding 0.250 in. into the pipe are easily possible, and ultrasonic testing may be impossible to perform without first grinding the pipe to produce a flat test surface.

• Taking Precautions Important

The potential for Corrosion Under Insulation (CUI) to develop can be minimized or eliminated given certain precautions when first installed.  Few options exist, however, once the insulation is in place and water has saturated the fiberglass.

Corrosion under insulation generally becomes more severe as the pipe temperature decreases, and the potential to condense and hold moisture increases.  Therefore, the colder supply side piping will typically show the greatest external wall loss – becoming the first location to check in any piping investigation.  Conversely, in dry and arid climates, exterior corrosion of roof level or outside pipe is far less of a problem due to less humidity and less rainfall.

Chill water pipe is especially vulnerable to CUI, as is any pipe which is constantly drenched by cooling tower overspray.  In some examples, corrosion under the insulation will deteriorate the steam or electric based heat tracing to the point of failure.

• Maintenance Failure

Surprisingly, the lack of proper maintenance to exposed pipe is the major cause of all such failures.  Rehabilitation is possible where the pipe has still acceptable wall thickness, although the outer surface corrosion must be addressed.  Cleaning the piping of all rust products using a mechanical wire wheel and re-coating it with a high solids paint or epoxy is critically important as soon as the problem is recognized.  Special attention to less noticeable areas, or those in close proximity to other building fixtures and pipe hangers and supports, is also important in order to ensure that the entire pipe circumference is protected.

Generally, a thorough visual inspection every few years is all that is necessary to identify such areas.  Given the almost 100% certainty of some degree of water infiltration will penetrate through outdoor insulation, we recommend the simple step of applying a strong waterproof protective coating to any new pipe prior to installation as a preventative measure.  There are other products and methods that can also be employed for the protection of pipe having a severe outer corrosion problem.

Another common problem for roof level piping, also shown in the above photo gallery, is the galvanic reaction which can occur between the pipe itself and the metal pipe hangers or supports in the presence of water.  This, we believe, is due to the small microvolt difference in ground potential which may exist between the condenser pipe and the building steel.  Whatever the precise cause, high wall loss is commonly found at pipe hanger locations, or at wall or roof level penetrations.  Steps to insulate the pipe and building metals from direct contact are therefore recommended, with many different products now available.

• Recommendations

A summary of general recommendations to minimize corrosion losses at exposed roof level piping areas are:

• • • • Maintain filled pipe with chemically treated water, if possible.  Insulate and heat trace if necessary.
      • Provide supplemental chemical protection to any fully or partially drained pipe.  Do not rely on an increase in standard chemical inhibitor to guard against corrosion.  Consider the use of highly effective VCI chemical inhibitors.
      • Seal off any open sections of the pipe when drained in order to minimize fresh air migration – such as exists at the cooling tower drip pans and sump.
      • Coat the outside of the pipe with a high solids paint or epoxy suited for exterior applications or marine environments.  Paint or corrosion protect the pipe whether or not it is insulated.
      • Repair any chipped or missing paint immediately.  Mechanically clean or sand blast all rusted pipe to a bare white metal surface before painting or coating.  Use a “rust reverter” or suitable primer prior to painting.
      • Follow established recommendations for installing insulation at exterior locations.  Always cover with a hard shell metal or plastic insulation, and extend the overlap of the covering at the joints in order to minimize moisture infiltration.
      • Minimize cooling tower overspray, leaks, or other water losses.
      • Repair all damaged or missing roof insulation immediately.
      • Add a waterproof mastic type sealant between the joints of every few sections of insulation to limit water travel if a failure occurs.
      • Prevent galvanic activity at the pipe hangers by isolating the pipe itself from the metal supports.
      • Perform a visual inspection of all roof level piping annually for signs of exterior rust.  Remove random sections of insulation to check for hidden signs of corrosion every few years.

• A Sign Of Further Problems

While the repair of an internal pipe corrosion problem is often difficult, roof level losses at the exterior side of the pipe can almost always be prevented with just a minimum of maintenance attention.

The presence of such obvious and easily corrected problems at roof areas always suggests that even worse mechanical problems may exist throughout a plant or building property due to a common lack of basic maintenance.  Given the much higher corrosion threat which exists today at more modern building properties and plant operations, any lack of proper response to a problem can allow just enough opportunity for significantly greater problems to develop.

© Copyright 2023 – William P. Duncan, CorrView International, LLC