Addition Agents in Electroplating Processes

Dr. R. J. Patolia SHREE HARI GROUP OF INDUSTRIES

09 May 2020

• They are absorbed on cathode surfaces during plating process to form a monolayer film and works as a barrier to diffusion of metal ions to cathode surface and there by control the plating rate to get fine uniform deposition.

• They are absorbed strongly on cathode surface at the high current density region thus lowering the plating rate at peaks by polarization and simultaneously increase the plating rate at valleys due to low polarization.

• They get attached to the metal ions in electrolyte and make the deposition process easier, control grain structure and increase brightness, hardness and other properties of the deposit.

• They lowers the surface tension of the Electrolyte at cathode surface and release the gas molecules from the cathode surface to reduce the tress and hydrogen embrittlement of the deposit.
• The wetting agents required are comparatively very high in case of acidic bath – such as Acid Zinc, Acid Tin and Acid Copper baths, while the brightening agents are very less because only a mild polarization is required to get levelled and bright electro deposits.

• To improve colour, brightness of the deposition at low current density area, specific metal chelating agents are used as a purifier to control or stop diffusion of unwanted trace metals. Eg., Cu in Ni. Cu, Pb, As in Zn.
• Now we discuss some plating processes with respect to different addition agents used during electro deposition of metals.

• Highly acidic and strong oxidizing nature of the bath react as, if you add any organic substance, it gets oxidized and lost totally. For brightness, fine grainment only Sulphuric Acid, Fluorosilicic Acid (H2SiF6 ) or Methane Sulfonic Acid are used for getting uniform, hard, bright deposits, plating efficiency is only 12 to 27% maximum, while rest current will utilized for hydrolysis of water to generate Hydrogen at cathodes and Oxigen at anodes surfaces. Accumulation of Hydrogen may leads to explosion. For fumes or mist suppressants Perfluoro Sulfonate are used.

• Highly Acidic Bath, having free Sulphuric Acid from 50 - 110 gm/liter. Only high molecular weight polyethylene Glycol (PEG) from 6000-20000 grade are used as a carrier and stable simple Thiosulfonic Acid derivatives such as SPS, DPS, ZPS, MPS etc. are used as a brightener.

• Acidic bath, where short chain Non-Polar wetting agents such as Octylpenol, Nonyl Phenol EO condensate are used. Cresols, Napthols Sulfonic Acid are used as a carrier. OCB, BAR are used as a Brightener.

• As a wetting Agents OP, NP or Beta Naphthol EO-20 Condensate are used. SB, DEG, IPA, TEMMOL (Phenol-Formaldehyde condensates) are used as a carrier OCB or BAR are used as a Brighteners.

• Polyhydroxy organic Acid or Phosphoric Acid are used as a Metal  complex formation, to slowdown the plating process. Methane Sulfonic Acid or Puridine-3-Sulfonic Acid are also used as a complexing agents to make process very slow and it result into fine grainment. For increasing the hardness of the deposit, cobalt, Nickle or Indium Complexes are used in required proportion. For more brightness of the deposit amines such as Diethlene Triamine (DETA), TETA, TEPA are used.

• This is stable, near to neutral bath of working pH 3.8 – 5.2. Nickel Sulfamates, Nickel Sulphates, Nickel Chlorides and Boric Acid are used for bath preparation. Wetting Agents like sodium Lauryl Sulphate and or Sodium 2-Ethylhexyl Sulphate are used in very less concentration of 2-5 mg/liter of bath. The chemistry of Nickel plating process is totally different from the other processed and it is used 65 to 75% as compare to all other processes.
• Unsaturated reactive organic derivatives with double or triple bond and or their sulphonate or mono-bis ethylene /propylene oxide condensates and Betaines containing positive and negative charged molecules are used as a brighteners.