Scanning Probe Microscopy Investigations of Microbiologically Influenced Corrosion on Naval Assets

T.T. Brown, J.S. Lee
U.S. Naval Research Laboratory,
United States

Keywords: scanning probe microscopy, atomic force microscopy, microbiologically influenced corrosion, coatings and surfaces, microbial deterioration, biofouling


Naval assets are utilized around the world, often in hot and humid environments. To protect aluminum aircraft skins and structural members from corrosion, organic coating systems are used. For example, in interior regions of aircraft where sunlight/ultraviolet light cannot penetrate and, exacerbated by heat and humidity, mold and mildew (fungal) spores may attach and proliferate leading to microbiologically influenced corrosion (MIC). Corrosion preventive compounds (CPC’s) and mildew inhibitors have been previously evaluated in prevention of corrosion and fungal growth, respectively. Hydraulic fluid (HF) is also present on most aircraft surfaces through unintentional deposition as a part of the normal operation of the aircraft. Fungi have been shown to be capable of degrading CPC’s and HF resulting in copious biofilms. The accepted procedure for removal of fungal biofilms during field operations is cleaning with isopropyl alcohol (IPA). The current study utilizes scanning probe microscopy (SPM) techniques to investigate bio-fouling of multiple coating systems, as well as cleaning procedures, on naval assets quantitatively and qualitatively. The effects of chemicals and processes used to remove and/or neutralize (sanitize) the biological growths and operational chemical residues were also examined, with respect to the removal of growths as well as to control for the effect of cleaning procedures on results of the various analytical results on the underlying coating surfaces. Cleaning chemicals used included IPA, acetone (ACE), ethanol (EtOH), methanol (MeOH), and commercially available cleanser (NavClean). Examinations of intact fungal mats were also performed using SPM in intermittent “tapping” mode in ambient air and liquid. The surface was investigated using commercially available silicon nitride (Si3N4) AFM tips. Further observations of various common cleaning solvents used on the fungal hyphae were shown to be relatively ineffective in removing the attached growths from the coating surfaces (IPA, ACE, EtOH and MeOH). It was observed that NavClean solution was the most effective in removing growths from organic coatings surfaces. Combinations of cleaning procedures may be required to remove both fungal growths and operational fluids which are acting as additional nutrients for microbiological propagation. Ongoing SPM investigations will aide in physical and dynamic observations of biofilm interaction with organic coating systems and surfaces.