MECHANISM AND CAUSES

PROFESSOR OF CIVIL ENGINEERING, COLLEGE OF ENGINEERING,

ANNA UNIVERSITY

INTRODUCTION

Corrosion is defined as the destruction (or) deterioration of materials due to chemical (or) electrochemical reaction with the environment, and means other than directly mechanical.

The corrosion of steel reinforcement is the depassivation of steel with the reduction in concrete alkalinity through carbonation.

Most of the materials undergo corrosion on exposure to natural environments (like atmosphere, water and soil) and to other artificial environments (like gases, liquids, moisture).

Corrosion is an electro-chemical process involving an anode, a cathode and an electrolyte. In steel, when favourable condition for corrosion occurs, the ferrous ions go into solution from anodic areas.

Electrons are then released from the anode and move through the cathode where they combine with water and oxygen to form hydroxyl ions.

These react with the ferrous ions from anode to produce hydrated ferrous oxide, which further gets oxidised into ferric oxide, which is known as the ‘red rust’.

Corrosion Mechanism

COLLECTION OF WASTE & SALT WATER ON SUNSHADE

COLUMN HEAD- PREFAB-STAIRS-CORRODED

Effect of corrosion

Deterioration of concrete due to corrosion results because the product of ferric oxide, brown in colour occupies a greater volume (more than 2 to 10 times) than steel and exerts substantial bursting stresses on the surrounding concrete.

The outward manifestations of the rusting include staining, cracking and spalling of concrete.

The progress of corrosion process will generally be in geometric progression with respect to time. Consequently, the c/s of the steel is reduced.

With time, structural distress may occur either by loss of bond between the steel and concrete (or) due to cracking and spalling of concrete, (or) as a result of the reduced steel c/s area.

This latter effect can be of special concern in structures containing high strength pre-stressing steel in which a small amount of metal loss could possibly induce tendon failure.

However, the process leading to the ultimate failure is slow and normally gives years of warning to the maintenance engineering squad.

Problems due to corrosion

Hazard to human life and economic losses occur due to premature deterioration and destruction of buildings, bridges, culverts, pipes, structures including marine and offshore structures, towers, water supply and sanitary fittings, carpentry and electrical fittings, implants for human body, etc.

Corrosion of steel reinforcement is the cause of deterioration of reinforced concrete which causes spalling of concrete due to increase in volume of oxidized compounds when compared with the volume of base metal dissolved.

This increase in volume causes tensile forces leading to cracks in concrete around steel reinforcement and further accentuates the effect of corrosive environments.

Therefore, a more complete picture of the behaviour of corroding steel in reinforced concrete members would be desirable.

Protection from corrosionProtection of the reinforcement from

corrosion is provided by the alkalinity of the concrete, which leads to passivation of steel. Formation protective skin around steel is shown in Figure

Typical Case of Potential Difference in the Ties of Column

Typical Case of Potential Difference in Concrete

The reserve of calcium hydroxide is very high, so there is no need to expect steel corrosion even when water penetrates to the reinforcement in the concrete.

Because of this, even the occurrence of small cracks (upto 0.1 mm in width) or blemishes in concrete need not necessarily lead to damage.

However, environmental influences and CO2 in particular, will reduce the concrete’s pH value from 12.6 to 8.0, and thus remove the passivating effect, in conjunction with existing humidity, the result is corrosion of reinforcement.

Electrochemical potential

The presence of an electrical potential is the pre-requisite for the occurance of corrosion . It may be created by:

Differential aeration – difference in concentration of oxygen on the steel surface.

Differential ion concentration – metal ions, dissolved salts and pH of concrete in the vicinity of steel.

Differential surface properties – small scale (or) breaks in coatings, impurities in concrete etc.

The reinforcement in concrete has a passivating layer of gamma ferric oxide (Fe2O3). Any breaks that occur in the protective film are quickly repaired in presence of sufficiently high hydroxyl ion concentration forming first ferrous hydroxide then cubic ferric oxide (Fe3O4). and gamma oxide (Fe2O3).

Corrosion Mechanism

There are two mechanisms of corrosion By direction oxidation; By electrochemical reaction.

The latter one is the most widely occurring mechanism. Fe Fe+ + +2e- (Anodic reaction)4e + O2 + 2H2O 4 (OH) (cathodic reaction)Fe ++ + 2 (OH) 2 Fe (OH) 2 (Ferrous Hydroxide)2Fe++ + 6 (OH) 2Fe (OH)3 (Ferric Hydroxide)2Fe (OH) Fe2O + 3H2O

(or)At the Anode :- Fe + 2(OH) Fe (OH) 2 + 2eAt the Cathode :-O2 + 2H2O + 4e 4 (OH)

It is observed that for corrosion to occur and to continue, we require oxygen and water.

There is no corrosion below 30% relative humidity. At 70 to 80% relative humidity corrosion rate is highest.

Chloride ion present in the cement paste surrounding the reinforcement from HCl which ultimately destroys passive protective film on the steal similarly formulation of CaCO3 after the reaction of Ca(OH)2 also destroys the passive environment by bringing down the pH from 12.5 to 9.0 and below.

Due to the phenomenon, the steel activated locally to form the anode and the remaining passive surface as cathode and localized corrosion in the form pitting ensues. Schematic representation of corrosion due to chloride ion is shown as

Fe++ + Cl- + H2O Fe (OH)2 + HClHCl Cl- + H+

Chloride Induced Macro Cell Corrosion of Steel in Concrete

STAGES OF CORROSION

LIME LEACHING RUST STAINING BULGING OF COVER (COLLECTION OF RUST PRODUCTS)

MICRO CRACKS MACRO CRACKS SPALLING OF COVER CONCRETE

The degree of protection against corrosion is provided by the pore fluid Ca(OH)2. The pH of Ca (OH)2 solution at 25C is 12.53 at maximum solubility of Ca(OH)2.

The pH still remains 11.27 when the concentration of Ca (OH)2 is only 5.5% of the maximum. Following Figure shows the quantity of corrosion products Vs time.

Quality of corrosion products Vs Time

Types of corrosion

Pitting corrosionCrevice corrosionBimetallic corrosionStress corrosionFretting corrosionBacterial corrosionHydrogen embrittlement

Pitting corrosion

As shown in Figure the anodic areas form a corrosion pit. This can occur with mild steel immersed in water or soil.

This common type of corrosion is essentially due to the presence of moisture aided by improper detailing or constant exposure to alternate wetting and drying.

This form of corrosion could easily be tackled by encouraging rapid drainage by proper detailing and allowing free flow of air, which would dry out the surface

Crevice corrosion The principle of crevice corrosion is shown in Figure the oxygen content of water trapped in a crevice is less than that of water which is exposed to air.

Because of this the crevice becomes anodic with respect to surrounding metal and hence the corrosion starts inside the crevice.

Bimetallic corrosion

When two dissimilar metals (for e.g. Iron and Aluminium) are joined together in an electrolyte, an electrical current passes between them and the corrosion occurs.

This is because, metals in general could be arranged, depending on their electric potential, into a table called the ‘galvanic series’. The farther the metals in the galvanic series, the greater the potential differences between them causing the anodic metal to corrode.

A common example is the use of steel screws in stainless steelmembers and also using steel bolts in aluminium members. Obviously such a contact between dissimilar metals should be avoided in detailing

Stress corrosion

This occurs under the simultaneous influence of a static tensile stress and a specific corrosive environment. Stress makes some spots in a body more anodic (especially the stress concentration zones) compared with the rest as shown in FigureThe crack tip in the anodic part and it corrodes to make the crack wider. This corrosion is not common with ferrous metals though some stainless steels are susceptible to this.

Fretting corrosion

If two oxide coated films or rusted surfaces are rubbed together, the oxide film can be mechanically removed from high spots between the contacting surfaces as shown in Figure.

These exposed points become active anodes compared with the rest of the surfaces and initiate corrosion.

This type corrosion is common in mechanical components.

Bacterial corrosion

This can occur in soils and water as a result of microbiological activity.

Bacterial corrosion is most common in pipelines, buried structures and offshore structures.

Hydrogen embrittlement

This occurs mostly in fasteners and bolts. The atomic hydrogen may get absorbed into the surface of the fasteners.

When tension is applied to these fasteners, hydrogen will tend to migrate to points of stress concentration.

The pressure created by the hydrogen creates and/or extends a crack. The crack grows in subsequent stress cycles.

Although hydrogen embrittlement is usually included in the discussion about corrosion, actually it is not really a corrosion phenomenon.

FACTORS INFLUENCING CORROSION

The factors which generally influence corrosion of reinforcement in concrete structures are:

pH value Moisture Oxygen Carbonation of concrete Chlorides and sulphates Ambient temperature and relative humidity Severity of exposure Quality of construction materials Cover to reinforcement

Permeability of concrete Initial curing conditions Formation of cracks High carbon content in reinforcement High stress levels Inadequate grouting of pre-stressed tendons Reinforcement corroded prior to embedment Alkali-aggregate reaction Potential difference associated with liquid Stray currents Periodical maintenance

pH value

The pH value of moist concrete is normally about 12.0, which is sufficient to passivate the reinforcement against corrosion. When it reduces below 8.0, carbonation of concrete takes place, in turn corrosion initiates. Following figure shows the schematic representation of pH value.

Schematic Representation of pH Value

Carbonation of concrete

Carbonation is a process by which CO2 converts the unleached lime to CaCO3 and water, thereby will reduce the pH further and as it reaches about 9.0 the passivation of the reinforcement is lost and corrosion will start with the availability of O2 and H2O.

The penetration depth of this pH is generally called “carbonation depth”. This process shown in Figure.

Carbonation Process

Carbonation time Vs w/c

Carbonation time Vs Time

The rate of carbonation depends on: Permeability of concrete CO2 concentration in the air Moisture in the gel and capillary pores Relative humidity of atmosphere As carbonation reaches the reinforcement, the

passivating influence of the concrete is lost and in presence of moisture and oxygen the reinforcement starts cooroding.

Schematic process is Ca (OH)2 + CO2 + H2O CaCO3 + 2H2O

Carbonation is controlled by Composition of cement Amount of cement per m3

Concrete mix-design including the grading of aggregate

Consolidation / compaction of concrete Curing of concrete Environmental condition in which concrete is

to live

Reaction with chloride

Presence of Cacl2 even in small percentage can lead to rapid corrosion of reinforcement as it reduces the electrical resistivity of concrete, and helps to promote galvanic cell action.

Presence of chlorides increases shrinkage cracks in concrete further accentuating corrosion of reinforcement in aggressive environment.

The ingress of chloride ions in excess of the threshold concentration value, reduces the alkalinity of the concrete and breaks down the protective film to set off the process of corrosion.

It increases the corrosion rate through their conductivity to the ionic activity.

Chloride salt can enter concrete in two ways:

chloride may be present in the concrete mix itself

chloride can penetrate into the hardened concrete wherever it is permeable and reaches the reinforcement at isolated points.

Chloride induced corrosion in steel reinforcement at isolated points.

Moisture

The presence of moisture in an R.C. member is due to two reasons.

Water that is used for making the concrete mix remains well distributed and enclosed in the concrete mass.

Water that finds its way into the hardened concrete from outside due to the subsequent absorption of water which may circulate freely in inside the mass.

Oxygen

The differential aeration cell, set up by differential concentration of oxygen, sets off the process of corrosion. Thus is r.c. members, region with least oxygen concentration becomes anodic while those with larger oxygen concentration becomes cathodic.

Ambient temperature and relative humidity

Rate of corrosion is directly proportional to the ambient temperature

Rate of carbonation increases with increase in temperature

The pH value limit, below which the corrosion is induced, decreases with increase in ambient temperature

Severity of exposure

Rate of corrosion is proportional to severity of the environment

Due to severe exposure, concrete in the cover region undergoes rapid deterioration and in turn the reinforcement looses its passivity and starts corroding.

Quality of construction materials

Quality of construction materials Use of construction materials which are contaminated

with a significant level of chlorides/sulphates, cause depassivation of reinforcement and sets off corrosion.Quality of concrete

It is one of the most common causes of early deterioration.

Good quality concrete will be dense and almost impermeable

The above property of concrete prevents easy access of external deteriorating agents to the reinforcement.

Cover to the reinforcement

Lack if adequate cover contributes very much to corrosion in an aggressive environment.

Well compact and continuous cover of good quality concrete of even small thickness on the reinforcement is sufficient to protect it from corrosion.For sever exposure - min 40mm thickFor moderate exposure - min 30mm thickFor mild exposure - min 20mm thickFor normal exposure - min 10 mm thick

Initial curing conditions

Process of corrosion depends on Permeability of the cover concreteInitial curing conditions

If the curing is sufficient then due to premature drying, the permeability of cover concrete increases.

High permeability of concrete to liquids and gases makes the alkalies in the hardened cement paste to combine more (or) less completely with CO2 and SO2.

Formation of cracks

The formation of cracks is dangerous for the protection against corrosion in due course of time.

Once concrete cracks, the external depassivating agents will have access to penetrate deep into the concrete and set off the process of corrosion.

Cracks running transversely to the reinforcement are less harmful than the longitudinal cracks along the reinforcement.

Thick concentrate cover increases the crack distance and the width

DAMAGES DUE TO CORROSION

Formation of white patches CO2 reacts with Ca(OH) 2 in the cement paste to

form CaCO3. The free movement of water carries the unstable CaCO3 towards the surface and forms white patches.

It indicates the occurrence of carbonation.Brown patches along reinforcement When reinforcement starts corroding, a layer of

ferric oxide is formed. This brown product resulting from corrosion may permeate along with moisture to the concrete surface without cracking of the concrete.

Occurrence of cracks

The increase in volume exerts considerable bursting pressure on the surrounding concrete resulting in cracking.

The hair line crack in the concrete surface lying directly above the reinforcement and running parallel to it is the positive visible indication that reinforcement is corroding.

These cracks indicate that the expanding rust has grown enough to split the concrete.

Formation of multiple cracks

As corrosion progresses, formation of multiple layers of rust on the reinforcement which in turn exert considerable pressure on the surrounding concrete resulting in widening of hair cracks.

In addition, a number of new hair cracks are also formed.

The bond between concrete and the reinforcement is considerably reduced. There will be a hollow sound when the concrete is tapped at the surface with a light hammer.

Spalling of cover concrete Due to loss in bond between steel and

concrete and formation of multiple layers of scales, the cover concrete starts peeling off.

At this stage, size of bars is reduced.Snapping of bars The continued reduction in the size of bars,

results in snapping of the bars. This will occur in ties/stirrups first. At this stage, size of the main bars is reduced

Buckling of bars and bulging of concrete

The spalling of the cover concrete and snapping of ties causes the main bars to buckle. This results in bulging of concrete in that region. This follows collapse of the structure.

When corrosion of reinforcement starts, the deterioration is usually slow but advances in a geometrical progression.

Corrosion can also cause structural failure due to reduced c/s and hence reduced load carrying capacity. It is possible to arrest the process of corrosion at any stage by altering the corrosive environment in the vicinity of the reinforcement.

PREVENTIVE MEASURES IN NEW CONSTRUCTIONS

Preventive measures for controlling corrosion of steel embedded in cement concrete use sound corrosion engineering principles directed towards:

Design factors Low w/c ratio High strength concrete Higher minimum cement content Higher concrete cover Proper detailing of reinforcement Moderate stress levels

Construction aspects Adequate compaction of concreteEffective curingProduction of impervious concreteEffective grouting of presented tendons

The various surface preparation methods are briefly explained below.

Manual preparation This is a very economical surface cleaning

method but only 30% of the rust and scale may be removed.

This is usually carried out with a wire brush.

Mechanical preparation This is carried out with power driven tools and

up to 35% cleaning can be achieved. This method is quite fast and effective.

Flame cleaning In this process an Oxy-gas flame causes

differential thermal expansion and removes mill scale more effectively.

Acid pickling This involves the immersion of steel in a

bath of suitable acids to remove rust. Usually this is done before hot dip galvanising (explained in the next section).

Blast cleaning

In this process, abrasive particles are projected at high speed on to the steel surface and cleaning is effected by abrasive action.

The common blast cleaning method is the ‘sand blasting’. However in some states of India, sand blasting is not allowed due to some environmental reasons.