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

831

2

Nomenclature A5

Elongation at fracture Constant amplitude tests

CAT

Diameter of machine hammer tool

d p

l p Track distance MHP M achine hammer peening NaCl Sodium chloride OCP O pen circuit potential PDP Potentiodynamic polarization R Stress ratio R m Tensile strength R z Mean roughness depth s Meander spacing TWAS Twin wire arc spraying v s Axial gun velocity ∆ E OCP Change in o pen circuit potential λ c Cutoff wavelength ρ i Impact density σ a Stress amplitude

© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of 23 European Conference on Fracture - ECF23 Keywords: Corrosion fatigue; Corrosion protection; Machine hammer peening

1. Introduction ZnAl coatings are used in various environments to protect components from corrosion. A common application due to the harsh conditions such as sea water, wind, tide, and waves is the coating of components used in offshore applications. In this application area, both mechanical and corrosive stresses are present and influence each other. Due to this and the impeded accessibility, the maintenance and repair costs for such components are very high. For offshore wind turbines, these costs can reach more than 50% of total investment, Tusar and Sarker (2021). ZnAl coatings primarily serve to protect the substrate from corrosion. During their service life coatings are also stressed by mechanical loads and surface wear, which can lead to cracks resulting in a negative impact on corrosion properties/protection. For offshore wind turbines, about 10 9 mechanical loading cycles can be expected for a lifetime of 20 years, Schaumann et al. (2011). On the one hand, the ZnAl coating acts as a physical barrier against the corrosive environment for the substrate, on the other hand, it also offers a passive protection effect due to the cathodic protection, which protects the substrate material even if the ZnAl coating is damaged. Cathodic protection is based on the more negative electrode potential of ZnAl in case of the galvanic couple ZnAl coating/steel substrate. If the coating is damaged, the ZnAl coating acts as a sacrificial anode and will degrade preferentially. For applications in marine environments organic coatings are used as top coating to enhance the corrosion resistance, Panossian et al. (2005). One approach to improving corrosion resistance is mechanical post-treatment, as this reduces porosity, increases density and produces a more uniform coating structure, Wielage et al. (2018). Hacini et al. (2008) showed the possibility of reducing porosity, increasing hardness, and introducing compressive residual stresses in near-surface areas by machine hammer peening (MHP). The adjustment of near-surface properties by MHP was also observed for ZnAl coatings, Tillmann et al. (2021) and Timmermann et al. (2021). The aim of this study was to verify whether mechanical post-treatment of the ZnAl coating with MHP has a positive effect on the corrosion and corrosion fatigue resistance. Potentiodynamic polarization (PDP) measurements to estimate the corrosion behavior and instrumented in-situ corrosion fatigue tests were performed.