Effect of inserting a-C layers on anticorrosion behavior of Ni-NiCr-NiCrAlSi composite coating on copper through magnetron sputtering for marine applications

Amorphous carbon film by magnetron sputtering was inserted in the NiCrAlSi layer to form NiCrAlSi/a-C multilayer with modulation period in a range of 131.6 nm to 240.2 nm in the Ni-NiCr-NiCrAlSi coating for a further improvement to copper on corrosion resistance aiming at application as a heat exchanger for mariculture in sea water. After structure characterization, the corrosion behavior of the samples with variant modulation periods of the NiCrAlSi/a-C multilayer keeping the same total thickness(1 μm) was compared using potentiodynamic polarization and electrochemical impedance spectroscopy. The polarization curves show that the corrosion potentials of the coatings with modulation periods of 240.2 nm, 198.4 nm, 158.4 nm and bilayer number 4, 5, 6 for NiCrAlSi/a-C layer are more negative compared to that with modulation period of 131.6 nm and bilayer number 7, which indicates that the smallest modulation period results in the highest corrosion resistance. The amorphous a-C sublayer in the multilayer coatings plays an active role in the improvement, for an introduction of the more interfaces between the NiCrAlSi and the a-C sublayers, as well as the compact structure and the chemical inertness to sea water (Cl⁻). In addition, the NiCrAlSi sublayer can be regarded as an interlayer to release stress in both the a-C sublayer and the NiCrAlSi sublayer, mainly from corrosion products, which also plays an important role for the improved corrosion resistance.