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Coatings & Corrosion Control Microbiologically Influenced Corrosion (MIC) By Bob Winters Recently, the use of certain environmental-friendly waxes or "semi-hard coatings", used by barge fabrica- tors to protect the internal void walls and floors of marine double-hulled steel barges from corrosion, have fallen under scrutiny by some barge owners/operators. It is believed that certain paraffin wax coatings act as a food source for certain living corrosive microorgan- isms. These single-celled organisms are associated with a phenomenon known as Microbiologically Influenced Corrosion (MIC) that can cause localized pitting and eventually leaks in steel barge voids. This following examines the biological and chemical research that has recently been performed on the Jotun Paints, Inc. "beeswax" coating by two independent lab- oratories. The research confirms that the wax coating is of the highest quality, effectively mitigates corro- sion, and does not introduce nor promote the growth of microorganisms when properly applied in barge voids. Allegations that beeswax coatings support MIC in barge voids must be with regard to other hydrocarbon type waxes. The void space between the outer and inner walls of double hull barges used to carry various cargos (i.e.. grain, aggregate, salt, coal and hydrocarbons) is often susceptible to condensation and resulting corrosion. The voids in barge compartments are accessible by opening sealed watertight hatches and climbing down fore and aft ladders for periodic floor and wall integri- ty inspections. Voids should be kept clean and dry. They are not recommended for use as ballast tanks dur- ing cargo loading and unloading operations. To help control corrosion, barge fabricators often apply coat- ings such as waxes, alkyds. vinyls, and epoxies to the cleaned, prepared metal surfaces in voids. Recently, some barge owners/operators have charged wax coat- ings applied in barge voids as being a food source for microorganisms and with the rampant spread of these bacteria associated with MIC. However, a certain wax coating composed primarily of "beeswax" has been researched by several highly regarded independent U.S. laboratories and found to be sterile of microor- ganisms in the virgin state and not susceptible to microbial degradation in barge voids. Mechanisms of MIC Microbiologically Influenced Corrosion (MIC), also known as biological or bio-corrosion, refers to corro- sion that is initiated or accelerated by microorganisms. It is commonly known that microorganisms (tiny one- celled organisms such as bacteria, fungi, and diatoms - unicellular alaae) are found living in almost every Typical configuration of void space in double-hulled barge. Minor localized pitting noted at damaged wax coating areas on the bottom of void space in the representative barge. Table 1 Microorganisms recovered and examined for and their relative corrosion Organism Aeromonas Pseudomonas Vibrio Clostridium Type Slime former Slime former Slime former Acid Producing Bacteria (APB) Corrosion Effect (0 - 4) 0 0 0 Sulfate Reducing Bacteria SRB Fusarium oxysporum Filamentous mould (fungus) 0-2 Comments Helps others but not a risk by itself Helps others but not a risk by itself Helps others but not a risk by itself Produces organic acids in small amts.; associated with other organisms Corrosive Some varieties have been associated with corrosion under certain conditions; most are Saprophytes. Recovered (frequency) Yes (all) Yes (all) Yes (all) Yes (2) No Yes (1) Note: This represents a subjective ranking as no work was conducted in this phase to examine the effects of these species on corrosion. aqueous environment on earth. MIC is caused by a few specific genera of microorganisms that attach to a firm steel surface to feed on and metabolize selective nutri- ents and other elements found in most types of water environments, to fulfill their life cycles. Microorganisms require water to propagate (live) .... no water, no MIC, no corrosion. A food source and specific environments can enhance the growth of microbes. The instant a metallic surface is immersed in water, a microbial mass or biofilm begins to form. However, this does not mean that all species in the biofilm are directly or indirectly corrosive to steel. Several types of microorganisms can play a role in MIC, but many do not. Free-swimming microbes (planktonic) float freely in the water medium and are not considered harmful to steel. On the other hand, when certain free-floating microbial cells attach to the steel surface and become sessile at specific metal receptor sites or inclusions, the initiation of a tubercule or nodule on the steel surface can be seen. Localized pitting or the MIC phenomenon occurs under the nod- ules. One of the most effective means of controlling MIC and the pitting of steel in barge voids is through the use of barrier coatings. The Jotun void wax is a barrier coating, that when applied in dry barge voids, has been a very good barrier and "first line of defense" in preventing general corrosion, bacterial, fungal and viral propagation and MIC. Natural substances found in this unique coating actually ward off microorgan- isms. Beeswax and Propolis A primary component of the Jotun void coating is beeswax. Beeswax is a tough wax (ester) formed from a mixture of several compounds secreted by honey- bees. In conjunction with the beeswax and honey, propolis is another one of the great products the bees produce. It is a resinous substance that the bees gather from tree leaves and bark, and combine it with nectar, beeswax, pollen, and bee bread to make a natural "glue" type substance. This glue is used to seal hive cracks and holes. It is also placed at the entrance to the beehive, where incoming workers have to brush up against it as they enter the hive. This sterilizes the bees from infection, and may disinfect them upon entry as well. Propolis is also used to line the wax birthing chamber where the queen lays her eggs, thereby pro- viding a clean, sterile environment for the developing eggs. Researchers have found that propolis contains all the known vitamins except for vitamin K. Of all the 14 minerals the human body requires for normal function, propolis contains all but one, sulfur. It contains a num- ber of unidentifiable compounds that create a perfectly balanced food substance. It also is composed of 16 amino acids that have been identified, and more biofla- vanoids (necessary for anti-inflammatory action within the human body) than found in oranges. The propolis found in beeswax is antibacterial, antiviral, antioxi- dant, antifungal, and anti-inflammatory. The Jotun beeswax has had a similar "no growth" effect on (Continued on page 50) Maritime Reporter & Engineering News
