A second exposure of the same metal samples reported in Testing Series B was performed over a longer period of 42 days. The purpose again was to measure the relative corrosion rate of the most commonly used CorrView models in 1-1/2 in. and 3/4 in. ASTM 1018 steel, and commercially available ASTM 1010 and 1018 corrosion coupons typically used in coupon racks to estimate pipe wall losses.
As previously defined, 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.
Coupons of ASTM 1010 and 1018 mild carbon steel, typically used in the corrosion rate evaluation of HVAC piping systems, were purchased from the open market as well as were samples of actual ASTM A53, A106, and A795 pipe; the samples of pipe fabricated into the approximate length and width of the corrosion coupons. Weights of the individual samples were not recorded, since an ultrasonic measurement of true wall loss would be used as the basis for corrosion rate, rather than weight loss.
All metal samples had been previously subjected to a salt spray fog chamber for a period of 15 days and reported upon in Testing Series B. After completion of ultrasonically testing the various metal samples, they were reintroduced back into the same salt spray booth for additional exposure.
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 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, similar to our 30 day exposur, thereby 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 31.2 MPY and 61.8 MPY were measured; higher and with a greater variance than those identified in Testing Series B and Testing Series C. 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. Also, corrosion activity typically accelerates as deposits increase at the surface - a time related function.
The higher corrosion rates 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 36.5% 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 22.5% - 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 1,000% in actual UT testing comparisons, shows the current 36.5 % 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, four 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 after a total exposure of 42 days. The far right photograph shows the same exposed sample after wire brushing.
Our testing produced an average corrosion rate of 45.8 MPY at the four samples of actual steel pipe - ASTM A106, A795, A135, and A53, and an average corrosion rate of all nine metals tested of 43.7 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. Corrosion coupons slightly under reported corrosion in comparison to the steel pipe samples; CorrView products both over and under reported the corrosion rates found at the steel samples during this test series.
The percentage of variation in corrosion rate from the average measured value of 45.8 MPY measured at the five steel pipe samples is also provided below. Further testing is planned.
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.
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