Monolayered versus multilayered electroless NiP coatings: Impact of the plating approach on the microstructure, mechanical and corrosion properties of the coatings

Electroless nickel-phosphorous (NiP) coatings were produced on low carbon steel substrates for a total plating time of 3 h. Different preparation modalities were pursued. Multilayered coatings were produced by stacking three layers of the same composition by successive electroless plating with rinsing steps in between. On the other hand, coatings termed ‘monolayered’ for the sake of comparison were deposited by one step electroless process, with and without undergoing bath replenishment of the electrolyte during plating. All the samples were subjected to thermal annealing at 400 °C for 1 h under argon atmosphere. The results show that the multilayer approach prevents crack propagation in the as-deposited coatings because the interfaces between layers block the advance of defects. Bath replenishment during monolayered coatings production creates pseudo-interfaces similar to those of the multilayered case but they are ineffective in terms of corrosion protection. Un-replenishment of the electrolyte promotes a change in the coating's microstructure from lamellar to columnar which severely worsens their performance. Upon annealing, the presence of interfaces, along with the recrystallization of the metallic matrix, promotes an upgrading of the corrosion performance of the multi-layered coatings. The corrosion products spread laterally at the interface where they stockpile. At a certain point, the accumulation of these by-products provokes the exfoliation of the outermost layer exposing the layer underneath to the corrosive media, thereby delaying the advancement of the corrosion attack. The results of this study highlight the importance of the plating approach selection, as well as the need for proper electrolyte maintenance during the production of high-performance electroless coatings.