Combined Effect of Sodium Lauryl Sulphate and Saccharin on Microstructure and Corrosion Performance of Electrodeposited Nickel Prepared from Modified Watts Bath

Abstract

Additives in Watts bath influence the surface properties of the electrodeposited nanocrystalline nickel. The changes in the properties are led by the alteration in the nucleation and plating over-potentials ($E_{n}$ & $E_{p}$). A galvanodynamic polarization technique was used to determine the $E_{n}$ & $E_{p}$ in the modified Watts bath. From all the commercially available additives, sodium lauryl sulfate (SLS) was used as an anti-pitting agent, and saccharin (SAC) was added as a grain refiner. The concentration of SAC was varied in the range from 1.5 to 10 ${\text{ml}}/{\text{l}}$ while keeping SLS concentration constant at 1.1 ${\text{g}}/{\text{l}}$ in order to see its effect on polarization potentials, surface roughness, and corrosion behavior. Refinement of surface roughness and crystallite size, corresponding to steady values of $E_{n}$ & $E_{p}$ was obtained for SAC at 3 ml/l and SLS at 1.1 g/l. The deposits with fine crystallite size showed minimal passivation current density and highest polarization resistance.

Appendix

1.1 (A) Polarization Study to Determine the Kinetic Parameters of Electrodeposition

To determine the kinetic parameters such as transfer coefficient (α), Tafel slope (b), and exchange current density $(i_{0} )$; cathodic polarization tests for all the samples have been done. The scan rate for all the testing is kept constant at 2 mV/s. As the electrodeposition process is cathodic and cathodic overpotential is much greater than anodic overpotential, the Butler–Volmer equation takes the form as follows (Eq. [A.1]):

$$ \eta = a + b\log i $$

(A.1)

$$ b = - \frac{2.303RT}{\alpha nF} $$

(A.2)

$$ a = \frac{2.303RT}{\alpha nF}\log i_{0} $$

(A.3)

Where $\eta$ is the overpotential in V, $b$ = Tafel slope in V/decade, $a$ = intercept, $F$. is the Faraday’s constant = 96485 C/mol, $R$ = universal gas constant and $T$. is the temperature of deposition bath in Kelvins.

The $a$ and $b$ for the cathodic polarization curves are determined by using EC lab software. Consequently, the value of α and $i_{0}$ are calculated using Eqs. [A.2] and [A.3].

1.2 (B) Model Based Evaluation of Nucleation Rate

Simple classical model (SCM) for nucleation was proposed by Volmer et al. and Becker et al. to quantify the kinetics of the nucleation process. The basic assumption is that the formed clusters have the same crystal structure and thermodynamic properties as the bulk material. Ain the electrodeposition process the critical nucleus is composed of few atoms, the assumption of same thermodynamic properties of the clusters and bulk material is not appropriate. To define the nucleation and growth process on a nanometric scale, atomistic models have been proposed. However, experimental validation of this theory is challenging due to the involvement of microscopic terms such as the number of nucleation sites and frequency of attachment/detachment of atoms to clusters. Hence, modified classical Gibbs models seem convenient to use to determine the nucleation rates. As per SCM, if the nucleation event is assumed to be the formation of spherical nuclei on the substrate, then we can use the following Gibbs free energy equation for the nucleation event.

$$ \Delta G = - V\Delta G_{v} \; + \;\Delta G_{s} $$

(B.1)

where $V =$ Volume of the nucleus, $\Delta G_{v} =$ Volume Gibbs free energy, $\Delta G_{s} =$ Surface Gibbs free energy = $A_{s} \gamma$, $A_{s}$ $=$Surface area of nucleus, and $\gamma =$ surface energy

The relationship between volume Gibbs free energy ($\Delta G_{v}$) and the overpotential ($\eta$) is given by the Nernst’s equation as follows:

$$ \Delta G_{v} = - \frac{nF\rho \eta }{M} $$

(B.2)

As we are talking about nucleation events, the use of nucleation overpotential is more appropriate. It is the difference between the equilibrium potential and the actual threshold potential for initiation of nucleation.

So, substituting all the values in Eq. [B.1]

$$ \Delta G = \frac{4}{3}\pi r^{3} \left( {\frac{nF\rho \eta }{M}} \right) + 4\pi r^{2} \gamma $$

(B.3)

By differentiating $\Delta G$ w.r.t $r$ and equating it to zero, the expression for the critical radius ($r^{*}$) of a nucleus and free energy for the formation of a critical cluster can be determined as follows:

$$ r^{*} = \frac{ - 2M\gamma }{nF\rho \eta } \;{\text{and}}\;\Delta G^{*} = \frac{{16\pi \gamma^{3} M^{2} }}{{3\left( {\rho nF\eta } \right)^{2} }} $$

(B.4)

However, the nucleation event is similar to the formation of hemispherical caps (contact angle, $\theta$= 90 deg) on pre-existing surfaces (Heterogeneous nucleation). Hence, an additional term i.e $f\left( \theta \right) = \frac{{2 - 3cos\theta + \cos^{3} \theta }}{4}$ is multiplied with the $\Delta G^{*}$ term and $r^{*}$ remain independent of $\theta$. For $\theta = 90^\circ$, $f\left( \theta \right) = 0.5$.

Now, the nucleation rate can be expressed as

$$ R = R_{0} \exp \left( {\frac{{ - \Delta G^{*} }}{RT}} \right) $$

(B.5)

where

$$ R_{0} = A_{s}^{*} Z\left( {\frac{{i_{0} }}{nF}} \right)\exp \left( {\frac{ - \alpha nF\eta }{RT}} \right) $$

(B.6)

$$ A_{s}^{*} = Surface\,area\,of\,critical\,nucleus\,=\,4\pi r^{*2} $$

$$ Z = Zel^{\prime}dovich\,factor = \frac{\eta Fn}{{\left( {12\pi K_{B} T*\frac{{16\pi \gamma^{3} M^{2} f\left( \theta \right)}}{{3\rho^{2} n^{2} F^{2} \eta^{2} }}} \right)^{0.5} }} = \frac{{1.732\rho n^{2} F^{2} \eta^{2} }}{{\left( {96\pi^{2} K_{B} T\gamma^{3} M^{2} } \right)^{0.5} }} $$

(B.7)

$i_{0} =$ exchange current density.

Although in some places $R_{0} and Z$ are considered as constant quantities, but there is a dependence of overpotential with both the quantities. The expression for both $R_{0} and Z$ is derived by considering 3-dimensional nucleation.

Putting the $\Delta G^{*} \& R_{0}$ values,

$$ R = A_{s}^{*} Z\left( {\frac{{i_{0} }}{nF}} \right)\exp \left[ { - \left( {\frac{\alpha nF\eta }{RT} + \frac{{16\pi \gamma^{3} M^{2} f\left( \theta \right)}}{{3RT\rho^{2} n^{2} F^{2} \eta^{2} }}} \right)} \right] = R^{\prime}_{0} \exp \left( c \right) $$

(B.8)

Hence, Eq. [B.8] evaluates the overpotential dependence of nucleation rate in the case of 3-dimensional nucleation.

See Table XI and Figure 13.

Table XI Nucleation (${\text{E}}_{\text{n}}$) and Plating Potential (${\text{E}}_{\text{p}}$) Values for Various Electrolytes Ordered as S1 to S5

Fig. 13

Potentiodynamic polarization curves for samples S1 to S5 after (a) 24 h and (b) 168 h of exposure in 3.5 wt pct NaCl. The exposed area in the case of cyclic potentiodynamic polarization, potentiodynamic polarization, and EIS is 3.141 cm² (as the internal diameter of the mounted glass tube is 2 cm). The exposed area in the case of galvanodynamic scan to determine $E_{n}$ and $E_{p}$ is 10 cm²