Corrosion studies on aluminium reinforced with uncoated and coated carbon fibres

Knowing materials behaviour under conditions of electrochemical attack is a n important necessity for engineering applications, especially in marine en vironments. The considerable deterioration of the electrochemical corrosion resistance which results from the reinforcement of aluminium with carbon f ibres demands preventative measures. In order to retain usual life times of applications, suitable corrosion protection has to be implemented. The pre sent study deals with an internal corrosion protection process resulting fr om the use of suitable fibre coatings, viz. a pyrolytic carbon (pyC) coatin g and a nickel fibre coating. However, neither the ceramic pyC coating nor the metallic nickel coating effect an improvement of the corrosion resistan ce in aqueous solutions of 3.5 wt% NaCl or 3.5 wt% Na2SO4. Although, the si mple immersion test confirmed the lowest anodic matrix dissolution for meta l-matrix composites (MMCs) formerly containing nickel-coated fibres, the po larisation of the MMCs to more positive potentials causes greatly accelerat ed electrochemical dissolution. Furthermore, the observed segregation of el emental nickel to the sample surface after the immersion tests in both elec trolytes indicate additional susceptibility to galvanic coupling. MMCs rein forced with carbon fibres obtaining a pyrolytic carbon coating show evidenc e of strongly accelerated electrochemical corrosion. Because the microstruc ture and the surface area of the pyrolytic carbon differs from that of the carbon fibres, a probable reason for the enhanced corrosion susceptibility may be the higher reactivity of the pyC coating. This result stresses the i mportance of taking into account the features of the carbon of the reinforc ing component and to distinguish between different carbon types. Special em phasis should be placed on the processing of pyC coatings in order to gener ate pyC coatings with few active sites and low reactivity. (C) 1999 Elsevie r Science Ltd. All rights reserved.