STUDY ON THE FORMATION AND SOME PROPERTIES OF ELECTROLESS NI-B-TL ALLOY COATING

In this research some properties of an electro less Ni-B-Tl coating (in chloride solution) on AISI 316 grade stainless steel was investigated. An alkaline bath having borohydride sodium as the reducing agent and thallium nitrate as the stabilizer was used to prepare the electroless Ni-B-Ti coating. Variations in micro hardness, deposition rate and weight percentage of the elements Ni, B and Tl with time and temperature were studied. In addition, variation in microstructure of the as deposited coating as a function of the amount of elements present in the coating was studied. The results showed that by addition of thallium nitrate, Tl co-deposited on the surface. An increase in the weight percentage of the elements B and Tl and decrease in that of Ni were found with increasing plating time, and a decrease in the weight percentage of the elements B and Tl and increase in that of Ni were found with increasing temperature. While micro hardness increased with temperature, it decreased after an increasing trend with plating time. XRD patterns from the post heat treated specimens at 450 ˚C for 1 h revealed the formation of crystalline nickel, thallium and nickel boride (Ni3B, Ni2B) phases. The increasing surface hardness of the coating could be related to the formation of the Ni2B and Ni3B phases.