Influence of the Microstructure of the Corrosion Performance of DP Steels

Influence of the Microstructure of the Corrosion Performance of DP Steels

Farias Moreno, D.E.

De Wit, J.H.W. (promotor)
Terryn, H. (promotor)

Mechanical, Maritime and Materials Engineering

Materials Science & Engineering

2014-10-03

Weight reduction of car bodies can be achieved by application of steel components with a lower thickness; however mechanical properties (for constructive and safety reasons) must be maintained, which can be achieved by using (U)HSS steels. These steels have been designed and optimized for improved mechanical behavior and therefore normally have a poorer or at least not well documented and understood corrosion resistance. One of these HSS steels are the so called Dual Phase steels, or DP steels. The aim of this work is to investigate the effect of the individual and combined ferrite and martensite phases in model DP steels on the corrosion performance and electrochemical behavior. For this, the first part of this work consisted in the development of a range of dual phase microstructures with different ferrite-martensite ratios ranging from virtually fully ferritic to fully martensitic. Controlled heat treatments were developed with the CASIM (Continuous Annealing SIMulator) and the dilatometer in order to obtain the desired microstructures. After the microstructures were developed, standard 3 electrode electrochemical setups were used to perform open circuit potential and potentiodynamic polarization measurements, approach which was then complemented by the use of Micro-Capillary cell. The use of the localized technique was focused on the possibility to perform single phase measurements which are not possible with the standard setups. The influence of the individual and relative amounts of ferrite and martensite on the electrochemical response of model DP steels was discussed. The first conclusions that were drawn from the electrochemical work was that due to the absence of a stable passive layer, performing measurements on standard setupsproved to be a challenge because crevice was a dominating factor in the system. Modifications to standard cell and Avesta cell were done in the attempt to avoid the presence of crevice. With the Micro-Capillary setup, even though crevice can also be an issue, reliable and reproducible polarization curves were obtained. A clear effect of sodium chloride concentration and the presence of a carbonate buffer on the electrochemical response of the model DP steels was observed. However, it was also found that the differences in the electrochemical behavior induced by the amount of martensite present are not significant enough to be distinguished from the reproducibility of the polarization curves. This was also the case for the base material sample, which even though composed of a highly deformed ferritic matrix with cementite-carbides islands, showed a similar response in the polarizations. The influence of the polarization system (setup, electrolyte, and microstructure) was discussed, as well as the importance of acknowledging the influence of crevice and reproducibility while performing electrochemical measurements.

DP
steel
micro capillary
polarization

https://doi.org/10.4233/uuid:a7ee5664-8c44-4cbc-b332-99b3f8671557

9789491909177

Institutional Repository

doctoral thesis

(c) 2014 Farias Moreno, D.E.