Plastic Coating For Corrosion Control In Petro And Production Industry
Authors: Russell J.W. 
Published: December 2007

Corrosion engineers and design engineers have been striving for years to develop methods to economically transport and contain corrosive fluid. Oil field production and chemical plants are becoming more sophisticated. As technology advances, environments have become more damaging and more difficult to handle. The need for coating with a higher degree of protection has increased. Coatings are no longer "bally hooed" as a cure. Instead they are formulated for specific operating conditions.

Introduction

The primary requirement of a coating in any service is that it be stable in that service, that is, that it be resistant to the environment contacting it under the service conditions expected. The important parameters controlling the environment are its chemical nature, e.g., acid, caustic, H2S content, etc., the temperature and pressures involved, and flow rates of fluids contacted or transported. Additional factors are the presence of solids, changes of environment and rate of environmental change.

Of equal importance are the physical properties of the coating and their relation to the normal and abnormal physical treatment to which the coating will be subjected during its designed service, as well as during transporting to and installation at the service area. These concepts are particularly important in oil field usage since handling procedures are generally "rough" and are carried out with a minimum of technical supervision. Among the physical and mechanical properties to be considered in determining the service of a coating are its resistance to impact, bending and abrasion, and its adhesion to the base metal. There is no coating available economically which possesses all of the desirable physical and chemical factors for all oil field services which can be encountered. A balance must be struck between the various properties to determine the optimum coating under specified environmental condition.

PURPOSES OF COATINGS IN OIL FIELD SERVICE PURPOSES OF COATINGS IN OIL FIELD SERVICE One of the purposes of baked-on plastic coatings for oil field service is to prevent the attack of corrosive fluids contacting metal equipment. The most commonly used metal in oil field service is carbon steel which is susceptible to corrosion by aqueous liquids containing dissolved salts and/or gases such as carbon dioxide, hydrogen sulfide, organic acids, e.g., acetic and oxygen. These fluids are often encountered during the normal production of oil, gas, and gas condensate and in the handling of fresh waters, as well as brine in water flooding and water disposal operations. In addition to these fluids which must be combated more or less continuously during producing operations, there are other corrosive fluids which may be encountered from time to time for comparatively brief intervals during the drilling and other operations. Examples are the highly alkaline fluids or muds used during drilling operations and the strong mineral acids used for various well completion and stimulation procedures. The alkaline muds often contain free sodium hydroxide and have pH values as high as 11.5. These high pH muds are very corrosive to aluminum, which is now finding increasing usage as drill pipe.

Mixtures of hydrochloric and hydrofluoric acid and, to a lesser extent, acetic acid are used for removal of drilling mud residues and for well stimulation through acidizing and water block removal. These mixtures are treated with either organic inhibitors or arsenic salts to reduce their normally high corrosion rates on oil field steels; however, the inhibitors become less effective with increasing temperatures and contact time of the acid on the metal. Furthermore, they are not satisfactory for protection of the high-strength steels such as N-80 and P-105 at elevated temperatures, even with short contact time.

Baked-on coatings are used for external, as well as internal protection of oil field tubular goods. In multiple completion wells with production through an annulus, protection is obtained by coating the external surface of the inner string as well as the internal surface of the outer string.