ABSTRACT
Although relatively few failures associated with sour top of line corrosion (TLC) have been reported in field, it has been recognised as a corrosion threat, particularly if methanol is present. The toxicity and flammability of H2S, however, puts significant health, safety and environmental (HSE) restrictions on test equipment for sour corrosion testing, especially at high H2S pressures. In this work, a specially made sample holder that can be fitted inside regular autoclaves was used for simulating sour TLC in the presence of methanol. The experiments showed that for 10 bar H2S + 10 bar CO2, the TLC mass loss rate was 0.2-0.3 mm/y of general corrosion without localized attacks. The presence of methanol (50% by weight in the bulk liquid phase) resulted in significant localized corrosion attacks with just a slight increase of the mass loss corrosion rate.
INTRODUCTION
Top of line corrosion (TLC) has been recognized as a corrosion threat since the 1980ies.1-11 Although the first reported cases of TLC were for sour conditions (CO2 + H2S),12-14 the major focus has been on sweet TLC (CO2 only). The number of sour TLC failures is limited compared to sweet TLC, indicating that sour TLC is a less common problem. Consequently, sour TLC has been given relatively little attention. However, several sour TLC failures in the field have been documented,12-28 and lately more research has been carried out on this topic.2,8,27-40
In contrast to sweet TLC, the condensation rate is less important for sour TLC.1 For sour TLC the steel surface temperature is important, and recent literature suggests that steel temperatures around 20 - 30 °C are more susceptible to sour TLC than at higher temperatures.27-29 It was assumed that this effect is related to the protectiveness of the iron sulphide, which is claimed to be better at high temperature than at low temperature. Furthermore, a recent literature review showed that methanol was present in 7 out of 10 cases of failures, suggesting that the presence of methanol can have a negative impact on sour TLC, but it is obviously not an absolute prerequisite for sour TLC to occurr.1
