A direct electroless process for Ni-P protective coating on aluminium

Ni-P coating is known to protect aluminium against corrosion under severe conditions of atmospheric exposure. Conventionally, Ni-P is deposited on aluminium after giving a thin layer of Zn followed by an overlay of Cu. However, recent years have witnessed direct Ni-P plating processes on aluminium without applying an intermediate Zn layer. In the present work attempt has been made to initially apply a thin layer of 99.9% Ni by using a hydrazine containing Ni - ethylene diamine bath followed by deposition of Ni-P with sodium hypophosphite from the same bath. This process involves fewer number of steps, since it eliminates both Zn and Cu coatings and hence faster and more cost effective. The Ni-P plated Al substrates have been found to withstand 48 hour salt spray as well as humidity resistance tests without deterioration