The Precipitation Hardenable (PH) Nickel alloys N09925, N07718, N09945, N09946 and N07725 are widely used for critical downhole oil field applications such as high strength tubing hangers and completion equipment.  The materials are particularly useful in High Pressure/High Temperature wells where high strength and corrosion resistance are required in H₂S containing production fluids. Over the last 20 years a limited number of field failure investigations in PH Nickel alloys have been related to the presence of sufficient amounts of intergranular precipitates promoting hydrogen embrittlement, which results in brittle cracking of UNS alloys N07718 ^3,4 and alloy N07725⁵ and N07716⁶. 

Precipitation Hardened (PH) Nickel alloys have been widely used in the Oil & Gas Industry for decades as these materials offer high strength and outstanding corrosion resistance in aggressive environments. They are commonly used in high-strength components in downhole wellhead subsea and Christmas tree equipment. However high profile failures of equipment have occurred including tubing hangers cross-overs and subsea bolts with alloys such as UNS N07718 UNS N07716 or UNS N07725. In all these cases the mechanism identified was Hydrogen Assisted Cracking (HAC) as the result of the interaction between atomic hydrogen adsorbed by the alloy and its microstructure.PH Nickel alloys are all subject to precipitation of secondary and tertiary phases which if processed improperly (particularly during hot working and heat treatment) may adversely affect the material properties required for the intended application. Despite the number of scientific and technical contributions produced over the last years the interaction between these complex microstructural features and atomic hydrogen is still not understood and is further complicated by variations in testing approaches used to simulate severe hydrogen charging conditions. The present paper provides insights on the HAC failure mechanism for API 6ACRA PH Nickel alloys comparing findings from numerous studies. In addition implications for currently adopted standards and emerging specifications are also presented and discussed.

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.