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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Tab. 2. Chemical composition of ZnAl4 feedstock wires according to the datasheet of the manufacturer. Element Zn Al Si Fe Pb Cu Sn wt.-% Bal. 3.5-4.5 ≤ 0.030 ≤ 0.005 ≤ 0.003 ≤ 0.002 ≤ 0.001

2.2. Material characterization Mean roughness depth R z , thickness, and hardness of the coatings were characterized before and after the MHP post-treatment for the conditions I-IV. Roughness measurements were performed using a white light confocal microscope Nanofocus µsurf (Oberhausen, Germany) with a magnification times 50 long objective and a robust Gaussian filter with a cutoff wavelength λ c = 0.8 mm. Substrate grain structure, coating morphology and thickness were investigated using an Olympus BX51 optical microscope (Hamburg, Germany) on metallographically prepared cross-sections. After each processing step of the corrosion fatigue specimens, three measurements of gauge length diameter were carried out using the optical micrometer Keyence TM-040 (Neu-Isenburg, Germany) and average values were calculated. Based on these measurements, stresses during corrosion fatigue testing were calculated. 2.3. Corrosion and corrosion fatigue testing PDP measurements were performed to characterize the corrosion behavior of the substrate and coating systems, Fig. 2a. Three-electrode setup and a potentiostat interface 1000 (Gamry Instruments, Warminster, PA, USA) were used to record and control the tests. The specimen served as working electrode, a silver chloride electrode as reference electrode, and a graphite electrode as counter electrode. Measurements were started 250 mV below the previously measured open circuit potential (OCP). The potential was increased to 500 mV above OCP with a scan rate of 0.5 mVs -1 . Stress-controlled constant amplitude tests (CAT) were carried out in the range of 280 to 360 MPa with a sinusoidal load-time function at a stress ratio R = -1 (fully reversed loading). A polyurethane coating covers the specimen except for a 9 mm wide area in the middle of the gauge length, which created a defined test area. Corrosion fatigue tests were performed in a self-designed corrosion cell using a 3.5% NaCl solution as electrolyte, which was continuously exchanged and tempered by a pump and thermostat, Fig. 2b. The same three-electrode setup and a potentiostat interface 1000 were used for PDP measurements. Prior to fatigue loading, specimens were exposed to the medium for 30 (I) and 60 min (II-IV) to reach their specific OCP. An extensometer with a gauge length of 89 mm was applied at the specimen shafts outside the cell. CAT ended with failure or reaching the limited number of cycles N limit = 2∙10 6 .

Fig. 2. Experimental setups: (a) Potentiodynamic polarization measurement; (b) corrosion fatigue test, Milz et al. (2022).