The effect of chlorine on the corrosion behaviour of different high-alloy stainless cast steels in seawater was studied. The investigations show that chlorine acting as a strong oxidizing agent is shifting the redox potential to very positive values. Because of the corrosion potential of the materials being influenced in the same way, the pitting potential of steels having a low pitting resistance equivalent may be exceeded. But first of all only the local inhomogeneities of the cast materials are attacked by pitting corrosion, which may stop after a while if the chlorine concentration is very low. Increasing chlorine contents make a repassivation of these spots more difficult or even prevent it, so these pits may be the starting points for stabile corrosion