Microbiologically Influenced Corrosion (MIC) is a result of many factors, including the formation of biofilms which can cause severe, under-deposit corrosion. This corrosion phenomenon is exacerbated in the cases where anaerobic conditions exist beneath the biofilm that can lead to the proliferation of sulfate reducing bacteria (SRB's). These SRB's metabolize sulfate to sulfide, which is known to be a strong aggressor against a variety of metals including higher alloy stainless steels.
Glutaraldehyde has a long history of established in-field effectiveness against SRB's and the penetration and removal of biofilms. Described herein are a number of laboratory and field test results that address control of MIC through the use of glutaraldehyde. In addition, a few significant case history examples will be highlighted.
Microbiologically Influenced Corrosion (MIC), sometimes referred to as biocorrosion, is a particularly aggressive form of under-deposit corrosion that has its genesis in microbiological presence and activity. MIC is a subset of a larger class of corrosion phenomena which includes electrochemical and galvanic corrosion; both of which are based on differential oxygen concentration cells. Recent research on MIC has identified many species of bacteria that can contribute to metal corrosion. Almost every common metal and alloy used today, with the possible exception of titanium, has been shown to be susceptible to MIC I.
The steps leading up to an MIC event are well defined and sequential (refer to Figure 1). Planktonic, or free-floating bacteria are prone to form sessile, or attached biofilms when their numbers are sufficiently high. The biofilm environment is one that is relatively impenetrable to dissolved oxygen in the bulk water and hence, creates an anaerobic condition. The anaerobic environment becomes ideal for the proliferation of sulfate reducing bacteria (SRB's) which metabolize sulfate ion into sulfide ion. The generated sulfide ion can then become protonated to form the corrosive acid, hydrogen sulfide. In addition to the SRB's, acid-producing and iron-oxidizing bacteria can also contribute to MIC related events.
Since the steps leading up to an MIC event are sequential, treatment of the problem early on in the sequence will reduce the chances for MIC to occur. Glutaraldehyde has been shown to be an effective treatment strategy for eliminating many of the steps leading up to an MIC event. Specifically, this paper will discuss how glutaraldehyde is particularly effective against biofilm formation and removal, elimination of both planktonic and sessile SRB's, and the removal of acid-producing and iron-oxidizing bacteria.
BIOFILM CONTROL
It's been well established that microorganisms that live within a biofilm community are much less susceptible to microbicides than are the same microorganisms living in the free-floating, or planktonic state. A prevailing theory on why this occurs is that the microbicide can't penetrate the biofilm due to poor diffusion within the biofilm matrix. However, biofilms are estimated to contain 98% water z with diffusion rates approaching 20-80% of those in pure water for solutes that are similar in size to common microbicides. 3 Biofilms are also known to possess channels within the matrix that allow solutes to move freely within them.
In many cases, barriers to biofilm penetration by microbicides occur when the microbicide r e a c t s within the biofilm before it can penetrate. This appears to be the prevailing mechanism for oxidants such as hydrogen peroxide and chlorine. 4 Another theory postulates that biofilm bacteria are in a slowo growth or non-growth phase, which makes them
