Plating , anodizing and Thermal spraying - pages93-100

Plating and Anodising
Plating
Typically this is Cr or Ni on steel – thicknesses of tens of microns to several mm plated. For
decoration alone, the plating will be just tens of microns. Silver and gold are also often used for
electroplating – for example silver plated stainless steel cutlery (EPSS) and EPNS. (EPNS stands
for electroplated nickel silver – ‘nickel silver’ is actually an alloy of 60 wt% Cu, 20 wt% Ni and 20
wt% Zn. The plating layer is pure silver).
Hardness values:
Cr 850 – 1250 HV; Ni 400 HV.
In electroplating (electrolytic plating) of Cr and Ni, it is important that the electrochemical
conditions used do not give rise to hydrogen embrittlement. The Pourbaix diagram of the process
predicts H2 production, and so organic additions are made to the electrochemical solution to alter
the nucleation and growth of the coating and limit H2 production.
The metal to be deposited is the anode in the process and the object to be plated the cathode. The
metal of the anode releases ions into the electrolyte and these are then plated out at the cathode.
Electroless nickel plating can also be used. In this process nickel ions from the solution and a
reducing agent (hydrogen phosphate for example) are used together to plate out nickel and
phosphorus (mostly nickel) onto the item of interest.
For Ni and P in the solution, Ni – 10 wt% P coatings have hardness values of ? 500 HV.
For Ni and B in the solution, Ni – 5 wt% B coatings have hardness values of ? 700 HV.
Both electroplating and electroless plating can be adapted to include second phases in the deposited
film (e.g. hard diamond, alumina or SiC particles, or even a solid lubricant such as PTFE). In this
way the coating can be engineered to have good wear characteristics or low friction.
Copper is often used as a thin undercoat for the plating of other metals such as nickel or silver.