CORROSION OF 13CR SUPER MARTENSITIC STEEL IN ARTIFICIAL SEAWATER UNDER ALTERNATING CURRENT AND ALTERNATING MAGNETIC FIELDS

Acknowledgement to Statoil Forsknings-senter, Trondheim, Norway, for supporting this research

AC corrosion is a familiar issue to different industries related to transportation and structure integrity. It can result from grounding loops i.e., the electrical current flows out of the path that was designed for it. If alternating current is conducted from the metal and through the electrolyte AC stray corrosion will occur. Alternating current consists of cyclic changes in the polarity of the electrode. Each half-cell reaction occurring during this cyclic polarization has different accumulative effects. Pitting is observed as a result of the anodic processes but also hydrogen charging is likely to occur due to the cathodic half-cell reaction occurring at the opposite half-cycle.

Measuring the current that causes the corrosion in the field can be difficult. It has been observed that AC corrosion produces a shift in the electrode potential, however the direction and magnitude of the shift depends on the amplitude of the AC more than other environmental conditions.

Studies have been conducted to explain the phenomena and the possible implications on the design of corrosion protection systems like cathodic protection.

Open circuit potential shift, electrochemical impedance spectroscopy and cyclic polarization have been used to determine the effect of different alternating current densities. Characterization of the surface after being exposed to AC with scanning electron microscopy of has also been used.

 

Key words: AC corrosion, pitting, potential shift