The corrosion of steel piping and its related components is a continuous and virtually unstoppable process. Even with the application of available countermeasures, pipe corrosion exists as one of the most potentially damaging threats to any private, industrial, or commercial property - second only to fire.

     Corrosion activity affects HVAC piping systems to varying degrees generally dependant upon the piping service, quality of the steel, age, its size and layout, joining method, chemical treatment protection, engineering design, and the specific corrosion mechanism involved.

     For many properties, the net result is an added maintenance problem, greater energy costs, unnecessary threat and liability, property damage, high remediation expense, and in the most extreme examples - the need for partial or total pipe replacement.

     Once established, most corrosion conditions are difficult, if not impossible to eliminate or even control. Repairs may be impossible and require total pipe replacement. Various investigative resources are required in order to firmly identify the problem - itself a major undertaking.

     A very first step toward resolving any corrosion problem is to establish the overall condition of the piping system through a general ultrasonic investigation. Clearly identified problem areas can then be further defined through the use of metallurgical and microbiological analysis. See Technical Bulletin P-7 for more about the level of information provided by ultrasonic testing.

     By comparing metallurgical against ultrasonic test results, it is often possible to identify the extent and severity of the corrosion condition within the piping system. Well documenting not only the corrosion mechanism itself, but the remaining integrity of the entire piping system, is critically important in order to minimize the potential damage from leaks caused by any planned corrective actions.

     Ten years of experience in ultrasonic pipe testing at hundreds of commercial office properties and plant operations has documented how environmental concerns and government restrictions, combined with less tolerant engineering practices and cost cutting, have greatly reduced the life expectancy of most new HVAC piping installations.

     Today's corrosion inhibitors have proven to be less effective under real world operating conditions when compared to prior chemical agents such as chromate and hydrazine. Under low flow or dead end conditions, corrosion and pitting often increases considerably. Whereas a 1 mil per year (MPY) corrosion rate for a condenser water system could be reasonably expected decades ago, a 5 MPY rate is now often considered acceptable, and costs significantly more to achieve.

     The greater corrosion susceptibility of present-day steel pipe products is another major factor in producing many high corrosion problems, and exists beyond any reasonable control of the property owner or plant operator. With many foreign sources of piping products well recognized for their lower corrosion resistance, specifying domestic steel is strongly recommended - although offers no guarantee of fewer problems.

     Together, these two elements alone can produce corrosion rates of above 25 MPY, which, virtually unheard of 30 years ago, now offers the potential to destroy any piping system within 10 years.

     A major factor seen at more recent commercial properties where piping failures have occurred relates to the specified piping schedule. Most early U.S. office buildings constructed prior to 1940 used extra strong pipe exclusively throughout. In the 1950's, thinner standard and schedule 40 pipe started being substituted for less critical services such as chill water, fire sprinkler, drain lines, domestic water, and secondary water systems.

     From 1970 onward, extra heavy or schedule 80 stock might only be found at more critical and corrosion susceptible condenser water and steam piping lines. Beginning in the 1980's, however, virtually all building services piping, excepting those serving excessive operating pressures, have been specified using standard or schedule 40 material.

     This move by design engineers and contractors from extra heavy to standard grade pipe has taken a further and more risky step in the past 10 years with the use of thin wall schedule 10 steel in many fire sprinkler and condenser water systems - often with disastrous results.

     For a section of 8 in. condenser water pipe that would have provided an extra heavy wall thickness of 0.500 in. for a 1950's property, or 0.322 in. at a facility constructed of schedule 40 in 1985, the frequently seen use of schedule 10 now provides only 0.188 in. of available wall thickness under substantially higher corrosion conditions.

     Advanced failures are therefore quite frequent where schedule 10 is employed, and easy to understand in viewing the above relative illustration of pipe wall thickness. View a more detailed comparison of various wall thickness values for carbon steel pipe.

     Not all piping failures can be attributed to a high corrosion rate however. To a great degree, many of the current problems seen at commercial office buildings and plant facilities are simply age related. With many U.S. building properties reaching 40 years of service or more, the cumulative effects of even a moderate and generalized corrosion condition will add up to produce substantial material losses.

     A low (and rarely seen) 1 mil per year (MPY) corrosion rate at 12 in. schedule 40 condenser or chill water pipe, while seemingly minor, actually results in the annual loss of 12.8 lbs. of steel for every 100 linear feet of pipe. Multiplied by the number of years in service and overall length - and the true magnitude of system corrosion takes on much greater significance than when reported simply as 1, 2, or 5 mils per year.

     For a 40 story office property built in 1965 and having 1,000 or more feet of 12 in. condenser riser piping, the total loss of more than 5,000 lbs. of steel into the circulating system can be shown. See the below table for the weight losses of various pipe sizes and corrosion rates. See Technical Bulletin # C-1 for the actual volume of rust deposits created.

     Often, a low to moderate corrosion rate will present no threat to the integrity of the pipe itself, yet may still damage the piping system after many years or decades of operation. At a constant 5 MPY corrosion rate, for example, 12 in. schedule 40 condenser water pipe will last well past 45 years before reaching 0.175 in. - its minimum acceptable wall thickness under most conditions. Yet, that same 5 MPY corrosion rate still presents a very serious threat.

     Since the corrosion of steel produces a significantly greater volume of less dense iron oxide, serious secondary problems can be created unless such deposits are continuously removed. Interior deposits can produce even more serious problems at closed systems - where no blowdown exists and no indication is given. Corrosion problems are often localized, and a gradual build-up of deposits in the lower floor horizontal lines due to a low 1 MPY system wide corrosion rate can produce random but severe wall losses at rates well above 25 MPY - leading to premature pipe failure.

     Unquestionably, removing all existing deposits of iron oxide, which can total in the thousands of pounds for even a medium sized commercial building system, should become the most important focus in addressing any corrosion problem. Aside from lost heat transfer, clogged strainers, and other operating problems, interior surface deposits greatly accelerate pipe loss by preventing corrosion control chemicals from reaching the base steel, and by initiating various secondary corrosion mechanisms.

     Due to the large volume of deposits typically produced by any corrosion condition, the addition of supplemental filtration is mandatory - with the option of filtering the greatest possible volume of water preferred over capturing the smallest micron particle. See Technical Bulletin W-4 regarding various types of filters available.

     Chemically removing the accumulated iron oxide by either dissolving it for blowdown or re-suspending it for filtration capture is typically employed, but often presents added threat to the piping and related system components depending upon the remaining integrity of the piping, cleaning agent, and clean-out procedure used.

     Greater maintenance demands in the form of punching heat exchanger tubes, cleaning strainers and tower pans, and added filter maintenance may be temporary, or in the case of a severe corrosion problem, may be a permanent addition to the operating schedule of the property.

     Increased chemical inhibitor and biocide levels are generally required, as are supplemental chemical dispersing agents. For a microbiologically influenced corrosion (MIC) problem, repeated sterilization and cleaning of the system will be necessary. Contracting an outside consultant to oversee and advise the chemical treatment or cleaning program is often necessary due to the complexity of various treatment options, conflicting claims and abilities of various chemical treatment contractors, and the potential threat to the piping system.

     Under the most severe conditions of a well established under deposit corrosion or MIC condition, it may be impossible to save the piping system from premature failure. In such cases, effort usually focuses on minimizing damage and operating problems, replacing pipe as necessary, and extending its service life as best possible.

     Avoiding such corrosion problems from first developing is clearly desired. Strict attention to the chemical water treatment through a reliable contractor is a high priority - especially in the earliest days and months of the initial start-up. Effective filtration and frequent cleaning of the piping system is mandatory, as is careful monitoring for corrosion and biological activity.

     Corrosion coupons exist as the most widely used form of corrosion measurement and monitoring today. However, installed within an isolated loop separate from the many electrochemical influences acting against the actual pipe surface, they at best offer an estimate of the corrosivity of the fluid, rather than a true measurement of the metal lost from the pipe itself. Read more about the limitations of corrosion coupons.

     Corrosion coupons can often produce falsely low corrosion estimates by a factor of 10 times or more, and where significant iron oxide deposits exist at the pipe surface, will not provide any indication of the deep under deposit pitting taking place. CorrView ^® monitors, removable spool pieces, and ultrasonic testing, therefore offer better and more reliable testing options.

     Maintaining trouble free HVAC piping systems can be readily achieved, although not with the ease found decades ago, and not without significantly greater expense and maintenance attention. New influences at many different levels exist today favoring higher corrosion rates - which must be recognized and effectively addressed by today's successful building professional and plant manager.

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