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Formation and breakdown on chromate conversion coatings on Al-Zn-Mg-Cu 7x75 alloys

Yoon, Yuhchae
http://rave.ohiolink.edu/etdc/view?acc_num=osu1101754567

Abstract Details

2004, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
The objective of this study was to characterize the formation and breakdown of chromate conversion coatings (CCCs) on aluminum alloys Al-Zn-Mg-Cu 7075 and 7475 with a focus on the effect of alloy temper, alloy purity and selected coating processing variables. Overall, results consistently pointed to a slight temper effect. Conversion coated AA7475-T7 was significantly more corrosion resistant than conversion coated AA7475-T6. In AA7075, there was only a slight difference in corrosion resistance between the two tempers. This was attributed to the effect of constituent particles on coating formation and breakdown, which are present to a much greater extent in AA7075 than in AA7475. The difference in the corrosion resistance between the T6 and T7 tempers in the coated and uncoated conditions is about the same suggesting that the origin of any “temper effect” in conversion coated materials is ultimately due to the intrinsic change in corrosion susceptibility of the alloy itself. Thicker coating formed on AA7475-T7 has the effect of increasing corrosion resistance, which could be associated with the 860 cm-1 Raman intensity band. Studies were also conducted with alloys in retrogression and reaged tempers and the W temper. Results with these tempers were mixed and no general conclusions could be drawn. In terms of electrochemically derived measures of corrosion resistance (electrochemical impedance, and pitting potential measurements), the magnitude of the temper effect was about the same as the effect due to the purity difference between AA7075 and AA7475. The temper effect was less significant than effects due to increasing coating time from 1 to 3 minutes, withholding certain substrate precleaning steps, or withholding key ingredients from the coating bath. Scanning probe microscopy, scanning Kelvin probe force microscopy (SKPFM) and scanning electron microscopy were used to characterize coating formation in the vicinity of constituent intermetallic particles (IMPs) present in the alloys. Coatings formed on IMPs exhibited different morphologies and were much thinner that coatings formed on the matrix. Coatings were thinnest on S phase particles, and in post-coating exposure to aggressive chloride environments coating breakdown was almost always associated with these particles. The difference in coating thickness between particles and the surrounding matrix was established within the first tens of seconds of coating. The thickness differential was then observed to remain constant for the remainder of the coating immersion time. Although coatings were thinner on IMPs, they did confer some level of protection. SKPFM measurements showed that the difference in the Volta potential measured between the matrix and the particles was strongly reduced by the conversion coating suggesting that intact coatings reduced any tendencies for microgalvanic coupling. Thin film analogs of two important intermetallic compound particles, eta (MgZn2) and S (Al2CuMg), were synthesized and analyzed using quartz crystal microbalance weight change measurements, potentiodynamic polarization and Raman spectroscopy. MgZn2 compounds were prepared with different levels of Cu to simulate the compositional change that occurs when 7X75 alloys are artificially aged. Results showed that these phases are electrochemically active and the conversion coatings reduce activity only slightly. The coatings that do form on these compounds are thin and may have lower hexavalent Cr contents than coatings that form elsewhere. With increasing amounts of Cu dissolved in the eta phase, coatings are even thinner and possibly more defficient in Cr6+. Multi-element electrode array experiments were used to electrochemically characterize coating formation and breakdown. Coating formation was characterized by an episode of measurable electrochemical activity on the array lasting for about 30 seconds. This was followed by a period of little measurable electrochemical activity. During the episode of electrochemical activity, individual electrode elements in the array exhibited transients that were predominantly anodic, predominantly cathodic or mixed. In subsequent potentiodynamic polarization of these coated electrodes in a chloride environment there was a weak dependence of the pitting potential on the character of the current transient with electrodes exhibiting cathodic transients having slightly more noble pitting potentials than electrodes that had anodic or mixed transients.
Rudolph Buchheit (Advisor)
282 p.
Engineering, Materials Science
Chromate conversion coating; AFM; XANES; Raman spectroscopy; EIS; Alloy temper

Recommended Citations

Citations

  • Yoon, Y. (2004). Formation and breakdown on chromate conversion coatings on Al-Zn-Mg-Cu 7x75 alloys [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1101754567

    APA Style (7th edition)

  • Yoon, Yuhchae. Formation and breakdown on chromate conversion coatings on Al-Zn-Mg-Cu 7x75 alloys. 2004. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1101754567.

    MLA Style (8th edition)

  • Yoon, Yuhchae. "Formation and breakdown on chromate conversion coatings on Al-Zn-Mg-Cu 7x75 alloys." Doctoral dissertation, Ohio State University, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=osu1101754567

    Chicago Manual of Style (17th edition)

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This open access ETD is published by The Ohio State University and OhioLINK.