Comparison testing was performed to measure the relative corrosion rate of the most commonly used CorrView model in 1-1/2 in. ASTM 1018 steel, and the most commonly installed pipe for HVAC and process water systems - ASTM A53. Testing was performed under water immersion for a period of 39 days. Measurement of wall loss was provided using ultrasonic measurement and a Panametrics 37DL instrument.
A wall loss based comparison between CorrView and standard ASTM A53 pipe steel was performed. Testing at both samples identified similar corrosion rate activity.
A standard 1-1/2 in. size CorrView corrosion monitor manufactured of ASTM 1018 mild steel was installed into a test assembly comprised of a 3 in. threaded length of ASTM A53 pipe via a 3 x 1-1/2 reducing adaptor. The outer surface of the 3 in. length of pipe was sanded smooth to minimize any error during ultrasonic testing, and to provide a reliable bonding surface.
An EPDM rubber template having die cut holes precisely matching the ultrasonic probe diameter was bonded onto the pipe using a high strength metal to rubber bonding adhesive - thereby ensuring precise re-orientation of the UT probe on subsequent evaluation. Duct tape was used to hold the template in place while the adhesive cured and left to remain.
The high resolution of ultrasonic measurement to 0.001 in. and normal variance of wall thickness as a steel surface corrosion makes any attempted comparison between two different ultrasonic tests over time, in even the most localized area of pipe, impossible to accurately compare. Only the general trend in corrosion can be normally provided. To solve this problem, CorrView International, LLC has developed a specialized corrosion monitoring procedure whereby a die cut rubber template is permanently bonded to the pipe surface in order to allow repeated and accurate wall thickness measurements as its internal surface deteriorates. A 7.5 Mhz high frequency probe dedicated to this form of testing and absent of any surface wear is used, and oriented in the same clock position at each die cut hole.
We have used this specialized procedure very successfully with our clients for over 15 years; whereby we can produce precisely accurate measurement of their pipe corrosion rate.
In effect, this procedure use the client's existing pipe as the "ultimate corrosion coupon." Under our procedure, repetitive wall thickness measurements and recalibration are performed until three individual measurements at each test site produce the same wall thickness value. Those values are then recorded for a comparison to new measurements taken at any future date the client requests. With the days or years between tests known as well as its precise wall thickness to 0.001 in. on those dates, ad precise estimate of corrosion activity is possible.
In our evaluation of the CorrView corrosion monitor using the same methods, a total of 15 sets of such precise wall thickness measurements were taken along two separate templates on the pipe, and recorded in a spreadsheet to provide a baseline of new pipe prior to any water contact. To define the beginning condition of the CorrView corrosion monitor itself, 15 wall thickness measurements were performed in a close circular pattern at the innermost 3/8 in. diameter of the front wear surface of its and similarly recorded.
The test assembly was placed on its closed end and filled with city water. Muriatic acid was added to reduce the pH of the water to approximately 5. Aeration of the solution was added to further accelerate the corrosion process, and the test assembly allowed to remain for a period of 39 days under ambient temperature conditions of approximately 70° F. At the conclusion of 39 days, a heavily rusted water solution was drained and ultrasonic testing was again performed at the 15 template sites along the pipe. The CorrView monitor was removed from the test assembly and cleaned of its rust product. It was then ultrasonically tested in the same area and pattern.
The 15 test locations along the ASTM A53 pipe showed a wall loss of between 0.000 in. and 0.002 in. each, for a calculated average wall loss of 0.000786 in. or 0.786 thousandths. Calculating that wall loss over its 39 days of exposure to the corrosive environment produced a corrosion rate estimate of 7.35 mils per year (MPY) to the interior pipe surface.
Measurement at the same front surface area of CorrView produced a wall loss of between 0.000 in. and 0.001 in., for an average thickness measurement of 0.000999 in. or 0.999 thousandths. Calculating this wall loss over the same 39 day exposure time produced an estimated corrosion rate of 9.36 mils per year(MPY).
This represents an approximate 20% difference in reported corrosion rates between CorrView and actual ASTM A53 pipe under identical conditions. We consider such results excellent considering that the difference in true pipe corrosion rate and corrosion coupons are often in error by 10 times or 1,000 percent or more. This is not due to any failure of the corrosion coupons themselves, but due to their placement in an isolated loop where the many other corrosive factors affecting the pipe are excluded from the testing procedure.
As the excerpt at left from an actual ultrasonic investigation shows, reported corrosion rates for a condenser water system of 0.3 MPY was revealed at actually 37 MPY at the steel pipe housing the coupon itself! During a period of exactly one year, a newly installed corrosion coupon rack assembled from 1 in. schedule 40 pipe, and having an original ASTM A53 specified wall thickness of 0.133 in., was found to have deteriorated to near 0.091 in. in areas.
That translated to a severe corrosion rate of 37.5 MPY which should have been obvious given the volume of rust deposits and prior failures throughout the piping system, but a full year of favorable corrosion coupon rates removed any concerns raised, and allow corrosion activity to continue unabated. Even thread leaks at the 1 year old corrosion coupon rack failed to offset the overwhelming relief corrosion coupons provided that no corrosion problem existed.
Only one of the hundreds of first hand examples where corrosion coupons totally failed in their ability to even remotely indicate piping corrosion activity.
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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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