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Environment
Air Microbes Gnaw
the Buildings
by VK
Joshi
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!
August 24, 2008
Image under license with
Gettyimages.com
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