Investigation of microstructure and corrosion behaviour of prior nickel deposited galvanised steels

By Sarkar, Kuntal; Mondal, Avik; Chakraborty, Anindita; Sanbui, Mohit; Rani, Nitu; Dutta, Monojit
Published in Surface and Coatings Technology 2018

Abstract

Structure and performance of hot dip galvanised coating can be improved by incorporation of different alloying elements like Al, Ni, Pb etc. in the coatings. Zn-Ni alloy based coating systems have already been proved to have high corrosion resistance along with improved adherence, weldability, mechanical properties etc. In the present study, the microstructural development of prior nickel coated galvanised coating is studied. The initial nickel coating thickness is about 6 ?m and the galvanising time is varied from 3 to 10 s. Nickel?zinc delta and gamma phases are formed within the coating and iron diffusion is considerably suppressed. Corrosion performances of prior Ni coated galvanised coating and conventional galvanised coatings have been evaluated with the help of electrochemical analysis techniques. The potentiodynamic polarization study shows that both the coatings have similar active dissolution behaviour. The EIS analysis reveals that the passivation behaviours of two different coatings are largely different for prolonged immersion in the electrolyte solution. The prior nickel coated galvanised alloy coating has been observed to develop a stable thin passive layer of oxide which is duplex in nature and the resistance to corrosion is increasing with immersion time. The exposure of nickel rich intermetallic layers with prolonged immersion is also acting as barrier to corrosive ions penetration and increasing the corrosion resistivity. On the contrary the pure galvanised samples first show increasing (up to 10 days) and then decreasing (10 to 20 days) resistance to corrosion due to the initial formation of the oxide film followed by its dissolution.

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