Behavior of mercury adsorbtion and desorbtion on metal surface used in natural gas production and exploration

Abstract

Liquid elemental Hg adsorption on the steel coupons cut from new petroleum subsea pipeline with the specifications of API 5L-X52 was investigated up to 90 days under controlled temperature of 25 0C. Examination of surface mercury adsorption morphology using SEM found that elemental Hg was in spherical droplets with sizes ranged <1 to >10 micron. The Hg droplets were observed either attaching on the steel coupon surface or falling into pits or crevices of irregular steel coupon surface. The distribution of Hg adsorption location is inconsistent which may be contributed by heterogeneous surface characteristic. EDS analysis of surface Hg concentrations revealed that they were not correlated with Hg adsorption periods. However, it provided only an indication of an increasing trend of surface mercury which begins at 30th until to 60th day before they stabilized and declined on 75 and 90 day, respectively. The following examination by XRD no later than a week found that Hg was found in HgO form on the surface. XPS surface analysis, undertaken no later than two weeks found that all Hg present on the steel surface was in HgO form but at very much lower surface concentrations compared to those initially present as elemental Hg. The results revealed potential loses of Hg through volatilization. The XPS analysis for Hg depth profile revealed that Hg was more consolidated on the top surface to the depth of about 10-20 nm. The Hg depth profile of different Hg treatment periods showed that they were consistent in term of the depth profile concentrations that they were all below < 1 %atom-Hg by XPS after the depth of about 20 nm from the top surface. The results of Hg decontamination by using I2/KI (at fixed KI of 2.0 M) found that at 0.2 M of I2 it worked well with different surface Hg levels ranging 20-34 %atom-Hg by XPS. It could reduce surface Hg levels to the final surface Hg levels at the minimum to maximum concentrations of 0.04% to 0.10 %atom-Hg by XPS, respectively. This I2 concentration was the minimum concentration which not only could remove Hg on the steel surface, but also in the depth profile with comparable performance with the higher I2 concentrations.