Electro- and electroless plated coatings for corrosion protection

Abstract

This chapter presents an overview of the fundamental aspects of electro- and electroless deposition process, the mechanism of deposition, alloy deposition, etc. and the utility of these coatings for corrosion protection. Both electro- and electroless deposition techniques are simple, cost-effective and offer unique advantages for preparing deposits with desirable qualities. In electrodeposition, the plating rate, stability of the bath and the number of turnovers are very high but the resultant coatings lack uniformity on complex shapes and blind holes and they need a post-finishing treatment to achieve the desired performance. In electroless deposition, the plating rate, bath stability and the number of turnovers are relatively less but the resultant coatings are more uniform and do not require post-finishing treatment. Electroplated coatings offer corrosion protection to the substrate metals in three possible ways: (i) cathodic protection; (ii) barrier action; and (iii) environmental modification or control. The corrosion performance of electroplated coatings is influenced by a variety of factors, which include structure, crystallographic texture, grain size, porosity, impurities and triple junctions, interactions involving metallic underplates and cleanliness or freedom from processing contaminants. Electroless nickel does not perform as a sacrificial coating in the same way that electrodeposited Zn or Cd performs on steel substrate to provide protection against corrosion. It behaves as a true barrier coating, protecting the substrate by sealing it off from the corrosive environments. Consequently, the thickness of the deposit and the absence of porosity are of great importance. The electroless nickel coating shows superior corrosion resistance compared to electroplated nickel coatings. The most important factors that determine the corrosion resistance of electroless plated coatings are: substrate composition, structure and surface finish; pretreatment of the substrate to achieve a clean, uniform surface; adequate deposit thickness to meet the severity and time of exposure to the corrosive environment; the properties of the deposit (composition, porosity, internal stress etc.) which depends on pH, formulation and prolonged use (turnover) of the plating solution; post plating treatments of the coating such as passivation and annealing; and the aggressiveness of the corrosive environment condition. Electro- and electroless deposited ternary/quaternary alloy coatings, composite coatings, duplex coatings, graded coatings and multilayer coatings are some of the promising developments to achieve improved corrosion resistance.