Electroless Nikel - Data Sheet

The principle of the catalitic deposition of nickel was discovered in 1845 by Wurtz who noticed that a solution of sodium hypophosphite made nickel fall from its salt. Further studies made by Roux many years after led in 1916 to issue a patent. But in this bath, once the reaction started,
it was uncontrollable and nickel deposited on walls and bottom of the container. Other research were made around 1940 in USA by Brenner and Riddel. Making an electrolytic deposition of nickel into a tube with an insoluble anode, they found a catodic yield superior to 100% when to the solution was added sodium hypophosphite and a deposition of nickel also to the external.
They deducted that, besides the electrolytic deposition, it occurred also a chemical deposition. We have to wait still some years to see processes at industrial level thanks to the studies of General American Transportation who in 1950 defined a process called Kanigen.
Since then other companies dealt with the new treatment and the baths were improved obtaining better speed of deposition and better characteristics of resistance to wear and tear and to corrosion. In Europe and Japan we have to wait till 1960 to talk about a true business of electroless nickel.

There are many applications of electroless nickel, either for the variety of interested industrial sectors, or for the different materials that can be treated. Last statistics say:

70%: ferrous metals
20%: alluminium and its alloys
  6%: special and stainless steels
  4%: ceramics and plastics

The tabel below shows the use of electroless nickel in the greatest countries in 1987:

WORK PROCESS
The process of chemical reduction of nickel salts is of autocatalytic nature, where reduction to metal is caused by the reaction between the sodium hypophosphite NaH2PO2* H2O and the objects soaked into the nickel bath which act as catalytic agents according to the following reaction

Ni⁺⁺ + H₂PO₂ + H₂O ---------> Ni + H₂PO^-₃+ 2H⁺

The iones hypophosphite in water solution are catalytically oxidized to iones phosphite with generation of hydrogen and at the same time the nickel cations are catalytically reduced to Ni metal by the sodium hypophosphite iones.
The result of reaction is the forming of a layer on iron, aluminium, copper and their alloys pieces, of a nickel-fosforo alloy, in which the content in fosforo depends on the fosforo concentration in the solution and on its PH. The bath goes on enriching in iones fosfito deriving from the oxidation of iones sodium hypophosphite, its deposition speed gradually reduces and nucleus phosphite form which are responsible of the deposit roughness and of the bath decomposition.

Besides nickel salt and sodium hypophosphite the bath is composed by:

- complexants: they stop a part of nickel iones and slow the precipitation of subproducts of reaction (hydroxy organic acids)
- stabilisers: prevent solution breakdown (heavy metal salts or cyclic compounds)
- acceleration agents: accelerate deposition (aliphatic dicarboxylic acids)
- wetting agents: increase wettability of surfaces to be coated help the detach of hydrogen bubbles from the pieces (mixture of cationic and anionic surfactants).

The bath, contained into a shaking stock tank, is taken by a pump that takes it to a filter, a heat exchanger controlled by a thermometric drill and to the tank into which the deposition occurs.
The tank has got a flinging pump that brings the bath again to the stock tank after cooling. Into the tank we continually add nickel salts and sodium hypophosphite which mantain the bath at the best concentration for deposition. Moreover we regulate PH and add the necessary quantity of stabilizer.
The pieces to be nickeled must be perfectly scoured and pickled before coming into the nickel bath. The deposited thickness is perfectly humiform on every part of the piece due to the soaking time. Once the desired thickness is reached, the pieces are washed, dried and put in oven at 200°C for 2 hours.
This thermic treatment, said dehydrogenation or degassing, is performed to remove hydrogen which is formed in the interface metal-coating and has the effect of improving the adhesion of the deposit, and reducing the hydrogen embrittlement of the material.

Scheme of one part of electroless nickel system

A: stock tank
B: pumps
C: filters
D: heat-exchanger
E: nickeling tank
R: regenerators

PROPERTIES OF NICKEL PLATING AND DEPOSIT UNIFORMITY
As this is a chemical process, the precision of nickel deposit is ±5% on the final thickness. This means that on a deposit of 20p thick we have a tolerance of 1p thus avoiding need to rectify the piece.

“NO EDGE BUILD UP”
Electroless nickel avoids the typical effect of the electrolytic treatment therefore pieces do not need to be refined after the treatment.

ADHESION AND TENSILE STRENGHT
The excellent adhesion of the deposit on surface can follow the eventual deflexion and expansion. Soliciting the piece to fracture does not occur any exfoliation.

RESISTANCE TO CORROSION
Test in salty fog show a better behaviour of the electroless nickel compared to electrolytic one, either for the low porosity or for its content in phosforus. The resistance to corrosion is very good in alcalyne, non ammonic, environment and also with most of organic acids in non-aerated environment, medium in inorganic acids and very low in cloridic acid in aerated environment.

RESISTANCE TO CORROSION OF ELECTYROLESS NICKEL
IN DIFFERENT SUBSTANCES

SUPERFICIAL HARDNESS
The thickness of deposit is influenced by its content in phosphorus and by the thermic treatment to which it undergoes. Non termically treated deposits have a shapeless structure with alterned more or less rich in phosphorus layers.
Their superficial hardness is of 500-550 HV (around 50 HRC). With a rise in temperature there is a gradual structure alteration that increases hardness till 1000-11 OOHV according to the content in phosphorus.
Maximum hardness is obtained with pre-heating at 290°C for 10 hours or at 400°C for 1 hour. A further rise of temperature causes a reduction in hardness.

Figura 2

Next figurere presents the relationship of superficial hardness of recooked at 400°C compared to other types of treatment.

Figura 3

Figura 4

The behaviour of electroless nickel in comparison with other types of treatment in Fig.5

Figura 5

WELDABILITY AND FOLLOWING TREATMENTS
Surfaces treated with electroless nickel are easily solderable. For next treatments the electroless nickel represents an ideal substrate for processes of gold and silver plating.

MULTIPLICITY OF TREATABLE MATERIALS
Electroless nickel can be deposited on almost all the ferrous alloys, cast irons and steels, stainless steels, alluminium and its alloys. Moreover it is applicable also to thermoresistant plastics to which it confers conducibility. The materials on which it is not possible to deposit electroless nickel are: lead and its

APPLICATIONS
The pieces treated with electroless nickel acquire excellent chemical-mechanical characteristics therefore it becomes possible to substitute esteemed materials as inox steel, copper, brass, with very cheaper and more malleable raw materials. Work cycles are thus optimized obtaining visible technical and mechanical advantages even for small and midium series production.