51313-02774-Pitting in the Water/Hydrocarbon Boundary Region of Pipelines—Effect of Corrosion Inhibitors

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Product Number: 51313-02774-SG

ISBN: 02774 2013 CP

Author: Guenter Schmitt

Publication Date: 2013

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Pitting has been observed in the water/hydrocarbon (HC) boundary region of some wet sweet and sour gas pipelines with a two-phase bottom-of-the-line (BOL) liquid. It was assumed that inhibitors and their phase behaviour may play a significant role in this corrosion phenomenon
which appears to be related to intermittent water/HC wetting of the steel surface during wavy flow of the BOL. This paper reports on experimental investigations in which two flat X65 steel coupons were positioned vertically in a water-HC two phase liquid with the water/HC interface in the middle of the coupons. To simulate the intermittent water/HC wetting of the steel surface one of the coupons was moved up and down with defined frequency and amplitude while the other coupon stayed quiescent. The corrosive medium consisted of brine with 200 g/L salt (NaCl + CaCl2; molar ratio Na : Ca = 10 :1 ) paraffinic oil in the diesel fraction range mixed with 10 vol-% of isomeric xylenes and 10 vol-% of isopropanol in equilibrium with 1 bar oxygen-free CO2. Inhibitors applied included talloil diethylenetriamine imidazoline (K1) and C12-C16- alkylbenzyldimethylammonium chloride (K2). The development of pitting was monitored by electrochemical noise (ECN) evaluated with a specifically developed software. It appeared that cyclic water wetting can influence the likelihood of pitting at the pipeline steel. Corrosive effects were strongly dependent on the presence type concentration and phase behaviour of inhibitors and additives. The efficiency of corrosion inhibitors was quantified via ECN measurements.