Buying an apartment in a high rise building in a coastal metro or constructing the apartments is not that safe in the long run as presumed. Threats of Natural hazards, like tsunamis, earthquakes, floods, cyclones etc affect the human population everywhere including the coastal habitats. Most of them are sudden events, leaving a trail of destruction. For some of the hazards like the floods and cyclones it is possible to forewarn the public to vacate the areas. Of course in most of the time in India residents face the ire of the nature at the cost of life and property.
Hazards like tsunami are also predictable, though the time available to the residents of the area of origin of tsunami may be much less. Earthquake is one hazard, which is not yet predictable. Whether on the coast or up in the mountains, people have to face it, when they are least prepared.
A new kind of hazard, which is like a slow poison, for the buildings appears to be looming large, especially over the coastal metros. That is the decay of the steel and concrete used for construction by atmospheric microbes.
A recent study carried out by the Indian scientists at the Central Electrochemical Research Institute (CERI), Karaikudi, Tamilnadu, has highlighted for the first time the role of air microbes in the corrosion of building materials and concrete by the atmospheric microbes.
S. Maruthamuthu, N. Muthukumar, M. Natesan and N. Palaniswamy of the Institute were able to actually 'trap' the microbes to catch them in 'action'.
Steel as we know is prone to corrosion in the atmosphere. Humidity in the atmosphere, sulphur dioxide and chlorine content affect the rate of corrosion of steel. Smoke belching chimneys of the industries enrich the atmosphere with sulphur dioxide (SO2). Thus corrosion experts found that the 'industrial air' rich in SO2 enhances the rate of corrosion. Similarly, the air of the sea coasts has higher chlorine content influences the rate of corrosion. Chlorine levels in the coastal air vary from the coast to coast and also upon the season and direction of the wind. In case the wind is blowing towards the sea, it is imperative the steel structures on the coast would face lesser corrosion.
Atmosphere enriched with SO2 starts a chain reaction upon prolonged contact with steel leading to formation of Iron Sulphate (FeSO4), which accumulates in tiny and discrete corrosion pits, 0.5 to 1 mm diameter, termed ferrous sulphate nests by corrosion experts. The corrosion process produces a film over the surface of the steel and the nests are sheltered underneath. Higher the concentration of SO2 greater the number of nests formed.
It is clear from the foregoing that air polluted with SO2 and chlorine causes more corrosion of steel. But then where do the microbes fit in the picture?
In order to reach the depth of the problem, we have to understand the environments in which the microbes thrive. They are so adaptable that they can live in many types of environments including water, soil and air where aerobic bacteria, fungi and algae can thrive with comfort. While studying biology in class ten we were taught that cockroaches are omnivorous and they can survive even on cement. Somewhat like them, many bacteria love to eat mineral ions from nitrogen, phosphorous, sulphur, iron, manganese and calcium. Not only that some of the bacteria love to sip sulphuric acid. Yes, it is a fact! Sulphur oxidizing bacteria and aerobic bacteria oxidize sulphur or sulphur bearing compounds and produce sulphuric acid as a by-product and thrive on it. Such bacteria include Thiobacillus ferroxidans and T. thiooxidans. They grow under acidic conditions well. It is common knowledge sulphuric acid is highly corrosive to many metals.
Then there are iron oxidizing bacteria. They oxidize ferrous to ferric salts and draw their energy from the process. These bacteria accelerate the rusting of mild steel and some of the forms include Gallionella, Sphaerotilus, Leptothrix and Crenothrix.
Maruthamuthu and his colleagues investigated the action of these bacteria on mild steel, and existing structures, viz. steel rods and concrete structures. Corrosion of metals by bacteria under water is well known. However, this is the first report of atmospheric corrosion being accelerated by bacteria, claim the authors.
Since bacteria are omnipresent, special precautions were taken to prepare the surfaces on which their action was to be studied. They used specially prepared, machine polished mild steel 6"X4" plates. A thin veneer of trichloroethylene was applied on the plates and rinsed with de-ionized water before being placed in the atmosphere of a petrochemical industry, five km from CERI, Karaikudi. Sulphur is used as the main raw material in this industry. In order to compare the action of microbes on the plates under different conditions some of the plates were exposed in the sheltered and unpolluted environment of their laboratory. Some material from concrete piles as also collected to ascertain the role of bacteria in corrosion of concrete.
After following the bacteriological procedures meticulously, Maruthamuthu and his colleagues were able to identify Bacillus, Micrococcus, Acinectobacter, Pseudomonas, Streptococcus, Staphylococcus, Moroxella and Anthrobacter from the existing steel rods and concrete.
Thiobacilii a group of bacteria that was found to be behind destruction of sewage pipelines in Germany by excreting sulphuric acid was also found present in the steel rods exposed to vagaries of sulphur dioxide rich atmosphere. It was seen that humidity and sulphur dioxide in the industrial area encouraged the proliferation of microbes on concrete and mild steel. The physiological activity of Thiobacilli may have reduced the pH of concrete (less pH indicates more acidic conditions) and deteriorated the quality of concrete and steel rod. The bacteria that oxidized iron and manganese reduced the strength of the steel rods.
The corrosion rate of mild steel exposed to the industrial air was 0.116 millimeter per year (mmpy) compared to 0.021 mmpy of the steel plate exposed in the sheltered area. Normally concrete is not affected by corrosion, since it has lime which creates alkaline conditions and hampers the growth of bacteria. However, the concrete in contact with the environment containing bacteria like Bacillus, Micrococcus, Acinectobacter, and Pseudomonas, affect the durability of concrete by dissolving in acid the Calcium present in the cement.
The study, though first of its kind has opened the floodgates of research in the field. Coastal towns anywhere are major business centers. Our metros like Mumbai, Chennai and Kolkata are on the coast only. They are industrial hubs as well. All these places are packed with high rise buildings. Corrosion in Kolkata due to high chlorine in water is well known. Incidentally the study of CERI scientists has highlighted the possibilities of higher rate of corrosion by the atmospheric bacteria, aided by chemical rich, industrial air of the coasts. There could be many more areas, which are industrially developed. One has to be cautious of all industries enriching the atmosphere with sulphur dioxide or chlorine, irrespective of their size or location.
Time is ripe for producing an 'Atmospheric Microbial Corrosion Hazard Map' of India because that will help the builders and planners to devise ways to counter the microbes. The maps can categories areas in to high hazard, medium and low hazard and also hazard free areas. Such maps need to be revised periodically. It goes without saying that in the hazard prone areas environment audit of the industries will have to be done every two years and strict penalties imposed upon defaulters.
Microbes as such need lots of research and this aspect of atmospheric corrosion by them being a new find needs to be researched thoroughly and systematically.
Microbes are everywhere. They are not always bad. Many of them help our digestive system to work smoothly. We need them. We generally understand microbes as cause for many of sicknesses. However, Maruthamuthu and his colleagues have opened new vistas. Now we have to be cautious before we decide to buy an apartment in a high rise building-who knows the microbes in the atmosphere might gnaw the steel rods and topple the building!