Electrodeposition of metallic ions from the electrolyte at the cathode is Electrowining or Electroplating. For example in case of Cu, the bath may be an acidic solution of high conc. of Cu++ ions , say CuSO4 . The cathode may be pure Cu plate or other non corroding metal . The anode is inert electrode like carbon or lead . The reactions are
At cathode Cu++ 2e= Cu E0= 0.34V
At anode H2O =2H+ +1/2O2 +2e E0=1.23V
The fig. at the left shows how O2 is evolved at anode and acid is produced due to anodic reaction. The cell potential can be determined from Nernst equation ,using above E0 values and activities of ions. For electroplating of very active metals like Al /Mg fused salt bath is taken , as in aqueous solution H2 is simultaneously evolved since reversible potential of H2 is much more noble than that of those metals.
Electrorefining is a technique of removing impurities from impure metal by making it as anode wherefrom the required metal is ionized into the electrolyte and redeposited electrolytically at cathode. For example impure Cu is taken as anode and pure Cu is plated at cathode as shown in the Fig. left). Cu++ ions diffuse from anodic site towards cathode and with time Cathode increases in dimension while anodes becomes thinner and thinner till it reduces to anode slime which contain a lot of precious metals like Pt, Au, Ag etc. which do not ionize into the electrolyte.
Electroless coatings are produced by the controlled chemical reduction of metallic ions onto a catalytic surface, the deposit/coating itself is catalytic to the reduction reaction and the reaction continues as long as the surface remains in contact with the bath solution or the solution gets depleted of solute metallic ions. The coating is uniform throughout the contours of the substrate because no electric current is used. Therefore, all parts of the surface area of substrate which are equally immersed in the bath have equal probability of getting ions deposited. Mainly two types of baths have been used : acidic and alkaline baths. Reducing agents used in electroless coating of nickel bath include sodium hypophosphite, amineboranes, sodium borohydride, and hydrazine. More than 70% electroless nickel is deposited from solutions reduced by sodium hypophosphite.
Electrochemical mechanism, where catalytic oxidation of the hypophosphite yield electrons at the catalytic surface which in turn reduces nickel and hydrogen ions is illustrated below:
H2PO2- +H2O = H2PO3- +2H+ +2e
Ni++ + 2e = Ni
2H+ +2e =H2
H2PO2- +2H+ + e = P +2H2O
Atomic hydrogen mechanism, where atomic hydrogen is released as the result of the catalytic dehydrogenation of hypophosphite molecule adsorbed at the surface is illustrated below.
H2PO2- +H2O = H2PO3-- +H+ 2Hads
2Hads + Ni++= Ni +2H+
H2PO2- + Hads = H2O + OH- + P
It is possible to electrodeposit two or more metals say Cu and Zn simultaneously to form an alloy,brass by maintaining a proper cell potential and current density. Cu++ and Zn++ ions of the electrolyte will get reduced at cathode. The proportion of Cu and Zn in the alloy deposition is dependent on ratio of cathodic current densities at the applied cell potential. The current density itself is a function of so many parameters, such as Tafel's slope, exchange current density , reversible potential which is again dependent on activities of the respective ions as determined by Nernst equation. So the nature of cathodic polarization curves of those two systems is the key to determination of process parameters for the alloy deposition.
If A is nobler to hydrogen electrode but B is not , co-deposition of hydrogen gas may cause porosity of the alloy formed unless hydrogen overvoltage is very high to retard gas evolution.
Anodizing is an electrochemical process of forming protective thin film of oxide layer by anodic polarization of the metals to be anodized such as Al, Ti ,W etc. in acidic baths viz. sulfuric acid, phosphoric acid , oxalic acid or mixture of these acids under certain cell potential. Cathode can be any inert electrode of Pb, graphite, Ni , stainless steel. Aluminum is unique among these metals in that, in addition to the thin barrier oxide, anodizing aluminum alloys in certain acidic electrolytes produces a thick oxide coating, containing a high density of microscopic pores. This coating has diverse and important applications including architectural finishes, prevention of corrosion of automobile and aerospace structures, and electrical insulation. When the circuit is closed, electrons are withdrawn from the metal at the positive terminal, allowing ions at the metal surface to react with water to form an oxide layer on the metal. The electrons return to the bath at the cathode where they react with hydrogen ions to make hydrogen gas. The reactions are
At the metal/oxide interface 2Al + 3O2- ==> Al2O3 + 6e (1)
At the oxide/electrolyte interface 2Al3+ + 3H2O ==> Al2O3 + 6H+ ( 2)
the anodic reaction 2Al ==> 2Al3+ + 6e (3)
The reaction at the cathode 6H+ + 6e- ==> 3H2 (4)
the overall reaction 2Al + 3H2O ==> Al2O3 + 3H2
The sealing reaction Al2O3 + 3H2O ==> 2AlOOH*H2O
It is seen from the reaction (2) that a portion of anodized oxide formed goes back into the solution , giving rise to pores on the oxide film. By controlling the above reactions different percentage of porosity can be generated. Pore size can be reduced to Nano range by a optimum combination controlling parameters : concentration, temperature, potential and current density.
Prepared by S Paul