Kinks, iron dissolution

In order to explain the number of inductive loops, Schweickert and CO-workers postulated a catalytic iron dissolution mechanism occurring at kink sites, the number of these active surface centers depending exponentially on the electrode potential. 

The next iterative analysis of the influence of adsorbed hydrogen on the iron dissolution kinetics included the possible correlation between the surface structure and the reactions occurring at specific sites, as revealed by experimental and theoretical studies by Allgaier and Heusler, " Lorenz and co-workers, and Keddam andco-workers. " All these authors agreed that the dissolution rate constant should be proportional to the weakness of the binding of the surface atoms to the bulk metal, thus decreasing in the order kink > step > plane. The rate of metal dissolution is proportional to the rate constant and to the number of atoms in the position concerned, which decreases in the order plane > step > kink. 

Figure 31. Theoretical polarization curves, computed on the basis of kinetic formulas taken from Tables 1 and 2 describing a preferential iron dissolution and hydrogen adsorption at kink sites concomitantly with a Volmer-Tafel hydrogen evolution reaction at the planar atoms, for ratios between the electrochemical rates approaching the limiting case II at the corrosion potential. Approximate theoretical characteristics at the corrosion potential and in the linear region at moderate polarization " = (2/er/5f)to[(C,Ci 4 )/(2A )]. A /ApH = -47 mV, = -0.60, = 30 mV dec", Voh" = U bf = 5i mW dec", vi, - = 0.5,...

Consequently, the result of the analysis of a mechanism based on iron dissolution at kink sites partially inhibited by adsorbed hydrogen is that the anodic hydrogen desorption can be responsible for the kinetic behavior of the iron-acid system, in a low range of overpotential. 

There are two concepts concerning the role of kink atoms in the iron dissolution kinetics and mechanism. One of them is that kink atoms are simply the atoms that dissolve according to the reaction... 

Then, Dr. Ileana-Hania Plonski from Bucharest, Romania, brings out the question of the mechanism of the dissolution of iron, clearly one of the more important reactions in technological electrochemistiy. Dr. Plonski s contributions differ from those of oth s in that she stresses the possible effects that arise from the presence of hydrogen on the surface, even under anodic conditions when it should dissolve, because it may be retained to kink sites and therefore influence the rate of iron dissolution and, eventually, corrosion. 

Figure 5.2 Dissolution of iron according to catalytic (Heusler) mechanism (a) adsorption of OH" at a kink site, (b) dissolution of a FeOH species with regeneration of FeOHads-...

Cobalt Cobalt seems to behave much similarly to iron and its dissolution is assumed to take place through the formation of (CoOH), a catalyst surface species produced in an initial step of dissociative adsorption of water analogous to Eq. (23). Regarding the structural description of the dissolution mechanism in term of kinks at the lattice surface, cobalt and iron apparently follow the same process in which atomic kinks are assimilated to catalytic sites (CoOH) i g. Unfortunately, in the case of Co, microscopic evidence of the changes of surface morphology and consequently estimation of the kink-kink distance are lacking. 

A model accounting for changes in the surface concentration of kinks was developed by Heusler " from quantitative observations of the steady-state morphology for iron surface vicinal to (112) during anodic dissolution. He described the model as follows ... 

---