Research On Low-phosphorus Electroless Nickel Plating For Aluminum Alloy

Aluminum radiator with characteristics of light weight and good thermalconductivity, are widely used in electronic products. But the disadvantage of poorweldability limite their further use. In this paper, using Al1060as substrate tosimulate the aluminum radiator, a low-phosphorus electroless nickel plating (ENP)have been developed to to improve the weldability of Al by plating Ni-P coatingwith about1μm thick.A low-P ENP which reduced the P content by using the complexing agents withweak complex ability, had been studied. The result showed that the bath turned toturbid, caused by the crystallization of Ni(OH)2and NiHPO3, during meatal turnover (MTO) test. It was neccesally to develop a low-P ENP with srong complexability. L8(27) orthogonal table was use to study the ENP with lactic acid andpropionic acid as main complexing agents. The research showed that an ENP with18g/L nickel sulfate could achieve high deposition rate, and the concentration ofsodium hypophosphite made the most significant effect on the deposition rate and Pcontent.Citrate acid, lactic acid, DL-malic acid, CA-A and CA-B were chosen as thecomplexing agents studying objects. The effects of the different complexing agentson deposition rate, P content of coating, SEM images of surfaces, the complexability were studied. The results indicated that the low-P ENP bath should use CA-Aas the main complexing agent, use lactic acid, DL-malic acid and CA-B as auxiliarycomplexing agents.Another orthogonal test was took place, and the basic low-P ENP was obtained:nickel sulfate18g/L, sodium hypophosphite21g/L, sodium acetate8g/L, CA-A6g/L,CA-B6g/L, DL-malic acid6g/L, lactic acid3g/L, thiourea (TU)1.5mg/L, SDS10mg/L, pH6.6, temperature87-89℃, and using air agitation. Testing on thebaisic ENP, the deposition rate was16.5-17.8μm/h, P content was about2.86-3.43mass%and pH buffering capacity and complexing ability were strong.The effects of process parameters (component concentrations, operatingconditions) and the additives (Pb2+, Bi3+, Ag+, ammonium molybdate, Ce4+, TU,urea, etc.) on deposition rate and P content were discussed. The results showed thatthe basic ENP was good enough and not needed to be optimized. But the use of airagitation could enhance the effects of TU. Bi3+, ammonium molybdate, and ureacould inhibit the precipitation of phosphorus as with TU. The antagonism between Bi3+and TU, urea and TU, caused an increase of P content; while the synergismbetween ammonium and TU caused a significal decrease of P content, but the use ofammonium molybdate could introduce Mo into Ni-P coating. As a result, TU waschosen as the only stabilizer in the bath.For the industrial practicability, the solution addition process and maintenanceprocess were studied. The addition solution composition was confirmed, and theaddition concentration ratio of sodium hypophosphite and nickel sulfate was1.1:1,the addtion concentration of TU was5.0mg/L per MTO.Zn2+did not affect the phosphorus content of the coating, but would poisonethe bath; the use of TU could increase the tolerance amount of Zn2+in bath; thedeposited of Zn and the carryout of solution could reduce the accumulation of Zn2+.The over consumption of agents can cause failure of the bath. The supplementedwith appropriate nickel sulfate, TU, SDS and Bath C could save the solution bymeasuring the residual concentration of nickel sulfate and sodium hypophosphite.The low-P ENP was studied by using the Al1060with double zincate assubstrate, and remaining high loading capacity (2.4dm2/L) and short plating time(3min). The treatment of double zincate affected the thickness of the Ni-P coating.Adjusting the operating time of first zincate, picking and second zincate, to get theZn layer with high quality on Al, could increase the thickness of the coating andreduce the risk of baring Al after electroless nickel plating. During the MTO test,the thickness of coating was0.7-1.2μm, the P content was1.4-6.1mass%in theformer3MTOs. The Al with low-P Ni-P coating had better weldability, but poorcorrosion resistance. Therefore, the low-P ENP needed further research.