Membrane nitrogen with up to 5 vol% O2 is widely used as blanket gas in the petroleum industry, including as blanket gas on rich and lean MEG tanks. The presence of oxygen, even in low concentrations raises questions with regard to potential corrosion damage in the carbon steel injection line as well as consumption of the dissolved oxygen and residual levels upon entering the pipeline. These aspects constitute the scope of this work.

A series of experiments were carried out with carbon steel specimens exposed to environments similar to service conditions encountered in MEG injection lines, including temperatures between 5 and 65 °C and various levels of alkalinity/acidity. The corrosion processes were monitored by electrochemical measurements. These allowed the empirical determination of a number of parameters governing the corrosion and oxygen consumption reactions. Based on the experimental findings and literature data, a comprehensive finite element model was developed and used for tracking the corrosion rate along an injection line of Lean MEG in the presence of dissolved oxygen under acidic, near-neutral and alkaline conditions. Consumption of dissolved oxygen along the injection line was accounted for via the bulk oxygenation of ferrous iron released during the corrosion process.

The experimental results revealed that the contribution of electrochemical oxygen reduction to the corrosion rates is minute; the oxygen is rather consumed in reaction with ferrous ions in the bulk. The model reproduced the measured corrosion rates well and made it possible to study the consumption of dissolved oxygen as a function of acidity/alkalinity, temperature and residence time. The results showed, for example, that the dissolved oxygen may not get completely depleted in the injection line at high flow rates.

The results presented here, as well as the approach used by the authors can be a useful aid in assessing the risk of corrosion damage in glycol injection lines and pipelines, associated with the presence of oxygen in the lean MEG.

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