Mass transfer study of the electropolishing of vertical cylinders with active ends under natural convection conditions

Abstract

Rates of electropolishing of vertical copper cylinders with active ends in H₃PO₄ were studied by measuring the limiting current under natural convection. Variables studied were H₃PO₄ concentration, cylinder diameter and aspect ratio. The rate of polishing of the whole cylinder was represented by the mass transfer equation

$$Sh = 0.33(Sc Gr)^{0.32}$$

for the range 1.17 × 10¹⁰ < Sc Gr < 5.11 × 10¹¹. Rates of mass transfer were measured also at vertical cylinder with insulated ends, and the upward facing surface (disc). Data for the vertical cylindrical surface were represented for the range 8.75 × 10⁹ < Sc Gr < 1.1 × 10¹² by the equation

$$Sh = 1.206(Sc Gr)^{0.255}$$

while data at the upward facing disc were correlated for the range 0.11 × 10¹⁰ < Sc Gr < 46 × 10¹⁰ by the equation

$$Sh = 0.17Sc^{0.396} (Sc Gr)^{0.146}$$

A comparison between the measured rate of mass transfer at the whole cylinder and the value calculated by adding the rates of mass transfer at the separate surfaces of the cylinder shows that the measured value deviates from the calculated value, the degree of deviation increases with increasing Sc × Gr. Deviation was attributed to flow interaction at the different cylinder surfaces.