Issue 48

X.-g. Huang et alii, Frattura ed Integrità Strutturale, 48 (2019) 48 1 -490; DOI: 10.3221/IGF-ESIS.48.46 485 According to the ISO 2063 standard, Zn and Cr coating are deposited by CMD-AS1620 arc spray system. Before spraying, the rust of samples has been removed, and the surface have been blasted by 0.5mm corundum to achieve the roughness of 50-80 µm. The blasting pressure is 0.6 MPa and the blasting distance is 150 mm. After several trials on spray conditions, the filamentary Zn and Cr are heated to the melting state, and blown into a mist by compressed nitrogen gas to form a uniform particle flow and sprayed on the substrate surface. The stand-off distance (distance between the nozzle and the substrate) is kept constant at 150 mm. The stagnation pressure is 0.7MPa and the powder feed rate is 27.78 g/min. During the spraying process, the process parameters are finely adjusted to obtain a constant thickness of 500 ± 30 µm. The cross- sections of AH 32 substrate and Zn and Cr coating interface are obtained by optical electron microscope, as shown in Fig.5. It can be clearly observed that a lamellar structure with lamellas parallel to the substrate surface, with a good bonding with close to no porosity. The coating-substrate interface is quite irregular, possibly due to the impact of the high velocity particles that constitute the coating, on the AH 32 substrate. Before deposition, it is evident that the substrate had undergone a severe plastic deformation during the coating process. However, such deformation enhances the mechanical bonding and adhesion of the coating to the substrate. Both Zn and Cr coating show a good build-up and had a final roughly uniform thickness in the range of 470 to 530 μm. Effect of coating on corrosion fatigue The corrosion fatigue tests of bare steel, and Zn and Cr coated specimens are carried out under the same test condition, and the S-N curves at 1Hz frequency are shown in Fig. 6. For comparison purposes, and to provide a better understanding of the effect of the corrosive environment on fatigue life of the AH 32, the S-N curve of fatigue testing is also shown in the same figure. It can be seen that both Zn and Cr coating can greatly improve the corrosion fatigue of AH 32 steel, and this effect enhances with the decrease of stress. This is attributed to the interaction of physical isolation, compressive residual stresses induced by arc spray and electrochemistry function of the coating materials [28-30]. An interesting phenomenon is that the effects of Zn and Cr coating on fatigue life extension are dramatically dependent on the stress level. The comparison of corrosion fatigue lives of Zn and Cr coated samples show that Cr coating has better performance than Zn coating at higher stress level, but the opposite is true at low stress amplitude. This can be explained from the contribution of these two coatings to crack nucleation and crack propagation at different stress levels. However, their contributions of each part to corrosion fatigue life is difficult to be quantitatively determined, and we will discuss in detail from the mechanism analysis. Figure 6: S-N curves of corrosion fatigue tests The pre-corrosion fatigue tests of Zn and Cr coated specimens are also carried out to investigate their corrosion resistance. The bare samples, Zn and Cr coated specimens are respectively emerged in the seawater for period of 15 d and 30 d, and the typical corroded samples with a period of 15 d are shown in Fig. 7. It can be seen the surface corrosion of test section of bare AH 32 samples is the most serious, followed by Zn coated and Cr coated samples. The fatigue behaviors of the pre corroded samples are tested under the same load as the corrosion fatigue, with results shown in Fig. 8. The fatigue lives of pre corroded Cr coating samples are apparently larger than that of Zn coated samples, and the difference between 160 200 240 280 320 10000 100000 1000000 10000000 S /MPa N /cycle f=1Hz(corrosion fatigue) f=1Hz(corrosion fatigue-Cr coating) f=1Hz(corrosion fatigue-Zn coating) f=1Hz(fatigue)