Issue 48

X.-g. Huang et alii, Frattura ed Integrità Strutturale, 48 (2019) 48 1 -490; DOI: 10.3221/IGF-ESIS.48.46 487 Material E corr (V) i corr (mAcm 2 ) b a (V.dec -1 ) b c (V.dec -1 ) R p (Ω.cm 2 ) AH 32 -0.510 0.0489 0.405 0.157 1005.962 Zn -1.103 0.0110 0.572 0.203 592.203 Cr -0.411 0.0078 0.128 0.281 4901.968 *Polarization resistance ( R p ): Rp= b a * b c /[2.3× i corr ×( b a + b c )] [31] Table 1 : The instantaneous electrochemical parameters from polarization curve. Test period Material 0h 15 d 30 d E corr (V) i corr (mAcm 2 ) E corr (V) i corr (mAcm 2 ) E corr (V) i corr (mAcm 2 ) AH 32 -0.510 0.0489 -0.557 0.0187 -0.564 0.0125 Zn -1.103 0.0110 -1.176 0.0073 -1.192 0.0065 Cr -0.411 0.0078 -0.427 0.0062 -0.430 0.0059 Table 2 : Comparison of corrosion current density of AH32 and coating materials in different immersion periods. Figure 10: Morphology of crack propagation zone (a. Zn coating, b. Cr coating). Through the above research, it is not difficult to find that the mechanisms of Zn and Cr coating to improve AH 32 corrosion fatigue resistance are different. The effect of Zn coating on corrosion fatigue prolongation embodies in not only crack nucleation but also crack propagation. Before crack nucleation, the Zn and Cr coating only acts as physical isolation. Because of the interaction of electrochemical process and fatigue, Zn coated specimens are more likely to form crack on the surface. In comparison, the surface corrosion of Cr coating samples develops more slowly for the better corrosion resistance, as obtained from pre-corroded tests. Therefore, Cr coated samples have longer crack nucleation lives than that of Zn coated ones, under the same corrosive load. Once cracks nucleate and propagate, Zn transform its role from physical isolation to sacrificial anode material, to a certain extent, to restrain the corrosion reaction of crack surface and crack tip and to avoid the acceleration effect of corrosion products on crack propagation. However, the electrochemical activity of chromium is not as good as AH 32, but worse than Zn. Cr coating cannot play the role as sacrificial anode to protect crack propagation of the substrate. Fig. 10 shows the crack propagation zone of Zn and Cr coated samples, respectively. In the fracture surface of Zn coated sample, the crack striations in crack propagation is very clear, and the corrosion status is not serious. While the corrosion at the fracture of Cr coated sample is serious covered with a layer of corrosion fatigue. What’s more, there also exists several deep secondary cracks. It has been testified that the corrosion products and secondary cracks have close relation with the crack propagation behavior [37-38]. Therefore, the phenomena in Fig. 6 can be explained as the superposition of the contribution of Zn and Cr coating to crack nucleation and crack propagation. At low stress level, corrosion fatigue crack propagation life is relatively longer, so the effect of Zn coating on a fatigue striations b corrosion product secondary cracks