Passive/active transitions in cyclic corrosion tests
Nowadays, cyclic corrosion tests are widely used to study new materials and coatings. They are more aggressive and can simulate various environmental conditions in a single test. The wet / dry transition in cyclic corrosion tests causes an active to passive transition which can temporarily generate conditions with significantly accelerated corrosion rates. Zinc coated steel samples with various types of pre-treatment were investigated in climate tests and tests with different cycle length. The tests showed similar trends except for those with insufficient drying time. Combined optical, electrochemical and spectroscopic investigations allowed linking the information accessible from each single method. This allowed a precise correlation between the spatiotemporal distributions of potential, current, pH, chemistry, delamination front as well as type, structure and amount of corrosion products. Segmented electrodes which were developed in the frame of this project proved to be a versatile tool not only for investigating the corrosion mechanism but also for controlling the corrosion progress in a desired way. Special emphasis has been put on the influence of the pH value. Not only the dissolution rates of zinc in certain solutions but also the semiconducting properties have been studied extensively and were understood much better. Based on these results the corrosion protecting effect of phosphatation could be understood as a buffering of the surface during drying into a range of stable zinc oxide or precipitate layers. The kinetic investigations demonstrated that delamination progress does not occur continuously but consist of periods with almost no progress in loss of adhesion and those of accelerated delamination. This behaviour is explained by other results that demonstrate spatial distributed fluctuations of the potential which can even mean an intermitting change in the sign of the potential, which is a change between anodic and cathodic conditions.
Bibliographic Reference: EUR 22392 EN (2007), 140 pp. Euro: 25
Availability: Katalogue Number: KI-NA-22392-EN-S The paper version can be ordered online and the PDF version downloaded at: http://bookshop.europa.eu
ISBN: ISBN: 978-92-79-04612-4
Record Number: 200719238 / Last updated on: 2007-07-05
Original language: en
Available languages: en