Advanced Corrosion Monitoring Overview

     Due to the combination of a wide variety of physical factors, building operations and management personnel are finding themselves faced with new and more complex corrosion related piping problems. Many are clearly related to the quality of the water treatment program or a past lack of corrosion protection, some are due to past problems which may have existed during construction and start-up.

     Many corrosion problems, in fact, are directly related to the quality and origin of the steel pipe itself, and whether or not it is of foreign manufacture. Review a summary of piping quality, operating, and design changes which have occurred.

     Addressing any corrosion problem requires accurate and reliable corrosion rate data in order to assess corrosion rate changes caused by the application of corrective actions. In response to such need to regularly monitor pipe corrosion, and due to the well documented failures of standard corrosion coupons to accurately reflect actual corrosion activity taking place, East Coast Industries, Inc. has developed this advanced corrosion monitoring solution.





The Limitations of Corrosion Coupons

     Corrosion coupons generally fail to produce corrosion rate values relative to actual pipe wall loss, and are typically placed at only one or two locations on a typical condenser water system. At best, they offer a rough estimate of the corrosivity of the fluid, rather than a true measurement of the metal lost from the pipe itself. Once a pitting condition or corrosion scenario other than mild generalized corrosion exists, the information derived from corrosion coupons becomes virtually worthless.

     The corrosion coupon rack itself, installed externally to the piping system, limits or eliminates many of the influences 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. Furthermore, corrosion coupons cannot be used to measure the significantly higher corrosion activity occurring during a winter drain down - documented in many cases to reach ten times that of water filled pipe. See Technical Bulletin # C-05 on the damaging effects of draining down pipe.

     Often ignored, the many different offerings or types of mild steel corrosion coupons will likely have quite a different chemical composition than most ASME specified pipe steels in service - which can vary between highly corrosion resistant A 72 wrought iron, and easily corroded A 106 foreign 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 - fundamental requirements of the corrosion process itself. 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 corrosion coupon surface is likely to be quite different than the surface 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 proportionately. 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 their thickness, impede contact between any water treatment chemicals and the base metal, and therefore reduce their effectiveness. Very heavy deposit buildup, however, will likely isolate the pipe from any chemical contact and protection whatsoever. Accumulated deposits often create a localized and severe metal loss known as "concentration cell," "corrosion cell," or "oxygen cell" corrosion, and may create conditions favorable to microbiologically induced corrosion, or MIC. See Technical Bulletin # C-06 regarding MIC.

     In examples where external ultrasonic examination can be verified with a view of the pipe's interior, we consistently find the highest wall loss hidden beneath the largest iron oxide deposits, commonly termed "tubercles." The below photograph clearly documents wall loss due to an aggressive under deposit corrosion condition, producing a localized wall loss of 0.150 in. seen at the left wall of the pipe. Such under deposit wall loss, whether generalized and shallow, or localized and deep, can never be identified or monitored via a corrosion coupon mounted in an external loop.



     Some other sources of corrosion coupon error include either a too long or too short test interval, varying intervals between successive tests, or the actions of the operating engineer. Extrapolating a thickness loss in mils per year (MPY) based upon the uneven weight loss of a six sided object adds even further error to the entire process. And, by the time the coupon may have finally acclimated to the conditions of the piping system, and may have begun to corrode relative to the actual pipe, it is replaced. The blatantly fraudulent practice of coating new corrosion coupons with a thin clear varnish or chemical protector has been documented.

     Not surprising, considering the many limitations of corrosion coupons, we have documented differences between coupon corrosion rates and ultrasonic testing as high as a factor of ten. We have repeatedly documented high wall loss caused by corrosion rates of 10 MPY or greater based upon actual wall loss, when the corrosion coupons results have consistently shown 0.8 MPY. For that reason we recommend corrosion coupons only as a method of verifying whether the water treatment chemicals are effective corrosion inhibitors, and that sufficient level of inhibitors exist in the system.


The Advantages of Ultrasound

     Identifying the current status of a piping system and the corrosion rate acting upon its metal surface is often difficult or impossible through other destructive and nondestructive means such as x-ray, corrosion coupons, spool pieces, selective pipe removal, or metallurgical testing, etc.

     Ultrasound technology, however, allows the precise measurement of the pipe thickness from the outside surface; thereby providing the means to produce a thorough corrosion evaluation at very reasonable cost. Ultrasonic testing differs from corrosion coupons, the most common form of corrosion monitoring, in that it summarizes the cumulative effects of all forms of corrosion over the lifetime of the pipe; providing a measurement of remaining wall thickness over a wide sampling of individual points. Most importantly, ultrasound measures wall loss at the pipe itself, rather than at a foreign metal sample installed externally to the actual operating conditions. See Technical Bulletin # C-01 for further information on the diagnostic limitations and inherent errors existing for corrosion coupons.

     The general advantages of ultrasound testing are many, and include:


Turning Your Pipe Into A Corrosion Coupon

     While ultrasonic testing is capable of producing extremely accurate wall thickness measurements, its narrow focusing width makes it difficult, if not impossible, to return to the same exact location for the future comparison of wall loss. A mark or photograph will more closely define a particular test site, but not accurately enough to ensure that the ultrasonic probe is precisely aligned and registered as before. Locating the general area may allow a rough comparison in evaluations where the piping surface is subject to relatively even corrosion activity. Where pitting exists, however, such a general comparison offers no useful information.

     Mounting a rubber guide or "template" to the proposed test area allows a reliable comparison of two ultrasonic measurements over time. By regularly measuring the wall thickness of those exact same points, we can very accurately determine corrosion rate, and the effectiveness of a 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 short term corrosion rate analysis - what we have termed, "fixed point monitoring."

     Whereas a general piping evaluation will estimate average corrosion rate over its accumulated time in service, this more advanced form of testing produces an absolute wall loss figure over one or more time intervals. A known wall thickness of 0.325 in. on 6/98, followed by a second test on 12/99 at the same location showing a wall thickness of 0.316 in. would therefore translate to an actual corrosion rate at the pipe itself of 6 mils per year (MPY).

     The data generated from such detailed testing provides a full range of corrosion rate and retirement date information for the following parameters:

     Similar to our other pipe analysis formats, all data is submitted in a fully detailed color report.

     For engineering departments, such a "fixed point monitoring" program offers significant benefits over most other forms of nondestructive testing. For one, it is 100% maintenance free. That means there are no corrosion coupons to replace, no flow requirements to monitor, no piping to install, and no timetables to accommodate. Fixed point corrosion monitoring can also be established in areas which could not, or would not, be normally addressed - such as in drained down areas, at the roof, at no flow areas, at dead leg piping, and at tanks and cooling tower pans. Other benefits of this program over corrosion coupon analysis include:


Common Applications

     Piping systems for which our fixed point corrosion monitoring is most frequently employed are:

     Other applications include:

     Generally, most other HVAC related piping systems do not suffer corrosion loss to any significant degree, and do not warrant this level of monitoring. Steam, steam condensate, and hot water piping, due to their higher temperatures, due to material and adhesive degradation, and due to the inherently higher level of measurement error caused by temperature fluctuations, make such a monitoring program difficult, if not impossible.

     Ultrasonic testing is suitable for determining the thickness of all piping materials, but is most commonly applied to:


Recommended Levels of Testing

     Setting up a fixed point monitoring program generally requires some knowledge of current corrosion conditions in order to place the unmovable rubber templates at the most appropriate piping locations. For a typical condenser water system serving a 35 floor office building, we would typically recommend the installation of between 4 and 6 fixed point monitoring sites. As for any form of testing - the more testing performed, the more accurate and representative the assessment of corrosion activity within that piping system.

     Actual test locations can be roughly assumed based upon commonly recognized corrosion trends - for example at horizontal sections, at distribution lines to HVAC units, at the top and bottom of the piping system, and in areas which have experienced periodic drain down. In many cases, interest to closely monitor corrosion activity follows a general corrosion or pipe testing investigation showing that such a problem exist, or after some form of leak. A previous pipe testing report is especially useful since it will suggest the most important areas to set up the fixed monitoring points.

     Further explanation of our general pipe testing and evaluation program can be found under the main heading of Testing Services - Ultrasonic Pipe Testing at the navigation bar at the left.

     Once the fixed point templates are set-up and the first baseline report is produced, follow-up testing can be performed on an annual basis or at any other time interval. Generally, we recommend more frequent tests at higher corrosion rates. Where generalized corrosion rates of 1 MPY exist, testing at a 1 to 3 year interval would be quite acceptable. At a 15 MPY corrosion rate, semi-annual testing would be advised. Most important, fixed point corrosion monitoring firmly establishes an important baseline that a specific amount of pipe wall thickness existed on a specific date.


Testing Accuracy

      The basis of each evaluation is, of course, the measurement of the remaining pipe wall at the locations tested. Accuracy alone of the equipment we employ, combined with our preparation of the pipe surface to remove any external sources of error, is approximately 1 mil or 0.001 in. for most applications.

     Our use of on-site verification of each ultrasound measurement ensures the accuracy of each thickness reading, and eliminates the questionable practice often used by other testing services of discounting the highest and lowest readings within any set of measurements - in some cases discarding as many as 25% to 50% of the wall thickness readings taken.


Reporting Criteria

     Rather than produce retirement date calculations based upon some arbitrary percentage of the original pipe wall, a commonly employed practice having no basis in fact or theory, we refer to established engineering formulas for minimum acceptable wall thickness which take into account pipe size, service or application, construction, pressure, material strength efficiency, and other operating factors. This is the same as our minimum wall thickness calculations used in our general piping evaluation.

     The result - a report conclusion that does not recommend premature or unnecessary pipe replacement, nor one that fails to recognize those operational factors which may lead to unexpected failure.


Field Testing

     Testing can usually be accomplished during normal working hours. Typically, it requires one day of field work to prepare the pipe, affix the rubber templates, set-up and produce the initial evaluation for 4 fixed point monitoring locations, and approximately 4-5 days to analyze the data and prepare the baseline report. A total of 12 individual test sites at each piping location define one fixed point monitoring location.

     Insulated piping requires the removal of small 12 in. areas for access to the pipe surface. Severely corroded outer surfaces may require mild grinding to produce an acceptable base measurement platform. To produce the more accurate measurements, all paint or coatings must always be removed in the immediate test area.

Follow-up testing is less involved, and only requires a return to each location for a new wall thickness measurement at each of the 12 individual sites. Ensuring that the rubber templates remain untouched, unpainted, or disturbed in any way is critical to any follow-up investigations.


Pricing Guidelines

     Fees are based upon a per location basis comprising 12 individual test sites, and include one copy of our full color report. Additional report copies are available, and all reports are archived by ECI for a period of 10 years.

     Our initial fee to prepare the pipe, set up the rubber template at 12 individual sites, take the initial wall thickness measurements and produce the first baseline report is $350- $400 per series of 12 points. All follow-up investigations are less involved, and are priced at $175 per test location. New test results are submitted as a new page of information into the original baseline report, and provide a chronological documentation of corrosion activity. Again, the above fees per each test location involves monitoring a total of 12 separate points at one general area on the pipe.

     Pipe testing that is located outside the immediate New York City area is priced at the same rate, but with the addition of actual expenses for travel and hotel accommodations.


Pipe Testing Report Samples

     A selection of reprinted pages from a typical client report can be accessed from the navigation bar at the left, and is provided in order to illustrate the level of detail provided in our fixed point corrosion monitoring program.

     In general, each follow-up test provides a new set of data upon which we can calculate current corrosion rates over that time interval. By definition, our first report provides a baseline of the existing pipe wall thickness, and an estimated corrosion rate only. Further testing then compares current values to past values for a far more accurate evaluation. Over time, this program produces a valuable trend analysis for each piping location tested.

     Graphed summaries are particularly useful for identifying various trends which may exist, and for comparing and evaluating the cumulative data derived from all test locations. In addition to the information presented at each individual test location, we also summarize the results according to each test parameter. Specialized piping problems and visually observed defects are documented with a photograph and commented upon.

     Each report offers both an executive summary of our findings. Written recommendations and corrective measures are also made addressing any piping deficiencies identified in the report. A photograph of each pipe location showing the actual test area is incorporated into each detail page of the report in order to better identify the test site to the reader, and to aid follow-up testing or future investigators.

     Since a high level of detail is provided in each ECI ultrasound report, and because we do not always have the opportunity to present our findings personally, we provide the written explanation and background information necessary to understand its content as best possible.

     Each report is preceded with a thorough explanation of the testing procedure, equipment used, basic theory of ultrasonic measurement, and necessary assumptions related to our testing and reporting procedure. Due to the fact that our corrosion monitoring reports are often submitted past a wide range of individuals including building owners, operating engineers, consulting engineers, and lawyers, among others, we have always strived to provide information ranging from the most basic conclusions to the most detailed technical information.

     For further, more detailed information regarding the actual testing procedures used, please view our Advanced Corrosion Monitoring Specifications and Procedures section at the navigation bar at the left. A wide variety of Technical Bulletins related to advanced corrosion monitoring specialties are also offered.


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