PSI - Issue 42

Michael P. Milz et al. / Procedia Structural Integrity 42 (2022) 830–837 Michael P. Milz / Structural Integrity Procedia 00 (2019) 000 – 000

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protection through cathodic protection. Also notable is the lower deviation in the measurements of MHP 2 (IV) compared to MHP 1 (III), which may indicate a more consistent result for the improved parameters of MHP 2 (IV).

Fig. 4. Potentiodynamic polarization measurements: ZnAl coating (II), machine hammer-peened MHP 1 (III), and machine hammer-peened MHP 2 (IV).

Fig. 5 shows the change in OCP ∆ E OCP during corrosion fatigue testing at all stress amplitudes σ a tested for MHP 2 (IV). In general, it can be observed that the OCP decreased at test beginning, because deformation created new surface areas of the ZnAl coating, which came in contact with the 3.5% NaCl solution. The later increase in OCP is due to the contact between the electrolyte and substrate resulting in a galvanic couple substrate/coating, with the steel substrate having the higher OCP, leading to a higher overall OCP and the preferential dissolution of the ZnAl coating. The maximum change in OCP increases with higher stress amplitude σ a , which is caused by the faster formation of new surfaces, resulting in a larger active surface area in contact with the electrolyte, as well as a faster decrease in OCP at higher stress amplitudes.

Fig. 5. Change in open circuit potential ∆E OCP for machine hammer peened MHP 2 (IV) at stress amplitudes 280 to 360 MPa.

The highest tested stress amplitude σ a at which a difference was observed between the performance of the uncoated sand-blasting (I) and the ZnAl-coated (II-IV) specimens was 300 MPa, Fig. 6. In the two tests, the number of cycles to failure N f for MHP 2 (IV) is 4.6 times higher than sand-blasting (I) specimen. Since only one material, the substrate, is in contact with the electrolyte for the testing of sand-blasting (I) specimen, there is only a decrease in the OCP throughout the duration of testing due to the formation of new active surfaces. On the basis of the S-N (Woehler)