Correlation of micro-galvanic corrosion behavior with corrosion rate in the initial corrosion process of dual phase steel

The initial corrosion process of dual phase (DP) steel produced by the continuous annealing process (CAP) in NaCl solution was systematically studied in this paper. The microstructure, nobility of the phases, and micro-galvanic corrosion behavior were characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) technique, scanning Kelvin probe force (SKPFM), and electrochemical impedance spectroscopy (EIS). Results indicate that there are two types of martensite, namely fresh martensite (FM) and tempered martensite (TM), embedded in the ferrite matrix. The two types of martensite are surrounded by dislocations and connect to each other as in a chain-like network at the ferrite grain boundary. Both FM and TM can act as a micro-cathode to promote the anodic dissolution of adjacent ferrite matrix since FM is up to 15 mV noble to the ferrite matrix and TM is to 30 mV noble. The initial corrosion process of DP steel can be divided into two stages. The first stage involves an increasing corrosion rate of the steel due to the successive emergence of cathodic TM and FM, while the corrosion rate decreases in the second stage because of the separation of martensite.