Corrosion can be defined as the deterioration of material by reaction to its environment. The corrosion occurs because of the tendency for most metals to return to their natural state; e.g., iron in the presence of water and oxygen (or even moist air) will revert to its natural state, iron oxide.
The development of biofilms and the role they play in corrosion and deposition processes may be the most misunderstood and baffling factor in the management of cooling and other industrial water systems.
Ask any water treatment professional about the major attributes of a typical corrosion cell, and you will usually get a reasonable explanation. Ask about biofilms, and you probably wonít get a response.
This review is designed to provide a basic understanding of what biofilms are, the problems they can cause, and what might be done to deal with them.
We need to begin by answering the basic question, what is a biofilm? Simply stated, a biofilm consists of microbial cells (algal, fungal, or bacterial) and the extracellular biopolymer the cells produce. Generally, it is bacterial biofilms that are most problematic in industrial water systems, since they are responsible for the blockage and fouling of heat transfer equipment. One should keep in mind that the more nutrients available in the form of useable organic carbon, the greater the diversity and numbers of organisms that can be supported. When dealing with cooling towers and spray ponds, algal biofilms are also a concern. Not only will algal biofilms foul distribution decks and tower fill, but algae will also provide nutrient (organic carbon) that will help support the growth of bacteria and fungi. Algae does not require organic carbon for growth but instead utilizes CO2 and the energy provided by the sun to manufacture carbohydrates. So, a cooling water system that begins with little or no organic carbon can generate a supply through the growth and dispersal of algal cells.
The second question that needs to be answered is, why does biofilm develop in some areas rather that others? In many instances, the piping and other apparatus serviced by a cooling tower is comprised of various and dissimilar materials. Figure 1 below is typical of most applications. Dissimilar materials in piping and process systems create galvanic corrosion which is
Fig. 1 - Dissimilar metals
the source of the food, energy, and oxygen needed for the multiplication of the microorganisms that generate the biofilm as a home. The bacteria which is the heart of the biofilm formations needs basically the same things that all living creatures need: oxygen, food, energy, and a home (shelter from predators [biocides]). Although there is no absolute relationship between existing galvanic action, which generates a food, energy, and oxygen supply, and the development of the microorganisms that create biofilm homes (shelter from predators [biocides]), the presence of a ready food, energy, and oxygen source certainly makes microbial development stress free for the ìcrittersî. However, one cannot categorically state ìif electrolytic corrosion is allowed to begin-then microbial corrosion will immediately followî, but a prudent facilities manager will not rest until the electrolytic corrosion has been neutralized.
Fig. 2 - Galvanic corrosion
As Fig. 2 illustrates, galvanic activity generates iron oxide and two free electrons. Due to the impurities in steel and also due to mechanical stress, steel does not have a uniform standard reduction potential across its surface. The result is that certain areas on the surface are more prone to pitting and corrosion than others. When dissimilar metals are in electrical or physical contact (or through an electrolyte, i.e. water), galvanic corrosion can take place.
The process is similar to a DC cell in that the more active metal (less Nobel) becomes the anode and corrodes, and the less active metal behaves like a cathode and is protected. Corrosion is often defined as the deterioration of material by reaction to its environment.
In microbiological corrosion (as shown in Fig 3), the iron reduces to Fe+++ + 1e- (unlike galvanic which is Fe++ + 2e-) and the electron is the energy for growth; the oxide is converted to oxygen for the bacteria. The secretion of the iron may be in any one of several forms, but all of the forms become housing- or sacrificial shelter- for the generating organism.
Microorganisms can influence corrosion in a variety of ways. Formation of localized differential cells, the production of mineral and organic acids, ammonia production, and sulfate reduction are just a few of the mechanisms by which bacteria, fungi, and algae can influence corrosion. The formation of localized cells is the primary mechanism of corrosion caused by iron oxidizing bacteria.
When iron and manganese oxidizing organisms colonize a surface, they immediately begin to oxidize available reduced forms of these elements and produce a deposit.
As the bacteria colony becomes encrusted with iron (or manganese) oxide, a differential oxygen concentration cell will develop, and the corrosion process begins. The porous encrustation (tubercle) may potentially become an autocatalytic corrosion cell or may provide an environment suitable for the growth of sulfate-reducing bacteria.
Corrosion may also develop when localized cells are formed, due simply to biofilms developing on metal surfaces (when no galvanic corrosion had ever existed). The oxidation of iron or manganese is not always a requirement for the development of a localized corrosion cell.
There are numerous factors that can contribute to localized corrosion on metal surfaces. The production of ammonia by the reduction of nitrates may lead to severe localized losses in copper based metallurgy. Inorganic acids, such as sulfuric acid produced by Thiobacillus sp., can also have detrimental surface effects. As biofilms develop, they will eventually achieve a thickness at which oxygen concentration is either very low or completely excluded. At a thickness of just 200 microns, the oxygen concentration within the biofilm is reduced to near zero ppm. When this occurs, facultative and obligate anaerobes can flourish. The import is that anaerobes must generate their own oxygen which means that corrosion is, and must be for sustenance and growth of the anaerobes, rampant.
Fig. 4 - Corrosion by SRB (sulfate-reducing bacteria)
Anaerobic sulfate-reducing bacteria, such as Desulfovibrio sp., is the bacteria most often considered when discussing microbial corrosion. This organism will seek out and colonize areas deficient in oxygen, such as those found within porous corrosion tubercles, within biofilms, and under debris. This bacteria is responsible for severe metal loss in industrial water systems. This type of corrosion is easily recognizable from the characteristic sulfide by-product present within the corrosion cell. Sulfate reducing bacteria primarily cause corrosion by utilizing the molecular hydrogen produced at the cathode, thereby depolarizing it. Systems that are sulfate limited will have less of a tendency to be attacked by SRB.
Fig. 5 - Fimbriae & multiple fimbriae floating through pipe
Microorganisms can be found in both the bulk water and on the surfaces of industrial water systems. Bacteria attach to surfaces by proteinaceous appendages referred to as fimbriae (Fig. 5).
Fig. 6 - Attachment of fimbriae to a surface
Once a number of fimbriae have "glued" the cell to the surface (Fig. 6 above), it makes detachment of the organism very difficult. One reason bacteria prefer to attach to surfaces is the organic molecules adsorbed there provide nutrients (oxygen & energy).
Fig. 7 - Attached fimbriae is surrounded by extracellular biopolymer
Once attached, the organisms begin to produce material termed extracellular biopolymer or "slime" for short. The amount of biopolymer produced can exceed the mass of the bacterial cell by a factor of 100 or more. The extracellular polymer that is produced provides a more suitable protective environment for the survival of the organism.
It can be seen that the growth of bacteria on surfaces in cooling and process water systems can lead to significant deposit and corrosion problems. Once this is understood, then the importance of controlling biofilms becomes quite clear. Too often microbiological control efforts focus only on planktonic counts, that is to say the number of bacteria in the bulk water. While useful data can be gathered from monitoring daily bacterial counts, monthly or weekly counts are simply time wasted and have little meaningful use if one is attempting to assess the presence and quantity of the microorganisms that produce biofilm. The microorganisms that produce extracellular polymer and hide under and within the slime are not borne freely through the bulk water to be counted as part of plankton count. Therefore, Planktonic counts do
Fig. 8 - The microorganism has begun to cover itself for protection
not have any correlation to the amount of biofilm that may be present. In addition, planktonic organisms are not generally responsible for deposit and corrosion problems. There are a few exceptions, generally within closed loop systems, where planktonic organisms may degrade corrosion inhibitors, produce high levels of H2S, or reduce the pH. Systems with low planktonic counts may have a significant biofilm problem and vice versa. Therefore, efforts should focus on general biofilm detection and control.
Fig. 9 - Multiple colonies develop as oxygen, energy and nutrients permit
Once bacteria begin to colonize surfaces and produce biofilms, maintenance and operational problems arise, including reduction of flow, reduction in heat transfer rates, fouling, corrosion, and scale. When biofilms develop in low flow areas, such as cooling tower film fill or piping ìdead legsî, they may go unnoticed for months or years, since they will not initially interfere with flow or evaporative efficiency. After time, the biofilm becomes more complex, accompanied by robust filamentous development. The matrix provided will accumulate debris that will initially impede flow and eventually completely block flow at heat exchangers and cooling jackets. Any sudden increase in nutrient supply in a previously nutrient-limited environment will send bacterial populations into an accelerated logarithmic growth phase with rapid accumulation of biofilm. All of the biofilms that develop, if left uncontrolled, will eventually interfere with heat transfer efficiency.
Fig. 10 - In time the original colony has insulated itself
Biofilm structure is often imagined as a coating of microbial cells and the secreted biopolymer spread evenly across a surface. In reality, biofilm structure is much more complex, including patchy and highly channelized development, which allows nutrient-bearing waters to flow through the extracellular polymer providing the microorganism with a ready nutrient supply. If allowed to multiply unrestrained, the organism eventually insulates itself in the shelter of slime it constructs and becomes almost impossible to destroy.
Algal biofilms will foul cooling tower distribution decks, tower fill, and basins. When excessive algal biofilms develop, portions eventually break loose and transport to other parts of the system, causing blockage and providing nutrients for accelerated bacterial and fungal growth. Biofilms can cause fouling of filtration and ion exchange equipment . As seen below, biofilm is the most damaging development that can occur in a heat exchange environment.
Thermal conductivity comparison of deposit-forming compounds and biofilm
A lower number indicates a greater resistance to heat transfer.
Coatings and deposits in the form of biofilm and biofilm with entrapped suspended debris are foulants that most of us can comprehend, but biofilms often lead to the additional formation of mineral scales as well. Calcium ions can attach to the biofilm by the attraction of carboxylate functional groups on the polysaccharides, or calcites may simply become trapped in the matrix. In fact, divalent cations, such as calcium and magnesium, are integral to the formation of certain extracellular polysaccharide gels.
Many of the microorganisms develop more rapidly in elevated temperatures and therefore seek out heat exchanger entrances and surfaces, which means the calcium ions are fixed in place at the heat transfer surface. This further complicates and impedes the heat exchange efficiencies because the ions are then positioned to react with carbonate or phosphate anions as they become available, which provides additional nucleation or crystal growth sites in and on the biofilm.
Biofilms can be controlled through the use of microbicides, biodispersants, and by limiting nutrient. Microbiocides, both oxidizing and nonoxidizing, can be effective in overall biofilm control when applied properly. The oxidizing microbicides, such as chlorine, bromine, chlorine dioxide, and ozone, can be extremely effective in destroying both the extracellular polysaccharide and the bacterial cells. When using oxidizing microbicides, one must be sure to obtain a sufficient residual for a long enough duration to effectively oxidize the biofilm. Unfortunately, there are those who are overly concerned with the corrosive nature of the oxidizing microbicides and fail to apply the needed residual oxidant required to control biofilm. Low residual oxidant levels may significantly reduce planktonic counts but may not be sufficient to control biofilm. The level of oxidant and duration required will vary from system to system. It is generally more effective to maintain a higher residual for a duration of several hours than it is to continuously maintain a low residual. Continuous low-level feed may not achieve an oxidant level sufficient to oxidize the polysaccharide and expose the bacteria to the oxidant.
Another misconception is with the use of chlorine at alkaline pH (> 8.0). It is often thought that chlorine is ineffective for controlling microorganisms at elevated pH. This is only half true. Certainly, the hypohalous acid form of chlorine (HOCl) is more effective at killing cells than the hypohalite form (OCl-). However, the hypohalite is actually very effective at oxidizing the extracellular polysaccharide and the proteinaceous attachment structures. Therefore, the use of chlorine in alkaline cooling waters can still be extremely effective. This is especially true when combining chlorine with bromine or with a compatible nonoxidizing microbiocide such as a polyquat. When this is done, you achieve both oxidation of the extracellular material and sufficient kill of the microorganism.
Nonoxidizing microbicides are also effective in controlling biofilm. Effective control is greatly dependent on frequency of addition, level of feed, and resistance of the incumbent population to the product being fed. Control cannot generally be achieved by once-a-week additions as is common in "full service" applications. Typical application for effective control may include a slug addition of product 2 to 5 times a week. As with oxidizing microbicides, frequency and dosage will depend on the system conditions. It is generally most effective to alternate nonoxidizing microbicides at every addition to ensure broad spectrum control. Most nonoxidizing microbicides will have little effect in destroying the extracellular polysaccharide found in the biofilm. However, many microbicides may be able to penetrate and kill bacteria found within the biofilm. Combining the use of nonoxidizing and oxidizing microbicides is a very effective means of controlling biofilm.
When using a nonoxidizing microbiocide in conjunction with an oxidizing agent, there should be no residual oxidant present in the system at the time of addition. Sufficient time should be allowed for the nonoxidizing microbiocide to work before resuming oxidant feed unless an oxidant compatible microbiocide is being used ( i.e., polyquat).
Biofilm control programs can be made more effective through the utilization of a biopenetrant/dispersant product. Products that penetrate and loosen the biopolymer matrix will not only help to slough the biofilm but will also expose the microorganisms to the effects of the microbiocide. These products are especially effective when dealing with systems that have a high TOC loading and a tendency to foul.
These products are typically fed in slug additions prior to microbiocide feed. Low-level continuous feed may not be as effective, since it often takes a certain threshold amount to produce the desired effect. Recent developments in biodispersant technology is making this approach more effective and popular than ever before.
Enzyme technologies that will break down the extracellular polysaccharides and degrade bacterial attachment structures (fimbriae) are currently being developed and patented. These technologies, although expensive, may provide biofilm control where microbiocide use is environmentally restricted or provide a means of quickly restoring fouled cooling water systems to a clean, efficient operable state. The importance of biofilm control must not be taken lightly. It is the fundamental basis for controlling a high percentage of deposition and corrosion problems in process and cooling water systems. Once these fundamentals are understood, effective treatment strategies can be developed.
The growth of bacteria and formation of biofilms may also result in another problem, that of corrosion. Microbiological corrosion may be defined as corrosion that is influenced by the growth of microorganisms, either directly or indirectly. To understand microbiological corrosion or MIC for short, it helps to have a basic understanding of corrosion chemistry. This document is not intended to provide that information, but any water treatment training manual will. In essence, corrosion occurs on a metal surface due to some inherent or environmental difference between one area on that surface and another. These differences will create anodic and cathodic areas, setting up a basic corrosion cell (Fig. 7). The anode is the area at which metal is lost. The electrons given up by the metal flow to the cathode to be consumed in a reduction reaction. Microbiological corrosion is electrochemical corrosion where in some manner the presence of the microorganisms is having some influence in the creation or acceleration of corrosion processes.