Organic additives in nickel and nickel alloy electrodeposition

The impact of additives on aqueous metal electrodeposition has fascinated the electroplating community for several decades. Additives can be classified into organic and inorganic compounds. The former are most common as they act as hydrogen permeation inhibitors, crystal growth modifiers, brighteners, levellers, wetting agents, and stress relievers. Even if added to the electrolyte in small amounts, their effect on coatings’ morphology and structure is remarkable, often without changing their chemical composition. For example, saccharine, a well-known sweetener to sprinkle onto food or added to coffee, is a typical grain-refining agent in metal electrodeposition. Saccharine is thus widely used in nickel plating as a brightener. Polyethylene glycol (PEG) is also added to nickel electrolytes to reduce surface roughness, refine grains, and promote twinning. In turn, the mechanical properties of the resulting coatings such as hardness greatly improve. Caution must be taken, though, regarding the incorporation of sulphur and carbon impurities that originate from these additives into the coatings.

 

Sodium lauryl sulphate (SLS) is another common organic additive in nickel baths. It has been used as an anti-pitting, misting, wetting agent since its misting effect was reported by Shinochiro et al. in 1979. As a wetting agent, SLS helps detaching hydrogen bubbles that originate as a side reaction during electrodeposition from the surface of the coatings during their growth, especially in acidic electrolytes. Hence, SLS prevents hydrogen bubble occlusion in the films and concomitant mechanical fragility due to the occurrence of porosity or internal voids.

 

The mechanism behind the effect caused by a given additive at the molecular level is often unknown and might require computational efforts like quantum chemical calculations and molecular dynamics simulations. Indeed, the knowledge acquired throughout History regarding additives in electroplating is of empirical scope and the number of publications that provide a clear-cut understanding of the phenomena are limited. When two or more additives are added to the electrolyte, synergistic effects may arise, which further complicates disentangling the effects imparted by each additive. Therefore, there is still a long way to go for a complete understanding of the science behind additives in metal electrodeposition, and nickel alloy plating in particular.