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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

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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

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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-

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Anodic reaction

Fe = Fe2+

+ 2e

(In presence of chloride ion)

Cathodic reaction

Fe + 2Cl

- = Complex (Fe

2+ +2Cl

-) +2H2O + 2e

-

Anodic reaction

Fe + 2Cl- = Complex (Fe

2+ +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.

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3 Mechanisms of corrosion

1) Pitting corrosion

2) Crevice corrosion

2) Crevice corrosion

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

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

becomes cathodic, leading to very

localized galvanic corrosion. The corrosion

penetrates the mass of the metal, with limited

diffusion of ions.

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.

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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.

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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.

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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

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(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