ABSTRACT
In order to investigate the influence of minor Ru on the electrochemical behaviour and structural characteristics of passive films on the surface of Ti-6Al-4V alloys under various oil and gas exploration conditions, electrochemical techniques, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and corrosion simulation tests were carried out. The results revealed that the oil and gas exploration conditions had a serious impact on the electrochemical behaviour and corrosion resistance of the tested alloys. The passivation film resistance and corrosion potential of the tested titanium alloys were significantly reduced with increasing acidity and temperature. With the addition of minor ruthenium, the potential of the passive film on the Ti-6Al-4V-0.11Ru alloy surface increased because of the high surface potential of the ruthenium element. The contents of metallic ruthenium and tetravalent titanium oxide TiO2 in the surface film of the Ti-6Al-4V-0.11Ru alloy both increased with increasing temperature, which led to increase the thickness, stability, corrosion resistance and repairability of the passive film on the surface of the Ti-6Al-4V-0.11Ru alloy being better than those qualities of Ti-6Al-4V. These results were also confirmed by corrosion simulation tests.
ABSTRACT
Titanium alloy has become a promising candidate material for oil country tubular goods (OCTGs) in harsh service environments owing to its high specific strength, low density, low elastic modulus, excellent toughness, excellent anti-fatigue and corrosion resistance. However, because the high-quality natural gas resources in China are mainly concentrated deep underground, titanium alloy tubing and casing will bear great external pressure loads underground, so the collapse strength of titanium alloy tubing and casing is very important for the safety of the string in the well. In this paper, a new collapse strength calculation model, the strength collapse criterion model (SCM), was proposed for titanium alloy tubing and casing. 35 different specifications of titanium alloy tubing and casing were selected for the full-scale collapse tests to verify the reliability of the established SCM model. Furthermore, the effect of different key parameters (such as strength, ovality, eccentricity and residual stress) on collapse strength of titanium alloy pipes were investigated systematically and compared with the same specifications of steel pipes. The strength collapse criterion model and analysis results can provide a technical reference for the design and use of titanium alloy OCTGs in the petroleum and natural gas industries.
