Martensitic stainless steels for OCTG materials are widely used in sweet and mild sour conditions. Environmentally-assisted cracking (EAC) is a major corrosion-related issue when using stainless steels as OCTG materials. The EAC in specific oil/gas well conditions with sour environments is defined as sulfide stress cracking (SSC) and stress corrosion cracking (SCC). The SSC is a type of cracking caused by hydrogen embrittlement, which is attributed to a cathodic reaction under acidic conditions, while SCC is associated with an anodic reaction. SSC testing for martensitic stainless steels for OCTG material is often carried out at or near ambient temperature under conditions simulating condensed water, and SCC tests are conducted at higher temperatures under conditions simulating formation water and/or the brine availability test.
ASTM Grade 29 titanium alloy (UNS R56404) has been traditionally used for oil and gas stress joints (TSJ). However, given the general difficulty of processing this type of alloy in the beta quenched condition and more recently the exorbitant increase in alloying costs due to the ruthenium, a new solution is required if titanium is to be considered for future applications. This 475 alloy was developed to meet geothermal requirements to replace Grade 29 seamless casing. The essential material properties of Grade 29 in bulk and welded condition as used for titanium stress joints were reported by Shutz et al.
Carbon and low alloy steels (CS and LAS, respectively) used for exploration and production in the oil and gas (O&G) industry are normally exposed to environments that may contain H2S in a wide range of concentrations. In aqueous solutions, H2S acts as a cathodic poison.1,2 A cathodic poison inhibits the recombination of atomic hydrogen to H2, and as a result, favors its absorption by the metal.1,2 In the presence of a susceptible microstructure and the simultaneous effect of applied or residual tensile stress, a crack can nucleate and propagate, when a critical concentration of hydrogen is reached in the metal.3 This environmentally assisted cracking (EAC) phenomenon is known as Sulfide Stress Cracking (SSC).2 SSC is commonly addressed as a case of hydrogen embrittlement (HE) damage.2
A model was built that describes stress field and hydrogen activity at the direct vicinity of a crack tip. A second model was based on the cohesive zone simulates the kinetic of a crack growth. Experiments using hydrogen permeation under stress on flat un-notched & notched specimens yielded data comparable to the simulations.
The sulfide stress cracking (SSC) resistance of carbon steels and other alloys is commonly addressed through testing according to NACE TM01771 or NACE TM03162. The Method A of the first standard is focused on tests using uniaxial tensile (UT) while the second standard considers 4-point bend (4PB) type of loads. A common way of qualifying a material according to these standards is the absence of failure of the specimens or SSC crack initiation at the surface of the material after a test duration of 720 hours (1 month). After testing, cross-sectional observations of non-broken specimens often reveal so-called “grooves” that can be significantly different in shape and depth depending on the test method, steel grade or environment considered.
Seamless X60QOS and X65QOS line pipes are widely used for offshore and onshore Sour Service applications. Sour Service refers to the risk of hydrogen related cracking as Sulfide Stress Cracking (SSC). The International standard (NACE MR0175 / ISO 15156) provides requirements for assessing the resistance to SSC, specifically on how to qualify for use in region 3 of the environmental severity diagram (Figure 1 in paragraph 18.104.22.168 of part 2). It is requested to expose materials in an acid solution saturated by 1 bar of H2S (NACE TM0177 Solution A) and to apply a tensile stress above 80% AYS by means of different methods: uniaxial tensile, C-ring or Four-Points Bend. However, for very sour fields presenting H2S partial pressures much higher than 1 bar, the preservation of the SSC resistance might be questioned and is presently a major concern for the O&G industry.
The present paper is dedicated to the evaluation of the SSC resistance of seamless quenched and tempered X65 grades, including the girth weld in the standard NACE TM0177 Solution A up to 15 bar of H2S partial pressure. Corrosion tests consisted of four-point bend tests performed in autoclave vessels. Different test configurations were investigated as specimen sampling locations through the wall thickness and surface state preparation.
Seamless X60QOS and X65QOS linepipes are widely used for Sour Service offshore and onshore applications. Sour Service refers to the risk of hydrogen related cracking, such as by Sulfide Stress Cracking (SSC). The International standard NACE MR0175 / ISO 15156 provides requirements for assessing the resistance to SSC, specifically on how to qualify for use in region 3 of the environmental severity diagram (Figure 1 in paragraph 22.214.171.124 of part 2). Qualification requires exposing materials in an acid solution saturated by 1 bar of H2S (NACE TM0177 Solution A) and to apply a tensile stress above 80% AYS by means of different methods: uniaxial tensile, C-ring or Four-Point Bend. However, for very sour fields presenting H2S partial pressures much greater than 1 bar, the preservation of the SSC resistance might be questioned and is presently a major concern for the O&G industry.