Autocatalytic tin plating in the fabrication of tin-coated copper tube
By Huttunen-Saarivirta, E. & Tiainen, T.
Published in Journal of Materials Processing Technology
2005
Abstract
Tin-coated copper tubes are water service tubes, which combine good properties of tin and copper, showing, for example, superior corrosion resistance in aggressive waters as compared to conventional copper tubes. Tin coatings are generally deposited by an immersion method, which introduces rather thin coatings. Autocatalytic tin coatings are capable of increasing thickness of the existing tin coatings and, therefore, of improving their properties. The present study reports fabrication of tin-coated copper tubes by a two-step coating process, which is composed of immersion tin-plating phase combined with an autocatalytic plating step, utilising a bath containing sodium hydroxide, sodium citrate and stannous chloride. Development of the coatings proceeds by alternate grain nucleation and growth processes; grain growth advances until a completely compact layer of tin grains is formed, which is followed by a new nucleation and successive growth phase. Bath temperature of 80 °C was found to be optimal for the autocatalytic tin coating development; here, grain growth is favoured, introducing an even coating layer with a reasonable thickness. Below 80 °C, discontinuities may be introduced to coating structure due to an excessive coating nucleation over a grain growth process. Above 85 °C, in turn, the reaction to deposit the coating starts to be uncontrollable, resulting in an unstable plating bath. 90 min plating time was found optimal to reach a reasonable coating thickness and avoid secondary nucleation processes, which may yield surface irregularities in the coating. Utilisation of the autocatalytic coatings yields improved corrosion performance in tin-coated copper tubes due to a more efficient shielding ability. Studied by potentiodynamic polarisation experiments in water showing moderate aggressivity to copper, utilisation of dense and continuous autocatalytic tin coatings was observed to shift corrosion current density value of an uncoated copper to less than one tenth of the original value and corrosion potential of copper by up to 350 mV in negative direction, which is much more significant than an influence caused by immersion tin coatings. Besides, the autocatalytic coatings of the present study show lower corrosion potential values than tin-coated copper tubes reported so far, which means an enhanced cathodic protection to copper.