ABSTRACT

CO2 corrosion behavior of carbon steel in presence of deposits found in coolers of a gas compression plant was observed. To carry out this study weight loss tests were done, using carbon steels filmed with such deposits, and also in presence of corrosion inhibitor. Afterwards, surface analysis was done to determine the kind of damage caused in presence of these compounds.

INTRODUCTION

Natural gas produced in the Northern part of Monagas State, in Venezuela, has a high content of liquids (water and hydrocarbons) and average concentrations of CO2 and H2S of 7% and 60 ppm, respectively in addition to some contaminants, like asphaltenes.

CO2 and H2S concentrations referred, besides presence of water, are factors that promote internal corrosion processes in gas pipelines. Although asphaltenes that are present in the mentioned gas streams have inhibitor corrosion properties 1' 2, 3, they could promote underneath corrosion due to the formation of a non-uniform asphaltene film. In order to mitigate such corrosive processes in gas installations, a corrosion inhibitor was being applied in gas gathering and compression plants.

After chemical treatment was initiated, solid deposits were found at the end of a 8274 kPa (1200 Psi) cooler causing many problems in the operation of a compression plant.

The composition of these deposits was determined by different analytical techniques, such as GC/MS (Gas Chromatograph-Mass detector), observing that a considerable portion of the solids could be constituted by corrosion inhibitor degraded thermally in some step of the compression process. Other components of the deposit are shown in Figure 1.

Once the solid was characterized, corrosion behavior of API 5LB steel in presence of such deposit and also the interaction of this system (carbon steel-deposits) with a corrosion inhibitor was studied. In this way, weight loss tests in static autoclave were done at the laboratory, imitating conditions of the 8274 kPa cooler and the 8274 kPa compressor.

EXPERIMENTAL PROCEDURE

Weight Loss Tests

Weight loss technique was used to evaluate corrosion behavior of an API 5LB steel in presence of deposits found at the end of a 8274 kPa cooler; besides, the effect of a corrosion inhibitor on the corrosion process of the system (carbon steel-deposits) was studied. In order to achieve these objectives, the plan shown in Figure 2 was followed.

Degraded inhibitor samples used in some of the tests were prepared through thermal degradation tests carried out in an autoclave. These tests entail exposing 35 ml of the product to an atmosphere of 662 kPa (96 Psi) of CO2 and 0.3 (0.05 Psi) kPa of H2S, at 162°C (maximum temperature could be reached at the gas compression plant).

API 5LB coupons were polished with abrasive paper to get a 600 grit roughness; then they were degreased and weighed. In those tests corresponding to coupons covered with the deposit, a mild mixture was obtained by diluting this solid in THF; thus, coupons were immersed in the mixture.

Specimens were introduced into 1 L of 3,5% NaC1 solution by a period of 7 days. After this time, coupons were cleaned, and introduced in Clarke's solution (ASTM G1 standard), rinsed, to then obtain weigh loss and corrosion rate.

Finally, coupons were observed with SEM( Scanning Electron Microscope) technique to identify the morphology of corrosion that took place through the tests performed.

RESULTS

Effect of temperature and corrosion inhibitor on CO2 corrosion rate of API 5LB steel Figure 3 shows corrosion rate of carbon steel as a function of the temperature and the surface condition, with or without deposits coveting the sam

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