Ni-B electrodeposits with low B content: Effect of DMAB concentration on the internal stresses and the electrochemical behaviour

• Published in: Surface & coatings technology 2018-06, Vol.344, p.190-196
• Main Authors: Lekka, M., Offoiach, R., Lanzutti, A., Mughal, M.Z., Sebastiani, M., Bemporad, E., Fedrizzi, L.
• Format: Article
• Language: English
• Subjects: Boron; Corrosion resistance; Electrochemical analysis; Electrodeposition; Hardness; Localized corrosion; Microcracks; Microhardness; Microstructure; Ni-B; Plating baths; Protective coatings; Residual stress; Residual stresses; Stress concentration; Stress relaxation
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2018.03.018

Ni-B coatings with low, increasing amount of Boron have been produced using a Ni sulfamate plating bath with the addition of dimethylaminborane. Coatings' characterization revealed that it is possible to produce crack-free coatings with a B content up to 0.12 wt%, with a thickness of about 40–50 μm. The introduction of an increasing amount of B changes progressively the coatings microstructure from columnar, to fine fibrous and then to lamellar. The presence of B as interstitial atom in the Ni elementary cell caused an increase of the residual stress, as measured by FIB-DIC method, which change from compressive to tensile as a function of B concentration. The microstructure refinement and the increase of the residual stresses caused a noticeable increase of the microhardness. On the other hand, the resistance to localized corrosion decreased by increasing the B content maybe due to the formation of micro-defects or micro-cracks on the Ni passive layer due to the residual stresses. To confirm this hypothesis, the Ni-B coatings have been annealed at 400 °C to achieve a complete stress relaxation. The stress relaxation caused a decrease of the hardness and a noticeable increase of the corrosion resistance. •Production of crack free Ni-B coatings with different B content by electrodeposition.•By increasing the B content the microstructure changes from columnar to lamellar.•The B content increases the residual stresses which change from compressive to tensile.•Microhardness increases and corrosion resistance decreases due to residual stresses.•Stress relaxation by annealing at 400 °C increases the corrosion resistance.