CORROSION:
            It is defined as the destructive
and unintentional attack on a metal. It is electrochemical and ordinarily
occurs at the surface1

OR

Corrosion is the deterioration of a metal as a result of
chemical reactions between it and the surrounding environment. Some of the
types of corrosion are;

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1)      Uniform Attack                                               6)
Pitting Corrosion2)      Galvanic Corrosion                                         7)
Intergranular Corrosion3)      Crevice Corrosion                                           8)
Selective Leaching etc.4)      Erosion Corrosion                                           9)
Stress Corrosion5)      Hydrogen EmbrittlementIn spite of the tremendous progress made in the field of
material sciences in the past few years, technological challenges remain. There
is a recognized need to find new and economical ways of removing shortcomings
like corrosion of the materials and to convert these shortcomings into our
benefits 

 EFFECTS:          Generally, effects of corrosion can
be classified on basis of type of material e.g. Metals, Ceramics and Polymers.
We will individually looks at the effects on each Material.1)  METALS:Metals are used everywhere in the
world for structural and mechanical purposes ranging from paper pins to blades
of turbines. They are also the most effected materials due to corrosion.
Therefore the understanding of these effects is extremely important in
analyzing the use of metals for various purposes. Different types of corrosion
can have significantly different effects on a structural member.
While corrosion has a marginal effect on the ultimate tensile strength of a
material, there is a strong correlation between corrosion and reduction in
ductility thereby increasing brittleness of the material, which in turn can
change the failure mode from a ductile to a much more dangerous brittle
failure.
In addition, reduced cross-section and stress concentrations caused by
corrosion can greatly influence the load carrying capacity of the particular
member and the structure as a whole. In the case of uniform corrosion,
its influence on the material’s structural strength is straightforward. One
should establish the reduction of the thickness and the weight-loss of the
given material and calculate the stress for a given load for a new, reduced,
cross-section of the specimen. The matter is far more complicated for localized
corrosion, where the localized stress fields play a much larger role and
directly affect the tensile strength of the material itself.In the area of metallic corrosion, bio-materials act of
paramount importance as they are required for the survival of human beings
suffering from acute heart diseases. In the treatment of these diseases, we use
implants that contain metals such as stainless steel, cobalt, chromium. The study
of corrosion on these metals is of utmost importance. The implants face severe corrosion
environment which includes blood and other constituents of the body fluid which
encompass several constituents like water, sodium, chlorine, proteins, plasma, amino
acids along with mucin in the case of saliva2.
As a material starts to corrode, the dissolution of metal will lead to erosion
which in turn will eventually lead to brittleness and fracture of the implant. Once
the material fractures, corrosion gets accelerated due to increase in the
amount of exposed surface area and loss of protective oxide layer. If the metal
fragments are not surgically extracted, further dissolution and fragmentation can
occur, which may result in inflammation of the surrounding tissues.

 

Biomaterial Metals

Effect of Corrosion

Nickel

Affects skin – such as dermatitis

Cobalt

Anemia B inhibiting iron from being absorbed into the blood
stream

Chromium

Ulcers and Central nervous system disturbances

Aluminum

Epileptic effects and Alzheimer’s disease

Vanadium

Toxic in the elementary state

Table 1- Ref 7: Aksakal B, Yildirim ÖS, Gul H.
Metallurgical failure analysis of various implant materials used in orthopedic
applications. J Fail Anal Prevent 2004; 4(3): p. 17.2)  CERAMICS:            They are
compounds between metallic and non-metallic elements, they are most frequently
oxides, nitrides and carbons. They are used for cook wear, cutlery and even
automobile engines parts. Ceramic materials may be thought of as already being
corroded. Corrosion of ceramic materials usually involves simple chemical
dissolution in contrast to electrochemical processes found in metals. The effects of corrosion on chemical and mechanical
properties and microstructure of four engineering ceramics materials namely
alumina, reaction bonded silicon carbide, sialon and PSZ zirconia were
investigated3
and characterized using a chemical H bed and sand water slurry erosion test
rig. Following conclusions were drawn from these experiments.·        
Corrosion of ceramic can
occur either by dissolution of entire top surface or by preferential
dissolution of sintering agents which leads to porous surface layer with inferior mechanical properties and reduced
surface hardness.·        
Different corrosion rates
lead to degeneration of mechanical properties on ceramic surfaces·        
The synergism in corrosion
is due to the formation of the outer porous layer which is mechanically weak. The corrosion process easily removes this
layer and creates cracks in the underlying material which is then easily corroded·        
The slow corrosion rate of
ceramic compared to metals and alloys requires a different approach to study
corrosion particularly in applications where the corrosion solution has time to
act on the surface.3)  POLYMERS:Corrosion on polymers, both plastics and rubber materials, is
in many cases similar to metals but in other cases it looks very different.
Corrosion attacks on polymers are often hard to discover, the material may look
normal but can in fact be embrittled and have lost its mechanical
strength. Mechanical stressed polymers in chemical environments may
initiate cracks on the surfaces. These cracks can thereafter propagate through
the material either as a result of the mechanical stresses or in combination
with continuing chemical attack. Corrosion of polymers can be divided into
either chemical reaction or physical interaction. Polymers consist of a network
with molecular chains mainly consisting of carbon, hydrogen and oxygen. Corrosion by chemical
reaction changes the configuration of the polymer chains. Listed below are some
of the environments that causes chemical reactions in polymers.1)      Heat                                                                4)
Water2)      UV- Radiations                                                5)
Chemicals3)      OzonePhysical effects on polymers are caused by interaction with
the environment. This may lead to swelling, dissolving or leakage of additives.
The interaction is dependent on diffusion of substances into the polymer, and
the process is in some cases reversible. Organic substances usually affect
polymers through physical interaction, while substances like strong acids or
bases normally result in an irreversible breakdown of polymers. PREVENTIVE MEASURES:It is
estimated that 5%4 of
an industrialized nations income is spent on corrosion prevention and the
maintenance or replacement of products lost or contaminated as a result of
corrosion reactions. Corrosion reactivity is affected by following items;1)      Heat transfer                                                  5)
Mechanism2)      Mass transfer                                                  6)
Surface to volume ratio3)      Diffusion
limited process                                7)
Temperature4)      Contact area                                                   8)
TimeAn effective
prevention system begins in the design stage with a proper understanding of the
environmental conditions and metal properties. Engineers work with
metallurgical experts to select the proper metal or alloy for each situation.
They must also be aware of possible chemical interactions between metals used
for surfaces, fittings, and fastenings.The most
obvious method of providing better corrosion resistance is to change the
materials but this can only be done to a certain extent. Exposed surface area
is a prime concern in corrosion, an obvious property to improve is the
porosity. Much work has been done in finding ways to make polycrystalline
materials less porous or denser. The most obvious is to fire the material during manufacture to a higher temperature.
Other methods of densification have also been used. These involve various
sintering or densification techniques: liquid-phase sintering, hot pressing,
and others. Alterations in major component chemistry may aid in increasing
corrosion resistance. Porous clay refractories were used originally for this
purpose. Various techniques have been used to lower the temperature of the
interface or hot face of the material (lower hot face temperatures mean less
corrosion). Improved corrosion resistance of
porous materials can be obtained by impregnating with either a material of the
same composition as the bulk or with a material that, in the case of SiC or
Si3N4, is later exposed to a carbiding or nitriding treatment.Corrosion resistance can sometimes be
improved by changing the processing method. Chemical vapor deposition (CVD) is
one of the most attractive methods to produce high purity dense materials
because the sintering process is not required if a bulk material can be
obtained directly from the raw vapors or gases. One method of minimizing
corrosion not widely practiced is that of coating the ceramic with a layer of
more resistant material. Probably the best method to coat a ceramic is by a
layer of CVD5 or
plasma-sprayed material of the same composition as the substrate6. 

1 Material Science and engineering and
introduction by William D. Callister, JR and David G. Rethwisch

2
Lawrence SK, Gertrude M. Shults. Studies on the relationship of the chemical
constituents of blood and cerebrospinal fluid. J Exp Med 1925; 42(4): 565-91.

3 Q. Fang, P.
S. Sidky and M. G. Hawking, dept of materials, Imperial College, Landon,
SW72BP, UK

4 Material Science and engineering and
introduction by William D. Callister, JR and David G. Rethwisch, Chapter 17,
Section 17.1

5 Davies,
G.B.; Holmes, T.M.; Gregory,  O.J. Hot
corrosion behavior of coated covalent ceramics. Adv. Ceram. Mater. 1988,  3  (6),
542–547.

6
Gogotsi, Yu.G.; Lavrenko, V.A. Corrosion protection and development of
corrosion-resistant ceramics.  Corrosion
of High-Performance Ceramics; 
Springer-Verlag: Berlin, 1992; 151–162. Chp. 7.

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