Abstract
Liquid mercury can be a natural component of natural gas, condensate, and crude oil production streams, so the potential for liquid metal embrittlement (LME) of hydrocarbon production metallic components must be considered. With increasing use of titanium alloys for deep sour high pressure/high temperature well oil country tubular goods (OCTG) and dynamic offshore riser components, titanium’s ability to resist degradation from liquid mercury contact warranted assessment under relevant hydrocarbon production conditions. The results of exposing UNS R56404 forging and pipe product forms to liquid mercury over the 25°-232°C temperature range while highly stressed at and beyond the alloy’s yield point via three differing loading modes are reported. These included sustained load 90 day C-ring tests, slow strain rate tensile testing, and rippled slow strain rate cyclic tensile loading. Although excessive plastic strain can allow substrate Ti metal wetting and low energy subcritical crack growth (i.e., LME), results confirm that titanium’s robust, non-mercury wetting surface oxide film is key to providing LME resistance to substantial plastic strain limits and beyond a reasonable range of surface mechanical damage expected in service.
Introduction
Trace amounts of elemental mercury (Hg) occur naturally in hydrocarbon reservoirs worldwide, which can be entrained in gas, condensate, and crude oil production streams. Concentrations of mercury in these streams are relatively very low in North America (e.g., Gulf of Mexico, USA – Overthrust Belt); but tend to be significantly higher in the Far East (e.g., Indonesia, Gulf of Thailand, Malaysia) and Africa.1,2 Mercury deposits are often associated with geological plate boundaries, fold belts, and areas of volcanic or hydrothermal activity. 1
Direct contact of certain metallic materials with liquid mercury may result in amalgamation, amalgam corrosion (i.e., liquid metal attack), galvanic corrosion, and/or liquid metal embrittlement (LME). 2,3 Aluminum, copper, and zinc based alloys are known to be highly susceptible to liquid metal attack and LME;2,3 whereas stainless steels and nickel alloys can exhibit variable degrees of LME susceptibility depending highly on heat-treatment and metallurgical condition. 2,3,4,5
