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|Title: ||A study of hexavalent and trivalent chromium conversion coatings on zinc surfaces|
|Authors: ||Chapaneri, Roshan|
|Keywords: ||Chromium conversion coatings (CCC)|
|Issue Date: ||2010|
|Publisher: ||© Roshan Chapaneri|
|Abstract: ||Physical, chemical and corrosion properties of a hexavalent chromium conversion coating
(CCC) and that of a commercial third generation trivalent chromium system; Tripass LT1500,
on zinc electrodeposited steel has been studied. Moreover, the role of additives has been
studied to elucidate film formation and corrosion resistance mechanisms. Micro-cracking and
self-repair corrosion protection behaviour commonly associated with hexavalent CCCs has
also been investigated.
Scanning Electron Microscopy (SEM) studies showed that for both hexavalent and trivalent
CCCs were in general, flat with a spherical-like structure and in the case of the former microcracked
beyond 122 nm conversion coating thickness. In general, the micro-crack pattern
observed e.g. a dense crack network, depended upon the underlying zinc substrate
morphology. The study has also demonstrated the effect of SEM imaging and prior specimen
preparation conditions on hexavalent CCC micro-cracking and blistering.
X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and Infrared
(IR) data has indicated that the hexavalent CCC film formation appears to be a
electrochemical / sol-gel mechanism given the lack of zinc content at surface and subsurface
regions within the conversion coating, presence of H2O and in particular the contribution of
Cr(OH)3 as opposed to Cr2O3. An alternative film formation mechanism may exist for
trivalent CCC given a higher proportion of zinc at surface and sub-surface regions, IR data
analysis indicating that chromium is possibly deposited from a chromium (III) complex ion
such as [CrC2O4(H2O)4]
, moreover as Cr(OH)3 and Cr2O3 compounds as indicated by XPS
data analysis. The role of cobalt nitrate during film formation is unclear given that cobalt was
not detected within the trivalent CCC from XPS and AES data.
Electrochemical LPR measurements, polarisation curves and XPS data has shown in general,
self-repair corrosion protection properties for hexavalent CCC to be lacking. Instead, it is
proposed that the corrosion protection behaviour for hexavalent and trivalent CCC to be
barrier. Polarisation curves and LPR data showed that the corrosion resistance performance
for trivalent CCC was higher than hexavalent CCC, in general. LPR data showed that the
omission of cobalt nitrate and increased addition of sodium molybdate content within the Tripass LT1500 treatment solution formulation was found to overall decrease corrosion
resistance within the trivalent CCC.
In addition, silica based topcoat and black trivalent CCCs was also investigated and
characterised using AES, SEM and LPR. Zinc whiskers was also observed from zinc
electrodeposits following exposure to thermal treatment (150°C for 1 h). Elemental analysis
and grain pattern investigations failed to help determine the cause of zinc whisker initiation.
Zinc whiskers was seen to protrude out of hexavalent and trivalent CCCs, with the latter
requiring a longer thermal exposure time.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Materials)|
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