Impedance measurements on Inconel and Monel alloys in Dead Sea water and sulphate solution

The electrochemical behaviour of Inconel 625 and Monel 400 alloys has been investigated in Dead Sea water and 0·5M K₂SO₄ solution using the impedance technique at several anodic potentials. In Dead Sea water, the Monel alloy is active at anodic potentials ≥ –100 m V; owing to pitting corrosion, while the Inconel alloy is passive up to 300 mV: Above this potential, destruction of the passive layer takes place leading to the formation of two maxima in the phase angle curves, indicating that two different oxide layers are present on the surface of the alloy. The radii of the pores formed at the surface of both alloys were calculated using the Kugel diffusion impedance element. In 0·5M K₂SO₄ both the Inconel and Monel alloys are passive, up to 500 and 600 mV respectively. Above these potentials both alloys became active. The passivity of the Inconel alloy is explained by the formation of chromium and silicon oxides.