CORROSION: of the types of corrosion are; 1)

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;

1)      Uniform Attack                                         6)
Pitting Corrosion

2)      Galvanic Corrosion                                   7)
Intergranular Corrosion

3)      Crevice Corrosion                                     8)
Selective Leaching etc.

4)      Erosion Corrosion                                     9)
Stress Corrosion

5)      Hydrogen Embrittlement

In 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.

A strong connection between
corrosion and ductility reduction is present which increases the brittleness of
the material, thereby changing the failure of the material from ductile
fracture to brittle one, which is dangerous because it occurs without warning.

Moreover, the load bearing capacity
of any particular member of the structure or the complete structure can be
influenced by the stress concentrations and reduced cross-sectional area. In the case
of uniform corrosion, its influence on the material’s structural strength is
straightforward.

The stress for a given load for new
and abridged specimen cross-section can be calculated by establishing the
linkage between thickness reduction and weight-loss of the material given. Conversely,
in localized corrosion, localized stresses directly affect the tensile strength
of the material.

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.

When material starts to corrode,
the metal dissolution results in erosion, which ultimately results in the
brittle structure and fracture occurs with warning. 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 normally involves simple chemical
dissolution in contrast to metals electrochemical reactions.

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:

It is often hard to
discover the attacks of corrosion on polymers because the material seems like a
normal one but with the passage of time, it might have become brittle and lost
its mechanical strength. Surface cracks are initiated by the mechanically stressed polymeric
materials like plastic etc. in chemical environments. In polymers, this crack
propagation takes place either through prolonged mechanical stresses or
stresses with chemical attack. Polymeric corrosion can either be through chemical
reaction or through physical interaction.  Polymers consist of a network with molecular chains mainly
consisting of carbon, hydrogen and oxygen. The polymer chains configuration
is changed by corrosion through chemical reaction. Listed below are some of the
environments that causes chemical reactions in polymers. Following are some
environments which cause corrosion by chemical reaction in polymers:

1)      Heat                                                                4)
Water

2)      UV- Radiations                                                5)
Chemicals

3)      Ozone

Physical effects on polymers are caused by interaction with
the environment. It can cause bulge, disbanding or trickling of additives. The physical
interaction depends upon the diffusion of additives into the polymer, and in
some cases, the process is reversible. Organic substances normally affect
polymers through physical interaction, while strong acids or bases like substances
usually cause an irreversible breakdown of polymers.

 

PREVENTIVE MEASURES:

It is assessed
that about 5% of a developed nation’s income is expended on corrosion
prevention and the conservation or replacement of lost products or products
that are contaminated due to corrosion reactions4.
Corrosion reactivity is affected by following items;

1)      Heat transfer                                                  5)
Mechanism

2)      Mass transfer                                                  6)
Surface to volume ratio

3)      Diffusion
limited process                                7)
Temperature

4)      Contact area                                                   8)
Time

An 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.