The CorrView corrosion monitor is a new product specifically designed in response to the inherent inaccuracy of corrosion coupons, and to the need to measure pipe wall loss under real world conditions. That is, under the same operating and environmental conditions as exist for the pipe metal itself. CorrView is also designed to provide long term and low cost wall loss monitoring where conditions or interests do not allow for any form of regular corrosion testing, and where no corrosion monitoring would be otherwise performed.
Unfortunately, corrosion coupons, the most common form of corrosion monitoring used today for condenser water or process cooling, fall far short of providing accurate corrosion rate data, and often produce wall loss estimates dangerously lower than what actually exist. While providing some limited benefit by showing upward or downward corrosion trends, we have seen such a high volume of failures to produce accurate corrosion rate data that we now consider them virtually worthless as a diagnostic tool for most applications.
In past ultrasonic investigations we have actually documented a 0.042 in. (42 mil) wall loss at the steel corrosion coupon rack itself over a one year period when the coupons themselves, located inside the same exact section of pipe, reported a 0.3 MPY corrosion rate. That's a staggering 1,400 % under reporting error!
During a recent investigation, building owners were surprised to learn their 7 year old condenser water system actually had a 5 MPY average corrosion rate, and moderate pitting to a rate of 8.5 MPY - especially a surprise given that coupons had never reported over a 0.4 MPY corrosion rate in the entire 7 years of close monitoring by two different corrosion authorities - two independent "corrosion experts."
Other testing methods, such as spool pieces and LPR electronic monitoring, are generally not practical on the scale necessary to produce sufficient useful data, and are often installed and employed improperly- thereby negating their potential benefit. Ultrasound, the best overall diagnostic tool available to determine corrosion activity and remaining service life, is often performed so poorly and so inaccurately as to again produce a totally worthless result. It is rarely employed before a corrosion problem arises.
A comparison between corrosion coupon results vs. actual pipe wall loss, as confirmed by ultrasonic wall thickness testing, consistently shows an under reporting of metal loss using corrosion coupons by 5 to 10 times - and in some examples by as much as 2,000 times or more. For building operators or plant engineers responsible for millions of dollars of mechanical equipment and infrastructure, a total reliance upon questionable corrosion coupon results can spell disaster - and often does.
This is not due to failure of the metal coupon per se, which well represents the corrosion potential of the liquid itself, but due to its placement externally to the piping system. The coupon rack testing configuration inherently eliminates most of the normal corrosion effects found within the pipe, and therefore less effect is shown by the coupon under lab analysis. Corrosion coupons often represent no relationship between the corrosion occurring at their surface, and the corrosion occurring at the pipe surface. Read more about the limitations of corrosion coupons at left.
Our own corrosion studies have documented the similarity in wall loss between corrosion coupons and common mild steel pipe samples under a controlled salt spray environment. Side by side testing has shown a general similarity in corrosion rate when under similar conditions - the key factor missing when coupons are installed in an isolated side stream loop.
At left we offer a more recent handout related again to corrosion coupon failures; providing a more detailed explanation of the evolution of meaning that takes place regarding corrosion coupons once a major corrosion condition finally becomes recognized. From the original interpretation that corrosion coupons report the wall loss occurring at the pipe wall, most building and plant operators will ultimately learn that they only suggest the potential corrosivity of the water itself - a significant difference! Read more about a case history of corrosion coupon failure.
In effect, CorrView functions as the direct opposite of standard corrosion coupon monitoring in two very important aspects. First, its wear surface is located directly within the piping system, whereas the coupon wear surface is located externally.
Second, time period of exposure is decided in days or months for corrosion coupons, and the amount of material lost through any corrosion activity is measured following that exposure. With CorrView, the thickness of material to corrode away is predetermined, and depending upon the existing corrosion activity, the time that it takes to wear away will change. Fixed time vs. varying metal loss for corrosion coupons, as opposed to fixed metal loss vs. varying time for CorrView.
Adequately monitoring a piping system for corrosion activity is generally prohibitive using any of the previously available testing methods.
Installed in an isolated loop and electrically insulated from the pipe, corrosion coupons never suffer the same corrosive and environmental effects as the pipe itself. They rarely provide accurate test results and in most cases provide nearly worthless information. Reliance on such worthless information then eliminates corrective measures, and usually allows a corrosion problem to continue hidden from view.
In many examples, even the most obvious signs of a problem such as clogged strainers, thread leaks, and tower pan debris are ignored due to low corrosion estimates provided by corrosion coupons. Different corrosion authorities may argue to the impact of a 1/4 MPY difference in their corrosion rate estimates, while at the same time ignoring the likelihood that both may be in error by 1,000% or more.
Hardened deposits, electrical activity, under deposit corrosion, micro biological buildup, high or low flow effects, and other common environmental factors typically do not exist for corrosion coupons. A flow requirement, by definition, prevents their installation in precisely those locations traditionally showing the highest corrosion threat, such as at dead end lines.
In addition, installing corrosion coupon racks at multiple points throughout a circulating system is not practical and is almost never performed - thereby forcing the unlikely assumption that the test results shown for one specific location are representative over the entire piping system.
Periodically cutting out samples of pipe for metallurgical analysis is extremely expensive, usually requires a system shutdown, and is rarely carried out for large diameter piping. For any critical or 24/7 operation is virtually impossible to perform. Combined contracting, maintenance, and metallurgical costs can easily exceed $5,000 per sample.
Metallurgical analysis does offer valuable information unavailable through any other means, and is especially useful in order to identify the specific cause of a corrosion condition or to define why a specific failure occurred, but is generally limited in use due to its cost and down time.
Unless a specific concern exists, a general ultrasonic survey is always a first best step in defining the most appropriate location to cut rather than leave the choice of location at random.
Metallurgical analysis plays an important role in explaining a corrosion problem, rather than discovering one.
Spool pieces, which are nothing more than removable sections of actual pipe within the system, provide valuable information regarding the actual net effect of corrosion activity against the pipe surface.
Unfortunately, they are only applicable for smaller diameter piping separate from the main lines. Properly installed, spool pieces offer a true inside look at deposits, surface pitting, inhibitor and cleanout effectiveness, as well as provide samples for micro biological cultures. They are especially useful in evaluating the effectiveness of a chemical cleanout program.
Defining the best and most appropriate areas to address requires some preliminary ultrasonic investigation. And like corrosion coupon racks, spool pieces are rarely installed throughout a piping system - therefore enjoying limited use.
Ultrasonic wall thickness testing provides the greatest volume of accurate and reliable data, and will typically produce a thorough corrosion evaluation as long as a sufficient number of test points are taken and the data is properly evaluated.
Ultrasound is often used as a prerequisite to other testing methods due to its low cost and wide coverage, or as a confirmation that wall thickness conditions known to exist in one area do or do not exist elsewhere within the piping system. For the same cost involved in metallurgically testing one pipe section, an ultrasonic evaluation can address 60 pipe sections and provide significantly greater information.
It is most often used as a tool to identify the extent of an already recognized leak or rusting problem. Long term corrosion monitoring using ultrasound offers further benefits, but requires establishing specialized testing procedures.
A wide variety of electronic techniques eixst to produce an estimate of corrosion rate generally based upon the principal of Linear Polarization Resistance, or LPR. In essence, the electrical conductivity of the fluid is measured from which an estimate of corrosion rate is produced.LPR provides the benefit of an immediate corrosion measurement that can be routed to monitoring electronics, or data logged for download, and offers an extremely useful corrosion measurement tool. LPR is generally expensive to install and maintain. Regular celaning and calibration is often required, and even then, results may not approximate true corrosion activity - especially if under deposit corrosion or MIC is active. LPR equipment is commonly installed in side stream loops to the main piping and even within PVC plastic lines - again raising the same inaccuracy found at corrosion coupons.
The insertion of a remotely controlled camera into a piping system offers a valuable though limited inspection option. Its use requires the system to be out of service over an extended period of time and drained, and is further limited by access into the piping system. Pipe size, physical configuration, valving, orientation, internal conditions, and length of travel offer further restrictions in its use. Remote Video Inspection (RVI) cannot provide any wall thickness data, but can quickly locate those internal indications that wall loss has occurred - such as tubercular deposits, exposed deep pitting, or suspected MIC growths.
RVI is extremely helpful in identifying areas of heaviest internal deposits under which deepest pitting will exist. By monitoring the length of travel through the pipe, ultrasonic testing can be performed from the outside to better define lowest wall thickness conditions.Combined with ultrasound or metallurgical testing, RVI can quickly and cost effectively document whether similar problem conditions exist in other areas of a piping system.
Obviously, pipe corrosion activity is a complex mechanism which cannot be reliably predicted by any one means. It is therefore necessary to establish a corrosion monitoring program that will not only provide accurate and relative information to what is actually taking place at the pipe, but to also address those areas of the system where elevated corrosion activity is likely to occur.
It is important to remember that while the variance in wall loss, corrosion rate, and pitting will fall within a certain range of values for most piping systems, that range of values will widen substantially when a serious corrosion problem exists. As corrosion rates and pitting activity increase, so does the probability that random piping areas may produce sufficient weakness in the pipe to cause a leak or operating problem.
This greater fluctuation in corrosion rate simply increases the existing danger of having a corrosion rate determination based upon only one monitoring technique, and at only one monitoring location.
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P.O. Box 8513Landing, NJ 07850 www.corrview.com Ph: 973-770-7764 Fax: 973-770-6576
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