The Main Source Of Nickel Ions

Published Date: 02 Nov 2017

CHAPTER 2

Faraday's Law Michael Faraday (1791-1867) was an English scientist who conducted research in the areas of chemistry, electricity and magnetism. Faraday formulated several laws but the one that concerns us most is his law regarding the relationship between current, time and the weight of electroplated metal. Plating is governed by Faraday's Laws that state the weight of a substance formed at an electrode is proportional to the amount of current passed through the cell. The weights of different substances produced at an electrode by the same amount of current are proportional to their equivalent weights (Electrolytics, Inc. 2004).

W = (I*t*A)/(n*F) Equation 2.1

where:

W = weight of plated metal in grams.

I = current in coulombs per second.

t = time in seconds.

A = atomic weight of the metal in grams per mole.

n = valence of the dissolved metal in solution in equivalents per mole.

F = Faraday's constant in coulombs per equivalent. F = 96,485.309 coulombs/equivalent.

The following illustration will describe how Faraday's Law can be used. Consider nickel electroplating. The electrochemical reaction at the cathode will be: 

Ni2+ + 2e- = Ni Equation 2.2

Equation 2.2 indicates that nickel ions in solution will plate out as nickel metal on the cathode when two electrons per nickel ion are passed into solution. If current of 1 ampere (1 coulomb/second) pass through the cathode for one hour (3600 seconds), the deposited nickel mass is: W = (1*3600*58.69)/(2*96485.309) = 1.09 grams (Equation 2.1). Usually though, we are more interested in the thickness of the electroplated metal. We can calculate this thickness using the following equation:

T = (W*10000)/(ρ*S) Equation 2.3

where:

T = thickness in microns.

ρ = density in grams per cubic centimeter.

S = surface area of the plated part in square centimeters.

10,000 is a multiplicative constant to convert centimeters to microns.

If Equations 2.1 and 2.3 are combined we have the following equation for plated thickness. 

T = (I*t*A*10000)/(n*F*rho*S) Equation 2.4

If we apply Equation 2.4 to our earlier example of nickel plating we find that the thickness of plated metal is: W = (1*3600*58.69*10000) / (2*96485.309*8.90*22.1) = 55.67 microns. Here we have assumed that the surface area of the part is 22.1 cm2.

Notice the importance of surface area in the equation. If all other parameters are kept the same, the plated thickness will increase as the surface area is decreased and vice versa.

The plating rate can be calculated by eliminating time from Equation 2.4 like so:

R = (I*A*600000)/(n*F*rho*S) Equation 2.5

where:

R = plating rate in microns per minute.

600,000 is a multiplicative constant used to make R come out in units of microns per minute.

Applying Equation 2.5 to our nickel plating example we get: R = (1*58.69*600000)/(2*96485.309*8.90*22.1) microns/minute. This makes sense because if one multiplies 0.9278 microns/minute by 60 minutes (3600 seconds) one gets a thickness of 55.66 microns which matches the earlier calculation.

2.2 Watts Bath

The Watts bath was introduced by Oliver P.Watts in 1916. This bath is the most common nickel electroplating solution. Electroplating operations in a Watts bath is low cost and easy to control. The Watts bath composition, operating conditions and mechanical properties of nickel electroplating given in Table 2.1.

Table 2.1: Watts bath composition, operating conditions and mechanical properties of nickel electroplating (M. Masoudi, 2012).

Electrolyte Composition

Nickel sulphate, NiSO4.6H2O

32-40 oz/gal (240-300 g/l)

Nickel chloride, NiCl2.6H20

4-12 oz/gal (30-90 g/l)

Boric acid, H3BO3

4-6 oz/gal (30-45 g/l)

Operating Conditions

Temperature

105-150 (40-65 )

Cathode current density

20-100 A/ft2 (2-10 A/dm2)

pH

3.0-4.5

Mechanical Properties

Tensile strength

50000-70000 psi (345-485 MPa)

Elongation

10-30 %

Hardness

130-200 HV

Internal stress

18000-27000 psi (125-185 MPa)

The main source of nickel ions is nickel sulphate because it is readily soluble (570g/l at 50ºC), relatively cheap, commercially available and is a source of uncomplexed nickel ions. In nickel plating solutions, the activity of nickel ions is governed by the concentration of nickel salts in solution, their degree of dissociation and concentration of others components of the solution. The presence of chloride has two main effects which are increases the diffusion coefficient of nickel ions and assists anode corrosion, thus permitting a higher limiting current density (Sònia Albert Calbetó, 2011). Boric acid acts as buffer to control the pH in the cathode solution interface (J. P. Hoare, 1986), also acts as catalyst which lowers the overvoltage for Ni deposition as well as considered as a homogeneous catalyst for Ni deposition in the Watts bath. The pH of the electroplating bath is adjusted to 4.0 by addition of NaOH to the electroplating bath (Tushar Borkar, 2010).

2.3 Metal Matrix Composite (MMC)

A material composite can be defined as a material consisting of two or more physically and chemically distinct parts, suitably arranged, having different properties respect to those of the each constituent.

Since work on Metal Matrix Composites (MMCs) began in the 1950s, researchers tried various combinations of matrices and reinforcements. In the last 20 years, MMCs evolved from laboratories to a class of materials with numerous applications and commercial markets. So as to enhance further the properties of MMCs more than two materials were added in the matrix such that to give birth to hybrid metal matrix composites. MCCs with one or more reinforcement phases are incorporated in a continuous metallic phase (matrix).

The characteristics of MMCs in term of physical and mechanical depend on the nature of the two components, the volume fraction of the adopted reinforcement and production technology. The nature of components included chemical composition, crystalline structure, in the case of reinforcement, shape and size. In general, MMCs utilize at the same time the properties of the matrix and of the reinforcement, usually ceramic. The properties of matrix are light weight, good thermal conductivity and ductility, while the properties of ceramic are high stiffness, high wear resistance and low coefficient of thermal expansion. By this way it is possible to obtain a material characterized, if compared to the basic metal component, by high values of specific strength, stiffness, wear resistance, fatigue resistance and creep, corrosion resistance in certain aggressive environments. However, cause to the presence of the ceramic component, ductility, toughness and fracture to the coefficients of thermal expansion and thermal conductivity decrease (S. Kumar, et al., 2008 & Harish K.Garg, et al., 2012).

2.3.1 Ni-Al2O3 Composite

The Ni-Al2O3 composite has almost a triple level of yield stress and double the hardness compared to pure nickel. These advanced mechanical properties make the Ni-Al2O3 MMC layer has a good wear resistant coat (A. Jung, et al., 2009). Due to the high corrosion and erosion resistance properties, Ni-Al2O3 can be used in a large amount of real applications. Ni-Al2O3 is considered as a superior protection layer for many industrial parts and components such as micro devices, the paddles of gas turbines and mixing blades for abrasive mixtures (H. Zhang, et al., 2008 & A. Jung, et al., 2009).

During the co-electrodeposition process, the presence of alumina particles affects the crystallization of nickel layer. The alumina particles decrease the grain growth and act as nucleation points. The grain size decreases in the presence of alumina dispersed particles and formed a finer crystalline structure under the composite coating process (H. Gül, et al., 2009).

The thermal stability of the Ni-Al2O3 MMC composite better than the pure nickel layer as the dispersed alumina particles prevent dislocations, grain boundary movements and recrystallization process at elevated temperature (H. Gül, et al., 2009). Due to the creation of a finer crystalline structure and increasing hardness, the Ni-Al2O3 composite shows a much higher wear resistance compared to a pure nickel layer (H. Gül, et al., 2009 & A. Goral, et al., 2010).

Figure 2.1: Ni-Al2O3 composite hardness versus current density and alumina content. (Q. Feng, et al, 2007)

A wear resistance test on Ni-Al2O3 composite layer has shown a considerable improvement against pure nickel layer. The particle content of coat increases as the wear resistance increases as shown in Figure 2.1. Since usually hardness and wear resistance have a direct relationship, the effect of current density and other parameters on wear resistance can be explained by considering their effect on the composite hardness (Q. Feng, et al, 2007 & H. Gül, et al., 2009).

2.3.2 Cu-Al2O3 Composite

Copper has been widely used in many industrial applications such as contact supports, frictional break parts, and electrode materials among others. Pure copper has relatively low mechanical properties. Alloying of copper with zinc or tin was used to improve the strength of the pure metal. Grain refinement strengthening or Hall-Petch strengthening was also used as a method of strengthening copper by changing their average crystallite (grain) size (J. R. Weertman. 1999).

Cu-Al2O3 composites have better properties than pure copper and precipitation or solid solution hardened copper. (R.H Palma, et al., 2003 & B. Tiang, et al., 2006). Recently Metal Matrix Composites (MMCs) and Nano MMC reinforced with ceramic particulates offered significant increase in strength over pure copper and their alloys. The strengths of these composites were found to be proportional to the percentage contents and fineness of the reinforced particles (G. B. Veeresh Kumar, 2011). MMCs with a uniform dispersion of particles smaller than 100 nm size exhibit more outstanding properties over MMCs and are termed Metal Matrix Nano-composites (MMNCs). The MMNCs are also assumed to overcome the shortcoming of MMCs poor ductility. It has been reported that with a small fraction of nano-sized reinforcements, MMNCs could obtain comparable or even far superior mechanical properties than MMCs. The main advantages of MMNCs include excellent mechanical performance, feasible to be used at elevated temperatures, low creep rate, good wear resistance, and so on (F. Shehata, 2011).

Dispersion strengthened Cu- Al2O3 composite materials are extensively used as materials for products, which require high-strength and electrical properties, such as electrode materials for lead wires, relay blades, contact supports and electrode materials for spot welding (V. Rajkovic, 2009). Electrode tips made of this composite material which operating temperature is approximately 800 demonstrate much higher softening (recrystalization) temperature than tips made of standard high strength and high conductivity copper alloys (D. W. Lee, 2001). A homogenous distribution and small size of oxide particles is the main requirement for structure of dispersion-strengthened materials.