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An electrochemical study of corrosion protection by primer-topcoat systems on 4130 steel with ac impedance and dc methodsTo investigate metal surface corrosion and the breakdown of metal protective coatings, the ac impedance method is applied to six systems of primer coated and primer topcoated 4130 steel. Two primers were used: a zinc-rich epoxy primer and a red lead oxide epoxy primer. The epoxy-polyamine topcoat was used in four of the systems. The EG and G-PARC Model 368 ac impedance measurement system, along with dc measurements with the same system using the polarization resistance method, were used to monitor changing properties of coated 4230 steel disks immersed in 3.5 percent NaCl solutions buffered at pH 5.4 over periods of 40 to 60 days. The corrosion system can be represented by an electronic analog called an equivalent circuit consisting of resistors and capacitors in specific arrangements. This equivalent circuit parallels the impedance behavior of the corrosion system during a frequency scan. Values for the resistors and capacitors, that can be assigned in the equivalent circuit following a least-squares analysis of the data, describe changes that occur on the corroding metal surface and in the protective coatings. Two equivalent circuits have been determined that predict the correct Bode phase and magnitude of the experimental sample at different immersion times. The dc corrosion current density data are related to equivalent circuit element parameters. Methods for determining corrosion rate with ac impedance parameters are verified by the dc method.
Document ID
19890010035
Acquisition Source
Legacy CDMS
Document Type
Technical Publication (TP)
Authors
Mendrek, M. J. (NASA Marshall Space Flight Center Huntsville, AL, United States)
Higgins, R. H. (NASA Marshall Space Flight Center Huntsville, AL, United States)
Danford, M. D. (NASA Marshall Space Flight Center Huntsville, AL, United States)
Date Acquired
September 5, 2013
Publication Date
May 1, 1988
Subject Category
Metallic Materials
Report/Patent Number
NAS 1.60:2820
NASA-TP-2820
Accession Number
89N19406
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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