Alternating current (AC) corrosion of mild steel in marine environments under cathodic protection (CP) condition was studied. Electrochemical studies at the two protection potentials namely -780 and -1100 mV versus SCE were examined by different techniques. DC polarization study was carried out for mild steel in natural seawater and 18.5 g/L NaCl solution to evolve corrosion current density. The corrosion rate determination, pH of the end experimental solution and surface morphology of the mild steel specimens under the influence of different AC current densities were studied. The amount of leaching of iron into the solution was estimated using inductively coupled plasma spectrometry. All these techniques revealed that AC influences the corrosion of mild steel in the presence of marine environments even though CP was given. Surface micrographs revealed that spreading of red rust products noticed on the mild steel surface. At -780 mV CP, red rusts are visually seen when the AC source was above 10 A/m(2) in both the media but red rusts are appeared after 20 A/m(2) in the case of -1100 mV CP. Weight loss measurements coupled with surface examination and solution analysis is a effective tool to characterize and quantify the AC corrosion of mild steel in marine environments. (c) 2006 Elsevier Ltd. All rights reserved.