Anodic Behavior of Aluminum in Glycine Aqueous Solutions at Various pH Values

At the present work, it was investigated aluminum (Al 99.99%) anodic behavior in glycine aqueous solutions (in the concentration range from 10^–4 to 10^–1M) at various pH values (pH 4.8 - 8.4). Reliability of the obtained experimental results was confirmed by use of a complex of independent physic-chemical methods (voltammetry, chronoamperometry, optical microscopy (МBS-2 (x7) and МIМ-7 (x500)), scanning electronic microscopy (SEM, x1000) with JEOL-6380LV and energy-dispersive X-ray analysis (EDXA) with INCA Energy-250).

The received data have shown that depending on the pH, the amino acid exhibits a dual effect. On the one hand, glycine zwitter-ions (pH 4.8–5.8) conducts one’s self  like an inhibitor slowing down the anodic process  and aluminum is in a stable passive state at these pH values in the vicinity of the isoelectric point of the investigated amino acid. Under this the probable reason of the observed effect consists of strong intermolecular hydrogen bridges which are formed between the amino and carboxy groups in the amino acid zwitter-ions predominate over it’s interactions with the oxidized aluminum surface. It follows from this that glycine zwitter-ions do not participate in the anodic process on aluminum, and the passive film on aluminum surface is formed upon metal–water interaction according to the following scheme  because of a high chemical affinity of aluminum to oxygen:

Al + yH₂O → [Al (H₂O)_y]_ads                                                          (1)

On the other hand, it has been shown that the glycine anions formed at pH = 8.4 stimulate pit formation (PF) on oxidazed aluminum surface in these solutions (pit depth was 6 μm at 10^{-1 M}). The choice of this pH value was caused by two reasons. In the first place, at pH = 8.4 the system contains both inactive zwitter-ions and active glycine anions. Secondly, solutions with higher pH might cause intense general corrosion of aluminum. The data about aluminum anodic behavior in glycine solutions (pH = 8.4) were analyzed on the basis of the nucleophilic substitution theory, according to which PF observed on aluminum involves nucleophilic substitution of ligands of adsorbed surface complexes by corrosive solution components with subsequent hydrolysis of a compound formed. Generalization of experimental data and literature data [1-3] allowed us to suggest the following multistage mechanism of the PF process:

Al + yOH^- = [Al(OH)_y]^z1-y _ads + z1e^-                                            (2)

 [Al(OH)_y]^z1-yads → [Al(OH)_y-k]^z1´-y_ads + k OH^-                            (3)

[Al(OH)_y-k]^z1´-y_ads + Gly^- = [Al(OH)_y-kGly]^z2-y_ads                          (4)

[Al(OH)_y-kGly]^z2-y_ads = Al^z3 + Gly^- + (y – k) OH^- + (z2-z3)e^-          (5)

According to the kinetic analysis data obtained in the present investigation, the limiting stage of the PF process studied is dissociation of the initial complex at  equation (3).This fact is confirmed by independence of the PF process rate from activator –glycine anions.  

References

1. T.A. Borisenkova, S.A. Kaluzhina // Int. J. Corros. Scale Inhib., 2, 122 (2013)
2. T.A. Borisenkova, S.A. Kaluzhina // Condensed matter and interphases, 13, 136 (2011).
3. T.A. Borisenkova, S.A. Kaluzhina // ECS Abstract of the 218^th Meeting, Las Vegas, №1227 (2010).