The risk of under deposit corrosion (UDC) in oil and gas pipelines where accumulation of solids can be expected is of key importance when CO2 corrosion mitigation strategies are assessed and qualified. In this work the authors present the results of a laboratory study aimed at evaluating the susceptibility for UDC when using either corrosion inhibitor (CI) or pH stabilization under field-relevant conditions, including a basic assessment of inhibitor adsorption on sand.

The tests were performed at ambient pressure in jacketed glass cells with a specially designed specimen holder and lid allowing in-situ deposition of a sand layer with a pre-defined thickness. The specimen assemblies comprised three carbon steel coupons, of which two were covered with sand. Galvanic coupling was established between one covered and the non-covered coupons. The behavior of the specimens was monitored electrochemically by continuous measurement of the galvanic current between the coupled specimens as well as regular linear polarization resistance measurements.

A parametric study was conducted including temperature, CO2 partial pressure, type of corrosion control chemical, solution chemistry, sand layer thickness and deposition scenario.

Inhibition was more effective when established before a sand layer got deposited. When sand was deposited first there was a delay and a reduction in inhibitor efficiency; pH stabilization was shown not to be as effective as inhibition under the test conditions. Galvanic effects increased the risk of both general and localized corrosion: underneath the sand deposit the coupon not galvanically coupled appeared better protected than the one galvanically coupled. Sand layer thickness did not have a significant effect in these tests. It can be noted, however, that sand may impede the CI from reaching maximum efficiency, regular maintenance pigging would therefore be advised. The inhibitor adsorption study confirmed that sand consumes CI by adsorption and this effect was pH dependent for the tested formulation.

Subject
Water, Sand, Corrosion rate, Iron carbonate, Adsorption, Under deposit corrosion, Partial pressures, Corrosion attacks, Chemical inhibition, Corrosion inhibitors, Galvanic current, Linear polarization resistance, Corrosion potential
Keywords:
Under Deposit Corrosion, Inhibition, pH-stabilization
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2020
Association for Materials Protection and Performance (AMPP)
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