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 pans 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.

     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 six 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 at the open pipe. See Technical Bulletin # C-3 about increased roof level corrosion activity during winter or temporary drain down.

     Lacking or ineffective paint or coating protection of 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.

     The below photo gallery, taken from CVI archives of previous corrosion and ultrasonic testing investigations, provides illustration of some very common exterior condenser water pipe conditions. 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.

     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 water, rain, and other moisture through the seams and joints of most insulation jobs, or to physical damage caused during maintenance work and everyday traffic.

     Water eventually saturates the fiberglass material to provide an untreated water condition at the exterior pipe surface - leaving the degree of pipe deterioration solely dependant 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, 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.

     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. See Technical Bulletin # C-9 about the threat of corrosion under insulation.

     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 - and becomes the first locations 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.

     Surprisingly, the failure to simply provide proper maintenance of exposed pipe is the major cause of all such failures. Scraping the pipe clean of all rust products 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 noticed 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 once per year is often 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 water protective coating to any new pipe prior to installation as a preventative measure. See Technical Bulletin # P-6 for more about exterior pipe corrosion. There are other products and methods that can also be employed for the protection of pipe having a severe outer corrosion problem. Contact CVI for further details.

     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.

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

• Maintain the pipe filled 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. See Technical Bulletin # C-9 for further information on VCI corrosion 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. Scrape 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.
  
  
• Add CorrView ^® to your corrosion monitoring program at the roof level or at any areas which may be drained occasionally. CorrView ^® will deteriorate at the typically higher corrosion rates associated with drained pipe to provide a more accurate assessment of wall loss.

     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 of 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.

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