The effects from Direct Electric Heating (DEH) on submarine pipelines have been investigated. The main focus has been to study the behavior of calcareous deposits when an AC voltage is applied. The main conclusions are: Pre-exposure in synthetic seawater using an AlZnIn sacrificial anode developed calcareous deposits on the specimen surfaces. Steady state protection current density stabilized at an expected level after approximately 3 weeks of cathodic protection. The measured steady state current densities were somewhat higher for the carbon steel test pieces used for large scale testing, than for the supermartensitic stainless steel (SMSS) specimens used for small scale testing. High alternating current on a cathodically protected SMSS and carbon steel surface destroys the calcareous deposits formed. Cracking of the deposits were observed and at 3 V_Ac the calcareous deposit fell off the steel surface. Most likely this is caused by increased gas evolution at the steel surface, underneath the deposits. Increased gas evolution was observed visually and is also supported from the cathodic polarization curves, where increased current densities at potentials more cathodic than -1.0 V_SCC were measured. Large scale testing using carbon steel test pieces with different coating thickness showed that the calcareous deposits are not so easily destroyed when a thick thermal insulation coating is used. The most likely reason for this is that the AC density on the steel surface is reduced by increased resistance from the calcareous deposits. Examination of the steel surface showed that an Mg-rich phase, believed to be magnesium hydroxide had formed during AC testing. The Mg-content increased with increased AC density. This was most likely caused by increased pH at the steel surface. The protection potential became more anodic with increased AC density. This was observed on large scale testing with carbon steel and small scale testing with SMSS. General corrosion in the range of 13 mm/y was observed on large scale testing with carbon steel at high AC densities of 250 mA/cm². From cathodic polarization under influence from AC it has been shown that the corrosion potentials for carbon steel and SMSS were shifted to more cathodic values. The oxygen limiting current on carbon steel is not influenced by AC. However, the hydrogen reduction reaction is shifted to higher current densities and the tafel slope is reduced under influence from AC. The increased current density can be a result from changes in the exchange current density or anodic shift of the reversible potential.