1. 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)

2. 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

3. 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).

4. 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)

5. 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

6. 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

7. 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

8. 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

9. Increasing loading density → increases corrosion at 98% RH
Maximum corrosion rate ≈ 25MPY

10. Optical micrographs (no ozone) Low LD → pitting attack High LD → towards uniform corrosion
70% RH
98% RH
70 μg/cm2
1000 μg/cm2

11. Increasing loading density → increases corrosion rate at 85% RH
Corrosion rate independent of loading density at 70% & 98% RH
Maximum corrosion rate ≈ 55 mpy

12. 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

13. 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

14. Increasing loading density → increases corrosion at 98% RH
Corrosion rate independent of loading density at 70% & 85% RH
Maximum corrosion rate ≈ 35 mpy

15. 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

16. 4ppm ozone is more corrosive than 8ppm
Corrosion rates highest for 98% RH µg/cm2

17. Effect of Ozone

18. 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

19. 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

20. 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.

21. Effect of UV Light