Practical Engineering of Corrosion Control
Authors: Baumgartner A.W. 
Published: December 2007

A great and wide variety of approaches to corrosion control exists today. This paper presents and discusses the corrosion prevention and protection techniques most frequently used. Material selection, protective coatings, cathodic protection, chemical inhibitors, mechanical handling of produced and injected fluids and other important factors are outlined. Various approaches to corrosion control are discussed from the standpoint of their applicability and limitations in controlling typical corrosion problems encountered in primary and secondary recovery of oil. These data are intended for use as a guide in establishing and achieving effective and economical corrosion control programs.

Introduction

Practical engineering of corrosion control requires first that the type and nature of the corrosion mechanism be defined. This is best accomplished through a study of the problem by an experienced investigator. Initial definition of the nature of the problem cannot be overemphasized. Analysis of water, the deposit, equipment and other samples by a trained expert is necessary. Following analysis and definition of the problem, corrosion control steps may be outlined and initiated. An evaluation program should be provided to determine the effectiveness of the action taken.

Review

All corrosion referred to in this paper shall be considered an electrochemical process. Three basic requirements must usually be met before corrosion can occur in any water or oil producing, gathering, or distribution system.

An anode (area of positive electrical potential) and a cathode (area of negative electrical potential) must exist. An electrical circuit or couple must be established betweenthe anode and cathode. The anode and cathode must be immersed in a solution thatwill conduct an electric current (electrolyte).

Satisfying these criteria will produce corrosion. However, presence of economically significant corrosion is dependent upon the magnitude of the potential difference between the anode and cathode and other factors that represent a resistance or hindrance to corrosion reactions. Briefly, these corrosion reactions occur as follows: metal at the anode loses electrons and enters the electrolyte as positively charged soluble ions. Electrons migrate through the metal to the cathode (current flow is described as being in the opposite direction through the electrolyte). The negatively charged cathode attracts positively charged hydrogen, which arises from the ionization or dissociation of water. At the cathode, hydrogen ions gain an electron and become hydrogen atoms. As a gas, the hydrogen may actually coat the cathode and render it passive (polarization).

When corrosion progresses at an economically significant rate some form of cathodic depolarization is active. That is, a reaction occurs to remove the hydrogen which has accumulated at the cathode. Effective cathodic depolarizers may be dissolved oxygen, sulfate-reducing bacteria, in an acid pH the evolution of hydrogen gas itself, or other factors. Corrosion control programs may be aimed in any of the following three major directions.

Removal of offensive chemical reactants from the water. In thecase of oxygen, for example, this may be attempted mechanically bypreventing its entrance into the system or by deaeration. Chemicaloxygen scavengers are often applied. Isolation and separation of the systems. Insulated flanges mayenable anodes and cathodes to be separated. Polar film formingchemicals are regularly added to satiate the negatively chargedcathode and insulate or shield the metal from the water. Plasticand cement linings are applied to mechanically isolate the metal(anode and cathode) from the electrolyte. Resistant alloys arealso used. Impression of an electric current to overcome or neutralize thecurrent established between the cathode and anode in the corrosionprocess has had success, while sacrificial anodes have hadwidespread application.

Corrosion Control Methods

Specific approaches to control of typically encountered problems are discussed in the following section.

JPT

P. 615ˆ