A new model for predicting the grain size of electrodeposited nanocrystalline nickel coatings containing sulphur, phosphorus or boron based on typical systems

Highlights

• Model is developed for Ni-S, Ni-B and Ni-P protective coatings.

• Understanding the influence of metalloid content in Ni coatings for corrosion protection

• Model correlating applied electrochemical deposition parameters to the grain size of the deposit

Abstract

Controlling the grain size in electrodeposited coatings for the prevention of corrosion is highly important. To understand the relationship with grain size and electrochemical performance many experiments need to be undertaken to vary the grain size of the deposit. In the present work the (crystallite) grain size of electrodeposited Ni coatings formed in the presence of metalloids such as boron (B), sulphur (S) and phosphorus (P) was estimated from analysing mass transfer at the cathode-electrolyte interface. A mathematical model has been proposed which indicates that the grain size of the deposit is directly proportional to current efficiency and the deposition rate while being inversely proportional to the current density and metalloid (B, S, P) content in the coatings. A simple relationship is developed which is in agreement with experimental data and data that is reported in the literature. The development of such a model should significantly decrease the amount of experimentation required to achieve the desired grain size in such systems (Ni-B, Ni-S, Ni-P coatings) obtained by electrodeposition.

Introduction

During the last two decades, electrodeposition has been proven to be an economical process to obtain virtually porosity free bulk nanocrystalline nickel (Ni) and Ni alloy coatings for corrosion and wear resistance [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. These coatings are either nickel phosphorus (Ni-P) [[13], [14], [15], [16]], nickel boron (Ni-B) [[17], [18], [19], [20]] or Ni containing sulphur (Ni-S) [4,5,[21], [22], [23], [24], [25], [26]] where each composition has individual physical properties as well as different applications. Electrodeposition parameters such as current density, additive (metalloid) concentration and temperature have an influence on the nanocrystalline structure of these coatings. In the final stages of electrocrystallization, after reduction, the means by which an adatom is incorporated into the crystal lattice will determine the crystal size of the deposit. Many models have been proposed to correlate these parameters with the obtained grain size through electrocrystallization [[27], [28], [29], [30]]. Budevski et al. [31] presented a review on electrocrystallization nucleation and growth phenomena in electrodeposited coatings. Wong et al. [30] proposed a mathematical model for electrocrystallization in pulsed electrodeposition mode using different types of waveforms and found that ramp-down waveforms can be employed to obtain fine grained structures. In a similar way, Molina and Hoyos [32] estimated the hardness of pulse current deposited nickel through the indirect effect of grain size on hardness. Choo et al. [28] qualitatively concluded that in order to have nanocrystalline grain sizes, during electrodeposition, a high negative over potential, high adion population and low adion surface mobility are required for a high nucleation rate and therefore reduced grain size. Rashidi and Amadeh [25] proposed a relationship between saccharin concentrations in the plating electrolyte with Ni grain size using a Langmuir type adsorption isotherm. However, this model does not give a relationship between grain size and the sulphur content in the coating. In another semi-quantitative study [8] these authors suggested that current density can lead to different grain sizes in Ni coatings depending upon the composition of the electrolyte. However, there was no direct quantification of the influence of current density on grain size. Indeed, many studies reveal conflicting data for the relationship between grain size and current density [33,34], however, many of these proposed relationships are qualitative in nature via interpreting direct experimental observation rather than developing a physical model behind the electrodeposition process.

During electrodeposition, there are many parameters (current density, current efficiency, deposition rate, alloying/impurity content etc.) for a given pH and temperature which can be experimentally chosen. We believe for a given experiment; all these parameters have a collective effect on the grain size of a Ni coating. The main aim of the present work is therefore, to study the influence of the above mentioned parameters to quantify the grain size of electrodeposited Ni coatings containing metalloid atoms such as sulphur, phosphorus and boron and develop a phenomenological model to predict the final grain size under specific conditions. This model can also be utilized to estimate the sulphur or phosphorus or boron content of electrodeposited Ni (Ni-S, Ni-P or Ni-B) coatings if current efficiency and grain size is known. The proposed model can also be used to predict hardness of the coatings through the estimated grain size utilizing the classical Hall-Petch relation.