The delamination of polymeric coatings from steel. II. First stage of delamination, effect of type and concentration of cations on delamination, chemical analysis of the interface

• Published in: Corrosion science Vol. 41; no. 3; pp. 579 - 597
• Main Authors: Stratmann, M, Leng, A, Streckel, H
• Format: Journal Article
• Language: English
• Published: 01.03.1999
• ISSN: 0010-938X

In the second part of this paper, the model presented in Part I is verified by several key experiments. For the simple, non-pigmented polymer used in this study, the migration of cations along the substrate/polymer interface is the rate-determining step for the overall delamination reaction. The delaminated area therefore increases with root time. The larger the size of the hydrated cation, the slower is the observed delamination rate, which increases in the order Li exp + < Na exp + < K exp + < Cs exp + . The type of the anion does not significantly influence the delamination rate. The delamination rate also depends on the concentration of cations at the defect: the higher the concentration, the faster the rate of delamination. This is interpreted by a threshold concentration of ions at the intact substrate/polymer interface, which is detected by the Kelvin-probe. The threshold concentration must be exceeded in order to galvanically couple the intact interface with the defect, and to allow the reduction of oxygen just in front of the delaminated zone, which will result in the loss of adhesion in this zone. The pH-change accompanied by the reduction of oxgyen is measured in situ by pH-sensitive pigments incorporated into the polymer film and by an Auger-analysis of the substrate/polymer interface. A large excess of cations in comparison to anions is observed. Finally, the first stages of delamination are analyzed. In contrast to steady state conditions, the delaminated area increases linearly with time, which is interpreted by charge-transfer controlled kinetics of the oxygen reduction.