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We start a course on degradation of materials. The course name is environmental degradation
materials. So, series of lectures will be taken on this topic. I am K. Mondal and I
am a faculty at material science and engineering department at IIT, Kanpur. This course is
under National Program on Technology Enhanced Learning, which is in short called NPTEL.
In this course, we will be concentrating on the degradation mechanism of materials when
the material is exposed to environment. Also, we would be looking at the characteristics
of the degradation of materials and we would like to see their different forms of degradation,
as well as their implication on economics as well as on the economy, on the safety of
the material of the equipment or the process, and when the material is undergoing degradation
and the processes that would be involved will be physical in nature or chemical in nature
or there could be a combination of mechanical effect or there could be mechanical effect
as well as chemical effect. So, all those things will be covered. We will
also try to cover all sort of materials because we know that there are four major varieties
of materials. Metals and non metals are the major groups. Then we have in the non metal,
we have polymers, we have ceramic and also, we will be considering composite, which is
mixture of two or more phases. Those phases consist of metal, non metal or metal, metal.
The kind of effect would be considering, which will lead to degradation, which could be a
mechanical heat or radiation or chemical action of chemical reagents like oxygen or moisture
or acid. All those things we will try to cover in this particular course.
So, what would be the broad essence of this course that we would try to see? Mechanism
of material degradation and then we would like to consider characteristics of material degradation, and then try to find
out what other different forms of degradation. Then we would also try to find out what are
the reasons. Reasons means we would like to see that what are the factors guiding degradation.
Now, in this also, we would like to, once we know the mechanism, characteristics, forms
and reasons; finally, we would like to see what are the control processes, so that, any
degradation will not be good for practical application. So, we would like to see that
how we can control that degradation, so that, the material can function for a longer period
of time. Now, before we come to little more about degradation,
let us see what are the things will be covered in this particular course. Mainly, corrosion
and oxidation of metals and alloys. Those are actually examples of material degradation.
That part would be covered in detail. The other things by which degradation can happen
is that, one is wear or radiation and those things we will touchup just, but we will just
see in brief. But the main consideration in this course would be corrosion and oxidation
of metals and alloys and the prevention mechanism.
Now, if we see the course content, now in the course content, it will start with the
definition, introduction, where we would consider definition and then we would like to see the
need for studying this particular course. In the introduction, we will see definition,
different forms and then would like to see cost of corrosion and then we would like to
briefly see the electro chemical nature of degradation, which is basically, mainly, corrosion and oxidation. Then what
would be our aim for this particular course. Then secondly, we will start with thermodynamics
of degradation. So, in the thermodynamics, we would cover
what are the other processes that will be happening at the interface. So, processes
at interface. Then we would see free energy consideration to understand the degradation,
rather to understand whether a material would like to go for degradation or not. What is
the driving force? Then we would like to consider e m f series because in corrosion, e m f series
would be very crucial. Then we would like to see important reactions, which could be
in aqueous medium or in gaseous medium. For example, in case of oxidation, we would discuss
what would be the reaction of the metal with oxygen or gaseous oxygen. In case of aqueous
solution, we would like to see what could be the reactions that would be happening at
the interface between the metal and the environmental species. For example, hydrogen ion or dissolve
oxygen or moisture or it could be a different metal cat ion, if those are pressure present
in the solution, what could be the effect of those different metal cat ions on the degradation
effect of or the corrosion effect of the metals and alloys.
Then, we would see reference electrode, cell potential reference electrode and then would also try to see Pourbaix diagram
to see that at particular p h level and potential level, what could be the phases that would
form on the surface of the metal in contact with environment. This is nothing but e versus
p h diagram, that is potential versus p h diagram and this is mainly considered in case
of aqueous corrosion.
Then, we would see the kinetic of corrosion. We would see then kinetics of degradation.
It could be oxidation, kinetics of oxidation or the kinetics of corrosion and there, would
like to discuss mainly the current density and then would like to discuss the rates.
Then, one important issue in corrosion is exchange current density. We would like to
discuss the polarization and finally, we would like to see what are the experimental techniques.
So, the experimental techniques would allow us to find out what would be the extent of
polarization, and what could be the current density of different polarization. For example,
cathodic or anodic polarization. Then once we understand these issues, we can get to
the mix potential theory. Mix potential theory which actually binds kinetics of degradation
and thermodynamics of degradation. This is the advance theory in order to understand
the corrosion of metals and alloys. Then, we would also see when a and then once
we know the mix potential theory, we can understand many of those corrosion effects from that
mix potential reference theory and then we would go to Passivation. Since, we know that
many metals when it reacts with environment, it forms a surface film. For example, iron.
It forms ferric oxide on the surface and chromium forms chromium oxide on the surface. That
chromium oxide gives protection to the chromium, to the metal because that or the film which
is actually passive film, that is forming on the surface and that demarcates, that prevents
that environment to further react with the metal. One particular example is stainless
steel, 1808 stainless steel, where we have 18 percent chromium, 1808 percent chromium
and 8 to 8 percent nickel. In that, since we have 1808 percent chromium, that chromium
forms a chromium oxide surface and that passivates the iron or stainless steel. That gives the
stainless property of the steel.
Then, once we understand these issues, then we will discuss different forms of corrosion.
In the different forms of corrosion, we would also discuss what are the different factors
that will effect corrosion. For example, we can think of factors like the stress factor,
we can think of the impurity factor, we can think of the presence of oxygen in the environment,
we can think of presence of impurity in the environment or conductivity or the salt contained
in the environment. Those factors will definitely guide or definitely affect the corrosion of
metals and alloys. Then, we will talk about corrosion measurement
corrosion measurement and failure analysis. Since, once we know different forms of corrosion
and different factors, then we can come to this corrosion measurement and failure analysis.
So, the corrosion measurement and failure analysis, in this case we would discuss different test methods and of course, we would
also try to see the test methods are either field exposure test or electro chemical test.
So, it could be field exposure test or plan test or it could be electro chemical test.
Now, once we know the corrosion measurement and failure analysis, then we can go for protection
mechanism or control mechanism in the corrosion control. Then corrosion control will be discussed.
That actually gives idea that how to go for protection of metals and alloys in a particular
environment, for a particular specific use. That has five different segments. One is,
it could be material selection, then we have design of equipment and then we can have change
in environment. For example, change in environments. Environment, we have dissolve oxygen. So,
if somehow we can think of some mechanism by which we can get rid of that oxygen, then
corrosion rate can be reduced to a great extent. So, that kind of change in environment, we
can also think of adding some external reagents. For example, inhibitor, which will take care
of the corrosion, which will reduce the corrosion rate to a great extent. Then we can think
of electro chemical waves of protection, which are mainly cathodic or anodic protection.
Then we can also think of coating. So, we will discuss coating. Now, once we
have all the topics till now and those topics will be covering metals and alloys. But at
the same time, we would briefly discuss the corrosion of or the degradation of the other
materials like polymer, composite or ceramic material, but that would be a very brief.
But mainly, the discussion will be concentrating on the corrosion of metals and alloys.
Now, once we have these things, then we will discuss oxidation or high temperature oxidation,
or high temperature degradation. In the high temperature degradation, the main discussion
would be on oxidation. Then we would also discuss a liquid metal corrosion and then of course, hot corrosion. In the
oxidation, we would like to see again the thermodynamics as well as kinetics of oxidation
will be discussed. There also, we will see what are the protection mechanism or the protection
rules we can employ in order to reduce the oxidation rate of the particular metal alloy
in a particular high temperature application, so that, the operation can be carried out
for a longer duration. Finally, we would definitely conclude by seeing what could be the effect
of this material degradation on the society or on the industry. So, these are the course
contents in brief.
Now, let us get into the first part, which is the introduction. Now, when we start talking
about material degradation, we need to define it. So, we also need to know why we want to
study that. These two things are to be needed first. First, let us start with why you want
to study. So, why to study degradation and what to study and then of course, how to study?
So, if we answer that, then we would have the purpose of this course or purpose of studying
this particular analyzing the degradation of materials.
Now, if we come to see the cost that is incurred by any country towards the protection or prevention
of material degradation, we have some number and that time, we would be able to appreciate
that it involves lot of money. For example, few data, if we see the cost of corrosion
in India is about 5 percent of total G D P A of our country. So, the 5 percent is, I
have one rough data; the cost of corrosion; that means, the corrosion related effect like
the prevention route, prevention mechanism or prevention ways we think of to operate
that particular material for a longer period or there could be loss of material or there
could be replacement of the material due to the degradation. So, that is around 2 lakh
crores. So, 2 lakh crores is the expenditure of India towards corrosion control or towards
this degradation of materials. Now, in case of US, one data, it says that
in the US, the cost of corrosion is about 260 billion dollars. Now, if we see this number;
that means, we need to worry about it. If we consider different segments where we have
say this corrosion problem, in case of US data, it says that the drinking water and
shiver system that involves maximum cost, which is around 36 billion dollar.
So, that means, it is actually a v in all sort of money. Then forget about money. If
we have money, then we can take care of it, but it also involves lot of damage. Now, if
we come to see damage, for example, if a person has gone for prosthetic replacement, let say
a bone is broken and there somebody replaces and that bone is replaced with bio material,
let us say the titanium alloy and that rod is fitted in order to give strength to the
rod and it is fitted in the place of that break or the location of the fracture and
if that rod pre maturely fails, then that person can even die. There could be failure
of bridges; there could be failure of airplanes because of corrosion. So, that is the loss
of human life, which is more important than the loss of money.
Now, we come to see what could be the direct cost and indirect cost. Then in case of direct
cost, we can see that in US data, 1998 and the data is taken from www.corrosiondoctors.org.
So, source is that web site. That data shows that, in infrastructure it involves around 22.6 billion dollars and
then in case of utilities, it is around 47.9 billion and then transportation it is around
29.7 billion. Like that, we have in case of production or manufacturing, it is about 17.6
billion and in government thing and then total is coming about around 137.9 billion dollars.
Now, this direct course involves, it could be what are involved in that. Expensive materials,
which can cater corrosion for a longer duration and which can be operated for a longer duration.
Then we have over design. Let us say one particular metal object is going through a continuous
or uniform corrosion; that means, all throughout the sections we have uniform dissolution.
So that time, in order to operate that material for a longer duration, generally the design
criteria that is taken, that increases in thickness of that material. So, that increasing
thickness of the material would also involve a lot of money. Then third is cost
of repair or replacement of material. Then fourth is employment of anti corrosion means,
so anti corrosion methods or means. So, these are the main parts which would lead to the
direct cost in corrosion.
Then, we can also think of indirect cost. In case of indirect cost and at that time,
it is also around close to 275.5 billion dollars. So, it also involves lot of money. What are
the issues that are involved in the indirect cost? One is cost of labor and then we have
loss of productivity. Let us say, one particular material needs to be replaced because it has
gone through lot of corrosion. So, that replacement time in the depletion replacement time, the
production needs to be stopped. So, there is a gap and there is a loss of productivity.
Then this is because of delays happening due to replacement failure.
So, those are coming under this particular; these are basically the direct region and
now this delay is leading to loss of productivity. Then it also involves cost of equipments,
which will be used for corrosion related activities. So, those are coming under indirect cost.
So, we see that, if we do not understand these environmental degradation materials, we cannot
think of some sort of protection mechanism or protection methods to control or to reduce
this amount of money that is spent on corrosion. So, we need to study this.
Now, coming to the definition part. If we come to see the definition of degradation
in general, it says that the loss of performance of an engineering system
and this loss of performance can be related to these parameters. One is, there could be
loss of strength, loss of mechanical strength, then loss of efficiency, then loss of life
time, and then loss of appearance. Loss of appearance means for example, if we go for
ornaments, if that is corroded, then that ornament becomes very dull looking. So, that
actually means, the loss of appearance. Then, it also involves wear and then there
could be expensive control system and there could be routine checkup. So, those involve the loss of performance. That is actually
the loss of performance of an engineering system and engineering system practically
involves materials. So, that is the definition, a general definition. When we consider degradation,
there could be any natural phenomena that can be involved in the degradation.
So now, we see what could be the losses. It could be loss of mechanical strength, it could
be appearance, or it could be loss of material. Now, we need to see what are the environmental,
overall what are the natural things that can lead to degradation. If we see a material,
if we look at this material, let say this is my material. This material can be used
on the surface of the earth crust or it can be used inside the soil or it can even be
used in the normal atmosphere. So, if we make a segment, now, in this segment,
we have one is soil and it can also involve an aqueous environment. Aqueous environment
means, let say the sea water application. Some material used in sea water application
involves aqueous environment and let us say the pipe line. Let say one pipe line is lead
just below the earth crust in the soil and that involves soil corrosion. So, in that
case, we have several factors, for example, temperature, then acidity, and then partial pressure of oxygen. We can also
have presence of chlorine and we can also have the presence of bacteria.
So, in the soil, due to these effects, corrosion can increase. Then in the aqueous environment,
the effects are even. Now, temperature and then p h level, and we can talk about partial
pressure of oxygen. Here we can talk about partial pressure of oxygen and here we talk
about the dissolve oxygen. Then we can think of presence of chlorine or presence of any
other metallic ions, for example, ferric ion. If it is present in hcl, then the corrosion
of zinc increases. Then we talk about flow velocity and then we can also talk about conductivity.
So, conductivity of the aqueous medium would guide the corrosion. Then this is the surface
and in the environment.
In the atmospheric condition, we have air and gas presence and temperature and then
of course, humidity, partial pressure of oxygen, and salt content in the environment. Salt
content would definitely affect the corrosion rate or corrosion. Then we can also think
about the radiation effect or even sunlight. For example, in polymer due to the effect
of radiation, there could be huge degradation of polymer.
So, these are the common effects which can affect the materials degradation. Now, if
we come to see that, whenever we talk about material degradation, we would see that this
degradation is actually a very, very natural phenomenon. Because if we come to see the
thermodynamics of this material degradation, we would all see if the material degrades,
that actually decreases the energy of the system. So, anything which changes the energy
towards the negative side; that means, if there is any decrease in energy, that process
is more natural than the increase in energy.
So, for example, one case, let us say in case of corrosion of iron, if we consider corrosion
of iron, the first thing if we see that what is the source of iron? The source of iron
in our environment natural sources is iron ore. So, we have iron oxide. Hematite, magnetite,
so those contains iron and that iron is take out from that hematite or magnetite by refining
route. Then once we go for refining, we can make pig iron and then we can go for steel
making. So, actually purification. So, this purification would decrease the carbon
content of pig iron and it goes to the steel production. Even we can go for pure iron making.
Pure iron we can make and this involves melting. So, we see that, all this involves a lot of
energy. So, energy consumption. So, this energy consumption involves this entire process from
iron oxide to pure iron because this process, actually if you see from this to this, it
involves change in energy in the positive side. So, we have to supply energy in order
to make iron from iron oxide. Now, if we have iron block in normal environment,
we will again see that if we have a iron block like this, we will again see that there would
be a gradual corrosion. So, gradual loss of material. So, the block thickness is reducing
and on all the side the thickness would reduce. So finally, from that point onwards, thickness
is reducing and finally, if you keep it for a longer duration, it will convert to, this
is iron, it will convert to again this. So, we see that this process, when iron is oxidized,
that involves lowering of energy. So, actually it has a natural tendency to
go to the iron oxide again and this is actually corrosion, which takes the iron to iron oxide.
So now, we see that this is a natural process. The corrosion is a natural process. Now, when
we talk about corrosion, we need to see proper definition of corrosion. So, we have talked
about the degradation. Now, let us see the proper definition of corrosion.
So, definition of corrosion, there could be a practical definition. It says that, the
tendency of a metal to revert back to its native state; that means, iron goes for change
to iron oxide. Now, if we come for the scientific definition, that says that, it is an electrochemical
degradation
of metal, if we consider only with respect to metal as a result of reaction with environment.
Now, the key factor is electrochemical nature. So, the corrosion, scientific definition of
corrosion involves electrochemical nature. What do we mean by electrochemical nature?
Now, electrochemical nature means, it actually involves the transfer of electrons. Now, if
we consider a zinc rod which is dipped in h c l, which is acid and let us consider that
this acid is very pure in nature. Also we consider that, this acid does not contain
any dissolve oxygen. If it does not contain any dissolve oxygen, if we come to see the
product that is coming out due to the reaction with zinc and h c l, we would see that the
reaction would be this. This reaction would happen.
Now, in this reaction, but before understanding this reaction, the first observation there
would be is bubble formation on the surface of zinc rod. If we analyze this bubble, we
will see that this bubble is nothing but hydrogen gas. Now, if we see this reaction, now if
we break this reaction into two halves, we will see that zinc goes to zinc plus plus
and h plus goes to h. Now, let us say 2 h plus and how we would be able to go from zinc
to zinc plus plus? So, it has to two electrons should be taken
out from zinc. So, that it goes to zinc plus plus and for this process, if we add two electrons
here, then it would go to this product, which is 2 h. Now, this 2 h will combine and then
form h 2 gas. Now, in this process if we see carefully, that this process is actually an
oxidation process or we can also see that this process, we can also define it as anodic
reaction. So, this is anodic reaction. Why because two electrons are taken out from zinc
atom and it goes to zinc plus plus ion. Now, whenever we have zinc plus plus ion;
that means, from the zinc rod, zinc plus plus ion is coming out. That is the only source
of zinc plus plus. So, zinc plus plus ions are coming out from zinc and that means, there
is a loss of mass zinc rod. That is happening because of electrons taken out from zinc.
So, that means, it is an electrochemical reaction. Same thing we need to satisfy the conservation
of charges, since two electrons are taken out. These two electrons will be consumed
by 2 h plus ion and this h plus ion is coming from the h c l acid, because h c l is acid.
You have the p h of h c l would be less than 7. So, we have lot of h plus concentration,
lot of h plus ions that are present in the solution. So, this 2 h plus will consume these
two electrons and it will go to h 2 and this is actually reduction process. This we can
also say that it is a cathodic reaction. Now, this is also electrochemical reaction.
Now, if we consider these two reactions, then we see that if we combine these two, we will
get to this. So, the final reaction actually involves two electrochemical reactions. One
is anodic reaction and one is cathodic reaction. This is also termed as Half cell reaction
and these two half cell are combining each other and then forming a complete reaction
is happening. This complete reaction leads to zinc chloride formation and this lead to
corrosion or degradation or the mass loss of zinc rod.
Now, if we blow it up, we will see that if this side zinc ion comes out, so here, we
have a small mass loss and these two electrons would go and this conductor will conduct these
two extra electrons between hubs; two half cell cathodic reactions. So, that means, it
is an electrochemical reaction and it involves two half cell reactions and also in total,
it is electrochemical degradation. So, this means the electrochemical degradation
of metal. So, whenever we consider electrochemical degradation, we must consider these two half
cell reactions. There could be many half cell reactions and we would come to know in consecutive
lectures. So, we see that there are two half cell reactions and also it involves those
reactions. Those half cell reactions are happening on the electrode, on the metal surface and
this zone; we have oxidation reaction or anodic reaction. That is why this is called anode
and where we have cathodic reaction, that part is called cathode.
So, we see that there are four basic components for any complete electrochemical reactions.
One is anode, second one is cathode and then we see that there could be ion movement in
the electrolyte. So, if we do not have this ion movement, so one hydrogen ion is taking
one electron and going to hydrogen atom. So, the next hydrogen ion would come here. So,
there would be always migration of ions in the electrolyte. So, we need one electrolyte.
Finally, we see these two electrons which are forming due to oxidation reaction, those
two electrons should go to the zone of cathodic reaction and there would be conduction of
these two electrons. So, we need a conductor. So, these are the four components of an electrochemical
reaction. Always we will see, any electrochemical reaction in the future lectures, we would
see that every time we have four different components in that particular complete electrochemical
reactions. And finally, we would see that, whenever we
talk about this corrosion, it involves four factors. One is metallurgical. So, if we see
the corrosion, so this is, let us say the corrosion part. So, then it would have mechanical
factor, it would have a metallurgical factor and then it would have thermodynamics and then finally, one side it would have electrochemical.
So, this involves rate, this involves stress and this involves material. So overall, this
is the basic understanding of corrosion or we would say that it is the basic scientific
definition of corrosion, which says electrochemical degradation of metal as a result of reaction
with environment with the metal.