Effect of ultrasonically induced cavitation on behaviour of nodular cast iron in sea water

Cavitation corrosion was induced on nodular cast iron used as marine globe valves by means of a 20 kHz ultrasonic vibrator at an amplitude of 25 µm. The rate of weight loss in natural sea water was determined for cavitated specimens under free corrosion and cathodic protection conditions as well as in the quiescent condition. The electrochemical corrosion rate was determined by means of polarisation resistance measurements. The electrochemical corrosion rate of the cavitated specimen was six times higher than that in the quiescent condition. The cavitation corrosion rate (the total mass loss rate) was about 75000 times higher than the electrochemical corrosion rate under cavitation. Elimination of the electrochemical corrosion by cathodic protection decreased the mass loss rate by 50%, Potentiodynamic polarisation curves indicated that the kinetics of the cathodic process were enhanced under cavitation compared with those in the quiescent state. It was concluded that the electrochemical factor had a synergistic effect that might have enhanced the rate of the mechanical removal of material. This was supported by the SEM results as no significant differences were observed between the morphology of the cavitation damage following exposure to sea water or distilled water, or under cathodic protection. Cavitation damage was initiated at graphitic nodules as well as in the ferrite phase. Material removal was by both brittle and ductile modes of failure on localised microscopic levels.