Electrochemical and Structural Characterization of Electrodeposited Tin: Influence of the Additive Alkoxylated β-Napthol

Abstract

Electrochemistry of tin has been studied for several decades. Nevertheless, the effect of the electrolyte additives on the electrochemical reaction and on the deposited structure is still an open field of research. Tin can be plated from both acidic and alkaline solution. Plating from the acidic electrolytes is more common and the electrochemical equivalent is twice that in alkaline solution. However, deposits plated from the acidic electrolyte without additives are often non-compact, porous and dendritic. Organic molecules are needed to add into the electrolyte in order to obtain homogeneous, compact tin coatings. One of the first additives introduced for this purpose in the tin electroplating is -naphthol. Despite of its main disadvantage, being relatively easily oxidized under plating conditions, it is used up to now. A more stable form of this additive would be akoxylated β-naphthol (ABN, C12H12O2). It influence on tin plating has not been described in literature yet. Within this work the influence of the ABN additive in the acidic electrolyte on the electrochemical reaction and deposited structure of tin has been studied. Electrochemical experiments were performed in tree electrode setup using Ag/AgCl as a reference electrode and pure tin as a counter electrode. Reaction kinetics was studied with the polarization measurements on rotating disc electrode and Tafel measurements. For this purpose chloride based (pH ~ 5) and methane sulfonic acid (MSA, pH ~ 0.5) electrolytes have been prepared. Electrodeposits of tin with the thicknesses of ca 7 µm and 25 µm were obtained on flat brass substrates. Surface morphology and preferred crystal orientation were studied and by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Comparison of the structural characterization of the thicker and thinner sample has been made. The average grain sizes were estimated and compared by two different methods. The Heyn lineal intercept method from the top view on unpolished samples was compared with the EBSD analysis from the fine polished samples. Results from the reaction kinetics showed a more pronounced inhibition effect of the ABN in the chloride electrolyte. In this solution suppression of H2 evolution and decrease in cathodic current density was detected with the increased additive concentration. In the case of MSA electrolyte, there was no difference in the measured cathodic current for the studied ABN concentration range. Further experiments showed that ABN inhibition effect in MSA solution was suppressed by the creation of tin- citrate complexes. Results from the exchange current density confirm the observation that the electron transport on the electrode-electrolyte interface is more influenced by the tin-citrate complexes than by the presence of ABN. In comparison to the chloride electrolyte, higher ABN concentrations are needed for blocking electrode active sites. An electrochemical quartz crystal microbalance was used in order to study the influence of the concentration of the ABN and the complexing agent in the electrolyte on the suppression of hydrogen evolution and thus the efficiency of the tin deposition. The EBSD analysis showed more complex structure and smaller grain size in comparison to the Heyn linear intercept method (HLIM) for the coatings deposited from the MSA electrolyte. According to HLIM, an increase of ABN concentration would lead to an increase in average grain size with no changes in the main crystal orientation. In the case of the coatings deposited from the chloride based electrolyte the results from both HLIM and EBSD analysis were in agreement. With the increased of the ABN concentration a grain refinement effect and changes of the main crystallographic orientation from (101) to (400) were detected. The addition of ABN influences the electrodepoition of tin differently in the two electrolytes investigated. In the chloride based electrolyte an inhibition effect is dominant while in the case of the MSA electrolyte this effect was overlaid by the presence of tin-citrate complexes.