Research - (2021) Volume 10, Issue 3
Microbiologically influenced corrosion (MIC) and biofouling both begin with an initial layer of bacteria accumulating on a hard surface exposed to the natural environment. These bacteria quickly form a biofilm which becomes the feeding source for marine life fouling and the root of both of these highly damaging, expensive types of corrosion. Preventative methods for biofilm development is an ongoing field of study due to critical necessity in many industries including healthcare, aerospace, and oil and gas. Today, biofilm inhibitors for the oil and gas industry may include regular cleaning or scraping of the affected surface, electrochemical processes, or biocide injections which have a negative impact on the environment and provide only temporary relief from MIC. This constant need for MIC and fouling remediation creates a great demand and thus market potential for long-term, more environmentally conscious methods to mitigate and control biofilm development. This study investigates the incorporation of well-known biocidal materials as well as one commercial additive into the fabrication process of underwater structures and surfaces. High Density Polyethylene (HDPE) and Fiber Reinforced Plastic (FRP) with antimicrobial additive were processed. Experiments were conducted per ASTM E2149-13a and F895 to evaluate antibacterial efficacy in the laboratory. Field tests were constructed per ASTM D3623 - 78a for material evaluation in offshore fouling conditions. The manufactured materials were tested against gram-positive and gram-negative bacteria and fouling microorganisms to analyze the effectiveness of biofilm prevention. Results showed positive efficacy of biocidal additives incorporated through the fabrication process in all cases including copper, multiple forms of zinc, and titanium dioxide. The commercially available additive produced the largest zone of inhibition and highest reduction of colony forming units in dynamic flow conditions. Fouling tests show that the incorporation of the additive into HDPE and FRP provides a surface protection and thus serves as an agent for material preservation. Results from this study demonstrate innovative and effective methods for surface protection from MIC and biofouling by incorporating antimicrobial additives into the structural matrix during the manufacturing process.
High Density Polyethylene, Fiber Reinforced Plastic, MIC
Microbes, specifically in the form of biofilms, are a pervasive threat in the oil and gas industry and often result in microbiologically influenced corrosion (MIC) [1-3]. These biofilms form when microorganisms join to form a group and attach themselves to a variety of surfaces including piping, aquatic structures, medical equipment, and even teeth (plaque) [4-7]. The biofilm will begin to react with the chosen substrate of attachment and a deterioration process will take place [8]. MIC clogs up pipelines, increases power consumption and results in equipment corrosion in water injection pipelines [9]. It is well recognized that both chemical and microbiological mechanisms contribute to corrosion and that an estimated 40% of all pipeline corrosion can be attributed to microbiologically influenced corrosion [10,11]. In this type of corrosion, microorganisms cause stress cracking in both metallic and non-metallic materials by forming colonies and eating away at the material surface [11]. MIC has been well documented in substrates exposed to a variety of aqueous environments including seawater, freshwater, soils, and fuels [12-14]. While MIC is commonly caused by sulfate-reducing bacteria (SRB) [15-17], these bacteria can combine with other bacteria and form a more complex, highly aggressive biofil m [1-3,18]
A related phenomenon that occurs when surfaces interact with natural environments such as soil and water is biocorrosion, also known as biofouling [19]. Biofouling was identified more than one hundred years ago, accounts for up to 20% of all corrosion costs, and occurs when a biofilm develops, and marine life begins to attach causing blockages or growths that develop on the hard surface [19-20]. These growths can be made up of microorganisms, algae, plants, and even animals [21]. Biofouling hinders efficiency and increases cost of operation by escalating drag and weight resulting in higher fuel costs and less efficiency as well as the additional cost to remove the buildup on a regular basis [22,23]. Each year, biocorrosion causes billions of dollars of economic losses in the USA [23].
Prevention of chemical corrosion (the most common and well known type) is managed using coatings, sacrificial coatings, linings, environmental controls, corrosion inhibitors, or specific designs and metal choices [24]. These methods are so easily implemented that it is rare to encounter an asset exposed to the environment in oil and gas, aerospace, and the marine industries that is not protected in some way [25]. In 2006, the Trans-Alaska Pipeline suffered from a leak that created significant environmental and economic impact because conventional methods for corrosion prevention were inadequate at prohibiting MIC [26].
Polyethylene is a widely used material in a variety of industries because of its low cost, chemical resistance, and long lifespan and was once considered to be a potential solution for MIC [27]. However, it has become clear that the limiting factor of polyethylene is the susceptibility of bacteria growth on the surface [28]. The rates of chemical and physical degradation are higher when compared the damage caused by microbes, but the effects of biodegradation can be more impactful [18,29]. Additionally, combinations of natural factors such as exposure to UV in conjunction with the presence of bacteria and fungi can rapidly accelerate the breakdown of polymers such as polyethylene [30,31]. Studies have shown the path of biodegradation of polymers as 1. attachment of the microorganism to the surface of the polymer, 2. growth of the microorganism using the polymer as the carbon source, 3. initial degradation of the polymer, and 4. Ultimate degradation [30]. Degradation of a polymer material such as HDPE leads to decrease in molecular weight, tensile strength, viscosity, and ultimately, failure. Recent research shows specific degradation rates of HDPE using a variety of offshore and onshore bacteria and fungi exposed to different environmental conditions. Results show that HDPE is susceptible to both MIC and biofouling and that the material failure is greatly enhanced by exposure to high temperatures, humidity, and UV energy [32-34]. Fiberglass has become increasingly accepted as a structural and corrosion solution for offshore and onshore applications because of the inherent qualities of fiberglass, some of which include, slip resistance, electrical and thermal non-conductivity, light weight, non-corrosive, and low maintenance repairs [35,36]. However, resistance to chemical corrosion does not imply resistance to biofouling. When barnacles and other hard shell marine life adhere to fiberglass, they continually grow and exert considerable pressure on the area where they are attached [37]. Fiberglass structures exhibit attractive mechanical properties such as hardness and toughness, but they are not protected against the destructive nature of the barnacle, mollusk, and a wide variety of microorganisms living in the soil and wat er [38].
For biofouling, the most accepted solution currently is treating the substrate with an antifouling paint or coating [37]. Antifouling paints contain biocides that repel fouling organisms when released at a controlled rate into the water adjacent to the structure [38]. The most common biocides incorporated into the coatings are copper, cuprous oxide, zinc, zinc oxide, and zinc pyrithione [39]. The rate of release of biocides is critical for efficacy; if it is too fast, the antifouling will fail prematurely, especially after a period of intense activity, while if it is too slow, the antifouling will be ineffective, particularly in areas with a high fouling challenge. The fouling organisms must be prevented from attaching and growing on the surface. Once this happens, growth is extremely rapid, and the organisms are beyond the influence of antifouling paints and can only be removed by scrubbing and scraping by underwater divers which is both costly and time consuming [37].
Overall, there is significant literature on studies and products that prevent biofouling through the use of a coating and/or biocide cleaning of the surface [7,10,15,38] The emerging theme, however, is that while the materials are capable of protecting surfaces against biofilm, they only provide protection in the short term and dependent upon the thermal and mechanical performance of the paint and coating and often mixed with complex chemicals that induce environmental risks [40]. This constant need for MIC and fouling remediation creates a great demand and thus market potential for long-term, more environmentally conscious methods to mitigate and control biofilm development [40]. This study investigates the development of such a method incorporating well-known biocidal materials as well as one commercial additive into the fabrication process of underwater structures and surfaces.
Biocidal materials
The biocidal materials used in this study were selected based on their documented bactericidal efficacy as well as their accessibility in powder form which was required for the fabrication process [7,10,27,41] These include zinc (44 micron, Belmont Metals, 8024A), zinc oxide (44 micron, ZoChem, ZOX-800), and zinc pyrithione (5 micron, TCI, M0633). The commercial product MIC-GUARD (90 micron, BTG Products, MG) is a MIC and biofouling inhibitor that is advertised as added directly into the coating or lining during the manufacturing process. This product is patent protected, and the technology is proprietary, but the data sheets describe it as a powder additive for biofilm prevention. The MIC-GUARD (MG) additive can be added to coatings and linings according to thickness of material, environment, and cost. This product claims to have enough versatility that it can be added to a variety of material at various percentages to prohibit biofilm development.
Microbiological testing
Microbiological testing was conducted at West Texas A and M University in a certified, third-party laboratory. The test facility used ASTM standards E2149-13a and F895 to evaluate the efficacy of biocides in FRP and HDPE.
ASTM F895 is a test method useful for assessing the cytotoxic potential of new materials and formulations and as part of a quality control program for established medical devices and components. Utilizing this testing method provided qualitative results of the potency of antimicrobial powders. Antimicrobial powders including zinc, zinc oxide, zinc pyrithione, and MICGUARD were poured into several wells that had been punched into the inoculated agar. The testing wells were then compared to one another based on the zone of inhibition of cell or spore growth each has created. The petri dishes were place in an incubator at 78 degrees Fahrenheit for 24 hours. The zones of inhibition each well had around itself were then compared. The top two antimicrobial additives from this test were used moving forward.
ASTM E2149 -13a is a test standard for determining antimicrobial activity of biocides under dynamic contact conditions. The 2 × 2 inch samples were placed in sterilized 500 mL flasks with 100 mL of dH2O. For HDPE, gram-positive bacteria, S. aureus was added to each flask at 3 × 106 VC/mL. For fiberglass, gram-negative bacteria, E. coli at approximately 1 × 106 cells/mL was added to each flask. Samples (3 × 0.1 mL) were taken immediately from each flask and spread on TSA plates. The flasks were incubated in a 37°C shaking incubator at 200 rpm. Samples (3 × 0.1 mL) were removed and plated at 1-hr intervals for three hours. Plates were left for growth at 25°C for 60 hr. Colonies were counted using Sphere Flash and the average VC/mL or colony forming units (CFU) were documented.
Sacred Heart Marine Research Center (SHMRC) is a primary test facility for immersion and is located in Karrapad Cove or Tuticorin Bay in south India. This facility is in close proximity to the floating test platforms in the protected bay area and enables SHMRC to expand its research and testing capabilities in marine coatings evaluation and marine research. Samples were sent to SHMRC to be submerged in a static immersion test per ASTM D3623-78a.
ASTM D3623-78a is a test is used to evaluate antifouling panels in shallow submergence. Static immersion remains a necessary step to validate the efficacy of coatings against fouling. The primary fouling organism is the barnacle, Balanus amphitrite amphitrite Darwin. This is also the most common fouling organism found in most parts around the world and likely distributed worldwide by seagoing vessels for many centuries [4]. The seawater temperature remains above 200 degrees Celsius all year and reaches as high as 350 degrees Celsius. The 12 × 6 inch samples are placed two feet below the surface of the water and inspected once a month for growth both qualitatively and quantitatively.
Substrate materials and methods
As the purpose of these experiments was to develop and examine a solution for long-lasting protection, the additive materials were incorporated directly into the substrate during the manufacturing process. This eliminated the dependence of the material performance upon an external coating as well as allowed for a larger matrix which will hold greater amounts of additive and increase the lifespan of the biofilm protection. Two different fabrication processes were used which include the rotational molding of high-density polyethylene (HDPE) substrates and the laying up of structural fiberglass components.
HDPE powder and manufacturing specifications were provided by a rotational lining company. The HDPE samples were prepared using a labscale rotational molder and each sample contained 900 grams of HDPE dry powder. The samples created include a control (no additive), zinc pyrithione at 10%, and MG at 10%. The powder was added to the dry HDPE, poured into a mold, rotationally lined at 350 degrees Fahrenheit for 8 hours, and then slowly cooled over two hours. The samples were cut to microbiological testing facility specifications at 2 × 2 × 0.5 and another set a t 12 × 6 × 0.25 inches.
Fiberglass samples were prepared by a company that specializes in fiber reinforced plastic. The additive was added while the plasticizer was in liquid form and before the fiber was added and curing process began. The production of these samples was done externally and unfortunately, only one set of the requested samples was produced. A control sample and MG at 10% was included in this set of samples which were cut to 2 × 2 × 0.5 and 12 × 6 × 0.2 inches.
This study investigates the development of an innovative method for incorporating zinc-based antimicrobial materials into the initial fabrication process of underwater structures and surfaces to protect against biofouling and microbiologically influenced corrosion. Both laboratory and field testing was used to demonstrate the efficacy of the materials against gram positive and gram negative bacteria. Agar diffusion testing highlighted zinc oxide, zinc pyrithione, and a commercially available material called MIC-GUARD as demonstrating the strongest resistance to bacteria based on the size of their inhibiting zone. These three additives were then used in dynamic shaker testing to evaluate the effectiveness of the additives exposed to water. Both zinc pyrithione and MIC-GUARD performed well under dynamic fluid conditions and were used for further field testing. High density polyethylene samples were rotationally molded with zinc pyrithione and MIC-GUARD additives. The samples were submerged in the Indian Ocean and marine fouling was observed over the course of 120 days. The MIC-GUARD additive protected the polyethylene samples from marine attachment and growth indicating that it preserves the substrate by preventing biofilm development. The MIC-GUARD additive was then incorporated into a fiberglass matrix and submerged in the ocean for 60 days. The fiberglass control samples had immediate attachment and growth of barnacles and oysters which increased over the course of testing. The fiberglass samples with MIC-GUARD have minimal barnacle attachment, zero oyster attachment, and show signs of detachment of marine life. This again indicates the lack of biofilm development on the surface of the MICGUARD treated samples.
Results from this study fill an increasing void as biofouling and microbiologically influenced corrosion are being recognized as a major problem in a variety of industries including oil and gas, aviation, and healthcare. Expensive coatings and toxic chemicals are being used as a short-term solution, but the ability to fabricate the substrate material with a ‘built-in’ antimicrobial presents a creative long term solution.
This work is protected under U.S. Patent Application Serial N o . 63/110,880
Journal of Material Sciences & Engineering received 3677 citations as per Google Scholar report
