Plating Formation and Bonding - Reaction Steps for Metal Electrodeposition

       Chapter 4 Formation and Bonding of Coatings



1 Reaction steps of metal electrodeposition



    Plating is a coating obtained by electroplating (or electroless plating). Usually electroplating (or electroless plating) is a process of forming a coating on the metal surface in a solution with a large number of electrolyte and solvent molecules, which is called wet plating, which is different from dry plating such as metal vapor deposition. As far as the crystallographic structure of the coating is concerned, the wet plating and dry deposition of metals show exactly the same physical phenomena, and have similar research methods and preparation methods. However, the formation and interfacial bonding mechanism of wet coating involves more disciplines such as physics, chemistry and metallography, and the progress of its theoretical research is completely dependent on the application and development of the theories and advanced technical means of the above disciplines. Since the wet plating system is more complex and has more varieties, the current research on the bonding mechanism of its plating is mostly specific plating systems, and there is a lack of comprehensive monographs. Therefore, this chapter mainly introduces some basic points of the theory of plating bonding in the electroplating process. .

 

    The electrodeposition process occurs at the electrode-solution interface, so to understand the principle of plating deposition, it is necessary to analyze the basic reactions of the electrode-solution interface and the various reaction steps associated with it. During electrodeposition, current flows from one solid-phase electrode through the interface into the solution, and then flows out of this electrode through the interface between the solution and the other electrode. Charge transfer is a complex process consisting of a series of different steps in series, and in some cases may also include side reactions in parallel. Due to the constraints of the series connection, the speed of each step in the whole process must be forced to be equal, so that the irreversible reaction speed on the electrode can enter a stable state, and the electrons can flow normally in sequence.



    The general process of metal electrodeposition can be divided into the following steps.

 

    1) During the mass transfer process, the particles participating in the reaction in the plating solution are transported to the interface for deposition needs.

 

    2) The pre-transformation of the surface transformation reaction particles before the interface charge exchange, that is, the preparation step before the reaction.

 

    3) The electrochemical step performs electron exchange at the interface.

 

    4) The metal ions after the phase formation discharge form a plating layer.

 

    The overall reaction rate of the above series of processes will be controlled by the most difficult or slowest step to react, ie the remaining steps will be forced to proceed at the same speed, thus the steps are both interrelated and interdependent. In metal ion electrodeposition, the general liquid phase mass transfer step tends to proceed relatively slowly, so this step often determines the proceeding speed of the entire electrode reaction.