Summary. Slow-strain-rate tests in ASTM seawater using specimens prepared from a failed nickel/copper-alloy bolt have shown that precipitation-hardened UNS N05500 (Monel K-500 trademark) is embrittled by cathodic protection with sacrificial aluminum anodes. Some loss of ductility also occurred when annealed UNS N05500 was coupled to aluminum anodes and when the hardened alloy was coupled to steel. Brittle fractures produced by slow-strain-rate tests were intergranular and were very similar in appearance to the field fractures. While the slow-strain-rate tests were conducted on an alloy from only one source, there is no reason to assume that UNS N05500 alloy from other sources would resist hydrogen embrittlement from standard cathodic protection systems. Introduction Bolts of alloy UNS N05500, a precipitation-hardening nickel/copper alloy commonly referred to as Monel K-500, failed in a brittle manner on North Sea platforms sometime before Aug. 1984. The failures, which originated at thread roots where the hardness was about Rockwell C (HRC) 39, occurred in bolts that had been hardened after threading. The failures were attributed to hydrogen embrittlement that was a result of cathodic protection. It was concluded that hydrogen embrittlement of Monel K-500 would not occur if the hardness was below HRC 35, which is the acceptable hardness of UNS N05500 in sour systems. To ensure proper hardness of the thread roots, it was recommended that annealing and hardening treatments be done after the threading. Since that time, subsea clamp bolts of UNS N05500 alloy with the recommended heat treatment and hardness have failed in a brittle manner on two North Sea platforms. These failures occurred while the bolts were coupled to steel and cathodically protected with aluminum anodes or a combination of aluminum anodes and impressed current. The bolts had been roll-threaded, annealed at 980 to 1050 de-grees C [1,795 to 1,920 degrees F], water-quenched, and precipitation hardened at 500 to 600 degrees C [930 to 1,100 degrees F] for 16 hours. This produced hardnesses of about HRC 25. The majority of the later failures occurred in bolts loaded to 344 MPa [49,890 psi] during installation. This was 59% of the specified minimum yield strength of the material. The bolts began failing after about 1 year of service, and they continued to fail with time. Because failures of the remaining cathodically protected subsea N05500 bolts were anticipated, the broken bolts and other cathodically protected alloy N05500 bolts used in critical subsea applications were replaced with steel bolts. The remainder of this paper discusses bolts that had been annealed and hardened after they were threaded. Results of slow-strain-rate tests of hydrogen-charged specimens from one of the failed bolts are presented, along with studies of fracture surfaces. Subsea Failures Because the more recent failures occurred in bolts that had the recommended heat treatment and were below the critical hardness level the material was traced through the various heat-treating and threading steps to the original alloy supplier to see whether the failures were restricted to the alloy from a particular vendor or processor. It was found that both sets of failed bolts were heat-treated and threaded by the same shops, but the material was supplied by two different companies. A few bolts purchased for use on the platforms had been heat-treated in an oxidizing atmosphere, but the failures were not limited to these bolts. Thus, it is thought that the failures are generic to the alloy and were not caused by improper manufacturing processes. Alloy compositions of the failed bolts were determined by several independent analyses; these analyses, plus heat analyses of all Alloy N05500 fasteners in use on the platforms, were studied to see whether the failures correlated to composition variables or to such tramp elements as sulfur or selenium. The failures occurred in bolts of the proper alloy composition, and were not caused by tramp elements or unusual alloy compositions. Typical microstructure of a failed bolt is shown in Fig. 1. All the failed bolts had grain growth at the thread surfaces, and some had detectable Ni (Ti, Al) precipitates, particularly in the larger grains. Applicable alloy specifications are given in Table 1. The cathodic protection system of the platform on which the most recent bolt failures occurred was designed to produce a current density of 130 mA/m2 using a combination of impressed current and aluminum anodes. Under normal operating conditions, the cathodic protection system delivers a current density of 35 mA/m2 at a potential of - 990 to - 1,140 mV vs. silver/silver chloride. Slow-Strain-Rate Tests Slow-strain-rate tests are popular for determining the susceptibility of materials to stress-corrosion cracking and hydrogen-induced environmental cracking. Tests specimens are subjected to a constant rate of extension until failure occurs. This test method promotes stress-corrosion cracking in alloy/environment systems that do not produce cracking in static tests. The data produced by the tests must be interpreted cautiously, however, because the absence of stress-corrosion cracking in slow-strain-rate tests is not sufficient evidence to eliminate cracking concerns. Slow-strain-rate tests were conducted in air and in circulating ASTM seawater that was exposed to the atmosphere. During the tests. the specimens were contained in plexiglass vessels equipped with two small anodes about 1 cm [0.4 in.] from the gauge sections of the samples. The slow-strain-rate tests were conducted in a 89-kN [20,000-lbf] -capacity load frame specially modified to achieve low strain rates. To calibrate the stiffness of the machine and test fixtures, the specimen strain over a 5-cm [2-in.] gauge length was monitored with an extensometer during the first test. For subsequent tests, the crosshead travel was set at the same speed, but an extensomerer was not used. Strain rates were on the order of 5 X 10-6 per second, For nickel-based alloys, this strain rate reportedly results in the maximum susceptibility to hydrogen embrittlement. Samples for the slow-strain-rate tests had 0.64-cm [1/4-in.] -diameter by 5-cm [2-in.] -long gauge sections. They were machined from the body of a 45-mm [1.8-in.] -diameter Monel K-500 bolt that had failed in the threads while in service on one of the North Sea platforms. This bolt, which had been in service about 1 year, had been threaded, annealed, water-quenched, and precipitation-hardened to HRC 25. Except for two samples that were annealed at 1010 degrees C [1,850 degrees F] and one that was baked at 175 degrees C [350 degrees F] for 6 hours after they were machined, the slow-strain-rate samples had the as-received heat treatment. SPEPE August 1988 P. 282^
Titanium alloy Ti-6AI-4V ELI is selected for a high-pressure drilling riser application due to its high specific strength, corrosion resistance, and favorable elastic properties. The qualification of this titanium alloy requires assessing its resistance to hydrogen embrittlement and stress corrosion cracking due to seawater with/without cathodic protection, evaluating its wear resistance against a rotating steel drill string, and studying the influence of service-induced defects on fatigue and crack growth behavior when subjected to the operating environment. The paper presents an overview of the design requirements for a titanium drilling riser, and the material properties of the Ti-6AI-4V ELI alloy proposed for this application. The paper also highlights recent efforts to merge titanium and composite technologies to develop cost-effective drilling risers. [S0892-7219(00)01001-3]
Summary Preferential weld corrosion (PWC) has long been a problem in the oil and gas industry for pipelines used in process facilities, seawater injection, produced-water service, offshore platforms, and downhole-production systems. This paper reviews recent PWC field failures in light of mitigation guidelines from existing literature. Environmental effects take precedence over chemical composition and microstructure in determining PWC susceptibility. Therefore, the primary approach to minimize PWC is continuous corrosion inhibition. The ideal PWC-prevention method would be the use of autogenous welds or welds made with matching consumables. However, this is not a practical option in the field where most pipelines are typically made from carbon (C) manganese (Mn) steels with broad chemical compositions, thereby making them susceptible to PWC. The feasible alternative is to use filler materials with strict compositional control concerning the following elements: nickel (Ni), silicon (Si), and chromium (Cr). In addition, increased-preheat and high-heat input welding processes should be employed to minimize PWC susceptibility.
The design, testing, and operational results from a subsea, electric-resistance probe system are discussed. Subsea monitoring provides the advantage of measuring the corrosion inhibitor efficacy at the point of injection, rather than inferring performance from platform measurements. The internal condition of pipelines can be monitored in a variety of ways. Electric resistance probes are one technique for acquiring subsea corrosion rate data. The optimum monitoring technique will change with pipeline age, location, accessibility, and operating conditions. More importantly, the applicable methods may change based on the type of information required. For evaluation of corrosion inhibitor performance a high-sensitivity corrosion monitor is required. A prototype dual-element, electric-resistance probe has been evaluated for pressure and temperature stability under subsea operating conditions simulating the Britannia field.The probe functioned well under all test conditions. As expected, temperature had the greatest impact on the stability of the corrosion measurements. Comparison of the relative response of the dual probes to the variety of test conditions is useful in evaluating the validity of field data and the functionality of the probe.The value of the field data from the subsea probes was apparent from an operational perspective. The development of a correlation between the topsides and subsea corrosion rates is a useful prediction tool. The probes were successfully installed, demonstrated the sensitivity to detect design corrosion rates, enabled evaluation of corrosion inhibitor performance, and confirmed proper location of the inhibitor injection point. However, the overall performance of the probes has not been as favorable as experienced during the evaluation program. Unfortunately, one of the two probes has failed for unknown reasons during the first year of operation. Plans for replacement are being developed.
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