Analysis of failure in a nickel coated brass dolphin striker component

Abstract

Brass is a versatile alloy suitable for making engineering components exposed to aqueous and marine environments. A failed Ni coated brass nut that was a part of a dolphin striker used in a sailing boat is analysed here. Preliminary investigation suggested that the fracture was brittle with no visible evidence of plastic deformation. Microhardness indicated that the general mechanical integrity was intact with hardness of 190 HV across random sections and this is consistent with expectation for high strength brass alloy. Point probe Energy Dispersive Spectroscopy (EDS) analysis provided evidence of dealloying through electrochemical loss of Zn. EDS of stains on Ni coating indicated the presence of Zn as corrosion and Cr as corrosion products. It was confirmed that a selective loss of Zn occurred from the underlying brass body through “gates” provided from damage of the normally protective Ni layer. Such selective loss of zinc due to corrosion attack would have led to loss of strength. However, the examination of fractured section suggested an intergranular brittle fracture. The finite element analysis (FEA) showed a maximum tensile stress of 90 MPa in the fracture region which is well below the expected fracture strength. It is therefore concluded that failure of the piece was due to a combined mechanism of dezincification followed by a stress provoked brittle fracture process. It is also proposed that such failures could be avoided by alloy design and surface treatment methods to harden the Ni coating.