The high strength and corrosion resistance of nickel-chromium alloys such as Alloy 718 and nickel-iron-chromium alloys such as Alloys 945 and 945X make them particularly good candidates for use in demanding environments in the upstream oil and gas industry. These materials generally perform well where resistance to sulphide stress cracking and chloride stress corrosion cracking is required. However whilst these alloys are considered ‘NACE compliant' environmentally-assisted failures can still occur.It is generally accepted that for hydrogen cracks to initiate there must be a critical combination of stress susceptible microstructure and hydrogen concentration. In this project the effect of microstructure is explored by heat treating Alloy 718 945 and 945X to standard and non-standard conditions. Tensile specimens were slow-strain-rate-tested in air and under CP to explore sensitivity to hydrogen embrittlement. Finally the effect of a severe stress concentration in the form of a sharp notch was used to determine whether there is an enhanced susceptibility to hydrogen embrittlement due to the presence of local stress raisers. The results are compared with tests undertaken by other authors under various hydrogen-charging conditions.

There is a significant use of Nickel based alloys in the oil and gas industry for high strength / high corrosion resistance applications yet there has been a lack of understanding of fracture toughness of these Ni alloys under seawater / Cathodic Protection (CP) environments. Furthermore this class of alloys has demonstrated a weakness following high profile failures where the failing mechanism identified was Hydrogen Assisted Cracking (HAC). This study examines several Precipitation Hardened (PH) Nickel alloys by the J-R Curve method (ASTM E1820) using side-grooved single edged notched bend (SENB) fatigue pre-cracked test samples in a simulated seawater environment under CP. The Ni alloys evaluated a good representation of those associated with the in-service failures reported in the past were UNS N07718 UNS N07716 and UNS N07725 together with other alloys more recently developed such as UNS N09945 and UNS N09955.The materials were tested in a 3.5%NaCl solution with applied potentials of -1.1V and -1.4V vs SCE at room temperature at a loading rate of 0.005 Nmm-3/2. The overall response of the alloys in laboratory air was elastic-plastic in nature while the behavior in environment shifted towards a linear-elastic response most likely associated with the embrittlement caused by the hydrogen adsorbed during CP. Scanning electron microscopy analysis was performed to obtain insights on the fracture morphologies. Amongst the alloys tested UNS N07718 showed the least reduction in fracture toughness in the environment in relation to air while alloy UNS N07716 showed the most susceptibility to the environment with the lowest performance.Key words: Ni Alloys Fracture Toughness J-R Curve Method CP environment seawater.