Issue 48

X.-g. Huang et alii, Frattura ed Integrità Strutturale, 48 (2019) 48 1 -490; DOI: 10.3221/IGF-ESIS.48.46 486 which become obvious with the prolongation of period of pre corrosion and the decrease of stress amplitude. It follows that corrosion resistance of Cr coating is stronger than that of Zn coating. With the prolongation of pre-corrosion time, the surface corrosion of zinc coating is more serious than Cr coating, and corrosion defects are more likely to induce fatigue crack nucleation under fatigue load, thus accelerating the fatigue failure of zinc coating samples. Figure 7: Morphology of the corroded samples. Figure 8: S-N curves of corrosion fatigue. Mechanism of Zn and Cr coating on corrosion fatigue To make insight into the mechanism of Zn and Cr coating improving corrosion fatigue resistance of AH 32 steel. The electrochemical measurements of the two coating materials and AH 32 substrate in the seawater are performed by a Perkin-Elmer M283 three-electrode-cell constant potential electrochemical testing system. Cyclic potentiodynamic Tafel polarization are measured starting from -250mV (vs open circuit potential), and scanned toward more positive direction with scanning rate of 0.5mV/s. Figure 9: Tafel polarization curves of coatings and substrate The instantaneous Tafel polarization curves of AH 32, Zn and Cr in seawater are depicted in Fig. 9, and the electrochemical parameters from polarization analysis are listed in Table 1. The corrosion current densities at different immersion periods are also listed in Table 2 for comparison. Compared with AH 32 substrate, Zn has a more negative corrosion potential than Cr, while Cr has a better corrosion resistance. Therefore, Zn is more suitable for cathodic protection, and often mixed with aluminum or magnesium to form an alloy coating for cathodic protection of engineering materials [32-34]. While Cr is often added to the substrate material as alloy element, to improve the corrosion resistance of the substrate [35-36]. bare AH32 Zn coating Cr coating 160 200 240 280 320 10000 100000 1000000 10000000 S /MPa N /cycle 15 d Zn coating 15 d Cr coating 30 d-Zn coating 30 d-Cr coating 15 d-bare 30 d-bare -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 -7 -6 -5 -4 -3 -2 -1 0 坐标轴标题 LGI (A.CM2) AH32 Cr coating Zn coating 2 Lg( ) /A.cm I SEC / V E