24-06-2014, 03:21 PM
EFFECT OF CORROSION IN STRUCTURES
INTRODUCTION
When a metal is attacked by substances around it, it is said to corrode and this process is called corrosion. Corrosion causes deterioration of essential properties in a material.
Billions of rupees are lost each year because corrosion and a huge amount of money is spent in prevention of corrosion and tarnishing of metals.
Corrosion involves the reaction of a metallic material with its environment and is a natural process in the sense that the metal is attempting to revert to the chemically combined state in which it is almost invariably found in the earth’s crust.
corrosion may be regarded as resulting in a variety of changes in the geometry of structures or components that invariably lead, eventually, to a loss of engineering function e.g. general wastage leading to decrease in section, pitting leading to perforation, cracking leading to fracture
CORROSION IN STEEL REINFORCED CONCRETES
Corrosion-induced deterioration of reinforced concrete can be modelled in terms of three component steps:
Time for corrosion initiation, Ti;
Time, subsequent to corrosion initiation, for appearance of a crack on the external concrete surface (crack propagation), Tp; and
Time for surface cracks to progress into further damage and develop into spalls, Td, to the point where the functional service life, Tf, is reached. Figure illustrates these schematically as a plot of cumulative damage versus time
CORROSION REACTIONS
Some of the anodic and cathode reactions that occur simultaneously on a metal surface in a "corrosion cell" are as follows.
A typical anodic oxidation that produces dissolved ionic product, for example for iron metal is:
Factors Associated Mainly with the Metal
Effective electrode potential of a metal in a solution
Over voltage of hydrogen on the metal
Chemical and physical homogeneity of the metal surface
Inherent ability to form an insoluble protective film
Factors Which Vary Mainly with The Environment
Hydrogen-ion concentration (pH) in the solution
Influence of oxygen in solution adjacent to the metal
Specific nature and concentration of other ions in solution
Rate of flow of the solution in contact with the metal
Ability of environment to form a protective deposit on the metal
Temperature
Cyclic stress (corrosion fatigue)
Contact between dissimilar metals or other materials as affecting localized corrosion
REASONS OF CORROSION
The two most common causes of reinforcement corrosion are
(i) localized breakdown of the passive film on the steel by chloride ions and
(ii) general breakdown of passivity by neutralization of the concrete, predominantly by reaction with atmospheric carbon dioxide.
Sound concrete is an ideal environment for steel but the increased use of deicing salts and the increased concentration of carbon dioxide in modern environments principally due to industrial pollution, has resulted in corrosion of the rebar becoming the primary cause of failure of this material.
The scale of this problem has reached alarming proportions in various parts of the world
Following are the contributing factors leading to corrosion
Cracks due to Mechanical Loading
Cracks in concrete formed as a result of tensile loading, shrinkage or other factors can also allow the ingress of the atmosphere and provide a zone from which the carbonation front can develop. If the crack penetrates to the steel, protection can be lost. This is especially so under tensile loading, for deboning of steel and concrete occurs to
some extent on each side of the crack, thus removing the alkaline environment and so destroying the protection in the vicinity of the deboning.
Stray Currents
Stray currents, arising for instance from railways, cathodic protection systems, or high voltage power lines, are known to induce corrosion on buried metal structures, leading to severe localized attack.
They may find a low resistance path by flowing through metallic structures buried in the soil (pipelines, tanks, industrial and marine structures). a cathodic reaction (e.g., oxygen reduction or hydrogen evolution) takes place where the current enters the buried structure, while an anodic reaction (e.g., metal dissolution) occurs where the current returns to the original path, through the soil
Moisture Pathwa
If the surface of the concrete is subject to long-term wetting, the water will eventually reach the level of the reinforcement, either through diffusion through the porous structure of the concrete, or by traveling along cracks in the concrete. Concrete roof decks, by their nature, are meant to be protected from moisture.
However, the presence of moisture on roofing systems may result from failure of the roofing membrane, poor detailing of drainage facilities, or lack of maintenance of drainage facilities
Low Concrete Tensile Strength
Concrete with low tensile strength facilitates corrosion damage in two ways. First, the concrete develops tension or shrinkage cracks more easily, admitting moisture and oxygen, and in some cases chlorides, to the level of the reinforcement. Second, the concrete is more susceptible to developing cracks at the point that the reinforcement begins to corrode
Conclusion
There are a number of ways of assessing on-line corrosion, involving electro chemical measurements or more direct assessments of effective section, but ins pection visual or otherwise, for all systems that may corrode has ramifications for the designer in ensuring that it is possible.
In some installations this may involve the incorporation of probes, coupons or test specimens exposed to the same environment as the plant and therefore simulating the corrosion of the latter, but in a form which allows easier assessment of the extent of corrosion.
Finally, but as an integral part of the total design and not as an afterthought, the means of corrosion control, by material modification or by chemical or electrochemical treatment, should be considered with as much care as is put into any other aspect of the design process.