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ETD Abstract Container

Abstract Header

IN-SITU MONITORING OF UNDERCOATING CORROSION DAMAGE BY DIRECT OPTICAL INTERROGATION (DOI)

Lopez-Garrity, Meng Tong
http://rave.ohiolink.edu/etdc/view?acc_num=osu1374004008

Abstract Details

2013, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
An approach referred to as “Direct Optical Interrogation” (DOI) has been developed as an extension of the thin film pitting approach developed and used by Frankel and others. Samples were prepared by depositing Al and Al-Cu alloy metallizations about 800 nm thick on glass substrates. These metallizations were then coated with various coatings and coating systems. Samples were introduced to aggressive environments and the progression of corrosion of the metallization under the coating was monitored in situ using low power videography. Because metallizations were thin, corrosion quickly penetrated through the metal layer to the glass substrate and then spread laterally. Measurement of the lateral spread of corrosion enabled non-electrochemical assessment of the corrosion kinetics. In Al-Cu thin films, both aged and as-deposited, corrosion sites are irregularly shaped because there is not enough cathodic current to propagate the entire corrosion site margin at equal rates. In a number of cases, corrosion propagates with a filamentary morphology resembling filiform corrosion. Cu played a strong role in determining under coating corrosion morphology and growth kinetics in experiments with Al-Cu thin films substrates. As-deposited Al-Cu metallizations were more corrosion resistant than aged metallization and both were more corrosion resistant than pure Al. Cu-rich dendrites were formed on the corrosion front. Corrosion rate (current density) was calculated using Faraday’s law by collecting corrosion site perimeter and bottom area. Systematic exploration of the effects of a chromate and chromate-free conversion coatings, chromate and chromate-free primer coatings and the presence or absence of a polyurethane topcoat confirmed the extraordinary corrosion protection by chromates. A commercial praseodymium-pigmented primer coating was not particularly effective in retarding undercoating corrosion site growth unless paired with a chromate conversion coating. The presence of a topcoat dramatically reduces undercoating corrosion and masks many deficiencies of a conversion coating or primer. DOI was used to compare undercoating corrosion that developed due to exposure in ASTM B117, ASTM G85-A5 and outdoor environments. Similar corrosion morphologies developed in ASTM B117 and static immersion exposures. A single and stable corrosions site nucleated and propagated with a filamentary morphology. In general, salt spray exposure was more aggressive than static immersion. ASTM G85-A5 exposure produced different corrosion morphologies. Corrosion sites were round rather than filamentary. Massive nucleation of small corrosion sites across the whole surface also occurred. ASTM G85 environment was mild compared with the ASTM B117 and static immersion exposures. In outdoor exposure testing carried out for 5 months, no signs of corrosion was observed. To assess the extent to which oxygen reduction occurring on the coated surface supported corrosion site growth, a universal pH indicator was added to agar gels or PVB coatings applied on top of metallizations. Color changes indicating pH changes associated with local alkalization or acidification due to local cell action were assessed visually. Overall, the evidence is consistent with the idea that both hydrogen reduction and oxygen reduction support local corrosion site growth. In practical embodiments of corrosion protection, every effort should be made to restrict oxygen reduction to slow corrosion growth rates.
Rudolph Buchheit (Advisor)
276 p.
Engineering
Corrosion; Coating; Undercoating

Recommended Citations

Citations

  • Lopez-Garrity, M. T. (2013). IN-SITU MONITORING OF UNDERCOATING CORROSION DAMAGE BY DIRECT OPTICAL INTERROGATION (DOI) [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1374004008

    APA Style (7th edition)

  • Lopez-Garrity, Meng. IN-SITU MONITORING OF UNDERCOATING CORROSION DAMAGE BY DIRECT OPTICAL INTERROGATION (DOI). 2013. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1374004008.

    MLA Style (8th edition)

  • Lopez-Garrity, Meng. "IN-SITU MONITORING OF UNDERCOATING CORROSION DAMAGE BY DIRECT OPTICAL INTERROGATION (DOI)." Doctoral dissertation, Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1374004008

    Chicago Manual of Style (17th edition)

osu1374004008
383
© 2013, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.