Corrosion of Carbon Steel in Sour Water From the Oil Industry

Among the corrosion problems, one of the most important present in the refining oil industry, is specifically the related to the sour water generated in primary distillation plants. Despite the efforts made in the industrial field, and in fundamental studies; the different operating conditions as well as different concentrations of pollutants, makes their control very complicated. 

The studies that have been trying to simulate the operating conditions in sour waters have increased in the last ten years, in which the characteristics of the raw obtained, have changed resulting in corrosive environments much more aggressive than before. The purpose of these studies was to try to find ways, in which various agents affect or minimize corrosion, with the purpose of determining the predominant mechanism of corrosion in such environments. 

On the other hand, carbon steel AISI 1018 is widely used as construction material in many industries due to its excellent mechanical properties and low cost . So, the evaluation of carbon steel corrosion in H2S environments is very important in the petroleum industry, as this phenomenon is responsible for costly social, economic and sometimes even human losses . The sour water is considered as waste water containing sulfides and chlorides, among other components; this kind of water is very harmful to metallic equipments that are in contact with it, producing high corrosion rates leading to failures and material drainage and spilling . An important factor involved in the process of corrosion, is the temperature, because the corrosion rate increases as the temperature increase in the corrosion medium on the reactions proceeding in pure acids.

However, this does not happen in all systems, for example, the steel AISI 304 SS; results indicated higher corrosion at high temperatures (more than 75 ºC), but at mid temperatures (between 50 and 75 ºC) corrosion rates were similar; and at low or moderate temperatures (between 25 and 50 ºC), the stainless steels are in true passive state below the transpasive region. From early studies in the same work on the corrosion behaviour of AISI 304 using anodic polarization method, it was observed, a smaller passive range at higher temperature and hence an unstable passive film. Finally, failures due to wet H2S cracking have been experienced extensively in production operations, in refining and in pipelines. Many commonly used steels and alloys are susceptible to cracking under the prevailing high temperatures, pressures and stresses.

Source:http://www.electrochemsci.org/papers/vol6/6105016.pdf

CORROSION PROBLEMS IN PETROLEUM INDUSTRY

Corrosion costs of the oil industry are billions of dollars in a year. Corrosion affects every aspect of exploration and production, from offshore rigs to casing. Methods of control and techniques to monitor corrosion, along with an explanation of the chemical causes of corrosion are discussed.

Corrosion in petroleum industry

The Petroleum industry contains a wide variety of corrosive environments. Some of these are unique to this industry. Thus it is convenient to group all these environments together. Corrosion problems occur in the petroleum industry in at least three general areas: 

(1) production

(2) transportation and storage

(3) refinery operations.

Production 

Oil and gas fields consume a tremendous amount of iron and steel pipe, tubing, pumps, valves, and sucker rods. Leaks cause loss of oil and gas and also permit infiltration of water and silt, thus increasing corrosion damage. Saline water and sulphides are often present in oil and gas wells. Corrosion in wells occurs inside and outside the casing. Surface equipment is subject to atmospheric corrosion. In secondary recovery operations, water is pumped into the well to force up the oil. 

Transportation and storage 

Petroleum products are transported by tankers, pipelines, railway tank cars, and tank trucks. The outside submerged surfaces of tanks on the outside surface of underground pipeline s are protected with coatings and by using cathodic protection. Cathodic protection is also applied to the inside of tankers to prevent corrosion by seawater used for washing or ballast. Gasoline-carrying tankers present a more severe internal corrosion problem than oil tanks because the gasoline keeps the metal too clean. Oil leaves a film that affords some protection. Tank cars and tank trucks are coated on the outside for atmospheric corrosion. The main reason for internal corrosion of storage tanks is the presence of water which settles and remains on the bottom. Coatings and cathodic protection are used. Alkaline sodium chromate (or sodium nitrate) has been found to be an effective inhibitor for corrosion of domestic fuel oil tanks. Internal corrosion of product pipelines can be controlled with coatings and inhibitors (a few parts per million) such as amines and nitrites. Ingenious methods for coating pipelines in place underground have also been developed. 

Refinery operations 

Most of the corrosion difficulties in refineries are due to inorganics such as water, H2S, CO2, sulfuric acid, and sodium chloride, and not to the organics themselves. For this reason, the petroleum industry has much in common with the chemical industry. Corrosive agents may be classified into two general categories: (1) those present in feedstock or crude oil, and (2) those associated with processes or control. Water is usually present in crude oils, and complete removal is difficult. Water acts as an electrolyte and causes corrosion. It also tends to hydrolyze other materials, particularly chlorides, and thus forms an acidic environment.

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