Electroplating hydrogen evolution

Fig. 27. Current-potential curves for hydrogen evolution in 5 M NaCl at 80 °C. (1) Ti (2) Steel (3) Ti and steel coated with electroplated Co-Mo alloy. After ref. 511, by permission of Chapman Hall.

Side reactions, mostly hydrogen evolution, play an important role in electroplating. As a rule, their effect is detrimental to the process, because of the loss of energy and possible hydrogen embrittlement. But side reactions can also be beneficial, improving the uniformity of plating, as we shall see. 

Electroplating generally employs very corrosive conditions creating an environmental problem that concerns the formation of corrosive mists above a bath surface due to competing hydrogen evolution processes. Several researchers have studied the effect of ultrasound on reducing some of these problems. 

In most electroplating baths of practical interest, hydrogen evolution occurs as a side reaction. Under cathodic overpotentials, the following reactions can lead to discharge of protons and adsorption of atomic hydrogen on the cathode ... 

Typically electroplated layers are some micrometers thick. The plating rate (i.e., the amount of deposited material per time) is proportional to the electrical current. The total amount of electroplated material is proportional to the passed electrical charge. The ratio of the plated material and the amount calculated from Faradays law is called the current efficiency. If there are no side reactions (e.g., hydrogen evolution), all the passed charge is used to plate out the material from the electrolyte and the current efficiency is 100 %. 

Theoretical developments are widely used in experimental investigation of real electrochemical systems (Chapters 8 and 9). The core part of the book deals with all important aspects of electroplating, including a systematic discussion of codeposition of metals and formation of alloys. It also discusses such related subjects as oxide layer formation (Chapter 10) and hydrogen evolution as a side reaction (Chapter 11). 

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