Considerations of Using Sodium Nitrite and Ammonium Bisulfite Solutions in Seawater Injection Facilities.

摘要

The effect of sodium nitrite on L-80 steel tubing under seawater injection (SWI) conditions was studied as part of a materials compatibility project. Ammonium bisulfite (ABS) is also used in SWI systems at a slight excess (～10 ppm) to remove remaining low levels of dissolved oxygen downstream of the de-aeration towers. It has been suggested that chronic use of this slight ABS excess fosters corrosion1. Both 1018 mild steel and L-80 coupons were used to investigate the corrosion characteristics of sodium nitrite injection into seawater in the presence of ABS at three different shear rates. Linear Polarization Resistance (LPR) studies done at low and medium shear (3.14 m/sec equivalent) revealed that nitrite injection into deoxygenated seawater, at levels between 1000 and 3000 ppm, yields a spike in corrosion rate that lasts for several hours depending on the prevailing shear. Following this spike, corrosion rates trend towards 1 mpy over a 24 hour period implying that a passive film forms. The effect of 1 ppm dissolved oxygen together with nitrite injection reduced the intensity of the nitrite-induced corrosion spike and hastened the trend towards a passivated state. Polarization scans in synthetic seawater (1000 mV above OCP) show a pitting potential only when nitrite is present at 3000 ppm under both aerobic and anaerobic conditions. High shear (9.77 m/sec equivalent) studies done with a rotating cage coupon assembly revealed pitting when nitrite is present at 3000 ppm under anaerobic test conditions as compared to only general corrosion under the same conditions without nitrite. The presence of low levels (10 ppm) of excess ammonium bisulfite (ABS) does not yield any corrosive activity at shear rates up to 3.14 m/sec. However, this work did identify a threshold of approximately 200 ppm where excess ABS becomes corrosive under anaerobic conditions.