Lifetime prediction for organic coatings on steel and a magnesium alloy using electrochemical impedance methods

Journal Article · Thu Sep 01 00:00:00 EDT 1994 · Corrosion (Houston); (United States)

DOI:https://doi.org/10.5006/1.3293547· OSTI ID:6909434

Scully, J R; Hensley, S T ^[1]

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1. Univ. of Virginia, Charlottesville, VA (United States). Dept. of Materials Science and Engineering

Electrochemical impedance studies were conducted on an epoxy polyamide-coated AISI 1010 steel (UNS G10100) and an epoxy/chromate conversion-coated magnesium (Mg) alloy ZE41A-T5 (UNS M16410). Results were compared to evaluate the general applicability of various impedance-derived measures of coating performance for radially different metal substrates. Both coating systems were immersed in room-temperature aqueous sodium chloride (NaCl) solution of near neutral pH. Correlation of impedance parameters obtained early in exposure with long-term visual appearance demonstrated that low-frequency impedance (< 10 mHz), coating resistance, breakpoint frequency, phase angle minimum forecasted the long0term performance of both systems. Impedance-based parameters adequately estimated the defect areas associated with corrosion sites that penetrated the organic coating but were unable to estimate the area associated with coating blisters. More conservative impedance-based criteria had to be used for the coated Mg alloy than for the coated steel in predicting long-term performance. It was hypothesized that differences in the critical impedance thresholds resulted from rapid Mg oxidation and reduction of water reaction rates that occurred at even the smallest defect sites and from the greater solubility of magnesium hydroxide (Mg[OH][sub 2]). These findings imply that more conservative coating performance criteria than traditionally envisioned for steel in neutral salt water might be necessary for Mg.

• OSTI ID: 6909434
• Journal Information: Corrosion (Houston); (United States), Vol. 50:9; ISSN 0010-9312
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
EPOXIDES
CORROSION RESISTANCE
MAGNESIUM ALLOYS
PROTECTIVE COATINGS
POLYAMIDES
SERVICE LIFE
STEELS
CORROSIVE EFFECTS
ELECTRIC IMPEDANCE
SEAWATER
ALLOYS
COATINGS
HYDROGEN COMPOUNDS
IMPEDANCE
IRON ALLOYS
IRON BASE ALLOYS
LIFETIME
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
ORGANIC POLYMERS
OXYGEN COMPOUNDS
POLYMERS
WATER
360604* - Materials- Corrosion
Erosion
&amp; Degradation
360105 - Metals &amp; Alloys- Corrosion &amp; Erosion