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Galvanic Corrosion of Coated Al Alloy Panels with More Noble Fasteners

Feng, Zhicao
http://rave.ohiolink.edu/etdc/view?acc_num=osu1436781873

Abstract Details

2015, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
A test sample incorporating a painted Al alloy panel, uncoated through-hole fasteners, and scribes has been shown to provide accelerated response during atmospheric corrosion testing in the field and in laboratory chambers. Several different aspects of this test sample and the behavior of different coating systems are investigated in this dissertation. The galvanic current between SS316 or Ti-6Al-4V fasteners and painted and scribed AA7075-T6 panels was examined during exposure in a salt fog chamber using a zero-resistance ammeter. The anodic current of the AA7075-T6 panel and the cathodic current of each of the four fasteners were monitored using different connection schemes. The anodic current of the panel depended on the number of fasteners connected. The total cathodic current of fasteners was approximately equal to the anodic current of the AA7075-T6 panel, which validates the accuracy of the current measurement. Furthermore, galvanic interaction between the fasteners was observed such that the cathodic current of other fasteners was decreased when a new fastener was added to the measurement. Scribes on a panel can interact with distant fasteners, not just the closest ones. The amount of corrosion as determined by charge and optical profilometry were close and indicated SS316 fasteners caused more corrosion attack than Ti-6Al-4V fasteners. The galvanic current of an AA7075-T6 panel coupled with mixed SS316 and Ti-6Al-4V fasteners was monitored using a zero-resistance ammeter during 3 weeks exposure in an ASTM B117 chamber or immersed in 5 wt% NaCl solution. SS316 fasteners provided more cathodic current than Ti in both environments and the current in ASTM B117 was higher than in 5 wt% NaCl solution due to greater oxygen availability. The integral of the anodic current with time and optical profilometery (OP) analysis were used to assess the corrosion attack quantitatively for two different coating systems. An acceleration factor was defined to represent the extent of accelerated corrosion for galvanically-connected fasteners. The acceleration factors were in the range of 20-50 for panels with SS316 fasteners and two different coating systems, both with and without a topcoat. The effects of SS316 fasteners were similar for the different coating systems even though the attack morphology was very different. The corrosion morphology and extent of attack of Al alloy panels with five different surface pretreatments prior to painting and scribing were compared after exposure to ASTM B117 with galvanic connection to uncoated stainless steel fasteners. The nature of the attack for samples with different surface pretreatment samples was found to exhibit two different trends: they either penetrated deeply into the substrate at the scribes or spread out under coatings. The galvanic currents between the coated Al alloy panel and the bare SS316 fasteners were monitored during 21 days exposure in an ASTM B117 chamber. The current trends measured for different surface pretreatment panels were consistent with Optical Profilometry (OP) results. However, the galvanic current and OP techniques both underestimated the extent of corrosion attack due to local H2 evolution and undercut attack, respectively. The results from both techniques were modified to compensate for these deficiencies. Acceleration factors associated with the galvanic interaction for the different surface pretreatment panels were determined. Corrosion of different coated galvanic panels was quantified after exposure to ASTM B117 and a beach field site. Galvanic currents were continuously monitored between the panel and stainless steel fasteners in the field and in the laboratory chamber, exhibiting current transients and stable high value currents, respectively, associated with the two different environmental conditions. The morphology and quantification of corrosion were consistent with the galvanic current results. Acceleration factors were determined to describe the influence of either galvanic coupling or an environment to accelerate coating degradation relative to the condition of no galvanic coupling or of another environment. An acceleration factor for the oxygen reduction reaction was also determined by comparing cathodic currents of a bare SS316 fastener in the different environments. The acceleration factor for different environments can be regarded as a correlation factor between lab results and field applications. Finally, electrochemical impedance spectroscopy (EIS) was used for over a hundred days exposure of coated Al alloys with different surface pretreatment in 5 wt% NaCl solution. The breakpoint frequency extracted from EIS spectra allows determination of small electrochemically active areas of coatings. It shifts from low to high values with increasing exposure time, due to the enlargement of active areas. A critical breakpoint frequency, in the range of 102 to 103 Hz, appeared after a certain exposure time corresponding to a drop of open circuit potential and Z0.01Hz, indicating visual pits initiation under the coating. The same chromate primer coated Al alloy with different surface pretreatments had an individual incubation period before reaching to the critical breakpoint frequency. The incubation time can be used as a criterion to evaluate the susceptibility to coating degradation during exposure. Localized corrosion attack was observed on stripped Al alloy after a long term EIS measurements. Optical profilometry analysis was also conducted to determine corroded areas of coatings that were a little larger than electrochemically active areas estimated by the breakpoint frequency method. The breakpoint frequency method is valid for a very small active area such as pit initiation, probably not able to quantify the large active area after pit propagation.
Frankel Gerald (Advisor)
Buchheit Rudolph (Committee Member)
Locke Jenifer (Committee Member)
Krissek Lawrence (Other)
270 p.
Engineering; Materials Science
Galvanic Corrosion, Al Alloy, Stainless Steel Fasteners, Coatings

Recommended Citations

Citations

  • Feng, Z. (2015). Galvanic Corrosion of Coated Al Alloy Panels with More Noble Fasteners [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1436781873

    APA Style (7th edition)

  • Feng, Zhicao. Galvanic Corrosion of Coated Al Alloy Panels with More Noble Fasteners. 2015. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1436781873.

    MLA Style (8th edition)

  • Feng, Zhicao. "Galvanic Corrosion of Coated Al Alloy Panels with More Noble Fasteners." Doctoral dissertation, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1436781873

    Chicago Manual of Style (17th edition)

osu1436781873
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© 2015, some rights reserved.Creative Commons License Galvanic Corrosion of Coated Al Alloy Panels with More Noble Fasteners by Zhicao Feng is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.