Potentiodynamic electrodeposition of paint (EDP)

Abstract

Potentiodynamic electropainting at a rotating iron disc electrode has been investigated with three different EDP resins, two anodic from the acrylate type and one cathodic from the epoxide type, and a wide variation of conditions. Voltage scan rate (νs=1 to 200 Vs−1), voltage range (40 to 200V) and electrode rotation speed (n=60 and 1000rpm) were the most important parameters. The (cyclic) voltammetric curves obtained generally exhibit three characteristic features: (1) The current rises steeply at the start of the experiment. Bath resistance transforms the potentiodynamic curve simultaneously into a galvanodynamic curve. After a transition time, τ, a critical pH is attained at the phase boundary and electrocoagulation occurs. This leads to a rapidly decreasing current density. The sharp c.d. maximum thus established has a peak voltage,Up, which increases withνs according to the relation logUP∼ 1/3 log νs in accordance with theory. (2) At high voltages, a limiting current density is observed, increasing with the square root of νs. This could be quantitatively interpreted in terms of dynamic growth of film thickness governed by Ohmic ion transport in the film. The preceding part of theU/j curve declines withj ∼t−1/2, which indicates the prevalence of space charge effects. (3) Ohmic lines are measured in the course of the first reverse scan and in all quasi steady state follow up cycles. They are flatter by a factor of 1000 in regard to the initial Ohmic line and reflect low voltage Ohmic behaviour of the EDP-film. At high voltages positive current deviations occur due to Child's law. The curves can be measured easily and reproducibly. Due to their salient features it is proposed to use them for characterization of EDP-paints.