During the last ten years high alloyed stainless steels with 6% molybdenum (6Mo) or 25%Cr (super duplex) have been the most popular materials for seawater systems on offshore installations in the North Sea. The basis for this material selection was to obtain maintenance free systems with long lifetime. However, practical experience has shown that corrosion failures can occour. This paper presents a simple and economical method to avoid corrosion problems internally in piping systems transporting chlorinated seawater. The method is called RCP – Resistor controlled Cathodic Protection. Principles of the method including protection potential, current density requirements and anode design in addition to different practical applications are described.