Mod-01 Lec-23 Forms of corrosion, Uniform Corrosion, Galvanic corrosion

Mod-01 Lec-23 Forms of corrosion, Uniform Corrosion, Galvanic corrosion


So, we see that there are several issues which are falling under metallurgical factors which can govern corrosion rates. Now, we have
environment related factors. In case of environment
related factors, there are mainly three subsections. One is concentration cell. This sort of problem
arises in case of localized attacked or rain water, rain droplet related attack. For example,
on a particular body, this is a metal. On that metal if one side is depleted in oxygen
and another side is excess of dissolved oxygen, say this is more dissolved oxygen and this
part is depleted, then in that case, this part will act as cathode and this part will
act as anode. So, this section, there will be corrosion and this is happening because
the change in concentration of dissolve oxygen. So, we are forming a concentration cell where
this part is cathode, this part is anode. So, the anodic part, this part will corrode
and this part will be protected, since the cathodic reaction will happen on this metal
surface of this section. So, this is an example of concentration cell formation. We will talk
about this when we will discuss the pitting or crevice corrosion. Now, there could be another issue which is
velocity related problem. Velocity of the corrosive, the electrolyte depending on the
velocity of the electrolyte, there could be change in corrosion phenomena. Let us say,
the pitting if we have low velocity, if we have the stagnant solution or in case of crevice,
if we have stagnant solution, the formation of pitting or crevice corrosion would be aggravated,
but if we increase the velocity, there could be a possibility of very low amount of crevice
corrosion or pitting corrosion, but if we increase velocity, it may happen that after
reaching some critical velocity, there could be more amount of erosion corrosion. So, velocity
is another factor. Now, there could be a factor of temperature.
More or less if we increase the temperature, the rate of any reaction increases, but and
in all the cases, most of the cases, if we increase the temperature, the corrosion rate
increases, but there could be some examples where there could be a reversal of polarity
and one such example is if we have steel iron and zinc iron. Zinc if we have coupling and
then below certain temperature, iron will act as cathode. Zinc will act as anode and
on zinc surface; zinc hydroxide will form below certain temperature, but if we increase
the temperature of the system, then the zinc hydroxide will convert to zinc oxide. Instead
of zinc hydroxide, zinc oxide will form on the zinc surface and zinc oxide is cathodic
to iron. So, in that case, iron will corrode and zinc will be protected since zinc oxide
layer has formed above some critical temperature, and that gives to corrosion of iron instead
of protection. So, the temperature is another issue. So,
if we increase the temperature, most of the cases the corrosion rate increases, but there
could be a possibility of polarity reversal due to the formation of different passive
layer. So, this is another factor. This is temperature factor. Now, if we consider metallurgical
and environment factor together, we will have, we can explain many failures due to corrosion. There is one more important factor that is
stress or pressure related factor. Now, most of the cases our engineering components are
under stress. So, if we have stress, at the time if we have corrosion phenomena combinedly,
we can have corrosion fatigue. We can have hydro gen embrittlement; we can have stress
corrosion cracking. All sorts of problems can arise. Now, what is about the pressure?
Now, pressure for example, in case of fretting corrosion, we have seen that two components
are under compressive stress and there is a very minute scale of a relative movement
between these two components and in the contact area, we have corrosion problem. That problem
is called fretting corrosion and that time; it is not the tensile stress. It is rather
the compressive stress and the relative movement or the vibration of the components with respect
to each other. That leads to corrosion that is pressure related problem and also, you
can have another factor that is if we have a differential difference in pressure, for
example, in case of propeller, one side is having higher pressure and one side is having
lower pressure. So, the higher side will have more corrosion compared to the low. See if
we have this part, so this part is high pressure and this part is low pressure. Now, when we
increase the pressure, when you decrease the pressure of water, there is a possibility
of bubble formation. Now, at the same time, if we on the cyclic process if we increase
the pressure at that point then bubble can collapse. So, we have seen this bubble is
forming and if we increase the pressure, bubble is collapsing and this formation will collapse.
This particular, these two phenomena’s, they can happen simultaneously and lead to
a cavitation problem, cavitation damage. So, pressure is another important issue. Stress
is another important issue clubbing with the corrosion phenomena. So, we see that there are major three factors.
One is metallurgical, another is environment related and another is a stress and pressure.
Now, we have two special corrosion phenomena. One is called microbial corrosion and another
is liquid
metal corrosion or liquid metal embrittlement. We will talk a little about this microbial
corrosion and liquid metal corrosion as we go ahead. Now, once we have discussed about
the different forms of corrosion in short and also the factors which can govern all
those forms of corrosion. Now, let us get into the individual forms
of corrosion. Now, first which is coming in the list is uniform corrosion. Uniform corrosion
the names suggest that there should not be any localized attack rather on a particular
object. The corrosion would be uniform throughout the entire cross section or the section surface
of the particular structure. For example, a car body corrosion or there are several
other examples, for example tarnishing of silver wire. For example, silver bowl and
all those stuff or the silver ornaments. They do get blackened as we use them. See this
dark black part is nothing but the most of the cases it is Ag 2 s. So, that A g 2 s can
lead to tarnishing which basically changes the luster or changes the shininess of the
silver wire. Now, there could be other example of rusting
of steel in air or corrosion of structural steel corrosion of structural steel. Now,
if you see it, when a body is exposed to atmosphere, there is a possibility of a uniform corrosion
all throughout the section. Now, if you consider particularly this example, the equations that
govern corrosion, we can see that gradually there are few equations. For example, one is iron goes to iron plus
2 ion. So, this is the anodic reaction. What is the cathodic reaction? In the atmosphere,
the atmosphere at this is anodic reaction and atmosphere, we have oxygen and it is almost
close to neutral media s oh 2 o plus 4 E goes to 4 OH minus. This is the cathodic reaction.
Now, these two can react forming f E o h whole 2. Now, once we have f E OH whole 2, it can
further react with the oxygen, that is there in the atmosphere that oxygen reaction along
with presence of moisture. It can go to 4 F E O H whole 3. So, this is hydrated ferric
oxide or f E o HOL 3. Even we can directly get F E to a hydrated F E 2 O 3 4 F E O H
whole 3 plus. Further, it reacts with oxygen and go to 2 F E 2 O 3 H 2 O.
So, plus 2 H 2 O. So, this is nothing but the rate rust what we see on the structural
steel. So, this is this reaction F E F E plus 2 and these reactions if we have this structure
all over the places, these reactions are happening. All over the places, this corrosion rust is
forming. So, gradually, you have the cross sectional changes because this rust might
get detached from the surface. So, gradually we will see that you have this much which
is left out after a long time. Now, how to avoid this particular corrosion?
This is a very general corrosion phenomena and one sort of protection is since you cannot
avoid corrosion. So, one protection is in the beginning instead of having the exact
dimension, we will have a little extra material in it. So, that extra material will take care
of that material loss due to uniform corrosion. Now, second thing is we can go for cathodic
protection. So, what are the protection mechanisms? Protection
we can employ in case of uniform corrosion. Protections are one is extra material we have.
The dimension will be little higher just to take care of this uniform corrosion. Second
is you can have cathodic protection. Paint will work nice here. Painting would be great
in case of taking care of this or controlling the uniform corrosion. So, these are the methods,
common methods which can protect this uniform corrosion.
Now, what are the factors? Now, if you see that the factors are very clear. Presence
of moisture, so humidity
is important factor. Second case let us say, tarnishing of silver wire. There we form a
G 2 S. So, that means, the presence of H 2 S or S O 2 in the atmosphere. That can decide
whether the silver wire will corrode faster or not even in the presence of S O 2, there
could be an extra corrosion of iron object because the S O 2 can dissolve in moisture
and it makes it acidic because there could be a sulphuric acid formation. So, that sulphuric
acid media iron can get corroded at a little faster rate.
Now, third is dew. Now, if there is a dew formation, then the possibility of corrosion,
uniform corrosion increases and if it is a dry place, then the uniform corrosion would
be less. So, these are the major factors which guide. Of course, other factor is temperature
or presence of salt, mainly Nacl. Let us say a metal or structure is exposed to media or
environment, where we have lot of Nacl content. Let us say the structure is near to the seashore.
So, since we have salt, the salt is always the corrosion phenomena. Now, again if we
have cold place and a hot place, the hot place in the presence of all these factors, other
factor, there could be extra corrosion. If there is no dew formation, if there is dry
place, then the corrosion rate would be less or if it is a wet place, corrosion rate would
be more. So, these are the common factors which guide uniform corrosion.
So, this is about uniform corrosion, but the more important corrosion phenomena would be
the localized corrosion. So, the localized corrosion that may arise, the first example
we would give that is the galvanic corrosion where the uniform corrosion. In fact, another
important issue is in case of uniform. We can predict what could be the life of that
particular structure greatly predict, but if it is localized corrosion, in most of the
cases localized corrosion are very difficult to detect by normal naked eye observation.
So, those corrosion phenomena are to be looked at very carefully and studied properly. So, let us get into first example in for localized
attack. First example of localized attack is galvanic or two metal corrosion. So, the
galvanic and two metal corrosion is very common form of corrosion and since, one particular
metal you cannot make use of it for structural applications, most of the time you need to
use many different metals or alloys into your structure. So, that case when you use it,
that time you have to join all those different parts. So, when you go for joining, that time
you definitely need to have a two metal contact and if the two metal contact is exposed to
electrolyte or corrosive, then there could be a possibility of galvanic corrosion or
two metal corrosion. For example, if we have stainless steel. If
we have stainless steel, then that stainless steel is connected to normal mild steel. Then
we have a galvanic contact around at this zone. So, at this zone, stainless steel will
act as and in case of let us say normal atmospheric corrosion or sea water corrosion, stainless
steel will act as cathode and mild steel will act as anode fine. So, the mild steel part
will be corroded faster than the corrosion of stainless steel part. So, this is basically
an example of galvanic corrosion. So, this sort of phenomena can arise if we have two
different metals in contact and one metal will act as cathode and another metal will
act as anode and the metal which will act as anode, that would be active in a particular
series and the cathode material will be acting as noble metal in the particular series.
Now, I am talking about a series and that series is called galvanic series and this
galvanic series for a particular solution, let us say Nacl solution or sea water solution
which is unpolluted, there if we dip let us say this is my sea water solution and if we
dip a particular metal object, it will have its own free corrosion potential. So, if we
measure the corrosion potential with a reference electrode, we can see that at a particular
potential, it stabilizes most of the cases and that corrosion potential is called free
corrosion potential. With respect to mixed potential theory, if it is log I verses potential
plot, if we do that time, we will see that cathodic line, polarization line and anodic
polarization line, they are cutting at a particular point which is basically the equilibrium point
where the rate at which cathodic reaction happens would be equal to the rate at which
anodic reaction goes on. So, this is my free corrosion potential and if we do polarization
experiment, we will see that the graph will look like this.
So, now for a particular metal m, let us say the free corrosion potential is E 1. Now,
another metal which is let us say this is for M 1 free corrosion potential is or the
corrosion potential is E 1. In this particular sea water solution, now another metal if we
take M 2, where the free corrosion potential is E 2. Now, if E 1 is greater than E 2 and
M 1 and M 2, these two metal parts is connected or welded or braised at this point or welded
at this point, then if we expose this to sea water, then this one will act as cathode and
this one will act as anode. Since, E 1 is greater than E 2, the reverse reduction potential
E 1 is greater than E 2. So, this one will be noble in the particular series and this
one will be active in the particular series. So, if you would like to put it in the galvanic
series, then M 1 would be on top and M 2 would be below this M 1. So, now in this case, we
have also seen one more series that is electrochemical series. That series if you see, there only
the pure metals are put there in that particular series according to their reversible reduction
potential in non-corroding position or non-corroding situation and also, you will see that case
you always see some reduction potential number. Let us say if we have reduction E M F series
or electromotive force series and if it is the reduction potential, if we consider m
plus n m, the reduction potential, then we will see that this hydrogen plus E H, this
has potential 0 volt with respect to hydrogen reference electrode and also, you can see
that the zinc plus zinc, there the reduction potential would be minus 0.761 volt. So, if
you see copper, copper plus copper, there also you will see E 0 E 0 would be equal to
0.34. This is plus, this is minus and this is 0.
So, according to this EMF series, we see that the copper, pure metal copper will be noble
compared to the pure zinc and that situation is arising when that is non-corroding situation.
That means, zinc ion concentration would be one or the activity of the zinc ion is one
and there is no other elements, no other species, but this situation is more or less ideal situation
since most of the cases we deal with alloy systems. Another situation is most of the
cases; we do not deal with the alloy element concentration which is one activity. Most
of the situations, they are away from this one activity or even the situation may arise
that elemental ion is not present in the corrosion. So, how to get some idea that which one would
corrode and which one would not corrode? That would be given by galvanic series. This can
give you the idea if those are pure and if we club copper and zinc, copper would definitely
not corrode rather copper deposition will happen and zinc would corrode.
Now, in case of this situation here, all those numbers are there, but in case of galvanic
series, we have to just see what is the relative behavior of those all those metals or elements
or different species, corrosive species. For example, oxygen dissolves all those species.
What is basically the relative position as per the free corrosion potential in a particular
solution? Let us say in case of Nacl unpolluted solution, iron will be placed, mild steel
will be placed like this and in that situation, the stainless steel, 18:8 stainless steel
will be on top of this and you will not find any value of potential. Any value of potential
is just the relative position in case of Nacl solution and in case of galvanic series, the
metal which has higher position than the mild steel that is actually the noble metal compared
to the other metal. So, if we club these two and if we have a galvanic coupling between
18:8 stainless steel and mild steel, then always this will not be corroded. This will
corrode. So, this is the essence of galvanic series like that you can place all the metals
as you want to have the information of different metals and alloys, you can form a galvanic
series. Now, another situation is for example, copper.
Let us say I have copper and I have a steel, mild steel. So, I have copper and mild steel.
This is a copper plate and this is mild steel plate. Now, if we would like to see what happens
in case of sea water situation, that situation will act as noble and this will act as active
and if we would like to take it to this situation, then of course, in the particular series,
copper will be placed at top and mild steel will be placed below this copper. So, this
is the position of copper and mild steel in the galvanic series in Nacl solution. So,
if we have this situation, this will corrode and this will not corrode and this will act
as cathode and point is that situation you do not see any copper ion or iron ion as you
have in case of EMF series. Now, what are the reactions then in case of
mild steel, since this is anode? So, always iron will release through electron and it
will go to iron plus ion and it will dissolve, but what would be the cathodic reaction since
these two electrons are to be supplied, are to be consumed. So, who will consume that?
So, cathodic reaction would be since sea water, we have dissolved oxygen and sea water is
neutral in nature, so oxygen in the presence of H2O except these for electrons and goes
to 4 OH minus. So, this is the cathodic reaction which will happen on this.
So, as we have already seen that the surface on top of which cathodic reaction happens,
that is nothing but getting protected. So, this will protect, this will be protected
and this will be corroding. So, this is the essence of galvanic series. We can have a
long series like this. Now, this is of course the solution dependent. If we change the solution,
the situation can vary one particular metal. For example, m 1 and m 2 in that situation
in Nacl, if it is M 1 is anode, m 1 is sorry m 1 is noble and m 2 is active in different
solution, it may arise that m 1 could be active and m 2 could be noble. So, that is what you
can have infinite number of series and different solutions. So, what are the characteristics
of galvanic series, since this galvanic corrosion or two metal corrosion depends on that galvanic
series characteristic? So, let us see the characteristics. One is
position of metals and alloys in galvanic series, agree closely to their constituent
elements, EMF series more or less it happens. So, if copper is noble, zinc is active. So,
in case of alloy system also, copper will act as noble and zinc will act as active,
but this is not. This may vary. So, position of a particular metal
is basically closely related to EMF series.
Now, second case passivity might or passivity always changes the position of the noble and
active nature of the particular metal. For example, 18:8 stainless steel and MS, both
the major component is iron, fine, but the iron component here we have chromium, around
18 percent chromium. So, because of this presence of 18 percent chromium, there could be a possibility
of formation of chromium oxide passive layer and once chromium oxide passive layer forms,
that chromium oxide passive layer acts as cathodic in comparison to a normal iron or
mild steel. So, since we have 18 percent chromium and it is passive, so this will be acting
as noble and this will be acting as cathode. This will be active metal in the couple between
mild steel, 18:8 mild steel and 18:8 stainless steel with mild steel. So, this is passivity,
of course changes the positions of different metals in the series.
Then, we have in the galvanic series, if you see carefully in a book, then you will see
some of the galvanic series. For example, let me put an example. Let us say 18:8 molybdenum
stainless steel, 18:8 stainless steel and both are in active state. Now, this you can
see in the book. It is in bracket. Now, there are two issues. One is why there is a bracket
and why I have written active here? Now, before until and unless the passive layer forms,
it cannot behave as passive metal. So, the passivity depends on the solution as well
as until and unless the passive layer forms, it will always act as active. So, that is
what when it is in active state, then these two cases, these two metals will always behave
similarly in that particular solution. Similarly means, both will have of the same level of
free corrosion potential. So, if they have same level of free corrosion potential, if
we combine them, then there would be a very little potential difference between these
two couple and if there is a very little potential difference, so there the situation will not
be galvanic in nature. There could be uniform corrosion over the two different metal surfaces.
So, that is what this bracketed and this active means. They have not gone into passive state.
Now, similarly we can have another bracket which is 18:8 molybdenum stainless steel,
where it is passive and 18:8 stainless steel. SS means stainless steel passive. So, there
also you will see a bracket. It means that these two metals or these two alloys will
act similarly. If we contact them, if we make a contact out of these two different alloy
system, so both the cases, they have reached to the passive state. So, that is what if
we combine them, club them, then there could be a less effect of galvanic. Galvanic effect
would be less since the free corrosion potential in the particular galvanic series or the solution
is same for both the cases. If they are almost same, so the potential difference would be
less or very small and since, the current, the corrosion is decided by the rate of current
flow or the rate of electron flow or the current density and if they have a similar potential
level, so there would be a less amount of galvanic coupling or galvanic affect. So,
the corrosion rate would be same for both the cases. So, that is what they have put
it as bracketed. So, this is the meaning of bracket in the galvanic series.
So, we see that in the galvanic series, we have the bracketed regions and in practical
use, this has a great implication. So, in those bracketed metals and alloys, they can
be clubbed together in practical operation without having much of galvanic corrosion
effect. So, uniform corrosion always would be better. So, there we can have uniform corrosion
if we combine all those metals in with the bracketed regions, bracketed metals and alloys
in the galvanic series. So, these are the characteristics of galvanic series. Now, let us get into some of the practical
examples of galvanic corrosion. One example is definitelya yacht with monel hull plus
steel rivet. So, if we have the monel hull and steel rivet, the monel hull can lose structure
and it cannot operate for a longer period, since monel hull will be cathodic and steel
rivet would be anodic and there is one particular issue that is governing there that is the
area factor. We will talk about that area factor. This is the major problem. If we have
steel rivet and monel hull, then the steel rivet cannot hold, cannot give strength to
the monel hull for a longer period. It will lose strength.
Second is same way the copper plate which is fastened with steel rivet or steel bolt.
That is also a very bad practice. So, copper plate will lose its strength. Third one is
an example of water tank. So, let us say this is the water tank and initially, this is made
with mild steel and the corrosive, the solution or the electrolyte is mildly corrosive, but
in order to protect, in order to have further protection, if we put a stainless steel bottom,
this is stainless steel and this is mild steel. In that case, this part where we have welding
between stainless steel or the cladding of stainless steel with the mild steel on top,
that part can develop leak. So, the metal, the solution can go out through this leak.
So, this is another example. Now, we see all those examples. Now, until and unless we see
the factors those are important in case of galvanic corrosion, we cannot explain much
about this. We see all those observations. Now, the factors. If we see there are three
major factors. One is environment effect, second is area ratio. Area ratio means cathodic
area and anodic area. Third one is distance effect. Now, if you see the environment effect
for example, temperature. It could be moisture; it could be salt content of the electrolyte.
Salt means Nacl or the chloride salt. Those are the three factors which can be deleterious
to the material and the corrosion would be more. Now, as we are discussing about the
temperature effect, let us say we have iron on top of that if it is galvanized. So, galvanized
means zinc coating and let us say some part of this galvanized coating is damaged. So,
in that case, since zinc is active in the series compared to iron, so iron will act
iron surface and this iron surface will act as cathode. So, cathodic reaction which is
nothing but O 2 plus in normal atmosphere O 2 plus or sea water O 2 plus H 2 O plus
4 E. It forms 4 O H. This is cathodic and anodic reaction will happen. Since, it is
anodic reaction, there would be zinc corrosion and this zinc plus will react with O H Minus.
It will form zinc hydroxide. Now, if zinc hydroxide is forming, that will be on the
surface and it would be a lose powder and still this will act as cathode and this will
act as anode and this will be cathodically protected.
So, we see that zinc hydroxide is forming. Now, this process that protection of iron
cathodically protecting as self protected and zinc corrosion, this phenomena will go
on till 180 degree Fahrenheit. Now, if it is more than 180 degree Fahrenheit, the situation
will be different. In that case, instead of zinc hydroxide, zinc oxide will form and this
is a very adherent layer on the zinc surface. Now, once we have zinc oxide on top of this,
zinc oxide is cathodic compared to iron. Then iron on this surface will act as anode and
this one will act as cathode. So, if zinc oxide surface, that means, the zinc oxide
is forming on top of this, if that is cathode and this is anode, so the corrosion will happen
in this part. So, we will not get any protection. So, this process is dependent on temperature
and this is happening because there is a reversal of polarity. What does it mean? Initially
iron was cathode, now after the temperature has reached beyond 180 degree Fahrenheit,
so this has become anode. So, this polarity difference can lead to corrosion to the material
which is subjected to corrosion protection because of the zinc coating. Now, this is
temperature factor. Another issue is moisture. If there is moisture,
of course then you have always a thin layer of electrolyte on the surface. If there is
a dry place, let us say this is dry and this is moist. So, in the moist case, if we have
a metal object and dry place if we have a metal object. On the moist surface, there
is a thin layer of moisture on top of the metal, but here you do not have. So, you have
the presence of electrolyte and in that case, if we have a galvanic couple M 1 and M 2,
then as per if the M 1 is on top of the M 2 in the galvanic series, then M 1 will not
corrode. M 2 will corrode. So, galvanic effect would be more vigorous if there is a moisture,
now salt content of course. Again salt increases the conductivity of the electrolyte. So, if
it increases the conductivity, so there would be easy current flow and ease in current flow
means there could be more corrosion. Now, it can be explained on the basis of mixed
potential theory. We see that if we draw mixed potential plot
log I verses potential, this is my corrosion point and this is my I corr and this is my
I corr, this is my E corr. Now, if there is no resistance; that means, the electrolyte
is highly conducting, then the situation would be like this. If the electrolyte is not that
conducting, if there is resistance to the flow of ions, then we have resistance drop
or resistance polarization. These resistance polarization is measured as I r drop. If there
is I r drop, then my corrosion current or the current density corresponding to anodic
dissolution is from here to here. So, initially, it was here when it was highly conducting,
the electrolyte was a highly conducting, but in this case, the electrolyte conductivity
has reduced or other way around. Electrolyte resistance has increased and there is I r
drop, so the current density corresponding to I a has decreased. So, the corrosion rate
has also decreased. So, this galvanic effect will also be less if we have less salt or
the moisture or the electrolyte is highly resistant.
Now, that is what if we have galvanic couple M 1 and M 2 and on top of that, if we have
electrolyte and that let us say that electrolyte is pure water and that pure water is de-ionized.
In case of de-ionized water, resistance is pretty high. So, in that case, galvanic corrosion
effect or the corrosion rate would be more or less, pretty low compared to the case where
the electrolyte contains lot of ions. Now, in this case, another situation can arise
that is the distance effect. Now, let us say this is the contact point between M 1 and
M 2 metal. Now, the galvanic couple actual coupling is here. Now, as you go away from
this, what happens is the ions has to move, ions have to move a longer distance. So, if
the ion is moving like this, so longer distance it always leads to pass through the resistant
path. So, the corrosion effect would be more near the junction and away from the junction.
Since, it crosses higher resistance path that I r drop would be more in this situation.
So, the corrosion would be less in this situation and this case, if we follow this particular
diagram, M 2 would be cathode and M 1 would be anode.
Since, I am showing the corrosion effect on the metal surface which is active in nature,
so the distant effect which is also important. So, close to the joint, the corrosion effect
would be, galvanic effect would be more and away from joint. Galvanic effect would be
less in case of the active metal in that particular couple. So, this is environment factor and
this is distance effect. Now, the more serious factor, which is the
area of ratio or the ratio between the cathodic part and the anodic part. The area factors
between these two particular sections let us say, this M 1 and M 2, if those are the
same area, then the galvanic effect would be there, but it will not be that serious,
but if the situation is like this. If M 1 is active and M 2 is noble and M 2 has larger
area compared to the M 1, then the situation would be very different and M 1 corrosion
rate would be manifold compared to this situation where the areas are same. Now, why this happens?
We have already explained this with respect to the mixed potential theory. Let me also
just briefly talk about that and then we will take care of this. We will explain these three phenomena.

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