Behaviour of Nickel Plated Alloy 800 in HTR Helium

Nickel plated alloy 800 creep specimens have been creep tested at CIIR Oslo, at 800°C and 900°C to examine whether nickel coatings can provide improved corrosion protection in HTR helium° Preliminary metallographic examination of 800°C exposed specimens have indicated that whilst nickel coatings of 150-200 microns have provided full protection against oxidation, the complete absence of protective oxide films has allowed diffusion of carbon from the helium environment through the nickel layer to carburize the underlying alloy 800 to a depth of 400 microns (see figs. 2 and 4). In localized areas near to the ends of the plated gauge length, the nickel coating is much thinner, and thick porous oxide has formed at the surface. This oxide is unprotective and allowed internal oxidation to a depth of 50 microns and considerable carburization of the alloy 800 (figs. 1 and 3). The limited creep strain data accumulated at both 800 C and 900 C had shown increased creep rates in the nickel plated specimens when compared with the unplated control specimens. It is suggested that loss of creep strength is caused by modification of the ageing mechanisms within the carburized zone in the alloy 800. This behaviour is very similar to the anomalous behaviour of alloy 800 heat treated in silica capsules (Report No. HTMP 1) therefore suggesting that non standard treatments can have an adverse effect upon the behaviour of alloy 800 in HTR helium. (author)