MDP454

Atmospheric Corrosion of steel

refinforcement
Student name ID
Kerolos Rafeek Farouk 20P6682
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Introduction:

Figure 1 corroded steel bar.

Carbon steel is a representative metallic material among metallic, ceramic, and
polymeric materials, and it has been widely used as a structural material,
industrial piping, and in a variety of other fields due to its low cost and good
mechanical properties.
Steel is susceptible to atmospheric corrosion because it is used in outdoor
applications. When steel is exposed to an atmospheric environment, several
environmental parameters can cause corrosion, which can lead to surface
cracking and thickness reduction, posing a serious problem for industry.
When steel is exposed to the atmosphere, atmospheric environmental factors
such as chloride, humidity, SO2, and NO2 cause corrosion. Atmospheric corrosion
occurs via a thin electrolyte (about 200 m) with numerous contaminants on the
surface, and the ambient temperature influences the corrosion. The coastal,
industrial, urban, and rural areas are the four types of atmospheric corrosion
environments. The corrosivity of urban and suburban surroundings is largely
influenced by air quality, which varies greatly from city to city and country to
country. (Y. R. Yoo, June 25, 2022).
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Atmospheric corrosion:
Atmospheric corrosion is the degradation of materials caused by air and the
pollutants contained in the air. It can be precisely defined as an electrochemical
process which depends upon the presence of electrolytes which may be rain,
dew, humidity or melting snow. The usual electrolyte is water which is a universal
solvent.
Atmospheric corrosion takes place under humid conditions, where the
atmospheric relative humidity exceeds the equilibrium relative humidity over any
saturated solution which is present on the metal surface.
n
K=∑ t n V k
t

where K = accumulated corrosion products

tn = time constant
Vk = average corrosion rate during the period of wetness.
The corrosion rate is controlled by time of wetness, temperature, and electrolyte
composition.
Atmospheric corrosion is the most predominant of all the other forms of
corrosion. No other form of corrosion affects the materials and equipment harder
than atmospheric corrosion. (ahmad)

Classification of atmospheric corrosion:

Dry Corrosion:
In the absence of significant water vapor, many common metals develop films of
oxide. In the presence of traces of gaseous pollutants, copper, silver, and other
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non-ferrous metals undergo film formation which is known as
tarnishing. The tarnishing of silver in the air is well-known. Tarnishing by hydrogen
sulfide may be retarded by moisture if present in very small amounts. (ahmad)

Damp Corrosion:
Damp corrosion would occur only when the relative humidity reaches 70% which
is considered generally as the critical value for the onset of corrosion.
The difference between the damp and wet environment is very narrow and it is
more representative of a climatic condition rather than the magnitude of
corrosion.
Damp environments promote the corrosion of most metals. Water saturated with
dissolved gases, such as CO2, H2S and SO2, causes severe corrosion of iron and
steels, copper, nickel, silver and other non-metallic materials and alloys.
For instance, silver loses its luster and develops a tarnished film of sulfide on
meeting H2S, and copper develops tints and becomes black. (ahmad)

Wet Corrosion:
This is the most frequently observed form of atmospheric corrosion, where the
water layers or pockets are formed on the metal surface, and the metal surface
remains constantly in contact with water.
The rate of corrosion would depend on the solubility of the corrosion product.
Higher solubility means a higher rate of corrosion because the dissolved ions
increase electrolytic conductivity.
In case of alternate dry and wet conditions, the dry corrosion product film may
absorb moisture from the air which increases the rate of corrosion of the metal by
bringing the moisture in contact with the metal surface.
corrosion of iron and steel structures are common examples of corrosion caused
by wet atmosphere. (ahmad)
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Theory of atmospheric corrosion:
Thin-Film Electrolyte and Electrochemical Reactions:
A fundamental requirement for electrochemical corrosion processes is the
presence of an electrolyte. Thin film "invisible" electrolytes tend to form on
metallic surfaces under atmospheric exposure conditions.
In the case of a completely uncontaminated atmosphere, at a constant
temperature, a perfectly clean metal surface would not be expected to undergo
corrosion damage at a relative humidity below 100%.
However, in practice, due to the presence of hygroscopic surface species,
impurities in the atmosphere and small temperature gradients between the
atmosphere and metallic surfaces, a microscopic surface electrolyte tends to form
at significantly lower humidity levels.
For iron, a critical humidity of 60% has been reported.
In the presence of thin-film electrolytes, atmospheric corrosion proceeds by
balancing anodic and cathodic reactions. The anodic oxidation reaction involves
the dissolution of the metal, while the cathodic reaction is often assumed to be
the oxygen reduction reaction.
It should be noted that corrosive contaminant concentrations can reach relatively
high values in the thin electrolyte films, especially under conditions of alternate
wetting and drying. Oxygen from the atmosphere is also readily supplied to the
electrolyte, under thin-film corrosion conditions. (ROBERGE)

Cathodic and Anodic Half-Cell Reactions in Atmospheric

Corrosion:
The Cathodic Process:
The reduction of atmospheric oxygen is the most important reaction in which
electrons are taken up. For atmospheric corrosion in near-neutral electrolyte
solution, the following oxygen reduction reaction is the most likely.
 MDP454 O2 +2 H 2 O+4 e
−¿→ 4 OH
−¿¿
¿

Two reaction steps may actually be involved [12], with hydrogen peroxide as an
intermediate species, according to
−¿¿
−¿ → H 2 O2+2 OH ¿
O2 + H 2 O+ e

H 2 O2 + 2 e−¿¿ → 2 OH −¿¿

If oxygen diffuses from the atmosphere through the electrolyte film to the metal
surface, a diffusion limited current density should arguably apply.
It has been shown that a diffusion transport mechanism for oxygen is only
applicable to an electrolyte layer thickness of approximately 30 um and under
strictly isothermal conditions.
In thicker layers and no isothermal conditions convective effects are dominant,
which increase the limiting current density to higher values than predicted by
simplified diffusion equations.
The predicted limiting current density of oxygen reduction in an electrolyte layer
thickness of 30 um significantly exceeds practical observations of average
atmospheric corrosion rates.
On this basis, it can be argued that the overall rates of atmospheric corrosion are
not likely to be controlled by the cathodic oxygen reduction process but rather by
the anodic reaction(s). (ROBERGE)

The anodic process:

The following simplified oxidation reaction represents the anodic half-cell for
atmospheric corrosion:
−¿¿
n+¿+ne ¿
M→M

The formation of corrosion products (metal oxides and hydroxides), the solubility
of corrosion products in the surface electrolyte, and the formation of passive films
affect the overall rate of the anodic metal dissolution process and cause
deviations from simple rate equations assuming activation control. (ROBERGE)
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Factors Affecting Atmospheric Corrosion:
Time of Wetness:
In atmospheric corrosion, the time of wetness refers to the duration that a metal
surface remains wet or exposed to moisture.
An increase in the time of wetness generally leads to an increase in corrosion.
Prolonged exposure to moisture provides more opportunities for corrosive
reactions to occur, which can result in an accelerated corrosion rate and more
severe corrosion damage.
Depending on:
Temperature: a high temperature leads to decreased adsorption.
Porosity voids: the larger the number of voids, the greater the volume of water
adsorption.
Degree of oxidation: lesser adsorption with higher oxidation.
rain boundaries: More adsorption with a larger number of grain boundaries.
Nature of surface: more adsorption on a rough surface. (ahmad)

Water Adsorption:
Water can be adsorbed onto a metal surface in either molecular or dissociated
form.
In the molecular form, the intact water molecule is adsorbed.
When adsorbed molecularly, the oxygen atoms bond to the metal substrate and
donate an electron pair, resulting in a transfer of charge from the water to the
metal.
In the dissociated form, the water molecule breaks apart into OH and H atoms
which then bond separately to the metal surface.
The film of hydroxyl (OH) groups that form via dissociated adsorption is protective
against corrosion.
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The adsorbed water layer acts like a solvent and can dissolve
corrosive gases in the atmosphere. This provides sites for corrosion to occur.
Once the adsorbed water layer reaches 3 monolayers thick 2.8 anstrons so we
have 8.5 x 10 -10 m, it takes on properties similar to bulk water. The relative
humidity is then close to the "critical humidity".
Above the critical humidity, the corrosion rate increases significantly. Below it,
corrosion is very low. (ahmad)

Effect of Atmospheric Factors on Atmospheric Corrosion:

Relative Humidity:
Atmospheric corrosion does not occur in dry air. The relative humidity is

RH = Amount of water vapor in air/ Amount of water vapor required to saturate the air

The corrosion rates of metals increases sharply beyond ‘critical relative humidity.

’ The level of the critical humidity varies with the nature of the metal and the type of contaminant. If the
atomssphere is clear and uncontaminated, corrosion is negligible at a relative humidity as high as 99%.
However, in the presence of contaminants, corrosion begins to increases around 80% RH, and this in
ideal situations must be to have sufficient number of ions , however if has contaminations the precent
decrease.

pH
Decreased pH on a metallic substrate caused by dissolution of certain contaminants like SO2− 4 may
lead to an acidification process and acceleration of atmospheric corrosion.

Dew and fog

They are more corrosive because of a higher concentration of atmospheric contaminants, hygroscopic
salts and a lower pH value.

Fog: Fog forms when the air becomes saturated with water vapor and cools to the point where the
water vapor condenses into tiny water droplets.

Fog typically occurs near the ground level and is most formed when warm, moist air meets a cooler
surface or when cool air moves over a warm surface.

Dew: Dew forms when the temperature of the surface cools down to the dew point ‫ درجه تكثيف‬of the
surrounding air.

As the surface cools, the moisture in the air condenses and forms small water droplets on the surface.
Dew typically forms on objects such as grass, leaves, or other surfaces during the nighttime when the
surface loses heat and cools down.

Dust and Soot:

Dust has an abrasive effect on metallic surface in combination with wind velocity in desert and on
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Impurities from emissions, such as CO2 and CO are adsorbed in dust particles and create
micro-corrosion cells. Dust particles being hygroscopic promote serious corrosion by
adsorption of salts promoting corrosion.

While dust particles can contribute to the initiation of pitting corrosion under certain circumstances,
their primary effect on atmospheric corrosion is not limited to pitting alone.

dust particles can act as physical barriers on the metal surface, preventing the access of oxygen and
moisture, which are necessary for the corrosion process. This can create a protective barrier, reducing
the rate of corrosion. However, if the dust particles absorb moisture from the atmosphere, they can
form a corrosive electrolyte layer on the metal surface, promoting corrosion.

dust particles may contain contaminants such as salts, sulfur compounds, or industrial pollutants, which
can accelerate corrosion by serving as additional electrochemical reactants.

Soot, on the other hand, is primarily composed of carbon particles CO2 resulting from incomplete
combustion of organic materials. Named black carbon. n complete combustion, the organic material
combines with oxygen to produce carbon dioxide (CO2) and water vapor (H2O) as the primary
byproducts.

However, when combustion is incomplete, there is insufficient oxygen available to convert all the carbon
in the organic material into carbon dioxide. As a result, carbon particles are formed and released into
the air as soot.

CO2 (g) + H2O (l) ⇌ H2CO3 (aq)

Soot particles are very small and can easily adhere to surfaces, including metal surfaces.

The carbon particles in soot can absorb moisture and create a thin layer of acidic condensate on the
metal surface. This condensate can lower the pH at the metal-electrolyte interface, leading to localized
corrosion.

Carbon particles may also contain adsorbed pollutants and contaminants, such as sulfur compounds,
heavy metals, or organic acids. These contaminants can react with moisture and atmospheric gases,
forming corrosive compounds that accelerate the corrosion process.

soot particles generally have a detrimental impact on atmospheric corrosion due to their ability to
absorb moisture, create acidic conditions, and catalytically promote corrosion reactions.

Wind Velocity
Wind velocity affects the concentration profile of salt particles in a particular area. It is responsible for
the transport of pollutants which may be deposited on a metallic substrate. It may also dislodge a
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protective layer formed on a metal surface and promote corrosion. Wind velocity, wind
direction and structural geometry are major contributors in areas subjected to strong
winds

Pollutants Chemistry and their Impact on Atmospheric Corrosion:

Chlorine Containing Compounds:
In atmospheric corrosion, certain gases and pollutants can accelerate the degradation of metals such as
steel. Two examples of such corrosive gases are chlorine gas (Cl2) and hydrogen chloride gas (HCl). The
rate at which steel corrodes is influenced by the relative humidity of the surrounding environment.

The critical humidity level refers to the threshold at which the atmospheric corrosion rate of steel is
significantly accelerated. In this case, the critical humidity level is stated as 70%.

Hygroscopic salts are substances that have the ability to absorb water from the atmosphere. When
hygroscopic salts absorb water, they can form an electrolyte solution. This solution can lower the
saturation relative humidity, which is the point at which the air is holding the maximum amount of
water vapor it can at a given temperature.

If the humidity exceeds the critical value for a particular salt solution, the saturated salt solution will
absorb water from the atmosphere until equilibrium is reached. Conversely, if the humidity falls below
the critical value, the salt solution will release water. This process of absorption and release of water is
influenced by the number of monolayers of adsorbed water formed on the metal substrate.

When the humidity in the environment exceeds the critical value for a specific salt solution, it means
that the air is holding more moisture than the salt solution can retain at that given temperature. In this
case, the saturated salt solution will absorb water from the atmosphere to reach equilibrium.

Equilibrium refers to a state where the salt solution has absorbed enough water to balance with the
moisture content in the air. At this point, the salt solution is saturated, and it cannot absorb any more
water. The absorption process occurs because the salt solution has a natural affinity for water
molecules.

Conversely, if the humidity in the environment falls below the critical value, it means that the air is not
holding enough moisture for the salt solution to remain saturated. In this situation, the salt solution will
release water back into the air to achieve equilibrium.

The process of absorption and release of water by the salt solution is influenced by the number of
monolayers of adsorbed water formed on the metal substrate. A monolayer refers to a single layer of
water molecules that are adsorbed or attracted to the surface of the metal.

As the relative humidity increases, more water molecules are adsorbed onto the metal surface, forming
multiple monolayers. This accumulation of water molecules on the metal surface can increase the
corrosion process, as moisture provides the necessary conditions for electrochemical reactions to occur,
leading to corrosion.

Conversely, when the relative humidity decreases, the number of monolayers of adsorbed water on the
metal surface decreases, and the water molecules may be released back into the environment. This
release of water reduces the moisture content on the metal surface, making it less prone to corrosion.
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In summary, the absorption and release of water by the salt solution are influenced by
the relative humidity in the environment and the number of monolayers of adsorbed
water on the metal substrate. The presence of water on the metal surface is a critical factor in the
corrosion process, as it provides the necessary conditions for corrosion to occur.

Rain
Rain plays a dual role, increasing the rate of atmospheric corrosion or decreasing it under certain
conditions.

Increases:

(a) Rain increases the thickness of electrolyte layers on surfaces. When it rains, more electrolyte solution
forms on substrates. Atmospheric pollutants like sulfur dioxide gas (SO2) in the air get washed down by
rain. They deposit as ions like hydrogen ions (H+) and sulfate ions (SO42-) in the electrolyte layer. This
accelerates corrosion by promoting corrosion reactions in the presence of the electrolyte.

(b) In less contaminated atmospheres, rain has a stronger effect in promoting corrosion. In clean air, rain
is more effective at depositing corrosive ions on surfaces due to fewer competing atmospheric
contaminants. This enhances electrolyte formation and corrosion.

(d) The nature of the substrate surface influences how much rain promotes corrosion. Rough, porous
surfaces hold electrolyte solution for longer than smooth surfaces. This prolongs corrosion reactions on
rougher surfaces compared to smooth ones, where water drains away more quickly.

Decrease:

Period of dry deposition: If a period of dry deposition of atmospheric contaminants is followed by rain, it
can decrease the degree of corrosion. The rain washes away deposits that built up during the dry period
before they can promote corrosion. This cleaning effect of rain limits corrosion.

If the contamination is still found with the moisture the corrosion will increase

Oxides of sulfur:
During the combustion of materials containing sulfur, such as fossil fuels, two main sulfur compounds
are formed: sulfur dioxide (SO2) and sulfur trioxide (SO3).

When sulfur dioxide is released into the atmosphere, a significant portion of it is absorbed by aerosol
particles. Atmospheric aerosols consist of small particles of solids, like dust, and liquids, like water,
suspended in the atmosphere. Atmospheric aerosols can be either emitted directly into the atmosphere
as a particle, like ash

Once absorbed, the SO2 undergoes oxidation to form sulfate (SO4) ions. This process can be
summarized as follows:

SO2(g) → SO2(aq) ⇒ SO42- (Acidification)

SO2 reacts with hydroxide ions (OH-) from the water on the surfaces of aerosol particles, leading to the
formation of bisulfite ions (HSO3-) that further convert to sulfuric acid (H2SO4):
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SO2(g) + OH- → HSO3- ⇒ H2SO4

It's important to note that SO2 is the primary contributor to atmospheric corrosion, and
the presence of SO2 is necessary for significant rusting to occur. Sulfuric acid is a powerful acid that can
corrode metals by attacking their surfaces. It can react with the metal oxides present on the surface,
forming soluble metal sulfate compounds and water. This process is known as acid corrosion or acid
attack. The presence of sulfuric acid, which is derived from SO3, can accelerate the corrosion of metals
exposed to the atmosphere. It can penetrate protective coatings or passivate layers on metal surfaces,
leading to the degradation of the metal. Sulfuric acid can also promote the formation of rust or other
corrosion products on the metal.

Rust cleaning from the steel reinforcements:

Rust cleaning has many methods like wire brush or sand blasting;

Sandblasting is a process that involves forcefully propelling fine particles of abrasive material,
such as sand, at high speed onto a surface to remove dust and other contaminants.

The abrasive particles strike the surface with great force, dislodging and removing the dust
particles from the surface. The particles of abrasive material act as tiny projectiles that physically
scrub away the dust, leaving a clean and prepared surface

Here are a few reasons why it is important to clean the rust from steel reinforcement before
construction:

1. Bonding strength: The rust layer reduces the bonding strength between the steel
reinforcement and the cement, compromising the overall structural integrity of the
concrete element. This can weaken the concrete and potentially lead to failure or reduced
load-bearing capacity.

2. Corrosion propagation: If rust is present on the steel reinforcement, the corrosion process
can continue even after construction. Once rust forms on a portion of the steel
reinforcement, it creates localized areas with different electrochemical potentials. This
difference in potential leads to the flow of electric current, causing corrosion to propagate
from the rusted areas to adjacent regions. Corrosion expands the volume of the steel,
leading to cracking and spalling of the surrounding concrete. This can cause significant
damage to the structure over time.

3. Durability: Rusting steel reinforcement can accelerate the deterioration of the concrete
structure. As the rust expands, it exerts pressure on the surrounding concrete, leading to
cracking and increased permeability. This allows moisture and aggressive chemicals to
penetrate deeper into the concrete, further promoting corrosion and reducing the
structure's durability.
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Prevention of the atmospheric corrosion:

The rusting of iron can be prevented by greasing, painting, galvanizing, anodizing, or oiling the surface.
These methods can be classified into the following categories:

1. Galvanization: Galvanized metal is coated with a thin layer of zinc to protect it against
corrosion. The zinc oxidizes when it is exposed to air creating a protective coating on the
metal surface.

2. Alloying: It is the method of improving the properties of a metal by mixing the metal with
another metal or nonmetal. When iron is alloyed with chromium and nickel in stainless steel is
obtained. Stainless steel does not rust at all.

3. Painting: Rusting of iron can be easily prevented by coating the surface with paint which
protects iron from air and moisture.

4. Greasing/Oiling: When some grease oil is applied to the surface of an iron object, then air and
moisture cannot come in contact with it, and hence rusting is prevented.

(BYJU`S, n.d.)

References
(n.d.). Retrieved from BYJU`S: https://byjus.com/question-answer/methods-for-prevention-of-
corrosion/

ahmad, z. (n.d.). Principles of Corrosion Engineering.

ROBERGE, M. T. (n.d.). Atmospheric Corrosion. academia.

Y. R. Yoo, S. H. (June 25, 2022). Atmospheric Corrosion Behavior of Carbon Steel by the Outdoor Exposure
Test . research gate.