Influence of Pre-Corrosion in NaCl Solution on Cavitation Resistance of Steel Samples (42CrMo4)

Marine applications often involve metallic materials, including steel, that must endure harsh conditions such as cavitation erosion (CE). This study investigates the CE behavior of 42CrMo4 steel, both in its original state and after pre-corrosion in a 3.5% NaCl solution for 120 days, simulating a simplified marine environment. Cavitation testing was conducted using an ultrasonic vibratory setup with a stationary sample, at intervals of 10 and 30 min, with a total testing time of 150 min. Mass loss (ML), mass loss rate (MLR), mean depth of erosion (MDE), and level of degradation (LoD) were calculated, while surface roughness (Rz) was measured using a TR200 tester. Surface changes were analyzed through field emission scanning electron microscopy (FESEM) and image analysis techniques. Morphological parameters such as the number of pits, average diameter, and total pit area were used to quantify surface damage. Results showed that pre-corroded samples exhibited a significantly higher erosion rate than non-corroded ones. Pre-corrosion introduced microcracks and surface defects that served as initiation sites for cavitation damage. These imperfections increased surface roughness and created favorable conditions for pit formation, leading to faster and deeper material loss. Image and FESEM analyses confirmed the presence of larger and deeper pits in pre-corroded samples compared to the smaller and shallower pits in non-corroded specimens. This study highlights the impact of pre-corrosion on the cavitation resistance of 42CrMo4 steel and demonstrates the effectiveness of combining mass loss data with morphological and surface analyses for evaluating cavitation damage under marine-like conditions.