Comparison testing was performed to measure the relative corrosion rate of the most commonly used CorrView models in 1-1/2 in. and 3/4 in., manufactured from ASTM 1018 steel, and commercially available ASTM 1010 and 1018 grade corrosion coupons typically used in coupon racks to estimate pipe wall losses.
In addition, comparison was simultaneously made against the most commonly installed pipe for HVAC and process water systems - ASTM grades A53, A106, and A795. Testing was performed by the accelerated salt spray method over a period of 15 days.
A short term comparison between CorrView products, standard corrosion coupons, and samples of carbon pipe steel were made. Results showed a relatively low variance in results between all three groups of materials.
In order to provide a more formal and more extensive evaluation of CorrView against standard coupons and mild carbon steel pipe, a salt spray test booth was constructed. Design of the test booth substantially followed ASTM Designation B 117 - 95: Standard Practice for Operating Salt Spray (Fog) Apparatus except that it was not temperature controlled. Coupons of ASTM 1010 and 1018 mild carbon steel, typically used in the corrosion rate evaluation of HVAC piping systems, were purchased from one of the major commercial suppliers of such coupons for corrosion measurement. Examples of new ASTM A53, A106, and A795 pipe were also acquired from commercial piping suppliers. Specimens were cut from each grade of pipe and fabricated into the approximate length and width similar to the corrosion coupons, but of the standard thickness of the original pipe. Weights of the individual samples were not recorded, since an ultrasonic measurement of true wall loss would be used as the basis to determine corrosion rate, rather than weight loss.The wall thickness of one end of each corrosion coupon and pipe sample coupon was measured ultrasonically at 15 individual locations according to a standard grid pattern of specific dimension. Both CorrView corrosion monitor models were similarly measured according to a specified grid at 15 individual locations at its center. All wall thickness data was recorded into a spreadsheet to establish a baseline.
All specimens were positioned on a wooden base for electrical isolation and placed into the salt spray fogging chamber. Specimens were physically arranged at an angle to the fog spray according to the requirements of ASTM B 117 - 95.
A 5% salt spray solution was introduced into the test chamber via ultrasonic atomization and an overhead header configured to provide uniform and indirect introduction of the salt fog. Our corrosion simulation procedure took exception to maintaining a test chamber temperature of 95° F or to maintain constant pH since the purpose of the test was to make a side by side comparison of different metal types rather than produce a standardized accelerated corrosion environment. All test subjects were equally exposed to the same conditions during the entire test period.
Testing was continued uninterrupted and the results observed. After an exposure of 15 days, the samples were removed and allowed to dry, then photographed. Deposits from the measured end of each test sample were then brushed free of rust deposits and the metal rinsed and again photographed in their corroded form. Following the removal of this area of deposits, ultrasonic testing was performed at the same area of each sample, and along the same grid pattern to provide a second set of wall thickness measurements. Wall loss and corrosion rates were calculated for each test specimen.
Results showed a general agreement of corrosion rate statistics within what we would consider reasonable limits. The salt spray environment produced an extremely aggressive attack at all metal surfaces as evidenced by visual inspection of the exposed metals, and of their underlying surfaces once deposits were removed (shown further below). Moderate to high pitting was found in all examples, requiring the use of "echo to echo" ultrasonic measurement technique in order to accurately measure the base dimension of such areas.
Corrosion rates of between 19.5 MPY and 33.3 MPY were measured at the different metal samples. The highly corrosive environment produced by the salt spray chamber would be expected to exaggerate minor differences in metal chemistry of the steel samples, and therefore produce a wider variation in result than under more typical conditions of a 1-3 MPY corrosion rate found at an open cooling tower system. The differences noted between the steel pipe samples alone help support this position.
Nevertheless, this series of tests showed a maximum variance of only 33.8% from the average corrosion rate measured, which we found reasonable - this highest variance found at sample # 5 for ASTM A53 pipe. The average variance of all samples tested from the mean corrosion rate was 13.9% - relatively minor in comparison to the large discrepancy often found between corrosion coupons and true corrosion losses.
Test results for the standard commercial corrosion coupons of ASTM 1010 and ASTM 1018 mild steels showed similar wall loss to the actual pipe samples measured, again supporting our long term argument that the underlying fault of corrosion coupons is their ASTM specified design and installation, bot the coupon itself. Installed within externally located corrosion coupon racks, however, their reported corrosion rates are typically far below what actually exists at the interior pipe wall - the result of their being substantially isolated from most of the corrosive effects existing within the actual piping system.
A corrosion rate determination for a condenser water system of 0.5 MPY using coupons, when the actual measured loss of pipe wall is 0.105 in. over 12 years or 8.75 MPY, is a substantial 1,750 % difference or under reporting of actual wall loss. Such extreme error, often found to exist between 100% and 1,000% in actual ultrasonic testing investigations, shows the current 13.9 % average variance in corrosion rate results of all samples tested to be nearly insignificant.
As the above graph illustrates, our salt spray accelerated corrosion analysis produced relatively similar results across the varied samples of metals tested. Such variance we consider well within normal limits.
The below set of tables offers a visual and statistical comparison of corrosion rates for the two corrosion coupons, four pipe sample coupons, and two CorrView products tested. Shown at the left of each set is the original metal sample prior to testing, and in its original form. The center photograph is the exposed and rusted sample after removal from the salt fog booth. The far right photograph shows the same exposed sample after wire brushing in preparation for ultrasonic testing.
Our investigation produced an average corrosion rate of 28.7 MPY at the four samples of actual steel pipe - ASTM A106, A795, A53, and A53, and an average corrosion rate of all eight metals tested of 27.3 MPY. For purposes of this evaluation, we then compared the corrosion rates of the commercially available steel corrosion coupons and CorrView products, finding an average 8.1 % and 12.7 % variance in their corrosion rates from the true pipe samples respectively. CorrView products slightly under reported the corrosion rates measured at the steel pipe samples.
The percentage of variation in corrosion rate from the average measured value of 28.7 MPY measured at the five steel pipe samples is also provided below. Further testing is is planned, and will be presented when available.
The CorrView corrosion monitor as been designed and constructed with safety and reliability as its first criteria, and well exceeds the pressure and strength demands of any cooling water or fire sprinkler application. It is manufactured and assembled in the United States using only the highest quality American made components.
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