The replacement of copperlbrass radiators in the automotive industry with radiators made
from aluminium components provided the basis of this research. Since aluminium is more
susceptible to corrosion than either copper or brass, factors that contribute to its corrosion
are of major interest and importance, and have been investigated. Three different
aluminium alloys were selected for study because of a special interest in their corrosive
behaviour by the automotive industry. These are the aluminium alloy AA 3003 (samples
A and B) and two supplier specific alloys (sample D containing Zn and sample E
containing Cu and Mg).
The various joining operations used in the automotive manufacturing process dictated the
preparation of the aluminium alloys used for corrosion studies. Mechanically Assembled
(MA) aluminium radiators use alloy samples as supplied by the aluminium industry and
hence suitable experiments were carried out on the 'as-supplied' (AS) samples used for
both finstock and tubestock material. The development of Composite Deposition (CD)
Technology to braze together finstock and tubestock material introduced new challenges to
corrosion research. To gain an insight into the corrosion of a Brazed aluminium radiator,
all samples were subjected to a thermal profile identical to that experienced industrially
under a Controlled Atmosphere Brazing (CAB) furnace. Two cases of interest emerged.
Firstly the 'heat-treated' (HT) samples were used to evaluate the effect ofheat treatment on
the alloy's resistance to corrosion. Secondly, alloy samples treated with a Composite
Powder Coating (CPC) and then subjected to the thermal profile provided a surface of an
AI-Si melt which represented the brazed joint. Experiments on these samples yielded
information on the AI-Si melt and the likely corrosion in a brazed joint.
The resulting corrosion of the AS, HT and CPC samples immersed in various corrosive
electrolyte solutions for 60 minutes was examined using two microscopic techniques.
Firstly, the actual surface pitting was examined using a Scanning Electron Microscope
(SEM), and secondly, cross-sections of the samples mounted in a resin, then suitably
polished and etched were examined using an optical microscope to further reveal the
nature of corrosion of the samples. The nature of corrosion was best revealed in an
acidified chloride solution. The AS samples showed delocalised crystallographic pitting
consisting of coalesced pits at localised regions of the surface. The HT samples showed
IV
localised crystallographic pIttIng consIstIng of many individual pits and intergranular
corrosion both at and below the surface. Intergranular corrosion was most severe for HT
sample E containing Cu and Mg. The CPC samples showed total corrosion of the surface
layer and eutectic AI-Si melt, some crystallographic pitting of the a-AI filler metal, and
crystallographic pitting including intergranular corrosion of the base alloy. The extent of
corrosion was found to depend on the chemical composition of the aluminium alloys, the
presence of Zn, Cu and Mg causing more severe corrosion of the aluminium alloys, with
the effect ofZn being most severe.
The electrochemical investigation involved the measurement of two fundamentally
important parameters. Firstly, the open circuit potentials (OCP) of the alloy samples
immersed in the various corrosive electrolyte solutions were measured as a function of
time. Secondly, the pitting potentials (Bp) of the alloy samples were measured using
anodic polarisation techniques by extrapolation of the resulting log i vs E plots. The OCP
and Bp of the AS samples were found to be influenced by the chemical composition of the
aluminium alloys. Heat treatment of the AS samples was found to change their
microstructure and solid solution composition which in turn affected the electrochemical
results. The effect of the Composite melt layer on the electrochemistry of the CPC
samples is discussed.
Micrographic and electrochemical results were used to assess the best combination of
finstock and tubestock material that would yield an aluminium radiator most resistant to
corrosion. The likely corrosion of the components in these combinations was assessed and
these results were compared with the actual results obtained industrially using the SWAAT
exposure test. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1999.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5580 |
| Date | January 1999 |
| Creators | De Leeuw, Barbara Marielle. |
| Contributors | Hawksworth., W. A. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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