Reactions, mechanisms, mto, bath life ...
"electroless nickel lives on it ..."
The chemically reductive course takes place in most cases under application of temperatures between 70 ° and 93°C as well as pH fields of 4,2 to 6,5 . Next to these conditions the electrolyte must be balanced regarding the content of nickel and reducing agent (sodium hypophosphite) and also regarding the concentration of stabilizer, accelerator and complexing agent. On condition of one cleanly pretreated and active surface as well as plant technical necessities we have three relevant reactions :
NiSO4 + NaH2PO2 + xH2O --> Nio + 2Hads + 2H+ + SO42- + NaH2PO3
2NaH2PO2 + 2Hads --> NaH2PO3 + P + NaOH + xH2O
3NaH2PO2 + NiSO4 + xH2O --> Nio + P + 2H+ + Na+ + SO42- + 2NaH2PO3
The decive reaction is the reduction of the nickel ions by the adsorbtive (atomic) hydrogen. The faster this course takes place, the fewer phosphorus is built into the layers. On the contrary, the slower this process, the more phosphorus is in the deposited layers. Phosphorus contents between 2 to 14 % are possible and characterize different properties.
Also important :
During the reactions the pH value decreases by constant formation of H+ ions so that with alkaline media as hydroxide or carbonate or ammonia the pH-value is kept in nominal value.
From necessary recoveries the salt products sodium or potassium sulfate as well as sodium orthophosphite result. The deposition of 1g nickel produces approx.4 g/l sodium orthophosphite. Does an en bath e.g. have 7 g/l nickel so after 6 mto approx. 70 g/l sulfate and approx. 150 to 180 g/l orthophosphite are produced.
Increasing quantities of salt products worsens the layer characteristics. Therefore electroless nickel solutions have a limited life - " bath life ". After approx. 5 to 9 mto a new make-up is carried out.
The operator can influence the speed defining parameters like temperature and pH value.
Topical notice : now is it possible to work without nickelsulfate, without nickelchloride, without nickelacetate. Look on the main page -> 15 mto and more.
Corrosion protection: The special feature of the corrosion protection of Ni-P-layers is based on the proportion of the phosphorus installation. Contrary to electrolytic layers lattice dislocations do not cause grain boundaries.
Type of bath: As a function of the desired phosphorus installation rate, one differentiates between fast-depositing types and so-called " high phos" systems. The required layer characteristics decide on the suitable process selection.
MTO: " metal turn over ", operates e.g. an EN bath in the debit with 7 g/l nickel and these in the context of the replenishment once is renewed, corresponds that to 1 mto.
"bath life": Between start and end one EN bath operates approx. 5 to 9 mto.
Litre charge: In EN baths should be available surfaces between 0.2 to 3.0 dm2/l . Being under it bad branching to behavior, over it reduction of the deposition speed.
pH correction: During the operation with alkaline solutions necessarily.
Deposition speed: As mentions under " type of bath ", fast-depositing baths are working approx. 15 to 25 µm/h. In order to achieve phosphorus contents over 10 %, fewer than 12 µm/h must be kept .
Replenishment: Nickel and reduction solution in many cases in the ratio 1:1. The nickel solutions contain usually 70 to 100 g/l Ni, the reduction solution 350 to 500 g/l sodium hypophosphite. In addition appropriate quantities of complex chemicals on base of different carbonic acids. Furthermore accelerators and stabilizers.
Phosphorus content: Depending upon deposition speed and suitable process between 2 to 14 %.
Further storages: Likewise in dependency of desired layer characteristics additionally hard materials like silicon carbide or boron carbide or diamond can be deposited with. Further teflon to approx. 20 %.
Transition of pressure to tensile strengths: Occur in Ni-P-layers also over 10 % P after 4,5 mto - new processes more than 10 mto, with fewer than 8 % P after already 2 to 3 mto.