Case Study Term Paper for Corrosion and Environmental Degradation of Materials

1. MT41013
CORROSION AND ENVIRONMENTAL DEGRADATION
OF MATERIALS
Term Paper
On
Corrosion of steel in concrete
Submitted by
Piyush Verma (09MT3018)
Supervisor
Professor S K Roy
Department of Metallurgical and Materials Engineering
Indian Institute of technology Kharagpur
West Bengal 721302
1

2. Contents
1 1 Introduction 3
2 2 Corrosion processes of steel in concrete 3
3 3 Mechanisms of corrosion 5
4 4 Reasons for corrosion 5
5 5 Prevention methods 7
6 6 Conclusions 9
7 7 References 9
2

3. 1 Introduction
Reinforced concrete structures, because of the high alkalinity of the pore solution in the
concrete and the barrier provided by the cover concrete against the aggressive species
from outside environment , the reinforcement has been believed to be” non corrodible”.,
i.e. the corrosion rate of the steel reinforcement has been believed to be too slow to be of
concern. However with passage of time some cover concrete would not be able to
provide good protection to the reinforcement due to the degradation of concrete and the
ingress of corrosive species from environment. It has been recognized that the concrete
cannot always be a non-corrosive medium to protect steel from corroding.
The corrosion processes are closely related to the concrete and environmental factors. For
example, the moisture content in the concrete depends not only on the relative humidity
of the atmosphere but also upon the temperature cycling during day and night. Also
variation of temperature has multiple simultaneous effects on different parameters which
may counter-balance each other. The oxygen content and the PH value of pore solution
decrease and the concentration of chloride ion increases when temperature rises.
2 Corrosion Processes of Steel in Concrete
Micro-structural Defects in Concrete
Micro-cracking is one of the most important defects in concrete that would be responsible
for serious corrosion attack of steel in concrete. It provides the short-cut for the ingress of
corrosive species from environment into the concrete. The aggressive species could
change the chemical properties of concrete seating a more aggressive environment in the
vicinity of the reinforcement. Cracks can be formed due to bleeding effects, rapid drying
of exposed surface of wet concrete, temperature difference in the core, freeze cycles and
external seasonal temperature variation.
Basic corrosion processes of steel in concrete
1) Depolarization reagent, i.e. O2 arrives at the surface through the medium surrounding
it, dissolved in the medium.
2) Electrochemical reactions at the interface of metal
(In presence of only oxygen)
Cathodic reaction
O2+2H2O+4e = 4OH-
3

4. Anodic reaction
Fe = Fe2+ + 2e
(In presence of chloride ion)
Cathodic reaction
- -
Fe + 2Cl = Complex (Fe2+ +2Cl-) +2H2O + 2e
Anodic reaction
- - - -
Fe + 2Cl = Complex (Fe2+ +2Cl ) + 2e = Fe (OH) 2 + 2H+ + 2Cl
Fe2+ can be further oxidized to Fe3+ under oxidizing conditions and can be
accumulated at the surface of steel rebar or be dissolved into the pore solution.
3) Accumulation of reaction products at the surface of metal.
Thin passive film of Fe (OH) 2 or Fe (OH) 3 can be formed on the steel surface due to
hydrolysis or oxidation of Fe2+. However under some conditions protective film cannot
be formed or would be broke down, this applies when due to carbonated concrete the Ph
value of the solution goes below 9, or when a certain amount of chloride ion has
penetrated into a concrete saturated with water and has reached the vicinity of the steel.
Hence the steel will dissolve and the cross-section will go on decreasing.
4

5. 3 Mechanisms of corrosion
1) Pitting corrosion
Pitting corrosion, or pitting, is a form of
extremely localized corrosion that leads to the
creation of small holes in the metal. The driving
power for pitting corrosion is the depassivation
of a small area, which becomes anodic while an
unknown but potentially vast area
2) Crevice corrosion
becomes cathodic, leading to very
localized galvanic corrosion. The corrosion
penetrates the mass of the metal, with limited
diffusion of ions.
2) Crevice corrosion
Crevice corrosion refers to corrosion occurring
in confined spaces to which the access of the
working fluid from the environment is limited.
These spaces are generally called crevices.
Examples of crevices are gaps and contact
areas between parts, under gaskets or seals,
inside cracks and seams, spaces filled
with deposits and under sludge piles.
4 Reasons for corrosion
1) Lowering in alkalinity due to loss of carbonates by CO2
The acidic gases react with the alkalis (usually calcium, sodium and potassium
hydroxides), neutralizing them by forming carbonates and sulphates, and at the same time
reducing the pH value. If the carbonated front penetrates sufficiently deeply into the
concrete to intersect with the concrete reinforcement interface, protection is lost and,
since both oxygen and moisture are available, the steel is likely to corrode. The extent of
5

6. the advance of the carbonation front depends, to a considerable extent, on the porosity
and permeability of the concrete and on the conditions of the exposure. In the case of
carbonation, atmospheric carbon dioxide (CO2) reacts with pore water alkali according to
the generalized reaction, Ca (OH) 2 + CO2 = CaCO3 + H2O . It consumes alkalinity and
reduces pore water pH to the 8–9 range, where steel is no longer passive.
-
2) Lowering in alkalinity due to Cl
The passivity provided by the alkaline conditions can also be destroyed by the presence
of chloride ions, even though a high level of alkalinity remains in the concrete. The
chloride ion can locally de-passivate the metal and promote active metal dissolution.
Chlorides react with the calcium aluminate and calcium aluminoferrite in the concrete to
form insoluble calcium chloroaluminates and calcium chloroferrites in which the chloride
is bound in non-active form; however, the reaction is never complete and some active
soluble chloride always remains in equilibrium in the aqueous phase in the concrete. It is
this chloride in solution that is free to promote corrosion of the steel. At low levels of
chloride in the aqueous phase, the rate of corrosion is very small, but higher
concentration increases the risks of corrosion.
3) 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 debonding 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 debonding.
4) 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. Metal loss results at the anodic
points, where the current leaves the structure; usually, the attack is extremely localized
and can have dramatic consequences especially on pipelines.
6

7. 5) Water-Cement Ratio
Concrete placed with a high water-cement ratio, as seen under Freeze-thaw cycles, is
more porous due to the presence of excess water in the plastic concrete. The porosity
increases the rte of diffusion of water and electrolytes through the concrete and makes the
concrete more susceptible to cracking.
6) 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.
7) Electrical Contact with dissimilar metals
Dissimilar metals in contact initiate a flow of electrons that promotes the corrosion of one
or the other, by a process known as galvanic corrosion. When two dissimilar metals are in
contact with each other the more active metal (lower on the list) will induce corrosion on
the less active. Such corrosion may induce cracking and damage in the concrete.
8) Corrosion due to difference in environments
Corrosion occurs when two different metals, or metals in different environments, are
electrically connected in a moist or damp concrete.
This will occur when:
1. Steel reinforcement is in contact with an aluminium conduit.
2. Concrete pore water composition varies between adjacent or along reinforcing bars.
3. Where there is a variation in alloy composition between or along reinforcing bars.
4. Where there is a variation in residual/applied stress along or between reinforcing bars.
7

8. 5 PREVENTION METHODS
1) Keep concrete always dry, so that there is no H2O to form rust. Also aggressive
agents cannot easily diffuse into dry concrete. If concrete is always wet, then there is no
oxygen to form rust.
2) A polymeric coating is applied to the concrete member to keep out aggressive agents.
A polymeric coating is applied to the reinforcing bars to protect them from moisture and
aggressive agents. The embedded epoxy-coating on steel bars provide a certain degree of
protection to the steel bars and, thereby, delay the initiation of corrosion. These coatings
permit movement of moisture to the steel surface but restrict oxygen penetration such that
a necessary reactant at cathodic sites is excluded.
3) Stainless steel or cladded stainless steel is used in lieu of conventional black bars.
4) FLY ASH: Using a Fly Ash concrete with very low permeability, which will delay the
arrival of carbonation and chlorides at the level of the steel reinforcement. Fly Ash is a
finely divided silica rich powder that, in itself, gives no benefit when added to a concrete
mixture, unless it can react with the calcium hydroxide formed in the first few days of
hydration. Together they form a calcium silica hydrate (CSH) compound that over time
effectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chloride
ions.
5) A portion of the chloride ions diffusing through the concrete can be sequestered in the
concrete by combining them with the tricalcium aluminate to form a calcium chloro
aluminate (Friedel’s salt). It can have a significant effect in reducing the amount of
available chlorides thereby reducing corrosion.
6) Electrochemical injection of the organic base corrosion inhibitors, ethanolamine and
guanidine, into carbonated concrete.
7) The rougher the steel surface, the better it adheres to concrete. Oxidation treatment (by
water immersion and ozone exposure) of rebar increases the bond strength between steel
and cement paste to a value higher than that attained by clean rebars. In addition, surface
deformations on the rebar (such as ribs) enhance the bond due to mechanical interlocking
between rebar and concrete.
8) As the cement content of the concrete increases (for a fixed amount of chloride in the
concrete), more chloride reacts to form solid phases, so reducing the amount in solution
8

9. (and the risk of corrosion), and as the physical properties improve, the extent of
carbonation declines, so preventing further liberation of chloride from the solid phase.
9) Electrochemical Chloride Extraction (ECE) is a relatively new technology for which
long-term service data are limited. This method employs a temporary anode that is
operated at current density 7 orders of magnitude higher than for cathodic protection,
such that anions, including chlorides, electromigrate away from the embedded steel
cathode. Repassivation can then occur, similar to what was discussed above in
conjunction with cathodic protection, although this occurs in a shorter period of time (1–2
weeks to several months). Not all chlorides are removed, but sufficient amounts are
displaced from the steel-concrete interface.
6 CONCLUSIONS
Common types of corrosion occurring are Pitting, Crevice and Intergrannular corrosion.
The two most common causes of reinforcement corrosion are chloride ions and
carbonation by atmospheric carbon dioxide. In wet and cold climates, reinforced concrete
for roads, bridges, parking structures and other structures that may be exposed to deicing
salt may benefit from use of epoxy-coated, hot dip galvanized or stainless steel rebar,
although good design and a well-chosen cement mix may provide sufficient protection
for many applications. Epoxy coated rebar can easily be identified by the light green
color of its epoxy coating. Hot dip galvanized rebar may be bright or dull grey depending
on length of exposure, and stainless rebar exhibits a typical white metallic sheen that is
readily distinguishable from carbon steel reinforcing bar. Cathodic protection can be
applied too.
7 REFERENCES
1) Guangling Song, Ahmad Shayan, Corrosion of steel in concrete: causes, detection and
prevention, a review report.
2) J L Smith and Y P Virmani, Materials and methods for corrosion control of reinforced
and prestressed concrete structures in new construction (2010)
3) Wikipaedia.com
9