The growth of fungi on the surface of metals has a great influence on their structural integrity and failure. The growth on metal surfaces is determined by their secreta metabolites, which enable them to adapt to new environments and nourishment conditions.
Although information on the capacity of fungi to adapt to metal surfaces is scarce, most fungi growing on metal surfaces alter the composition of the metals involving it in the process of functional growth. Metabolism changes in the composition and colour of the metals are some of the evidence confirming that fungus has penetrated the metal surface and uses it to satisfy its nutritional need with resultant corrosion.
Fungi can influence corrosion from different perspectives ranging from the role of biofilms, corroded media generation by fungus metabolism processes by fungal growth on metal surfaces.
In addition, no single mechanism conclusively explains all forms of fungal influenced corrosion because every mechanism is unique and applies to individual fungus, its metabolic or growth pattern.
Fungi are a very important factor in environmental ecology, which often determine the duration of the effective use of metal and their alloys. Fungi affect metals and structural parts of building acceleration of such materials, the action of which can lead to loss of esthetic value, integrity and collapse of such structures. They mostly find their way unto metal surface under the action of adhesion forces and start to act even at the lowest moisture. In such a way, they form chemical bonds with the metal.
However, not all fungi can survive on a metal surface. Most of them die from stress, unfavourable conditions on the metal surface, such as alternating moisture, temperature, physical, chemical and technical parameters. Only fungi that are able to incorporate the metals as a link into their activity chain connecting them with their environment and whose functioning helps reduce the tension between the metal and their vital needs can survive changes. In the composition of the surface layer of metals or metal alloys is one piece of the evidence confirming that fungi have penetrated the surface layer of metals or metal alloys and use it to satisfy their nutritional needs.
For example, the formation of Aspergillus Niger colours and their colour are determined by oxygen concentration on the metal oxide layer. They further say that the pH decreases with an increase in the quantity of oxygen, which is mainly pronounced in the one where fungi are growing. This causes defects and pinholes on the surfaces of mild steel and changes in the colour of aluminium coupons exposal to score Aspergillus fumigatus, Acremonium Kiliense and Penicillium Chrysogenum. In order to reveal the response of fungi developing on metal surface rich in organic substances exposure to aluminium, iron, copper and zinc plates in close contact with peat in close compost, the results obtained suggest that fungi developing in the environment actively participate in the process of surface damage and destruction of the metal.
Generally, any factor that influences anodic reaction, cathodic reaction or both of a metal can affect the corrosion rate of such metal or the overall process. The factors that can change corrosion rate include pH, temperature and the presence of micro-organisms such as fungi. Unlike bacteria, fungi are the most desiccant resistant microorganism with the ability to remain active at water activity (aw) of 0.6 while few bacteria can remain alive at water activity of 0.90.
Fungi corrosion via acid generation is always accompanied by the degradation of organic materials such as lubricants, and where there is relatively high level of organic material and water. Fungi are able to generate a wide range of acids under aerobic conditions. These acids allow the sequestration of metals from the environment. The impact of organic acids on the corrosion of radioactive waste containers was reviewed, which indicated that both formic and acetic acid were able to promote steel corrosion, particularly if present as a vapour.
Numerous reports document fungi growth in passenger compartments of an in-service aircraft coated with polythene paint. There are documents that the following eight fungal are generally associated with aircraft: pastolotia, trichoderma, epicocceum, phoma, strephylium, hormodendrum (also known as cladosporium), penicillium and aurebasidium. These organisms were cultural from virtually all interior surfaces including primer, a polyurethane – coated 2024 T-6 aluminium UNS A92624, fibreglass structures, calicking, synthetic fibres, wiring and air conditioning ducts.
According to them, the concentration of organisms depends on the availability of nutrients. They maintained that fungi derived nutrients from large quantities of organic acids, which persists under anaerobic conditions established under fungal mat. Many metals are essential for fungal growth and metabolism.
Nevertheless, metals exert toxic effects in many ways: they can inhibit enzymes, displace essential metal ions, cause disruption of membranes and interact with systems that normally protect against harmful effects of free radicals.
The objective of this document is to present an overview of fungal influenced corrosion, which is essentially aerobic in nature and medental by fungal metabolic activities. Anaerobic corrosion fungi are commonly found in connection with aerobic corrosion. They colourise the metal surfaces, creating an oxygen-free environment for anaerobic bacteria, especially exopolymeric substances (EPS). Excretal by fungi and some bacteria may contain organic acid high concentration that may cause metal deterioration.
Some groups of aerobic bacteria, for example, produce strong inorganic acids and become very corrosive toward iron:
40H- + 4Fe(OH) + O + 2H2 4Fe(OH)3
– Cathodic:
Ferric iron is further hydrolysed in the presence of oxygen. In the presence of biofilms generated by aerobic microorganisms, the corrosion can be enhanced by metabolic reactions taking place in these biotic heterogeneities near the metal.
There is no known universal mechanism of microbial influenced corrosion, instead many mechanisms exist and some of them have been described and quantified better than others. The mechanism of fungal corrosion of metals is evident on how the growth, adhesion and production of metabolites interfere with the corrosion process. It is known that mechanical and structural factors all play a role, both biotic and abiotic in nature and these may all work in concert to cause and sustain microbial influential corrosion.
Formation of biofilms that enables the emergence of aeration cells, fungal and other microorganisms influence the corrosion of metal oxygen at different locations on the metal surface can be caused by active consumption of oxygen at different locations on the metal surface in biofilms not uniformly distributed on the metal surface. It can also be caused by different concentrations of oxygens at different locations on the metal surface, and can be caused by the active consumption of oxygen by micro-organism in biofilms not uniformly distributed on the metal surface.
It can also be caused by a passive mechanism in which oxygen access for some area is physically obstructed. The location where the oxygen concentration is higher will have a higher potential (more cathode) than the location where oxygen concentration is lower (more anode). The difference in potential will give rise to current flow from the anodic location to cathodic locations and the establishment of corrosion cells. This is the mechanism of differential aeration cells and the prerequisite to this concentration of oxygen varies among locations.
Abu Lamidi, Chief Conservator, Department of Conservation, National Museum and Monument, Benin City.
