In follow-up to Corrosion Evaluation B, a second series of comparive 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. over a longer time period - 30 days. ASTM 1018 steel, and commercially available ASTM 1010 and 1018 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 A53, A106, A135, and A795. Testing was performed by the accelerated salt spray method. A sample of ASTM A135, commonly found at fire protection systems, was added to this series of testing.
We employed the same test set-up defined previously in Evaluation B. Following completion of our wall thickness measurements used to define corrosion rates in Evaluation B, newCorrView models, corrosion coupons, and steel pipe samples we introduced into the same salt spray mechanism. Accelerated corrosion testing was performed for another 15 days for a total of 30 days accelerate corrosion testing at all samples.
In order to provide a more formal and more extensive evaluation of CorrView products 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, A135, and A795 pipe were also acquired from commercial piping suppliers. Specimins 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 30 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 30 day 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 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 34.5 MPY and 54.7 MPY were measured; significantly higher and with a wider dispersion than those identified in Testing Series B . 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.
As we have documented and argued to our many clients, corrosion activity typically accelerates as deposits at the surface increase to initiate cell type corrosion. This would be expected for samles which have had double the exposure.
However, we can also raise the possibility that variances in temperature of the test setup between both series of tests, and possibly other different physical conditions, may have contributed to this difference. More important than differences between series of tests, however, is that the set of samples comprising each test set were exposed to identical conditions and therefore offer a comparative view of their respective corrosion susceptibility.
Nevertheless, this series of tests showed a maximum variance of only 23.0% from the average corrosion rate measured, which we found reasonable. This highest variance in result was again found at sample # 5 of ASTM A53 pipe. The average variance of all samples tested from the mean corrosion rate was 14.2% - minor in comparison to the large discrepancy often found between corrosion coupons and true corrosion losses.
Test results for the standard corrosion coupons of ASTM 1010 and ASTM 1018 mild steels showed similar rates of corrosion to the actual pipe samples measured. Installed in 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 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 in corrosion coupon rates, often found to exist between 100% and 2,000% in actual UT testing comparisons, shows the current 23.0 % variance in corrosion rate results of all samples tested to be nearly insignificant.
The below set of tables offers a visual and statistical comparison of corrosion rates for the two corrosion coupons, five pipe sample coupons, and two CorrView products tested. Shown at the left is the original metal sample prior to testing, and in its original form. The center photograph is the exposed and rusted sample as removed from the salt fog booth. The far right photograph shows the same exposed sample after wire brushing.
Our testing produced an average corrosion rate of 43.3 MPY at the five samples of actual steel pipe - ASTM A106, A795, A135, and A53, and an average corrosion rate of all nine metals tested of 43.3 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 20.4 % and 9.8 % variance in their corrosion rates from the true pipe samples respectively. Both the corrosion coupons and CorrView products slightly over reported the corrosion rates found at the steel samples during this test series - opposite of what was found in Testing Series B.
The percentage of variation in corrosion rate from the average measured value of 43.3 MPY measured at the five steel pipe samples is also provided below. Further testing 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 USA using only the highest quality American made components. Comparative corrosion testing shows a relative similarity to common mild carbon steel used in most HVAC and fire protection systems. A similar accelerated corrosion comparison based upon a 60 day exposure is provided in Comparison D.
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