Localized Corrosion Testings of Stainless Steel in Low-Dose Hydrate Inhibitor by Cyclic Potentiodynamic Polarization
Authors: Tsang Ying Hang , Garza Tim , Burger Chelsea Ann , Neptune Dwight , Menendez Carlos M. , Stead Paul , Lehmann Marc 
Published: October 2011

ASTM electrochemical methods are reviewed for application to assess the impact of low-dose hydrate inhibitor (LDHI) to cause localized corrosion in select alloys. Experimental results are presented to support recommendations for the use of a modified cyclic potentiodynamic polarization (CPP) method based on the ASTM G-61 method, a procedure that was originally developed to evaluate the potential of pitting corrosion of stainless steels (SS) in chloride environments1. The modified method was utilized to show the effects of methanol (MeOH) levels in LDHI formulations on the pitting potential (Epit) vs. open circuit potential (OCP) for 316L SS and 2507 Super Duplex (SD). By comparing these potentials to the determined repassivation potential (Erepass) vs. OCP it was found that by decreasing MeOH concentration in the LDHI, both Epit and Erepass improved for both alloys. The effect of oxygen on the OCP of the alloys immersed in LDHI was assessed.

Aeration of LDHI formulations were found to increase the OCP in the electropositive direction and typically increased from 50 to 160mV when switching from a deaerated environment to an aerated solution. This observation was contrasted to results from CPP and OCP tests and used to rationalize where the intrusion of air into chemical delivery systems may explain why pitting corrosion events were observed under certain field environments. The developed electrochemical method offers a rapid prescreening method for measuring the compatibility of LDHIs against different alloys used for umbilical and capillary deployment. Recommendations are given as to where the method may be applicable and suggestions as to where it may be extended to screen other oilfield chemicals.

Introduction

This study was carried out to address recent concerns of potential pitting corrosion in offshore umbilicals by incumbent lowdose hydrate inhibitor. A series of Design of Experiments (DOE) has been performed to determine the root cause of the pitting observed on stainless steel. Methanol concentration and initial pH were varied systematically to examine the effect of each on pitting corrosion. As a result, a novel low-dose hydrate inhibitor anti-agglomerant (AA) was developed to address the identified issues. An ASTM method that was originally designed for evaluation of pitting potential of SS in chloride environments1 is modified to test the susceptibility to pitting corrosion in presence of the various LDHIs and possibly other oilfield chemicals.

A root cause failure analysis was commenced to evaluate the pitting corrosion observed in umbilical application offshore of equatorial coast of Africa2. The analysis found that the internal pitting corrosion observed was associated with the current LDHI in use. Before any production chemical (LDHI, corrosion inhibitor, scale inhibitor, etc.) is used in subsea applications, a rigorous qualification process is employed to certify it suitable for subsea umbilical and downhole capillary delivery, which ensures quality, reliability, and performance. This suite of FATHOMTM tests typically involve multiple stages of product testing and certification and encompasses product stability at low and high temperatures, performance, fluid compatibility, high-pressure viscosity, corrosivity, materials compatibility, capillary injection approval, umbilical injection approval, and ultra-clean filtration for umbilical injection. These test methods are routinely updated to reflect increasingly challenging conditions and feedback from field performance.