Transcript Corrosion - ThaparNotes

Materials Science & Engineering (UES012)
Corrosion is the disintegration of metal through an unintentional chemical or
electrochemical action, starting at its surface.
Corrosive environment is needed for it
All metals exhibit a tendency to be oxidized, some more easily than others. A
tabulation of the relative strength of this tendency is called the galvanic series.
Knowledge of a metal's location in the series is an important piece of
information to have in making decisions about its potential usefulness for
structural and other applications.
The driving force that causes metals to corrode is a natural consequence of
their temporary existence in metallic form.
To reach this metallic state from their occurrence in nature in the form of
various chemical compounds (ores), it is necessary for them to absorb and
store up for later return by corrosion, the energy required to release the metals
from their original compounds.
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Standard EMF series
This table is for reduction reactions. For oxidation reactions, the corresponding
voltage will change sign.
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Corrosion in metals
It is the process by which metallic atoms leave the compounds in the presence of water or gas. Metal atoms leave
the metal until it fails or erodes from oxidation.
The tendency for oxidation is dependent on the metal. Metals used in construction, such as steel and copper alloys,
are both subject to corrosion. Metals are chemical combinations of multiple elements. These metals are highly
vulnerable because of the high energy content of the elements used in metallic form
Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as
its oxide, hydroxide, or sulfide due to environmental conditions such as pollution, moisture etc
For metallic materials, the corrosion process is normally
electrochemical.
Electrochemical reaction
- a chemical reaction in which there is transfer of electrons from one chemical species to another.
Metal atoms characteristically lose or give up electrons in what is called an oxidation reaction.
M Mn++ ne-
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Reduction Reaction
The electrons generated from each metal atom that is oxidized are
transferred to and become a part of another chemical species.
This is a reduction reaction.
Reduction of hydrogen ions in an acid solution
Reduction reaction in an acid solution
containing dissolved oxygen
Reduction reaction in a neutral or basic
solution containing dissolved oxygen
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Electrode potential (Principle of Corrosion)
Not all metallic materials oxidize to form ions with same degree of ease.
If the iron and copper electrodes are connected electrically,
reduction will occur for copper at the expense of the oxidation
of iron.
Cu2+ + Fe  Cu + Fe2+
In terms of half-cell reactions:
Fe  Fe2++ 2eCu2+ + 2e-  Cu
Cu2+ + Fe  Cu + Fe2+
Cu2+ ions will deposit (electrodeposit) as
metallic copper on the copper electrode,
while iron dissolves (corrodes) on the other side of the cell and
goes into solution as Fe2+ ions.
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Standard
half-cell
Galvanic cell
When externally connected, electrons generated from the oxidation of iron
flow to the copper cell in order that Cu2+ be reduced.
In addition, there will be some net ion motion from each cell to the other
across the membrane.
This is called a galvanic couple - two metals electrically connected in a
liquid electrolyte wherein one metal becomes an anode and corrodes,
while the other acts as a cathode.
An electric potential or voltage will exist between the two cell halves.
For example:
Cu2+ + Fe  Cu + Fe2+
Potential: 0.780 V
Fe2+ + Zn  Fe + Zn2+
Potential: 0.323 V
Various electrode pairs have different voltages !
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Standard EMF series
Cell voltages represent only differences in electrical potential.
 establish a reference point, or reference cell, to which other cell halves
may be compared.
Reference cell = standard hydrogen electrode
Electromotive force (emf) series
Connect a standard half cell of various metals to
the standard hydrogen electrode.
Measure the voltage of the cell.
Rank the metals as per this voltage.
H2 gas
1 atm
pressure
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Corrosion rate
The corrosion rate, or the rate of material removal as a consequence of the
chemical action, is an important corrosion parameter.
Corrosion penetration rate (CPR), or the thickness loss of material per unit of
time.
where W = weight loss after exposure time t ;  = density, and A = exposed specimen area,
K is a constant.
There is an electric current associated with electrochemical corrosion reactions.
Corrosion rate r can be written as
r = i/nF
i = current density (the current per unit surface area of material corroding)
n = number of electrons associated with the ionization of each metal atom
F = 96,500 C/mol.
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Forms of corrosion
Uniform Attack
- Occurs with equivalent intensity over the entire exposed surface
- Often leaves behind a scale or deposit.
Examples: General rusting of steel and iron, Tarnishing of silverware
Galvanic Corrosion
- occurs when two metals or alloys having different compositions are electrically
coupled while exposed to an electrolyte.
- The less noble or more reactive metal in the particular environment will
experience corrosion; the more inert metal, the cathode, will be protected from
corrosion.  Lower in the series experiences corrosion
Example:
Steel screws corrode when in contact with brass in a marine environment;
If copper and steel tubing are joined in a domestic water heater, the steel will
corrode in the vicinity of the junction.
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Forms of corrosion
Crevice Corrosion (a narrow opening especially in a rock or wall/ crack)
In small opening where solution is not in flow and is stagnent
- Arises as a consequence of concentration differences of ions or dissolved gases in the electrolyte
solution, and between two regions of the same metal piece.
- For such a concentration cell, corrosion occurs in the locale that has the lower concentration.
Example: occurs in crevices and recesses or under deposits of dirt or corrosion products where the
solution becomes stagnant and there is localized depletion of dissolved oxygen.
The crevice must be wide enough for the solution to penetrate, yet narrow enough for stagnancy
galvanic corrosion two connected metals + single environment
crevice corrosion one metal part + two connected environments
This form of attack is generally associated with the presence of small volumes of stagnant solution in occluded
interstices, beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads.
It is a localized form of attack, where there is a breakdown of the surface passive layer, in crevices or on
'shielded' areas beneath surface deposits. Engineered or 'designed in' crevices can be set up at bolted and other
joints, beneath flanges or between flanges and gaskets or other contact areas such as valve seats.
Crevices make a chemical environment which is different from that of freely exposed surfaces and therefore
accelerate corrosion. This environment keeps moisture, traps pollutants, concentrates corrosion products.
.
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Forms of corrosion
Stress Corrosion
- results from the combined action of an applied tensile stress and a corrosive
environment; humidity, pollution both influences are necessary.
- In fact, some materials that are virtually inert in a particular corrosive medium
become susceptible to this form of corrosion when a stress is applied.
- Small cracks form and then propagate in a direction perpendicular to the stress,
with the result that failure may eventually occur.
- Failure behavior is characteristic of that for a brittle material, even though the
metal alloy is intrinsically ductile.
- Furthermore, cracks may form at relatively low stress levels, significantly below
the tensile strength.
- Most alloys are susceptible to stress corrosion in specific environments,
especially at moderate stress levels.
Example:
Most stainless steels stress corrode in solutions containing chloride ions,
A bent wire will corrode easily at the bend.
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Hydrogen embrittlement
The embrittlement of a metal or alloy by atomic hydrogen involves the ingress of hydrogen into a component,
an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic
brittle failures at stresses below the yield stress of susceptible materials.
Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and
hydrogen dissolved in the metal.
Examples of hydrogen embrittlement are cracking of weldments or hardened steels when exposed to conditions
which inject hydrogen into the component. Presently this phenomenon is not completely understood and hydrogen
embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem. Hydrogen
embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium
alloys and aluminum alloys.
Sources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in
welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment
that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be
produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be
added to reactor coolant to remove oxygen from reactor coolant systems.
Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarized
below:
by some manufacturing operations such as welding, electroplating, phosphating and pickling; if a material subject
to such operations is susceptible to hydrogen embrittlement then a final, baking heat treatment to expel any
hydrogen is employed
as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction described
here acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather
than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be
thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into
the component, the cracking phenomenon is often termed “sulphide stress cracking (SSC)”
the use of cathodic protection for corrosion protection if the process is not properly controlle
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Erosion corrosion is a degradation of material surface due to mechanical action, often by impinging
liquid, abrasion by a slurry, particles suspended in fast flowing liquid or gas, bubbles or
droplets, cavitation, etc. The mechanism can be described as follows:
mechanical erosion of the material, or protective (or passive) oxide layer on its surface,
enhanced corrosion of the material, if the corrosion rate of the material depends on the thickness of
the oxide layer.
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Protection against corrosion
• Use of noble metals – least susceptible to corrosion (from emf series)
• Avoid physical contact between dissimilar metals – No galvanic cell formation
• For dissimilar metal contacts, anode should have a larger surface area than
cathode. Corrosion rate is dependent on current density.
Ex: Cu nut & bolt on large steel plate
• Materials with two phase structure – formation of galvanic couple, i.e., one
phase acts as anode while another one acts as cathode.
Ex: cementite in steel is more noble (cathode) than ferrite.
 Protection by passivation layers
Ex: Aluminum coating on both sides of duralumin sheets
• Use of inhibitors – formation of a protective coating
• Use of non-metallic coating such as enamel, oil, paint, etc.
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So far…
Classification of materials
Crystal Structure
Chemical Bonding
Structure of Solids:
Crystalline and non-crystalline materials, Inorganic solids,
Silicate structures and its applications.
Crystal Imperfections:
Point defects, Line defects, Surface defects, Movement of
Dislocation, Dislocation energy.
Diffusion:
Laws of diffusion, Temperature dependence of diffusion coefficient,
Determination of activation energy.
Mechanical Properties:
Elastic, Anelastic and Viscoelastic behaviour, Plastic behaviour,
Critical shear stress, Twinning and slipping phenomenon, Creep.
Equilibrium Diagram:
Solids solutions and alloys, Gibbs phase rule, Isomorphous
and eutectic phase diagrams and their construction, Lever- arm rule,
Application of phase diagrams, Zone refining.
Corrosion Process
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Properties of Materials
One goal of materials engineering is to select materials with suitable
properties for a given application.
Mechanical properties
A. Elasticity and stiffness (recoverable stress vs. strain)
B. Plasticity
(non-recoverable stress vs. strain)
C. Strength
D. Brittleness or Toughness
E. Fatigue
Electrical properties
A. Electrical conductivity and resistivity
B. Semiconducting materials
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Properties of Materials
Dielectric properties
A. Polarizability
B. Capacitance
C. Ferroelectric properties
D. Piezoelectric properties
E. Pyroelectric properties
Magnetic properties
A. Paramagnetic properties
B. Diamagnetic properties
C. Ferromagnetic properties
Optical properties
A. Refractive index
B. Absorption, reflection, and transmission
C. Birefringence (double refraction)
Corrosion properties
Biological properties
A. Toxicity
B. bio-compatibility
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