Corrosion behaviour of electroless high boron-mid phosphorous nickel duplex coatings in the as-plated and heat-treated states in NaCl, H2SO4, NaOH and Na2SO4 media

The corrosion behaviour of electroless nickel-boron and nickel-phosphorous coatings, as monolayers and duplex systems was investigated with and without heat-treatment (400 °C - 1 h, non-reactive atmosphere). Monolayer coatings with 20 μm of thickness were compared with duplex coatings composed of two 10 μm layers. Four distinct configurations of duplex coatings were prepared: NiB/NiP; NiP/NiB; NiB/NiB and NiP/NiP. The salt spray test showed that visible differences exist in terms of general corrosion: corrosion starts after 24 h for the NiB samples and after 48 h for the NiP samples. In addition, image analysis was used to quantify the corroded surface after salt spray. The duplex coatings presented intermediate behaviour when compared to the monolayer systems. Furthermore, the corrosion behaviour was evaluated by means of potentiodynamic polarisation technique. Results clearly suggested that monolayer NiP exhibits higher corrosion resistance than monolayer NiB. In all cases, polarisation tests showed that a NiP layer is preferable as the outer part of the coating rather than a NiB layer. Finally, cross-section analysis allowed better understanding of the corrosion properties.