Issues Surrounding Accelerated Testing of Corrosion

1
Issues Surrounding Accelerated Testing of
Corrosion

• R. G. Kelly
• Dept. of Materials Science and Engineering
• University of Virginia

2
The Problem

• Salt spray testing methods of various flavors
  have had limited success in predicting corrosion
  damage in field
• Rates
• Morphology
• Dependencies, e.g., alloy composition
• Connection to field use is challenging
• No fundamental framework to predict or
  rationalize behavior

3
Example for Coatings Testing

• Comparison of Salt Fog, Prohesion, and Field
  Results
• Test of scribe creep for 20 different coating
  formulations
• Correlation is non-existent
• Similar experiences with other forms of corrosion

M. Hoeflaak, TNO Pub 040323 (2004).
4
Possible Origins of Disagreement

• Variability of atmospheric conditions and service
  environments
• Only rudimentary characterization of service
  environments
• Chemistry of surface solution is dynamic and
  poorly characterized

5
Can Use Post-diction to Rationalize Behavior
Alclad 2024-T3 scribed, exposed to DAB for 12
months
AA 1230 clad AA 2024
Segment mounted for metallographic analysis
Find model conditions that best reproduce
observed behavior
Cui, et al. Corr J., (2006)
6
Abbott Field Observation 1 The Chloride Mystery

• Abbott at Battelle has measured effective Cl
  distribution at land-base sites and in lab tests
• Exposes silver coupon
• Galvanostatic reduction to determine Reactive
  Chloride
• Results
• Reactive chloride at all land sites tested
  (highest in coastal areas, but measurable in
  desert)
• Virtually no production of AgCl in laboratory
  Salt Fog

Abbott (2000)
7
Possible Reason

• Sea salt aerosols are not simply NaCl
• There exists a rich and complex chemistry

Keene, 2002
Finlayson-Pitts (2000)
8
Sea Salt Aerosols Have Low pH

• Small particles have lowest pH
• All pH lt 5
• Transported for long distances inland

Keene (2002)
9
Sea Salt Aerosols Have Strong Oxidizers Due to
Interaction with UV and O3
Molecular Halogens at Artic Polar Sunrise
O3
Br2
Irradiance
BrCl
Finlayson-Pitts (2002)
10
Atmospheric Solution Layer Chemistry is Far More
Complex than NaCl

• Interactions with atmospheric chemists appears
  critical
• Need to define aqueous chemistry on surfaces and
  electrochemical behavior of materials in those
  solutions
• Information applicable across all forms of
  corrosion when structure is exposed to
  atmospheric conditions rather than full immersion