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Accelerated Atmospheric Corrosion Testing of Steel
ECS San Francisco, CA October 30th , 2013 Accelerated Atmospheric Corrosion Testing of Steel Piyush Khullar, Robert G. Kelly Center for Electrochemical Science and Engineering University of Virginia Acknowledgements: This work is funded by the Office of the Undersecretary of Defense, Corrosion Policy and Oversight Office (D. Dunmire)
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Problem Accelerated lab tests are widely used for material selection, quality control and service life prediction Lab tests provide acceleration factors to rate materials and environmental conditions Limitations on Acceleration Factors (AF): Poor correlation between lab accelerated tests and field exposures (“superior” materials can fail lab testing while “inferior” materials can pass the lab testing) Marine atmospheric sites like Cape Canaveral have atmospheric corrosion as high as 42 mpy whereas standard lab tests like ASTM B117 are capable of reproducing rates only as high as 35 mpy The corrosion attack and morphology in lab tests does not match the field exposures Lab testing does not tie corrosion rates with the environmental variables like Ozone, UV, Salt Loading density and Relative Humidity
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Approach Discrepancy between field exposures and lab tests was studied in detail for silver[1] The field exposures at every site produced corrosion, but no measurable corrosion found during B117 exposure (4 months) UV + Ozone produced the most corrosion Ozone acts as a proxy for natural oxidants Increasing ozone increased the amount of silver corrosion UV alone (no ozone) corroded silver slightly [1] Wan, Macha, Kelly, Corrosion, 68 (3), (2012).
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Summary of Silver Corrosion
Standard salt spray chamber modified (MB117) to introduce and control ozone and UVA light MB117 recreated the corrosion of silver (same corrosion product and damage morphology) observed in the field Accelerated factors (AF) for different versions of the MB117 test (assuming linear kinetics)
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Types of atmosphere and corrosion rates of low carbon steel[2]
Location Corrosion Rate (mils/yr) Marine Kennedy Space Center, FL (Oceanfront) 42 Galeta Point Beach, Panama, CZ 27 Kure Beach, NC (80ft from Ocean) 21 Point Reyes, CA 19.7 Daytona Beach, FL 11.6 Kure Beach, NC (800ft from Ocean) 5.7 Tropical Marine Limon Bay, Panam, CZ 2.4 Rural Marine Esquimalt, BC, Canada 0.53 Industrial East Chicago, IL 3.3 Pittsburgh, PA 1.2 Urban Cleveland, OH 1.5 Rural State College, PA 0.9 Desert Phoenix, AZ 0.18 Test duration: 2 years [2] S. Coburn, Atmospheric Corrosion, in Metals Handbook, 9th ed, Vol. 1, 1978, p.720
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Objectives Determine the corrosive effect of critical environmental variables that are “missing” in standardized tests (e.g., UV light, ozone, salt loading densities, cyclic humidity or TOW) Develop a quantifiable test akin to MB117 for steel that: can be tuned to match multiple service locations simulates atmospheric corrosion matches corrosion morphology for various testing locations simulates relative performance rankings of materials observed in the field
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Experimental Setup Carbon steel samples of size ¾” square cut and polished till 600 grit Small chamber used to control the environment Humidifier to produce an environment of 70% to 98% RH UV arc lamp ozone generator to introduce ozone in the chamber NaCl salt sprayed using aerosol spray with known deposition rate (salt loading density) Cleaned and weighed samples exposed in the atmospheric corrosion chamber for 100 hours After 100 hours, samples cleaned as per ASTM G1-03- Standard practice for preparing, cleaning and evaluating specimens-before measuring mass loss
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Results The optical micrographs show the extent of corrosion produced in 100 hours The graphs show the corrosion rate in mils per year on a constant y-axis Variables “tuned”: Relative Humidity (RH) 70% 85% 98% Salt Loading Density (NaCl) 70µg/cm2 400µg/cm2 1000µg/cm2 Ozone 0 ppm 4 ppm 8 ppm
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Increasing loading density → increases corrosion at 98% RH
Maximum corrosion rate ≈ 25MPY
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Optical micrographs (no ozone) Low LD → pitting attack High LD → towards uniform corrosion
70% RH 98% RH 70 μg/cm2 1000 μg/cm2
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Increasing loading density → increases corrosion rate at 85% RH
Corrosion rate independent of loading density at 70% & 98% RH Maximum corrosion rate ≈ 55 mpy
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Ozone acts as a surrogate for all natural oxidizers Increases both OCP and icorr
Ozone was bubbled in 2.8M NaCl bulk solution No Ozone
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Optical micrographs (4ppm ozone) Low LD → pitting attack High LD → uniform corrosion High RH → uniform corrosion even at Low LD 70% RH 85% RH 98% RH 70 μg/cm2 400 μg/cm2 1000 μg/cm2
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Increasing loading density → increases corrosion at 98% RH
Corrosion rate independent of loading density at 70% & 85% RH Maximum corrosion rate ≈ 35 mpy
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Optical micrographs (8ppm ozone) Low LD → pitting attack High LD → uniform corrosion
70% RH 85% RH 98% RH 70 μg/cm2 400 μg/cm2 1000 μg/cm2
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4ppm ozone is more corrosive than 8ppm
Corrosion rates highest for 98% RH µg/cm2
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Effect of Ozone
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Visual comparison of field exposed samples[3] with lab accelerated tests
Point Judith (3 months) Coconut Island (3 months) Charlottesville (2 months) ASTM B117 2 days 1 ppm ozone,90% RH printed 1000μg/cm2 6 days[3] 8 ppm ozone, 98% RH 1000μg/cm2 100 hours 8 ppm ozone, 85% RH 400μg/cm2 100 hours 4 ppm ozone, 98% RH 400μg/cm2 100 hours [3] Y. Wan, R.G. Kelly, NACE Corrosion 2012, March 2012
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Summary Long term field testing is not always possible
Accelerated lab tests are important to predict corrosion behavior in a limited period of time Ozone is a strong oxidizer and leads to accelerated corrosion 8ppm ozone can lead to passivation of steel Higher relative humidity leads to accelerated corrosion Higher salt loading density leads to more uniform corrosion
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Future work Role of cyclic humidity and TOW (as per Eric’s work)
Role of salt deposition method and particle size on corrosion rates (as per Bailey’s work) Continue work on Ozone and RH Conduct field exposures to get corrosion rates and morphology at different service locations In situ electrochemical experiments to measure atmospheric corrosion Develop improved accelerated lab test that characterizes morphology and corrosion rate with environmental variables.
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Effect of UV Light
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