Optimization of electrodeposition processes for tin coatings
Abstract
The mechanism of nucleation and growth of tin electrodeposits was investigated. Also the influences of the operating parameters on the morphology, texture, and microstructure as well as the corrosion resistance of tin coatings were studied in order to propose optimum deposition conditions. Tin electrodeposition on glassy carbon and steel from acid sulfate electrolytes proceeds via the 3D (Volmer-Weber) mechanism. On low carbon steel substrates, 3D tin crystallites are initially formed preferentially on step edges, followed by fast deposition of other sites with much smaller and densely packed crystallites of about 150-180 nm in diameter. It was discovered that hydrogen co-evolution and gelatin have a synergy effect contributing to the fast and complete coverage of steel substrate with tin. The cathodic potential oscillation of tin (II) reduction in the tin electrolyte is attributed to the depletion and restoration of tin ions on the cathode surface. It has been found that an increase in current density improves the coverage of the substrate or the covering power. Without agitation, the deposits are powdery at high current densities. In order to obtain smooth and compact coatings, agitation is crucial when the applied current density is well above the limiting value. Tin coatings with two different fibre textures, (100) and (301) were produced by electrodeposition. At a lower current density of 100 A/m 2, (301) fibre was obtained. At the current densities of 100 and up to 400 A/m2, only (100) fibre texture was observed. An increase in current density leads to a decrease in grain size. At the same current density, the grain size of tin coatings increases with increased temperature. The influence of temperature (20, 40, 60 and 80°C) on texture is small. The corrosion resistance of tin coatings increases with a decrease in grain size. The corrosion resistance of tin coatings with (301) fibre was higher than that of the tin coating with (100) fibre texture. The optimum operating conditions are 200 A/m2 and 20°C for tin deposition in the investigated electrolyte. It is demonstrated that the chronopotentiometry is a very useful and efficient tool to study the deposition process when combined with SEM.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2005
- Bibcode:
- 2005PhDT.......189W