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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3. Results and discussion 3.1. Coating system characterization

The cross-section of the substrate material was etched with 5% Nital solution, where the ferritic areas appear white and the pearlitic areas dark. The substrate steel S355 J2C + C had a ferritic-pearlitic microstructure, Fig. 3a, b. The rough and plastically deformed specimen surface is the result of the sandblasting process. All coated and MHP post treated (II-IV) specimens showed a continuous coating, Fig. 3c-h. Visually, a reduction in pore number and size can be observed between ZnAl-coated (II) and MHP post-treated (III+IV) specimens. MHP led to an increase in coating hardness and a decrease in coating thickness and mean roughness depth R z (Tab. 3), which should improve fatigue resistance. The adjusted MHP parameters of MHP 2 led to a further improvement of these properties.

Fig. 3. Cross-section: (a) Overview substrate microstructure (I); (b) substrate microstructure (I; (c) overview ZnAl4-coated (II); (d) coating of ZnAl4-coated specimen (II); (e) overview machine hammer peened MHP 1 (III) specimen; (f) coating of machine hammer peened MHP 1 (III) specimen; (g) overview machine hammer peened MHP 2 (IV) specimen; (h) coating of machine hammer peened MHP 2 (IV) specimen.

Tab. 3. Substrate and coating hardness, thickness, and roughness.

Condition

Substrate hardness HV10

Coating hardness HV0.1

Average coating thickness [µm]

Coating mean roughness depth R Z [µm]

ZnAl-Coating (II)

206 207 210

34.8 45.5 51.3

225 215 209

29.1 25.2

MHP 1 (III) MHP 2 (IV)

7.0

3.2. Corrosion and corrosion fatigue testing The PDP measurement resulted in an average OCP of 665 mV for the substrate. The measurements show a more negative OCP for post-treated MHP (III+IV) coatings in comparison between the as-sprayed ZnAl coating (II), Fig. 4. The difference of more than 500 mV for all coating systems compared to the substrate shows the suitability of the ZnAl coatings for cathodic protection. OCP has shifted to even lower values after MHP, which should benefit passive