Abstract
The influence of iron pick-up on the corrosion properties of overlay and butt welds to NiCu30Fe/steel clad materials was investigated by electrochemical tests in aerated artificial sea water (ASTM Standard D1141-75) at 25, 50 and 80 deg. C and by exposure to sea water in a sea water test rig on Helgoland. NiCu30Fe plates with defined Fe contents between 0 and 14% were used for comparison. When the Fe was distributed homogeneously, no negative influence of the Fe on the pitting resistance was noted in aerated artificial sea water of 25 deg. C up to an Fe content of 10%. The pitting resistance decreases with rising temperature (50 and 80 deg. C) and the Fe influence increases slightly. On exposure to sea water on Helgoland, the corrosion in the sub-tidal zone is mainly caused by marine growth, resulting in the formation of shallow pits. In the tidal zone, Fe contents of more than 6.7% cause rust pittings at very low integral corrosion rates (<0.002 mm/ y). With integral Fe contents of between 0.5% and 3% in the final pass, the overlay welds exhibit the same good corrosion resistant properties of commercial NiCu30Fe (alloy 400) plate material. The compound butt welds exhibit slight pitting in the weld metal and the heat affected zone (HAZ) which cannot be exclusively attributed to the Fe pick-up, but is due to the difference in free corrosion potentials of the NiCu30Fe cladding material. In the tidal zone, where alloy 400 is commonly used, corrosion rates are less than 10 mpy for all specimens regardless of welding method. Iron contents of up to 8% which can be met by all welding methods when welded in two layers have no negative effect on the corrosion properties.
