ELECTROLESS NICKEL PLATING OF IRON

L. M. Kurvyakova, I. V. Pimenova, and E. P. Zakharova

P O W D E R

UDC 621.35:669.248T621.762

The object of the work desc r ibed below was to study the e l e c t r o l e s s nickel plating of iron powder and de t e rmine conditions under which its pa r t i c l e s become evenly coated with nickel and the ex t rac t ion of nickel f rom a solution is a max imum [1-5]. Nickel deposit ion was p e r f o r m e d with intense agitat ion ( s t i r r e r speed 500-800 rpm) in a solution containing 25 g / l i t e r NiCI 2 �9 6H20 , 25 g / l i t e r NaH2PO4, 50 g / l i t e r Na4P2OT, and 20 ml /Li te r NH4OHconc" at pH 10 and a t e m p e r a t u r e of 18-25~ PZhCh-1 pure reduced iron powder, to TU 1 4 - 127--90--77 technical specif icat ion, and e lec t ro ly t i c iron powders produced under l abora to ry conditions by the e l e c t r o l y s i s of aqueous solutions were chosen fo r investigation.

The pa r t i c l e s ize of the powders co r responded to that of a f rac t ion pass ing through a No. 005 s ieve (with an ave raged size of 25 tim). Weighed s a m p l e s of the powders were f i r s t pickled in 1% HC1 solution and washed until the w a t e r gave a neut ra l reaction~ To prevent powder lo s ses due to s t i r r ing up during the washing, a magnet ic base was employed. Aider nickel plating the powders were ca re fu l ly washed f ree f rom the solution, dewatered with acetone, and dried in a d e s i c c a t o r at 40~ The degree of nicke] ex t rac t ion f rom the solution was a s s e s s e d pho toco lo r imet r i ca l ly by the change in the intensity of co lor of nickel in a solution with dimethyl g lyoxime. The nickel contents of powder s a m p l e s were de te rmined by weighing the l a t t e r before and a f t e r nickel deposit ion. The dis t r ibut ion of nickel on the par t ic le su r faces was studied with an MIM-7 meta l lographic m i c r o s c o p e . Th i s involved p repa r ing m a s s e s containing meta l powder s amples and polymethyl me thacry la te in the ra t io 1 ." 10. The m a s s e s were p r e s s e d into g lass tubes and po lymer i zed in boiling water , a f t e r which s t an- dard mic ro sec t i on s were p repa red . Before being examined, the mic rosec t ions were lightly etched with 4% HNO 3.

The r e su l t s obtained were employed for plotting cu rves showing the effect of plating t ime on the nickel contents of the powders (Fig. la) . With increas ing plating t ime , the ra te of the p r o c e s s dec reased , indicating s lower en r i chment of the powders in nickel (curves 1-4). It will a lso be seen that, as the specif ic charge weight grew, the t ime requ i red to r each this condition was shor tened (curves I t and 1).

30

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,• , , / , , : , ~ S . : x 2 - - - - , 2 f f eo i , / m ~

, ~'/_, ')"~.. ,-~ .~ 50

20 a

io

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,r

b ~x

t5 ~n t5 rain 5 r 5o #5 rain

Fig. 1. Var ia t ion of nickel content of powder (a) and yield of nickel f r o m solution (b) with plating t ime and powder charge weight: 1, I t) 100; 2, 2 w) 50; 3, 3 t) 20; 4, 4 t) 10 g / l i t e r . I ron powders: 1-4) reduced; lY-4 t) e lec t ro ly t ic .

D. I. Mendeleev Moscow Chemicotechnological Insti tute, Novomoskovsk Branch. Trans la ted f rom P o r o - shkovaya Metal lurgiya, No. 5(197), pp. 15-17, May, 1979. Original a r t i c l e submit ted July 11, 1978.

0038-5735/79/1805- 0295507.50 �9 Plenum Publishing Corpora t ion 295

Fig. 2. Distr ibution of nickel on iron par t ic le su r - faces , • 340.

As e l ec t ro l e s s nickel-plat ing solutions are not ve ry stable, b reak down fa i r ly readi ly and a re difficult to co r r ec t , it is best to use them only once, ensuring a high degree of nickel extract ion. In our work the yield of nickel f rom the solution grew with increas ing specif ic charge weight and p roce s s t ime (Fig. lb). Examination of the distr ibution of nickel on the par t ic le sur faces made it possible to de termine conditions ensur ing full and even coating with nickel at a maximum ext rac t ion of meta l f rom the solution. At a charge weight of 100 g per l i t e r of the solution (curves 1 and 1'), in 30 rain 95-98% of then icke l in the solution was t r a n s f e r r e d to the pow- der . M i e r o s t r u c t u r a l examinat ions demonst ra ted tha t plating for 15-20 rain at this specific charge weight, a f te r which the powder had a nickel content of 7-8%, resu l ted in the format ion of even nickel coatings of 2- to 3 -#m thickness on the par t ic le su r faces (Fig. 2). Under these conditions the yield of nickel f rom the solution was 90-92%.

The p roce s s of e l ec t ro l e s s nickel plating was found to be affected also by the surface condition of the powder being t r ea ted . With the reduced powder, the amount of nickel in the powder and its yield f rom the solu- t ion were lower, o ther things being equal, than in the plating of the e lec t ro ly t ic powder (Fig, la and b). The reason for this was that in e l ec t ro l e s s nickel plating an important part is played by the catalyt ic action of the metal surface . A cata lyt ical ly active surface promotes the decomposit ion of the hypophosphite by water . A s a resu l t of this reac t ion e l ec t rons a r e l iberated, which reduce nickel ions to the metal . Elec t ro ly t ic i ron powder, being pu re r than reduced powder, apparent ly has g r e a t e r ca ta ly t ic power. As a resul t , the percentage amount of nickel is slightly h igher in e lec t ro ly t ic than in reduced powder.

C O N C L U S I O N S

A study was made of conditions ensur ing full and even coating of i ron powder par t ic les by nickel at a maximum ex t rac t ion of the metal f rom an e l ec t ro l e s s plating solution. The yield of nickel f rom the solution is h igher with e lec t ro ly t ic than with reduced iron powder.

L I T E R A T U R E C I T E D

1. I . M . Fedorchenko, "Scope and tasks of powder metal lurgy in the formulat ion of new mater ia l s , n Poroshk. Metall . , No. 1, 3-6 (1963).

2. Yu. A. Sidorenko and P. I. Kukin, "Phospho rus - f r ee e l ec t ro l e s s nickel deposits on powders," Poroshk. Metall. , No. 2, 29-31 (1977).

3. V . I . Pavlenko and D. S~ Yas' , "The copper plating of graphite powders in the manufacture of c o p p e r - graphite mate r ia l s , " Poroshk. Metall . , No. 2, 9-13 (1976).

4. N . L . Kctovskaya, A. V. Pomosov, and L. M. Kurvyakova, NSilver plating of coarse copper powder, n Poroshk. Metall . , No. 8, 16-19 (1971).

5. S .A . Vishenkov, Chemical and ChemicothermalMethods of Depositing Metal Coatings [in Russian], Mashinostroenie , Moscow (1975).

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