In situ Raman microscopy of chromate effects on corrosion pits in aluminum alloy

In situ, ex situ, and microscopic Raman spectroscopy were used to examine the interactions of dilute chromate solutions with actively corroding pits in aluminum alloy used for aerospace applications (AA-2024-T3). Approximately 10(-3) M Cr(VI) in salt solution greatly reduces alloy corrosion, and soluble Cr(VI) is a major component of chromate conversion coatings (CCC). The interactions of dilute Cr(VI) with corroding alloy are likely to underlie the "self-healing" property of CCCs which make them so useful. Raman microscopy revealed that 10(-3) M Cr(VI) was transported into an active pit and formed a chromate corrosion product (CCP observable by its distinctive similar to 850 cm(-1) Raman band. This band was concentrated in pits and difficult to observe on the unpitted surface. Detailed spectroscopy of the CCP and several synthetic analogs revealed that the CCP is a mixed oxide of Al(III) and Cr(VI). Electrostatic bonding between the Al(OH)(chi) matrix and Cr(VI) is favored when the Al(OH)(3) is cationic at low pH, while covalent bonding is also possible at higher pH. The spectra of the CCP formed in AB 2024 pits implies a local DH in the region of 6 to 7. The implications of these findings to the corrosion protection mechanism of Cr(VI) on aluminum are considered. (C) 1994 The Electrochemical Society. S0013-4651(99)01-089-7. All rights reserved.