ABSTRACT
Recent efforts by manufacturers of OCTG have led to the development of several new grades of modified or ?supermartensitic? tubulars. The main feature of these products is improved corrosion performance at high temperatures (above 13OC), i.e. in conditions which standard 13Cr, based on type 420 chemistry, corrodes at a rate too high to permit its use. In order to study these new materials, laboratory corrosion tests have been conducted, in conditions in which the standard 13Cr (API 5CT L80) suffers severe corrosion. The H,S partial pressure in these tests was in the range 0.01 - 0.1 bar (1 - 10 kPa). It was found that the general corrosion rate of the modified alloy was approximately one-tenth of that of the standard 13Cr, and the pitting rate was reduced by a factor of 3 - 4. Observation of the samples after test revealed the presence of coloured interference films on the modified materials, whereas the standard 13Cr was different in having a corrosion product on its surface. This was less adherent and protective than the film on the modified matensitics. Electrochemical testing has confirmed that the corrosion behaviour of the modified matensitics falls between that of the standard 13Cr (which tends to corrode generally) and that of 22Cr duplex in the same conditions (which is fully passive).
Of the corrosion resistant alloys used downhole in tubular form, 13Cr (per API 5CT L80) is much the most widely used, with in excess of 2.5 X 106m installed? in the period 1975 - 1993. By 1995 this alloy represented two-thirds of new North Sea completions?; this situation was brought about by the increasing corrosivity of the production fluids encountered. This alloy has an excellent track record worldwide3*4 in sweet and slightly sour conditions. However, in the North Sea and elsewhere, operating companies are turning to deeper, hotter and more sour conditions. Current high pressure - high temperature wells in the North Sea have downhole conditions typically 19OC, 15,000 psi (100 MPa), high chloride and sour (per NACE MRO175 definition). These conditions require duplex or superduplex stainless steel tubing, but consideration is now being given to a category of materials intermediate in price (and corrosion performance) between those of standard 13Cr martensitic (abbreviated here to 13Cr) and the duplex alloys. This category is termed supermartensitic or modified 13Cr martensitic stainless steel (abbreviated here to Ml 3Cr). The 1990s have witnessed the development of low carbon 13Cr alloys with additional alloying elements Ni, MO and N. One objective for the manufacturers was to create a weldable martensitic stainless steel, since standard 13Cr is not readily welded. However, reducing the carbon content has also resulted in a significant improvement in corrosion behaviour. Two published corrosion indices illustrate the strongly detrimental effect of carbon on corrosion performance:
GCI = Cr -12C + 0.75 Ni + 10 N (ref 5)
CCI=Cr-lOC+2Ni (ref 6)
Reducing the carbon content from 0.2% (that of standard type 420 chemistry materials) to nearer zero reduces the corrosion rate by 50% in sweet conditions in the temperature range 120 - 150°C?. Adding molybdenum to the alloy further reduces corro
