Recent research has shown that sour top of line corrosion (TLC) is different from sweet TLC. The steel surface temperature is the most important factor for sour TLC while condensation is less important. Failures due to sour TLC are relatively rare, but some cases have been documented the last decades.
In this work, sour TLC was systematically studied in autoclaves with a special holder that exposed one side of the TLC coupon while the back side was exposed to cooling liquid. The coupon surface temperature could be controlled by adjusting the temperature of the cooling liquid. The effect on TLC of various parameters like bulk temperature, condensation rate, organic acids, MEG and methanol was studied.
Methanol was clearly the most important factor affecting TLC corrosion. Although the mass loss corrosion rates increased somewhat with methanol, the major effect was that it resulted in significant localised corrosion compared to all other conditions that were tested. This is important because methanol sometimes is used in pipelines to prevent hydrates. A related literature review showed that methanol was involved in seven of ten field failure cases, further supporting the negative effect of methanol on sour TLC.
Top of line corrosion is a corrosion phenomenon that has been known for since at least the 1950s.1-3 Although the first reported TLC cases were for sour conditions, most research has been on sweet TLC for which the problems seems to be most common. A literature review on sour TLC revealed that only a few field failures have been described in the public literature during the last 60 years.4
Top of line corrosion is occurring when moist gas in pipelines is cooled so liquid water is condensing on the pipe wall. The phenomenon is only occurring for stratified and stratified wavy flow regime for which the top segment of the pipeline is not in contact with the liquid phase flowing on the bottom of the line. The condensing liquid will be acid since it contains sour gasses CO2 and H2S, and possibly also organic acids. The acid condensed water is very corrosive to carbon steel and it is frequently referred to as ”hungry water”. TLC is a dynamic process where the condensed water gradually becomes buffered with alkaline corrosion products (HCO3” and HS”) while at the same time it is drained to the bottom of the line. Since the top segment is constantly fed with ”fresh” condensed water this area becomes most susceptible for TLC. Furthermore, the traditional film forming corrosion inhibitors that are added to the bulk aqueous phase does not work since the top segment is physically separated from the liquid in the bottom of the line (for stratified flow). Batch inhibition using a spray pig or a large liquid plug is sometimes used to inhibit the top segment, but the process must be repeated at regular intervals.