Corrosion resistance of zinc electrodeposits in an ultrasonically agitated bath

Abstract

Corrosion control is a topic of increasing interest in the metal-finishing industry. Ultrasonics has been used during plating to try to enhance the corrosion resistance of electrodeposits. Zinc was plated on mild steel, with and without ultrasonic agitation. The corrosion resistance offered by the coating was evaluated by polarization studies. The corrosion rate was found to be lower for the ultrasonically prepared samples. It was also observed that the weight loss in a total-immersion test was lower. This is attributed to the reduced porosity of the electrodeposit in an ultrasonic field. The conservation of materials has received a great deal of attention in many industries. The field of surface finishing is no exception, and electroplaters must constantly strive to increase their output and to reduce unnecessary costs. Any reduction in the plating time is advantageous. Obtaining a metaldeposit thickness which is as uniform as possible on complicated shapes is also important. Many modern plating solutions use agitation in one form or another to improve the conditions of operation. Ultrasonic agitation of the plating solution is the most recently developed method. The term "ultrasonics" decribes any vibratory wave motion with a frequency above the audible limit of the human ear, which is estimated to lie between 16000 and 20000 Hz. Ultrasonic waves produce rapidly alternating regions of high and low pressure when passed through an electrolyte. Since liquids generally have low tensile strengths, liquid particles are actually "pulled apart" or fractured into thousands of tiny cavities by ultrasonic waves. These cavities explode with great force, which is concentrated over very small areas [1]. The net result, as far as the plating industry is concerned, is agitation of an extremely intense nature; this agitation is much more thorough than is possible by any other agitation method. Zinc coatings are used on iron and steel products which require corrosion protection but which do not require a decorative finish. Such products include nuts, bolts, screws and nails. The microhardness of electroplated zinc was observed to increase when it was plated in an ultrasonic field [2]. Ultrasonic agitation improved the corrosion resistance of electroless nickel coatings [3]. Zinc was electrodeposited on mild steel using a zinc acid bath. The composition of the samples is given in Table I, and the composition of the plating solution is shown in Table II. The samples were tested for microhardness in a TABLE I Composition of the mild-steel specimens used for electroplating