Technical Bulletin # C-01


An Accurate, Low Cost, And High Tech Corrosion
Monitoring Alternative To Standard Corrosion Coupons



THE PROBLEM:

     One of the most potentially destructive events that any building owner or operator can face is the weakening or failure of their HVAC piping system(s) due to advanced corrosion problems. This may be caused by an excessive general corrosion rate, galvanic activity, localized pitting, cell corrosion, inferior pipe quality, microbiologically influenced corrosion (MIC), or other causes and events.

     Aside from the claims of chemical water treatment companies to provide the highest level of corrosion protection available, the fact remains that a high number of properties have had their HVAC piping systems virtually destroyed due to the failure of those services.

     In many cases, a severe corrosion problem is only discovered after first producing noticeable cooling tower deposits or other operating difficulties - often in clear contradiction to repeated field tests and lab analysis reports showing acceptable chemical levels and other water system parameters.

     While the causes may vary between poor maintenance, ineffective corrosion control, inaccurate water test reporting, and even outright fraud, responsibility for such conditions ultimately rest upon the building’s mechanical operators for their failure to adequately monitor the corrosion rate of the affected systems.

     Corrosion coupons, the standard method of corrosion monitoring used throughout the HVAC industry, is performed by measuring the metal loss of a mild steel sample over a given period of time. While providing some basic information about the level of corrosion activity in a system, corrosion coupons only provide information relevant to a short interval of a few months, and for one localized area of concern.

     Corrosion coupons, shown below in their holders, generally fail to produce corrosion rate values relative to actual pipe wall loss, and are very rarely used on steam, condensate, domestic water and other piping systems. At best, they offer a rough estimate of the corrosivity of the chemically treated fluid, rather than a true measurement of the metal lost from the pipe itself. Most importantly, the mild steel of a typical corrosion coupon will have quite a different chemical composition than most ASME specified pipe steels - and can vary between highly corrosion resistant A 72 wrought iron, and more easily corroded A 106 or a A 795 foreign pipe.



     The corrosion coupon rack, installed externally to the piping system, limits many of the corrosion influences normally acting against a circulating water system. Variations in water flow can dramatically influence corrosion estimates by as much as five fold, as can materials of construction, layout, pipe size, filtering, and even the physical distance from the main piping. Important for many properties, corrosion coupons cannot be used to measure the significantly higher corrosion activity occurring during a winter drain down - documented in many cases to reach two to ten times that of water filled pipe.

     Since corrosion coupons are typically isolated from any metal to metal contact through the use of a galvanic insulator, they are totally unaffected by the many anode/cathode electrochemical reactions always present in an established piping system. As a result, the corrosion mechanism responsible for the majority of pipe loss in most piping systems is never measured.

     Having a smooth polished surface, which minimizes the adhesion of dirt and microorganisms, corrosion coupons are rarely attacked in the same manner as an aged piping system having an irregularly worn and pitted surface. In addition, any passivating layer of iron oxide at the coupon surface is likely to be quite different than that which has been established on the actual pipe.

     While the above factors highlight some of the inherent problems in measuring pipe metal loss via standard corrosion coupons, the existing conditions at the pipe wall itself often provide the most serious obstruction to an accurate corrosion rate measurement. It is well recognized that as surface deposits increase, the correlation between the actual corrosion rate and the corrosion coupon measured rate decreases.

     Once a solid layer of iron oxide or scale deposits form on the pipe's interior, an entirely new set of corrosion mechanisms may form which simply cannot be duplicated, nor measured, by any corrosion coupon. Mild deposits may, depending upon thickness, impede contact between any water treatment chemicals and the base metal, and therefore reduce their effectiveness - whereas a very heavy buildup will likely isolate the pipe from any chemical contact and protection whatsoever.

     Accumulated deposits may create conditions favorable to extremely destructive microbiologically influenced corrosion, or MIC. See Technical Bulletin # C-06 for further information on MIC corrosion. It more often creates a localized and severe metal loss known as "concentration cell" or "oxygen cell" corrosion. The is well documented in the below photograph, which shows generally even and very acceptable remaining wall thickness throughout most of the pipe wall, but with severe and localized deterioration reducing the wall by 50% at one upper area where heavy interior deposits exist.



     Some other sources of corrosion coupon error include either a too long or too short test period, varying intervals between successive tests, or the actions of the operating engineer. Extrapolating a thickness loss in mils per year (MPY) based upon the weight loss of a six sided object adds even further error to the entire process. And, by the time the coupon has finally acclimated to the conditions of the piping system, and may have begun to corrode relative to the actual pipe, it is replaced.

     Over hundreds of ultrasonic evaluations, ECI has documented a wide variance between actual pipe metal loss and the estimated corrosion rate as based upon corrosion coupon analysis. We have found errors in the under-reporting of corrosion activity by corrosion coupons by as much as a factor of 10 times - a 1 MPY corrosion coupon estimate equalling an actual 10 MPY or higher corrosion rate at the pipe metal itself.

     The below photo illustrates a good example of the hidden threat in relying on corrosion coupons. Here, our 1996 ultrasonic investigation estimated a 12.4 MPY average corrosion rate and high pitting consistently at 30 different locations over the entire condenser water piping system. Lowest thickness measurements at the threaded joints showed minimal service life remaining and the possibility of failure. Yet, years of corrosion coupon results by the water treatment contractor showed no more than 1 MPY of wall loss at any period of time, with reported wall loss measured as low as 0.38 MPY - a virtual impossibility.



     Two years later, a pipe failure, predicted in our earlier report, forced the replacement of the threaded piping. Corrosion products as the result of the high corrosion rate had, in fact, restricted areas of piping by approximately 50%. Corrective actions, which could have helped reduce the high corrosion activity, were not taken due to the reliance on corrosion coupon results - results which were, in fact, inaccurate.


THE SOLUTION:

     By regularly measuring the wall thickness of the exact same points of a piping system, ECI can very accurately determine the effectiveness of any chemical treatment program over any given period of time.

     In what can essentially be viewed as a "permanent" form of corrosion coupon analysis, ECI can ultrasonically measure the pipe wall thickness at each test site; evaluating each reading in relation to previous measurements for the purpose of producing a thorough and reliable corrosion rate analysis. Similar to our other pipe analysis formats, all data is submitted in a fully detailed color report.

     This corrosion analysis program offers significant benefits over standard corrosion coupons and most other forms of nondestructive testing, such as:

     Our full series ultrasonic pipe testing survey, while ideally suited for evaluating the cumulative effects of corrosion over the lifetime of any commercial or industrial property, cannot provide detailed corrosion rate information over a period of one year or a few months. See Technical Bulletin # P-01 on general pipe testing.

     In addition to monitoring the corrosion activity at piping systems, fixed point template testing is also suitable for:

     Setting up our fixed point ultrasonic testing program begins with a walk through evaluation of the piping system along with a review of building history and corrosion measurement concerns. Preliminary testing to locate any critical piping areas is advised. At each selected test site, ECI will fasten one or more permanent pipe templates having perforations of the exact size to match our ultrasonic probe. In doing so we are assured that every wall thickness measurement will be taken at the exact same location and orientation as all those prior.

     This method of analysis effectively eliminates any significant test error and provides building operating personnel a detailed and continuing history of important corrosion rate information.

     The following photographs of actual test locations show the application of the rubber template permanently bonded to the condenser water pipe surface, and is typical of most piping installations.



     A far more detailed explanation of our fixed point template testing and reporting procedures can be found under the heading Testing Services - Advanced Corrosion Monitoring at the Testing Services menu.



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