Raman spectroscopic analysis of the speciation of dilute chromate solutions

The relative concentrations of various Cr-VI species are important to sever al mechanisms for corrosion inhibition by chromate conversion coatings and chromate containing primers. The pH dependent conversion of CrO4-2 to Cr2O7 -2 is well known, but the existence and importance of bichromate, HCrO4-, i s more controversial. Past reports have described the pH and concentration dependence of the CrO4-2/HCrO4-/Cr2O7-2 distribution, while recent reports have concluded that HCrO4- does not exist in aqueous media. The current wor k shows that HCrO4- does indeed exist at a pH below 6 and at low total Cr-V I concentration. The Raman signature of the Cr-O-Cr bond in dichromate decr eases relative to the Cr-O symmetric stretch as [Cr-VI] decreases. The resu lts are quantitatively consistent with previous reports on the CrO4-2/ HCrO 4-/Cr2O7-2 equilibrium, and establish that HCrO4- is the dominant Cr-VI spe cies for low [Cr-VI]. In the low concentration and slightly acidic conditio ns likely to be present during corrosion inhibition by Cr-VI in the field, HCrO4- is the most prevalent form of Cr-VI in water. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.