ABSTRACT

Susceptibility of Grades 23 and 29 titanium Tapered Stress Joint (TSJ) forgings to gas phase methanolic Stress Corrosion Cracking (SCC) was measured in deaerated, dry 0-0.5 wt.% and saturated methanol-containing methane gas environments at 25°C in a two phase laboratory program. Phase 1 utilized slow strain rate (SSR) tensile testing to aggressively identify any SCC behavior as a function of increasing methanol vapor content; whereas Phase 2 focused on fracture toughness testing with slow-strain rate rising K loading to determine the effect of methanol vapor on component fracture resistance via KJ value determination. SSR tensile results for both Ti alloys revealed nil-minimal methanolic SCC at dilute methanol levels, and a measurable drop in reduction in area (but not total elongation) values, indicating minor SCC susceptibility, at saturated gas levels. Grade 29 Ti KJ values remained well above 68 MPa vm at all methanol levels tested, and displayed no discernable, consistent trend with methanol content.

INTRODUCTION

Titanium TSJ Export Gas Compatibility

The current guidelines for minimum water content in injected methanol liquid established for Grade 23 and 29 titanium TSJs1,2 are generally based on scenarios of injection into produced well fluids, and/or direct, sustained exposure of the TSJ bore to the injected methanol liquid (e.g., riser flooding during shut down). As such, it is required that the factory-direct, neat liquid methanol (approx. 99.6 wt.%) be initially modified with 3 wt.% or 10 wt.% minimum water addition for Gr. 23 or Gr. 29 titanium TSJs, respectively, to totally preclude susceptibility to methanolic stress corrosion cracking (SCC) under any conditions.

The water in the methanol simply provides available oxygen atoms for repair, rehealing and repassivation of titanium protective TiO2 surface oxide film2-5 according to Reaction (1):

equation

Insufficient water content (i.e., dry, near-dry methanol) allows formation of a much less protective and semi-soluble Ti methylate film which renders these Ti-6Al-4V-based alloys susceptible to transgranular stress cracking under tensile stress.2-5 Typical SCC fracture mode for these alloys in dry liquid methanol consists of transgranular quasi-cleavage and/or well-defined flutes, as pictured in Figure 1 at higher magnification. This stress cracking susceptibility is greatest within the 18°-30°C temperature window, and diminishes at higher and lower temperatures.2,3,4 Factors which are known to influence liquid phase methanolic SCC in titanium alloys are discussed elsewhere.2,3,4

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