ELECTRO-PLATING: BASICS - MicroChemicals

1 Chapter01 MicroChemicals Fundamentals of of : BASICSThe following section would like to explain the physical and chemical basis for electroplating to the extent as it is useful for understanding the following Metal PotentialA Metal in Aqueous SolutionIf a metal is dipped in an aqueous solution (a salt solution, a diluted acid, or only water), some of the metal goes into the solution as positive ions, where the metal is negatively charged by the electrons re-maining left (Fig. 127).The system consisting of metal and solution strives for minimally free enthalpy H = U - T S (U = internal energy, T = temperature, S = entropy). The change in the internal energy aff ects both the lattice energy of the metal atoms required when leaving the solid state and the energy released during the hydration of the dissolved , the greater degree of freedom (= increase in the entropy) of a metal ion in solution previously bound in the solid state, and the stronger localisation (= decrease in the entropy) of the previously freely mobile water molecules bound to the metal ion during hydration, contribute to the entropy two phase transitions, the dissolution of the metal atoms and their replacement in the solid state, remain in equilibrium.

2 The potential diff erence between the dissolved phase and the solid state in the equilibrium state is called metal potential. Physical Description: The Nernst EquationThe Nernst equation can be described as the function of the metal potential E of certain environmental conditions:Where E0 is the standard electrode potential (see next section), T is the temperature, R is the molar gas constant, F is the Faraday constant, ze is the degree of ionisation of the dissolved metal atoms, and CM+ is the concentration in metal potential increases accordingly with the temperature (by more powerful thermally activated diff u-sion of the hydrated metal ions, they can go more away from the electro-chemical double layer in solution), as well as - for the same reason - with the concentration of dissolved metal ions. The increase in the metal po-tential is weaker in metals with a higher ionisation stage since, from the electro-chemical point of view, a dou-ble-charged metal ion builds up the electro-chemical boundary layer to the same extent as two single-charged ions.

3 ()++=MeCFzRTEEln0 Fig. 127: Formation of an electro-chemical double lay-er on the surface of a metal dipped in an aqueous solu-tion: Atoms from the metal go as positive ions hydrated in solution, the negatively charged electrons (blue) re-main in the metal or on its (elementary)Hydrated metal ionsNegatively charged boundary layerAqueous solution Chapter01 MicroChemicals Fundamentals of of The Electropotential SeriesTheoryThe metal potential between the negatively charged electrode and the electrolyte enriched with positive metal ions is not measurable itself, however the potential between two diff erent electrode potentials order to be able to compare the relative size of the metal potentials of diff erent metals, a measure-ment with a counter-electrode is carried out under standard conditions (25 C, kPa, one-molar solu-tion of the metal under investigation). A platinum electrode bathed by H2 serves as a counter-electrode, on the catalytically active surface of which the H2 gas dissociates into atomic hydrogen, which forms an atomic layer (normal hydrogen electrode) on the Pt Potentials and Their ImportanceIf such a measurement is carried out under standard conditions for certain elements by (Fig.)

4 128), its standard potentials are listed in Table higher the standard potential of a metal the more noble the metal: Metals with a positive standard potential are, by defi nition, noble metals, which are not attacked by acids (except oxidising acids). Base metals with negative standard potential dissolve in acids under H2 Electropotential Series of Neutral ElementsThe following table lists the standard potential of several elements or electrode reactions. The more neg-ative the standard potential, the less noble the reac-tionNormal potential (V)ElementElectrode reactionNormal potential (V)LithiumLi Li+ + Tin (IV)Sn Sn4+ + 4e-+ PotassiumK K+ + AntimonySb Sb3+ + 3e-+ Ba2+ + ArsenicAs As3+ + 3e-+ StrontiumSr Sr2+ + CopperCu Cu2+ + 2e-+ Ca2+ + CopperCu Cu+ + e-+ Na+ + Iodine2I- I2 + 2e-+ Mg2+ + SilverAg Ag+ + e-+ Al3+ + PalladiumPd Pd2+ + 2e-+ Mn2+ + MercuryHg Hg2+ + 2e-+ 128: Diagram of the determination of the standard potential of a metal (in this example, copper, left basin) relative to a standard hydrogen electrode (right), which is a platinum electrode in diluted hydrochloric acid bathed with electrodePt electrodeH2H2 HCl1-molar CuSO4 solutionIon bridge Chapter01 MicroChemicals Fundamentals of of Zn2+ + PlatinumPt Pt2+ + 2e-+ Cr3+ + Chlorine2 Cl- Cl2 + 2e-+ (II)Fe Fe2+ + GoldAu Au3+ + 3e-+ Ni2+ + GoldAu Au+ + e-+ Pb2+ + Fluorine2 F- F2 + 2 e-+ H+ + e- Table 7: The standard potential of several elements or electrode reactions.

5 Because measurement is done in the reference to a hydrogen electrode, its standard potential is 0 ExchangeDip Deposition: A Metal in the Salt Solution of a Noble MetalIf a less noble metal such as iron (see diagram below) is dipped into an aqueous solution of a metal salt ( copper as a CuSO4 solution), atoms are fi rst dissociated from the iron as ions in solution (Fig. 127). For each dissolved Fe2 + ion, two electrons in iron remain, which bond with a Cu2 + ion from the solution to Fig. 129: The charge exchange between two metals (the less noble metal in a solution of the nobler metal) with diff erent standard potentials is carried out until the nobler metal has formed a coating on the less nobler metal which prevents further dissolution of the less noble + onsCu2 + and Fe2 + ionsCuSO4 solutionIronIron with copper coatingFig. 130: In the case of a metal powder (shown as blue) in the solution of a nobler metal (represented in gold colours), this replaces the less noble metal through charge exchange.

6 Chapter01 MicroChemicals Fundamentals of of copper. This copper deposits as a thin fi lm on the iron, preventing the further transfer of iron ions in solution and ultimately stopping the process of charge exchange between iron and currentless mechanism of the charge exchange is applied on an industrial scale such as in the cop-per- plating of iron, or the silver- plating of copper or brass in a silver nitrate : A Metal Powder in the Salt Solution of a Noble Metal If the principle of dip deposition is not applied to a metal rod, but to a powder of a metal in a salt solu-tion of a nobler metal, the conversion may be complete : If the size of the metal particles of the powder is comparable to the thickness of the dissolved layer or the thickness of the coating of the nobler metal, the less noble metal particles dissolve completely, and until then serve as substrate for the growth of a particle of the nobler this way, for example, elementary copper can be obtained from a copper solution, or elementary gold from a cyanide gold solution with zinc dust (schematically shown in Fig.)

7 130. The Galvanic CellTwo Metals in An AcidIf, as shown in Fig. 131, two diff erent metals (one less noble than the other, iron and copper in this exam-ple) are dipped in an acid (in this example sulphuric acid), this is referred to as a galvanic element. From the time of dipping in the acid, the following electro-chemical reactions take place:From the electrode with the less noble metal, iron in this example, iron ions are converted into solution via Fe Fe2 + + 2e-. The two electrons migrate through the electrical connection of both electrodes to the nobler copper electrode, where they form neutral hydrogen via 2 H3O+ + 2e- 2 H2O + H2, which rises in gas form. The reaction does not come to a standstill until either the iron electrode or the acid is long as the reaction takes place, the voltage measurable between the two electrodes is the diff erence between the standard potentials of both metals (approximately V in this example), which is the basic principle of a undesirable eff ect of the principle of galvanic cells is electro-chemical corrosion: If components made of, for example, iron and aluminium, or iron and copper are electrically conductive, the less noble metal begins to corrode as soon as the components are wetted by ( rain) 131: A galvanic cell con-sisting of two connected electrodes of diff erent met-als in an acidic currentH2 formationSulphu-ric acidFe2 + in solutionH3O+ ions Chapter01 MicroChemicals Fundamentals of of Metals in the Salt Solution of a Third MetalIf two electrically conductive, diff erently noble metals ( iron and aluminium in Fig.)

8 132) are dipped in the aqueous solution of a salt of a nobler metal ( copper), iron as well as aluminium begin to dissolve in solution. In accordance with the principle of the charge exchange, elemental copper deposits on both aluminium, which is less noble than iron, is thus faster to dissolve, which would lead to a more rapid charge of the aluminium. This potential diff erence causes an electron fl ow from the Al to the Fe electrode. Once there, the electrons can neutralise Cu2+ ions so that the copper fi lm grows even further on the iron when the iron is already copper-plated over the whole process ends when the aluminium is coated over the entire surface by copper. In this state, no fur-ther Al ions can dissolve, correspondingly no further e- can fl ow to the iron and neutralise Cu ions. For a technical application of the copper- plating of metals, the less noble metal is thus provided with a much larger nitions: Electrodes, Electrolyte and ElectrolysisThe previously treated electro-chemical processes between metals and solutions, such as the formation of a metal potential, the charge exchange or the processes in a galvanic element, take place without an external voltage or current the metals dipped in solution are connected to a voltage source, they act as electrodes (negative cathode or positive anode).

9 The liquid (an aqueous salt solution, a diluted acid or base), which functions as an ion current conductor between the electrodes, is called electrolyte. The resulting electro-chemical processes and those which are treated in the following sections on the surfaces of the electrodes are designated as 1: Electrolysis of Hydrochloric Acid with Insoluble AnodeDiluted hydrochloric acid as a strong acid is almost completely dissociated in the form of oxonium (H3O+) ions and chloride (Cl-) ions. If an electrode pair, for example, platinum or graphite, which is connected via a voltage source, is dipped in such a solution and a direct voltage is applied to the latter, two reactions take place (Fig. 133):The negatively charged chlorine ions release their electrons on the positive anode and are converted to neutral chlorine gas via 2 Cl- 2e- + Cl2. On the negatively charged cathode, the oxonium ions are con-verted via 2 H3O+ + 2e- 2 H2O + H2 to water and hydrogen gas.

10 The process only ends when the hydro-chloric acid is 132: Diagram of the copper- plating of iron with the help of aluminium in a CuSO4 solution. Left: Immediately after the dipping of the metals in the solution; right: The process stops when the aluminium is copper-plated over the entire currentThin Cu fi lmThick Cu fi lmCuSO4 solution Chapter01 MicroChemicals Fundamentals of of 133: Diagram of the electrolysis of hydrochloric acid: After applying a DC voltage to the electrodes in an electrolyte of diluted hydrochloric acid, the H3O+ ions migrate to the cathode, where they are neutralised to H2, the Cl- ions to the an-ode where they form chlo-rine (Cl2).Platinum electrodePlatinum electrode- +H2 formationCL2 formationH3O+ ionsCl- ionsExample 2: The Daniell ElementThe Daniell element, which is to be listed here as a historical example of a galvanic element, consists of two electrolytes connected via an ion bridge: One with a solution of zinc sulphate, into which a zinc electrode is dipped and one with a solution of copper sulphate, into which a copper electrode is dipped.