Corrosion and protection of heterogeneous cast Al-Si (356) and Al-Si-Cu-Fe (380) alloys by chromate and cerium inhibitors
In this study, the localized corrosion and conversion coating on cast alloys 356
(Al-7.0Si-0.3Mg) and 380 (Al-8.5Si-3.5Cu-1.6Fe) were characterized.
The intermetallic phases presence in the permanent mold cast alloy 356 are
primary-Si, AlB5BFeSi, AlB8BSiB6BMgB3BFe and MgB2BSi. The die cast alloy 380 is rich in Cu and Fe
elements. These alloying elements result in formation of the intermetallic phases AlB5BFeSi,
AlB2BCu and Al(FeCuCr) along with primary-Si. The Cu- and Fe-rich IMPs are cathodic
with respect to the matrix phase and strongly govern the corrosion behavior of the two
cast alloys in an aggressive environment due to formation of local electrochemical cell in
their vicinity. Results have shown that corrosion behavior of permanent mould cast alloy
356 is significantly better than the die cast aluminum alloy 380, primarily due to high
content of Cu- and Fe- rich phases such as AlB2BCu and AlB5BFeSi in the latter.
The IMPs also alter the protection mechanism of the cast alloys in the presence of
inhibitors in an environment. The presence of chromate in the solution results in reduced
cathodic activity on all the phases. Chromate provides some anodic inhibition by
increasing pitting potentials and altering corrosion potentials for the phases. Results have
shown that performance of CCC was much better on 356 than on 380, primarily due to
inhomogeneous and incomplete coating deposition on Cu- and Fe- phases present in alloy
380. XPS and Raman were used to characterize coating deposition on intermetallics.
Results show evidence of cyanide complex formation on the intermetallic phases. The
presence of this complex is speculated to locally suppress CCC formation.
Formation and breakdown of cerium conversion coatings on 356 and 380 was also
analyzed. Results showed that deposition of cerium hydroxide started with heavy
precipitation on intermetallic particles with the coatings growing outwards onto the
matrix. Electrochemical analysis of synthesized intermetallics compounds in the presence
of soluble cerium cations showed that of anodic and cathodic activity was not as strongly
inhibited as was observed for chromate ions. Overall cerium conversion coating showed
good performance on Al-Si (356) ally, but poor performance on Fe- and Cu-rich alloy
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:aluminum alloys protective coatings corrosion and anti corrosives
Date of Publication: