Microbiological Corrosion What Causes It and How It Can Be Controlled
Authors: Baumgartner A.W. 
Published: December 2007

BAUMGARTNER, A.W., ASSOCITE MEMBER AIME, BRADFORD LABORATORIES, DIV. OF HAGAN CHEMICALS and CONTROLS, INC., ABILENE, TEX.

A synopsis of conditions that must be present for corrosion of ferrous metals to occur is presented. These criteria are discussed as they are encountered in oil producing and gathering systems and in water-storage, transfer, treating and injecting equipment. The role of bacteria in corrosion is described in detail. Special emphasis is placed on sulfate-reducing "Desulfovibrio" bacteria, since these microorganisms are directly responsible for practically all corrosion attributable to bacteria in the oil-producing industry. typical situations that should lead operating personnel to suspect the presence of these microbes, as well as more specific methods for their detection, are given. Techniques for controlling undersirable bacteria are outlined. Critical considerations for both chemical and mechanical methods of eliminating or correcting microbial corrosion problems are also stressed.

Introduction

In defining oilfield corrosion Stiff"' says, "It is the holes in the industry's pockets". Many petroleum engineers prefer to define corrosion as a process that tends to increase the horizontal permeability of pipe, tubing, or casing. Actually, corrosion is a complicated process resulting in metal loss from wet metal surfaces, This paper primarily concerns the function of bacteria in contributing to corrosion of oilfield equipment, how to detect and evaluate their action, and some critical considerations in effectively controlling microbial corrosion. To appreciate and understand the role of bacteria in oilfield corrosion, it is necessary to be familiar with the basic fundamental concepts of corrosion of all iron metals.

Criteria for Corrosion

Today, most authorities explain the corrosion mechanism as an electrochemical process. For this process to take place, three requirements must be fulfilled.An electromotive force or potential difference must be present. Before a metal can corrode it must have an anode or area that has a positive potential which attracts negatively charged particles or ions (anions), and a cathode or area that has a negative charge or potential to which positively charged particles or ions (cations) are attracted.There must be an electrical circuit or couple established between the anode and the cathode.The anode and cathode, electrically connected, must be in contact with a solution that will conduct a current (electrolyte).

Water containing some dissolved salts serves this need. While satisfying all of these criteria will produce some corrosion and while the magnitude of the potential difference (electromotive force) determines the tendency for corrosion to occur, the actual rate of corrosion depends upon other factors that contribute varying degrees of resistance or inhibition to the process. Every new piece of oilfield iron product whether it be tubular goods, pumps, rods, pressure vessels, etc. contains anodic and cathodic areas. Normalizing or heat treating may reduce these areas somewhat, but a number always remain. Mill scale, an oxide of iron (Fe3O4), is cathodic to the anodic iron and represents another source of potential difference. The use of dissimilar metals is still another common occurrence of "built-in" anodes and cathodes. Generally, metals or alloys at the top of the electromotive or galvanic series (Fig. 1) are corroded when coupled to metals below them providing the other requirements for corrosion are met. In other words, a metal in this list is usually anodic to metals below it and cathodic to those above it. Of course, the second criterion for corrosion is easily met in most oil-producing or water-injection equipment because differences in potential may be found in the same section of metal or in different parts installed in intimate contact with each other. Use of insulated flanges can reduce or eliminate a significant portion of these electric couples.

JPT

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