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

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 mu m. 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 degrees C to achieve a complete stress relaxation. The stress relaxation caused a decrease of the hardness and a noticeable increase of the corrosion resistance.