Effects of Alloying Elements in Aluminum Alloys and Activations on Zincate Treatment and Electroless Nickel-Phosphorus Plating

Abstract

Zincate treatment and electroless nickel-phosphorus plating for aluminum alloys of A1100, A2017, A5052 and A7075 were researched from the viewpoints of alloying elements and activation conducted prior to zincate treatment. Surface morphologies of zincated surfaces, electrochemical properties during zincate treatment and depth profiles on activated and zincated surfaces were investigated by microscopy and measurement of temporal changes of electrode potential.Adhesion strengths of the plated films were measured by 90 peeling test, which showed that double zincate treatment improved adhesion strength especially when ferric ion was added into the zincate solution except for the case of A7075. Temporal changes of electrode potential during zincate treatments and morphologies of the zincated surfaces showed that copper and zinc promoted uniform precipitation of zinc and resulting rapid termination of conversion reaction. Excess zinc was shown to dissolve into the plating solution and cause the formation of lowdensity area or gaps between plated films and substrates.Activation for A1100 by various conditions showed that formation of oxide film had a remarkable effect on precipitation of zinc during zincate treatment. When a mixture of nitric and hydrofluoric acid was used for activation, finer particles of zinc were formed near etch pits compared with the cases of hydrochloric or nitric acid. On the other hand, the surface was uniformly covered with fine zinc particles in the case of the activation with a solution of sodium hydroxide. This indicates that uniformity of oxide film on substrate before zincate treatment is thought to bring about uniform precipitation of zinc.

Formation of Zincate Films on Binary Aluminum Alloys and Adhesion of Electroless Nickel-Phosphorus Plated Films

“…4,5) The activation was conducted in a solution of nitric acid (69 mass%) and hydrofluoric acid (46 mass%) whose mixing ratio was HNO 3 : HF = 3 : 1 in volume. The solution for the zincate treatment was either the basic solution or the ferric ion-added one.…”

“…The former was prepared by dissolving zinc oxide (40 kg/m 3 solution) into a solution of sodium hydroxide (240 kg/m 3 solution), 7,8) and the concentration of the ferric ion in the latter was Zn : Fe = 40 : 1 in atomic ratio. 4,5,911) The substrates underwent the zincate treatments for 30 s at 12) The averaged thickness of the plated film was 20 µm for the 5.4 ks plating.…”

“…Based on the previous reports, 4,5) zinc particles, which excessively precipitate on commercial aluminum alloys, vigorously dissolve into the solution of the electroless nickel-phosphorus plating as soon as the zincated surface is immersed. The electrons generated by the dissolution of zinc are consumed by the formation of the plated films and by hydrogen gas which leaves voids between the plated films and the substrates.…”

“…According to previous reports, 4,5) thin and uniform zincate films are formed by both the first and the second zincate treatments for commercial aluminum alloys which contain copper or zinc (A2017, A7075). On the other hand, coarse zinc particles are formed only by the first zincate treatment on the aluminum alloys which contain no meaningful amount of copper or zinc (A1100, A5052), then uniform zincate films are obtained by the second zincate treatment.…”

Effect of Alloying Elements and Generation of Hydrogen Gas on Zincate Treatment and Electroless Nickel-Phosphorus Plating of Aluminum Alloys

“…On the other hand, a zincate solution, which contains ferric ion, was used prior to plating since the amount of precipitated zinc is reduced and the adhesion between the substrate and plated film is improved. 4) Generation of gas during the zincate treatment was visually observed. The zincated substrates were immersed in the 363 K plating solution, and plating for a substrate of 99.5 mass% zinc was also performed to investigate the behavior of excessively precipitated zinc during the zincate treatment.…”