Final Report - Strategic Environmental Research and Development ...

6. Concluding Summary
In this section, conclusions will be drawn for each task in turn.
Task 1. Fundamental Studies of the Trivalent Chrome Process (TCP).
Trivalent chromium process (TCP) coatings were studied on AA2024, 6061 and 7075
alloys that were first degreased and deoxidized. The TCP coating on all three alloys is
approximately 50-100 nm thick. It is primarily hydrated zirconia with coprecipitation of
Cr(OH) 3 . Under the zirconia layer is a K- and F-rich fluoroaluminate interfacial layer
(K x AlF 3+x ). The Cr-rich regions of the coating are in and around pits.
The layer forms because of an increase in the interfacial pH caused by both dissolution of
the oxide layer and localized oxygen reduction at Cu-rich intermetallic sites.
TCP coating provides both anodic and cathodic protection by physically blocking Al-rich
sites (oxidation) and Cu-rich intermetallics (reduction), resulting in 10x greater
polarization resistance and suppressed anodic and cathodic currents around the open
circuit potential in potentiodynamic scans.
TCP coating can release chromium into solution, which can then form a film on the
surface of an uncoated alloy surface exposed in close proximity. Artificial scratch
experiments showed that the polarization resistance of the uncoated surface near a TCPcoated
surface is higher than uncoated controls, which indicates the TCP coating can
provide active corrosion inhibition.
At least in the short-term (4-h in mild and aggressive electrolytes) the structure and
chemical composition of the coating are stable as evidenced by unchanging
electrochemical properties. Preliminary, evidence indicates that ageing (3-7 day air dry)
can have some beneficial effects by increasing the barrier properties of the coating.
Raman spectroscopy generated evidence for transient formation of Cr(VI) species in the
coating after some period of drying and or electrolyte solution exposure. Cr(VI) likely
forms due to the oxidation of Cr(III) oxide by locally produced H 2 O 2 , which is a product
of oxygen reduction. This transient Cr(VI) might be the source of the active corrosion
inhibition.
Task 2. Mechanisms of selected inhibitors.
The inhibition performance of common inhibitors found in commercial chromate-free
primers. i.e. molybdate, silicate, and praseodymium, was studied. A mechanism for the
protection provided by each inhibitor was postulated.
Molybdate, inhibition is optimal at near-neutral pH at a concentration of 125 mM in 0.1
M NaCl solution. MoO 3 imparts anodic inhibition by ennobling the pitting potential. The
inhibition mechanism involves an oxidation-reduction process whereby MoO 2 is formed
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