Macrothrowing power

The properties of electrolyte are usually characterized by electrolytic conductance, covering power, macrothrowing power, and microthrowing power. 

Macrothrowing power predicts the ability of an electrolyte to lay down as nearly as possible a uniformly thick deposit across the surface of a workpiece. A good covering power is a prerequisite for good macrothrowing power. Other factors that affect macrothrowing power include the current distribution and current density, electrolyte composition, electrolytic conductance, and electrolyte agitation. 

Microthrowing power indicates the extent to which metal electrodeposition occurs at the outer plane of the substrate or at the base of valleys or cracks. Microthrowing power can be improved by activating the surfaces at the base of valleys or cracks to promote electrodeposition there, while inhibiting the outer surfaces by using inhibitors preferentially. In many cases, microthrowing power is inversely related to macrothrowing power. 

The throwing power is said to be micro (Micro-throwing Power) if the metal deposit occurs preferentially in pores and scratches and macro (Macrothrowing Power) when a dejiosit of relatively uniform thickness is produced on an irregularly-shaped cathode. 

Figure 19.2 shows that at the current densities of interest in the plating industry, the Wagner number is less than unity, making it difficult to obtain good macrothrowing power. Hence uniform plating will not be attained on complex shapes of the cathode. 

Mass transport plays an important role in pulsed metal deposition. On the one hand it limits the maximum rate of deposition and influences the structure and properties of deposits. On the other hand it effects the macrothrowing and microthrowing power. Under dc conditions, the maximum deposition rate is given by the limiting current density, fg, where the metal ion concentration... 

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