Introduction
• Mostly metals are unstable in ordinary aqueous
environments.
• Metals are usually extracted from ores through
the application of a considerable amount of
energy.
• Certain environments offer opportunities for
these metals to combine chemically with
elements to form compounds and return to
their lower energy levels.
Introduction contd...
• Corrosion is the primary means by which metals
deteriorate.
• Most metals corrode on contact with water (and
moisture in the air), acids, bases, salts, oils,
aggressive metal polishes and other solid and liquid
chemicals and gaseous materials like acid vapors,
formaldehyde gas, ammonia gas and sulfur
containing gases.
• The best known case is rusting of steel.
• Corrosion processes are usually electrochemical in
nature.
Introduction contd...
• When metal atoms are exposed to water molecules they
can give up electrons, becoming themselves positively
charged ions provided an electrical circuit can be
completed.
• This effect can be concentrated locally to form a pit or,
sometimes a crack or it can extend across a wide area to
produce general wastage.
• Localized corrosion that leads to pitting may provide
sites for fatigue initiation.
• Pitting corrosion also occurs much faster in areas where
micro structural changes have occurred due to welding
operations
Introduction contd...
• The corrosion process (anodic reaction) of the
metal dissolving as ions generates some electrons,
that are consumed by a secondary process
(cathodic reaction).
• These two processes have to balance their
charges.
• The sites hosting these two processes can be
located close to each other on the metal's surface,
or far apart depending on the circumstances.
Theory of corrosion
• The problem with iron as well as many other metals
is that the oxide formed by oxidation does not
firmly adhere to the surface of the metal and flakes
off easily causing "pitting".
• Extensive pitting eventually causes structural
weakness and disintegration of the metal.
• Certain metals such as aluminum, form a very
tough oxide coating which strongly bonds to the
surface of the metal preventing the surface from
further exposure to oxygen and corrosion).
Theory of corrosion contd…
• Corrosion occurs in the presence of moisture.
Eg: when iron is exposed to moist air, it reacts with
oxygen to form rust, (Fe2O3 . XH2O)
• The amount of water present also determines the color
of rust, which may vary from black to yellow to orange
brown.
• The formation of rust is a very complex process which is
thought to begin with the oxidation of iron to ferrous
(iron "+2") ions.
Fe -------> Fe+2 + 2 e-
Theory of corrosion contd…
• Both water and oxygen are required for the next
sequence of reactions.
• The iron (+2) ions are further oxidized to form ferric
ions (iron "+3") ions.
Fe+2 ------------> Fe+3 + 1 e• The electrons provided from both oxidation steps
are used to reduce oxygen as shown.
O2 (g) + 2 H2O + 4e- ------> 4 OH-
Theory of corrosion contd…
• The ferric ions then combine with oxygen to form ferric oxide
[iron (III) oxide] which is then hydrated with varying amounts of
water. The overall equation for the rust formation is:
4Fe+2 (aq) + O2 (g) + [ 4 + 2 XH2O (l) ] ------> 2Fe2O3 . XH2O (s) + 8H+ (aq)
rust
• The formation of rust can occur at some distance away from the
actual pitting or erosion of iron.
• This is possible because the electrons produced via the initial
oxidation of iron can be conducted through the metal and the
iron ions can diffuse through the water layer to another point on
the metal surface where oxygen is available. This process results
in an electrochemical cell in which iron serves as the anode,
oxygen gas as the cathode, and the aqueous solution of ions
serving as a "salt bridge" as shown below.
Theory of corrosion contd…
Theory of corrosion contd…
• Many factors affect the rate of corrosion.
moist conditions, presence of salt (dissolved salt
increases the conductivity of the aqueous
solution formed at the surface of the metal and
enhances the rate of electrochemical corrosion).
Factors Influencing Corrosion
Reactions
Factors associated mainly with the Metal
• Effective electrode potential of a metal in a
solution
• Overvoltage of hydrogen on the metal
• Chemical and physical homogeneity of the metal
surface
• Inherent ability to form an insoluble protective
film
Factors Influencing Corrosion Reactions contd...
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.
Types of Corrosion
Corrosion could be widely discussed under the following:
• Inorganic Corrosion
• Organic Corrosion
Inorganic Corrosion:
• It is co
o ly ide tified as rust , is defi ed as
destruction of metal or alloy by chemical or electro
chemical reaction with its environment.
• Though corrosion is a complex function of many factors,
the three most important are –
- A voltage differential between pure and impure areas.
- Physical conditions of temperature and humidity
- Oxygen in the air.
Types of Corrosion contd...
With iron or steel, the ferrous ion may react with
hydroxyl ion in water to form Fe(OH)2 and with
oxygen to produce Fe(OH)3 (rust).
Accelerating and Intensifying Factors
• In pure air almost no iron corrosion occurs, at
relative humidities even upto 99%.
• However, pure air is seldom encountered in
practice. Contaminants present in the air like
sulphur dioxide, particles of charcoal etc. enhance
the corrosion rate.
Types of Corrosion contd...
Organic Corrosion
• High temperatures along with high moisture level
speed up the growth of bacteria spores.
• Micro-organism growth is injurious to material:
as it not only results in decomposition but also
mechanical weakening of the products like
leather and other organic material.
• In most cases bacterial growth can be arrested if
RH is maintained below 35%.
Corrosion Types or Corrosion Forms
• Although there is only one fundamental mechanism of
corrosion, the electrochemical cell, there are several
Corrosion forms or Corrosion Types that can occur.
• Each form of attack has a specific arrangement of anodes
and cathodes and the corrosion which occurs has a
specific location and pattern.
• Each form of corrosion can be effectively controlled during
design if it is anticipated.
• By understanding the various forms of corrosion, the
conditions under which they occur, and how they are
quantified, they can each be addressed and controlled.
Corrosion types
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Uniform Corrosion
Concentration Cell Corrosion
Inter-granular Corrosion
Atmospheric Corrosion
Fretting Corrosion
Dealloiyng
Formicary Corrosion
Microbiological Corrosion – MIC
Galvanic Corrosion or Bimetallic Corrosion
Pitting Corrosion - Crevice Corrosion
Stress Corrosion Cracking
Cavitation Corrosion
Hydrogen Embrittlement
Immunity
Corrosion Fatigue
Corrosion types contd...
1. Uniform Corrosion
• Is the attack of a metal at essentially the same at all exposed
areas of its surface.
• At no point is the penetration of the metal by corrosion twice as
great as the average rate.
Eg: Rusting steel in the atmosphere and the corrosion of copper
alloys in seawater. Steel submerged in seawater can also corrode
uniformly but can also suffer from non-uniform attack under
some circumstances.
2. Concentration Cell Corrosion:
• Is accelerated by differences in environment between separated
areas on a single metal.
• The rates of attack experienced in concentration cell corrosion are
affected by relative anode/cathode areas in the same manner as
in galvanic corrosion.
Corrosion types contd...
3. Inter-granular Corrosion:
• Just as most engineering metals are mixtures of one or
more metals, they consist of large numbers of individual
metal crystals called grains that are joined together at their
surfaces or grain boundaries. As there can be differences in
composition at or adjacent to these grain boundaries,
selective corrosion can occur at these sites.
Eg: Aluminum alloys and some nickel alloys are susceptible
to inter-granular attack, usually the type that is caused by
segregation of impurities at the grain boundaries.
Corrosion types contd...
4. Atmospheric Corrosion:
• The atmospheric environment varies drastically with regards to corrosivity
depending on the geographical location.
• The electrolyte is moisture from precipitation, fog or dew, sea spray, or
other sources.
• The three factors that have the most influence on the corrosivity of the
atmosphere at a given site are:
(1) The amount of time that exposed surfaces remain wet at the site,
(2) The amount of chloride from the sea that reaches the surfaces
(3) The amount of industrial pollutants (mainly acids) that reach the
surfaces.
• Corrosion of most metals in atmospheric environments is not limited by
the amount of oxygen present and can proceed rapidly when the
electrolyte is present. The corrosivity of the atmosphere varies drastically.
Corrosion types contd...
5. Fretting Corrosion:
•
When surfaces move in relation to each other, this relative
motion can result in abrasion.
• This abrasion can increase the attack at these fraying surfaces.
Eg: Fretting has largely been eliminated through welded
construction. Fretting is, however, still encountered in bolted
joints and flanges where there is not enough bolt tension to
eliminate movement in the joint. Thermal expansion with
frequent cycling can also result in fretting attack. Any combination
of corrosion and wear will almost always be worse than the action
of either one separately.
Corrosion types contd...
6. De-alloying:
• Most of the commonly used metallic materials are alloys formed from
mixing two or more metals.
• Pure metals are usually too soft and weak to be used structurally.
• In this form of corrosion, de-alloying, corrosion selectively attacks one
or more constituent of the alloy mixture.
Eg:The dezincification of brass and the graphitization of cast iron are
common examples of de-alloying.
7. Formicary Corrosion:
• Formicary corrosion differs from typical copper oxidation in that the
corrosion manifests itself as an ant nest (i.e. formicary) pattern within
the metal.
• The end result of this tunneling effect can be pin-hole leaks in copper
tube.
Corrosion types contd...
8.
Microbiologically Influenced Corrosion:
• Microbiologically Influenced Corrosion (MIC) is, by far, the most severe
and threatening form of corrosion to HVAC piping systems.
• An MIC presence usually signals a very severe threat to the entire
system - requiring extensive cleaning and sterilization at great expense.
• For many affected systems, MIC cannot be eliminated, and an elevated
corrosion and pitting condition will exist for the remainder of system
life.
• MIC produces large and deep pits due to the microorganism's utilization
of iron as an energy source (often as an alternative to oxygen), and
through the production of strongly corrosive metabolic by-products
such as sulfuric acid - which further assists the microorganism in
dissolving pipe metal.
• MIC exists to varying degrees of severity, and is not exclusive to carbon
steel piping systems or open condenser water systems.
Corrosion types contd...
9. Galvanic corrosion / Bimetallic corrosion:
• It is a localized mechanism by which metals can be
preferentially corroded.
• This form of corrosion has the potential to attack
junctions of metals, or regions where one construction
metal is changed to another.
• Frequently this condition arises because different metals
are more easily fabricated into certain forms.
• Galvanic corrosion is well known to most designers,
specifiers and fabricators, but often the only rule in
force is "don't mix metals".
Corrosion types contd...
10. Pitting Corrosion - Crevice Corrosion:
• Under certain specific conditions, particularly involving chlorides (such
as sodium chloride in sea water) and exacerbated by elevated
temperatures, small pits can form in the surface of the steel.
• Dependent upon both the environment and the steel itself these small
pits may continue to grow, and if they do can lead to perforation, while
the majority of the steel surface may still be totally unaffected.
Crevice Corrosion:
• A special case of pitting corrosion, but one where the initial "pit" is
provided by an external feature; examples of these features are sharp
re-entrant corners, overlapping metal surfaces, non-metallic gaskets or
incomplete weld penetration. To function as a corrosion site a crevice
has to be of sufficient width to permit entry of the corrodent, but
sufficiently narrow to ensure that the corrodent remains stagnant.
Corrosion types contd...
11. Stress corrosion cracking:
• It is the inter-granular or trans-granular cracking of a material due
to the combined action of tensile stress and a specific environment.
Examples:
• Many materials, particularly high strength materials, are susceptible
to stress corrosion cracking when exposed to a specific
environment. For example, cold worked brass, which is found in
ammunition cartridges, is susceptible to stress corrosion cracking
when exposed to an environment containing ammonia. In chloride
containing environments, titanium alloys, aluminum alloys, and high
strength stainless steels are susceptible. The stresses required to
initiate and propagate cracking are often low and many failures
occur due to residual stresses rather than applied stress.
Corrosion types contd...
12. Cavitation corrosion
• Enhanced through the formation and collapse of gas or
vapor bubbles at or near the metal surface.
Eg: Cavitation is commonly encountered in pumps and
in high speed propellers. It is enhanced when entrained
air is present in systems with high velocity flow.
Cavitation can also occur on or near high intensity
sound generators.
Sainless steel, some nickel alloys, and titanium alloys
are highly resistant to cavitation damage but even
these will be attacked under severe conditions.
Corrosion types contd...
13. Hydrogen embrittlement:
• Hydrogen embrittlement is the severe loss of
ductility of a metal when hydrogen has been
introduced into the metal structure.
• Examples:
– Ferritic and martensitic (magnetic) steels,
particularly those with a yield strength in excess of
130 ksi, are particularly prone to hydrogen
embrittlement
Corrosion types contd...
14. Immunity:
• Immunity is the lack of measurable attack on a metal when
exposed to operational environments.
Examples:
• Gold and platinum are examples of metals that are normally
immune in a wide variety of environments due to the low
energy content of their metallic state.
15. Corrosion Fatigue:
• Corrosion fatigue is the reduced ability of a metal to
withstand repeated stress when exposed to the combined
action of stress and a corrosive environment as compared to
the effects of stress alone.
Prevention and Control of Corrosion
• Methods of preventing corrosion could be
summarized as follows:
1. Methods based on modification procedure:
– By attention to design
– By application of cathodic and anodic protections
2. Methods based on modification of the environment:
– By addition of corrosion inhibitors
– By de-aeration or adjusting the pH of the environment
– By purification or dehumidification of air
Prevention and Control of Corrosion contd...
3. Methods based on modification of the metal:
– By addition of alloying element
– By heat treatment
4. Methods based on protective coatings:
– Coatings by reaction product (chemical or
electrochemical treating of metal surfaces)
– Organic Coatings (paint, resins, etc.,)
– Inorganic coatings (enamels, cements)
– Metal Coatings
– Temporary protective
Prevention and Control of Corrosion contd...
(a)Corrosion prevention by attention to design:
• Proper planning and careful attention to protection at
the design stage is essential to minimize corrosion
and simplify future maintenance.
• Avoid sharp edges, crevices, rough welds, corners,
depressions and pockets.
• Whilst it is realized that some types of structures will
provide difficult areas for coating after erection, the
design authority should be mindful of painting before
erection.
(a) Corrosion prevention by attention to design contd...
• Sharp edges should be eliminated wherever possible. Coating materials
tend to run away from an edge.
• Welds must be given special attention when coatings are specified. One
of the major difficulties along the welds occurs because of weld
splatter.
• Brackets and other temporary fabricating aids are frequently welded on
the surface of structures during construction. They are sometimes left
in place after the job is completed.
• Skip welding consists of welding a 5cm bead and then skipping from
5cm to 30cm before welding another 5cm bead. Skip welding is used
mainly for reinforcing purposes when a continuous weld is not
considered necessary. Structures, which will be exposed to a corrosive
environment, should have continuous welds.
(a) Corrosion prevention by attention to design contd...
• Lap welding consists of continuous welding on
the outside surfaces only, leaving the steel plates
lapped on the inside thus forming crevices, which
are difficult to coat properly.
• Pipe supports, flanges, threaded joints and pipe
hangers are all potential points of corrosion.
• Crevices are formed in threaded couplings, which
allow the penetration of moisture.
(b) Corrosion prevention by application of Anodic and
Cathodic protections
(i)
Cathodic protection
• Is the most widely applied electrochemical corrosion
control technicque.
• This is accomplished by applying a direct current to the
structure which causes the structure potential to
change from the corrosion potential (Ecorr) to a
protective potential in the immunity region.
• The required cathodic protection current is supplied by
sacrificial anode materials or by an impressed current
system.
• Most metals in contact with an aqueous environment
having a near neutral pH can be cathodically protected.
(b) Corrosion prevention by application of Anodic
and Cathodic protections contd...
(ii) Anodic Protection
• For metals exposed to strongly alkaline or acid
environments, where the metal exhibits
active-passive behaviour, anodic protection
can be applied to control corrosion.
• Here the metal potential is shifted to the
passive zone from the active region by the
application of a direct current.
(c) Corrosion prevention by addition of corrosion
inhibitors:
• Generally two broad categories, organic and inorganic.
(i) Inorganic inhibitors
• Most often used in cooling tower water, heating/cooling
mechanisms, dehydration glycol and sweetening amine
solutions.
• Usually metal salts, which act to passivate the metal
surface. They have limited use because they require
constant concentrations, are often pH sensitive, and
usually don't work if there are chlorides present.
(c) Corrosion prevention by addition of corrosion
inhibitors contd...
(ii) Organic corrosion inhibitors
• Used in oil, gas and water wells, water and gas systems and
flow-lines.
• The majority of these are "organic film forming inhibitors".
These are organic chemicals with a polar ("water loving")
head and a long hydrocarbon ("oil loving") tail.
• When applied, these compounds align with the polar head
towards the metal and the tail towards the outside,
effectively establishing an oil-wet film on the metal surface.
• This inhibitor film breaks the corrosion cell by separating
the metal surface and the electrolyte containing water.
(d) Methods of preventing the atmospheric corrosion of
metals fall into two broad categories
• The choice of a suitably resistant metal or alloy
or combination of materials, or the provision of a
protective coating which supplements or
enhances the protection given by the air-formed
oxide film on the metal surface.
• The control of the environment by the exclusion
of water or aggressive contaminants, or by the
introduction of a corrosion inhibitor.
(e) Method of preventing organic corrosion
• The ideal method to retard bacteria growth
thereby limit organic corrosion is again to
maintain relative humidity levels below 3540% RH as well as keep temperature low.
(f) Corrosion Control with Organic Inhibitors
• Inhibitors are commonly defined as substances which, when
added in small amounts to the corrosive environment of a metal,
will effectively decrease the corrosion rate.
• Where dissolved oxygen, salts, and weak acids comprise the
corrosive environment inorganic inhibitors have been applied
successfully to minimize corrosion of metals, particularly iron and
steel.
• Examples of such inhibitors are soluble hydroxides, chromates,
phosphates, silicates, and carbonates. However, where strong
acids (dilute or concentrated), acid brines, high temperatures,
and microbiological action are constituents of the corrosive
environment, it has been found that polar organic compounds
and colloidal organic materials are more effective corrosion
inhibitors.
Protective coatings
• Protective coatings have been traditionally used to
provide corrosion protection.
• The life of such coatings, however, critically depends on
the surface preparation.
• The surfaces on which these coatings are applied have
to be free from all impurities including rust, mill scale,
salts, etc. and most critically moisture.
• To produce such surface various techniques are
employed including sand blasting, shot blasting, etc.
• This activity not only clears the impurities but also
creates an anchor profile, which improves the adhesion
between the coating and the substrate.
Protective coatings contd...
• When we blast the surface i.e. the metal surface,
then the naked pure metal is exposed to the
atmosphere.
• And if surface temperature of the metal is lower
than the dew point of the air surrounding it.
• Immediately condensation and subsequently
flashing of the metal occurs and the metal
surface is covered with rust bloom.
• If coating is carried over such a surface then the
entire exercise if surface preparation goes to a
waste.
Protective coatings contd...
• Corrosion prevention organizations have formulated
certain basics to be adhered to while carrying out
surface preparation and coating.
• It has defined that a differential of at least 5°F be
maintained between the surface temperature and
the air dew point while carrying out such
operations (with the metal temperature being
higher).
Control of Relative Humidity
• Dehumidification comes to the rescue here and all
structures (with existing vapor barriers and where they
can be created with ease) must be dehumidified while
being blasted or coated so as to prevent all the
problems mentioned above.
• Dehumidification helps in maintaining a differential of
at least 5°F even during night (while surface
temperature drops) and during high humidity periods
like Monsoons etc. The coating life is increased
dramatically and quality achieved excellent.
Control of Relative Humidity contd...
• The second method, to prevent is to control relative humidity to
a level where it not only prevents condensation but is reduced
to a critical level below which corrosion does not occur.
• The controlled space is to maintained at a specific relative
humidity and temperature so that the products or material in
the controlled space cannot absorb moisture from the
surrounding atmosphere.
• As a general rule, rust and other oxidizing type corrosion
reactions will not occur if the relative humidity in the area is
maintained at or below 40% RH.
• Materials or product residing in this atmosphere need not be
coated with oil or other surface protecting materials, and can
thus be kept in ready-to-use condition.
Control of Relative Humidity contd...
• Some of the corrosion prevention applications do not have
coincidental temperature control.
• Humidity control is maintained to achieve protection of the area
or products in the area, as this constitutes a much more
economical method than air conditioning.
• An example of this is a waste pumping station or sewage plant,
where the primary humidity control problem is the maintenance
of a dewpoint within the space which is lower than the
temperature of the liquid which is being circulated or stored
there.
• In this way, condensation of moisture on the outside of pumps
and piping is eliminated, thus significantly reducing the cost of
painting and other maintenance normally required to control
corrosion.