This paper deals with the phenomena of corrosion by acetic acid and carbon dioxide at the top of a horizontal pipeline under dewing conditions. The effect of different parameters known to influence the Top of the Line Corrosion (TLC), such as the condensation rate and the bulk temperature is studied. The free acetic acid concentration varies from 0 to 1000 ppm, the bulk temperature, is set mainly at 70oC, the condensation rate is varied while the partial pressure of carbon dioxide and the gas velocity are set at a fixed value. The presence of acetic acid increases the corrosion rate both at the top and at the bottom of the line in different ways. The condensation rate influences strongly the top of the line corrosion when it has a small influence at the bottom. Evidences of localized corrosion are found at the bottom of the line. The corrosion at the top is uniform in the experiments conducted.
Top of the Line Corrosion occurs in the transportation of wet gas when significant heat exchange between the inside of the pipe and the outside environment is present causing the water vapor carried by the wet gas to condense on the interior wall of the pipe. Due to gravity forces, most of the condensed water drains to the bottom of the pipe and a thin film of condensed water forms on the sides and at the top of the internal pipe walls (see Figure 3). Continuous injections of inhibitors can prevent corrosion at the bottom of the pipe but at the top and on the sides, it is not possible. Therefore the condensation of wet gas can lead to a very corrosive environment.
In 1999 Gunaltun1 presented a complete visual inspection of a TLC case under stratified flow regime. He divided the pipe wall in three different areas:
- The bottom of the pipe where the corrosion is uniform and where the rate can be lowered with the use of inhibitors.
- At the sidewall of the pipe where the condensed water drains to the bottom. The corrosion is also uniform but inhibitors are not efficient.
- At the top of the line where a protective iron carbonate layer can be formed in
certain cases. Inhibitors are not effective and localized corrosion occurs.
Many papers4-7 have been published describing the main parameters influencing TLC due to carbon dioxide. In each of them, the parameters are the temperature of the gas bulk, the temperature of the pipe wall, the total pressure of the system, the partial pressure of carbon dioxide, the gas velocity and the condensation rate. All of these parameters influence the corrosion rate in a complicated way but a common behavior is generally observed:
- At low condensation rates and at a bulk temperature higher than 70C, a protective film of iron carbonate appears at the surface of the pipe, which lowers the corrosion rate. The formation of this film is explained by the saturation of the liquid film and the precipitation of corrosion product, which locally lowers the pH. This film still exists but is no longer protective at 50 C and below due to the increase of the solubility iron carbonate.
- At high condensation rates, saturation cannot be reached and the corrosion rate can reach several mm/y. The corrosion rate is governed by the rate of the corrosive reaction and the rate of condensation. The former increases the amount of iron in the liquid film while the later decreases it. The corrosion rate depends on the balance of these two counteracting effects.
In 2002, Vitse2, 3 completed a study on the TLC due to carbon dioxide, in the same experimental loop used in this work. He presented sever
