THE CORROSION CHALLENGE
That India is geographically surrounded by a 7000 km. Coastline is a fact. That India is investing in industries and construction, involving metal usage in a big way, is also a fact.
Put the two facts together and the conclusion is inevitable – that corrosion and its various fallouts will continue to be a major source of worry for the construction industry in India. So let’s acknowledge the truth – that corrosion is a fact of life that cannot be wished away. So what exactly is this corrosion that strikes so much fear in the construction industry? Corrosion is an electro-chemical action, where one metal is changed into a chemical or is simply eaten away. Thus in the presence of air, water or salt, non-precious metals like Steel, Zinc, Aluminium and Copper undergo a process called oxidation (rusting), which slowly disfigures the metals, rendering them unsafe and useless. Humidity and air pollution are the other factors that contribute to hastening the corrosion process.
THE DIFFERENT TYPES OF CORROSION
GENERALISED CORROSION (OXYGEN TYPE)
This is the most common and prevalent type of corrosion, where humidity and moisture results in electrolytic reactions developing on the steel surface, leading to progressive corrosion. The rate of corrosion increases rapidly in the presence of other pollutants and raised humidity levels.
GALVANIC CORROSION (HYDROGEN TYPE)
In this case, the combination of two dissimilar metals with an electrolyte (moisture) is all that is needed to form a reaction, where the less noble metal migrates and dissolves into the solution. Galvanic corrosion is also used for corrosion prevention when a less noble metal like zinc is used to protect the carbon steel.
BIOCHEMICAL CORROSION
When metals dissolve in acid and caustic solutions of different strengths, chemical corrosion occurs because metals tend to combine with oxygen to form oxides. The less noble the metal, the stronger the tendency. The attacking acids often come from the atmosphere. For example, the sulphuric acid that comes from the sulphur dioxide emissions from burning fossil fuels, is found in urban and industrial areas; nitric oxides, chlorine, hydrogen chloride, formic acid, acetic acid, etc. are found in the vicinity of industrial plants, while chloride and sodium chloride especially, are common atmospheric pollutants in coastal regions.
AERATION CELL CORROSION
An oxygen deficiency can develop in a damp thermal insulation or in any other situation when moisture is trapped and gathers pollutants. Corrosion will occur in the area of restricted oxygen supply, even in a non-polluted area of high PH value. Where other pollutants are present as in a lower PH value are, the corrosion will increase.
THE COST OF CORROSION
Corrosion significantly affects the structural integrity of a construction. Premature deterioration or failure, results in greater maintenance, repair and replacement of damaged equipment. Besides this, costs also escalate, due to lost production hours and increased downtime of equipment; all arising from corrosion problems. Talking of cost, in India, the cost of corrosion has been estimated to be 3% of its GDP and that the roofing industry in particular, incurs a loss of no less than Rs. 360 crore annually.
