Let us begin again. We ended up in discussing corrosion protection by electro chemical methods. We have also seen that there are two sections,
one is cathodic protection and another one is anodic protection. We started cathodic
protection, and we saw that where we can go for cathodic protection and some of the places
we cannot go for cathodic protection. For example, we have seen that the locations where
the structure is above water line or above the surface of the earth that cannot be protected,
because since there is no electrolyte. Now, another place where if we have a shielding
effect, in that case we cannot go for cathodic protection. Another case where we have non-conducting
solution, for example, oil which is non-conducting, we cannot go for cathodic protection. Now,
another place where the system is highly corrosive, if the electrolyte is highly corrosive, then
we should not go for cathodic protection. So, in order to have cathodic protection there,
we need to spend lot of current and we need to spend lot of money. Now, we saw those two cases. Now, let us start
with the principle of
cathodic protection. Now, while we discuss the principle of cathodic protection, we will
take the help of mix potential theory again. Now, we have seen that, if the structure,
this is the metal object which is to be protected cathodically. We have an auxiliary electrode
and we connected to a cathode terminal of external source, so that we make this metal
object which is to be protected, a cathode. Now, we see that electron will flow this way
and current will flow this way and the circuit will be computed like this. The current will
flow like this and it will go like this. So, positive to negative terminal and the current
will go out of the auxiliary electrode and enter into the metal, when we have cathodic
protection. Now, since this is cathode and the current
is entering into the metal object, so that means, another criterion for cathodic protection
is we have to see that the metal object, the current should enter into the metal object
which is to be protected. So, there are three cases. One is cathode metal object is to be
made, cathode; second is the metal object is to be connected to a cathode terminal for
of an external DC source; and the another criteria for cathodic protection is we should
see that the current should enter into the metal object, so that the metal object is
protected. So, another is electron should flow into the cathode or the metal and third
is the current will enter into the metal. Now, if we come to mix potential theory, let
us get into this particular board, we will plot potential versus log of current density.
Now there, let us take a simple system where both the cathodic and anodic polarization
is activation controlled. So, this is my anodic polarization line and this is my cathodic
polarization line. Now, we see that this is the free corrosion potential. Now, if we keep
this is M M, this is actually n plus. This is anodic side, so that is what I have written
anodic reaction metal is going to the metal line and this is E reversible potential, if
the metal ion consentation is unity activity. So, we can write it as E 0. This is my i 0,
which is the exchange current density at the metal surface. That is what I have to write
M i 0 of M. This is another reaction, which is i 0 for the cathodic reaction and C for
cathodic reaction. If it is hydrogen evolution reaction, then i 0 should be written as i
0 bracket on M. That means, hydrogen evolution is taking place on the metal surface.
Now, this is my free corrosion potential, this is E corr and this is my i corr. If we
keep it like this, so the system will corrode maintaining this current density at a particular
temperature and pressure. Now, if we would like to introduce cathodic protection; that
means, we have to supply electrons to the system. To this system, if we can supply electron
or if we can introduce negative current or i c, if we increase the i c, this is my i
c, which is cathodic current density or cathodic current density, during the cathodic polarization
of particular cathodic reaction and this is my i a and this is my i c.
Now, if we increase i c, what would happen? If we increase i c, the system, the polarization
will be enhanced. The polarization line, the cathodic polarization line will try to take
the direction as shown in this figure. Now, let us say this is my corrosion potential.
If we measure the potential of this electrode at this condition, we will see this potential
is shown. Now, this point onwards, we will have extra cathodic polarization because the
current value, we are supplying cathodic current. So, let us say, I have reached to this potential
and this is my polarization and the value of the polarization. Now, at this point, my
i c is, this is my i c and this is my i a and i a is nothing but the corrosion rate
because i a is indicating anodic current density. So, my applied current would be this much
and this is my i applied which is nothing but i c minus i a. Since i c is considered
to be negative current, so I put a mode value. So that, this is the value of i applied. At
this value of i applied, I will have this much polarization and this is my current density
for the corrosion. So, this is my new corrosion rate. Actually,
my corrosion rate was here, which is i corr. Now i a, which is the new one, let us say
this is new; n means new. So, this is new. So, i a new is less than i corr. So, I have
reduced the corrosion rate of the metal object in the particular solution. This is the very
concept of cathodic protection. So, I have to somehow take the potential, the corrosion
potential to a lower value compared to the E corr value and we can reduce the corrosion
rate to a great extent. Since this is a lock scale, so if we reduce it by, we can reduce
it by may be 2 to 3 order magnitude. So, we can have a very good corrosion protection.
Now, if we keep going along this line, and this is my position of i 0 and E r and i 0
is nothing but the current density corresponding to the reversible reaction for the forward
and backward reaction. Now, if we keep going like this, I keep decreasing the i corr, i
a, but at the same time, we are increasing the i applied. We are increasing the i applied
and this i applied will be increasing with respect to the lock scale. So, i applied would
be a very large value, if we keep going down. Finally, we can reach to this value, which
is nothing but the i corr would be i a would be equal to i 0 of metal or metal surface.
Fine? That point, we can see that the corrosion rate would be minimum for the metal object
and this particular point would be called as the complete protection. Now, beyond complete
protection; so complete protection means a very large i applied.
So, I have to send a huge current huge to the system, huge negative current to the system.
That negative current, the amount of negative current would be so large it could be of the
order of 10 to the power of 4 to 5 ampere per centimeter square, which is a large current.
So, we should not do that. Rather, we should stop somewhere in between in order to have
a realistic corrosion control. Now, what happens if we go down? If we go
down, you can go down, if we go down what would happen. I will see that it is becoming
cathodic reaction. This is also cathodic. So, this part of this metal polarization is
a cathodic polarization of metal and is going to elemental metal. So, below this complete
protection, instead of having a better corrosion protection, there could be a situation for
ampheteric metal for aluminum or zinc like this. There could be corrosion again. The
corrosion could be like this, z n o plus h 2 o plus 2 o h minus, it can go to z n o h
4 minus 2. So, instead of having protection, zinc ion
can go into this complex ion, which is dissolving again. So, instead of having corrosion protection,
we can have corrosion and this is could be termed as the cathodic corrosion. So, we should
not go beyond that. Actually, we should stop in between in order to have a realistic protection
at a realistic amount of money which is to be spent. So, corrosion protection by cathodic
means, is basically taking the potential to a negative side and at the same time sending
the negative current and taking the i corr i a value, which is the anodic current density
to a much lower value. This is the principle of cathodic protection on the basis of mixed
potential theory mixed potential theory. Now, once we know this, we should see that how
many ways we can send negative current to the system. Now, there are two ways; one is sacrificial
and second one is
impressed current cathodic protection. This, in short we call ICCP. So, let us see first
this particular method. In this method, what we do is, we have let us say a pipeline, which
is to be protected and this pipeline is buried pipeline. This is the arc surface and this
is the pipeline. You know, we have to protect this pipeline from corrosion or control the
corrosion rate. What we do? We connect it to another anode material, which could be
magnesium or zinc or zinc or aluminum. We can connect it to the magnesium electrode
or zinc electrode or aluminum electrode and this is the steel; the material of the pipe
is steel. Why we connect it to this magnesium and zinc
and aluminum is because those are highly electro negative compared to steel in the electro
chemical series. Fine? Now, once we connect this thing, now this is forming a galvanic
series. Now, this becomes anode and this become cathode. So now, we have made it cathode in
this system. Here, we are not supplying any external current. So, the current is coming
into the system. So, this is anode. So, electron will go this way and current will pass this
way. So, the concept of having cathodic protection, that electron should go to the material which
is to be protected and the current should enter into the system from electrolyte. So,
that is what we are having. So, this material is cathode and also, we
are supplying current. We are sending current and the current is entering into the system
and the electron is entering into the system through the conducting wire. So, this system,
where we are not using any external current source or power source and this system is
called sacrificial anode. Why? While we have this protection, this magnesium zinc or aluminum,
they will corrode and the steel pipe will be protected. Now, this is exactly similar
like why we use zinc coating on steel sheet; that means, galvanization. We use galvanized
steel. There we have a steel plate and on top of that, we use zinc coating and zinc
is acting as anode and steel will be acting as steel surface will be acting as cathode.
This is protection, which is called sacrificial anode because the anode material is getting
sacrificed and giving protection to the steel pipe. Now, in this case, this kind of situation,
we can have it in protecting water tank, that is hot water tank. The system would be like
this. The water tank, we can also have a protection. For example, this is my water tank and I need
to protect it using sacrificial anode method. Now, this is also a steel material. This is,
let us say some hot water is kept. Now, we can do it like this. We can have hood, which
is also made of steel and from that hood, we can hang one magnesium rod. This is magnesium
rod. So now, once we hang magnesium rod, this magnesium will be acting as anode and this
would be cathode. Current will go like this and enter into the steel tank and the steel
tank will be protected. Now, when we see that the steel pump, steel tank is protected because
the current is entering from the anode through the electrolyte to the steel object. But at
the same time, wherever the current is leaving the magnesium rod, that part will be corroded.
So here, the current is leaving, so this part is corroded. So, one more point. Please remember
this one that, when current leaves the metal object, so that point is vulnerable for corrosion.
Now, this is one more situation. Now, once we have this knowledge, that we can use it
for sacrificial anode method, the aluminum, zinc of aluminum, zinc aluminum magnesium,
so we need to see what are the characteristics of those anode material. Because one point
is, when we are using this, we need to know that what point we can go for this method
and where we should go for this method and how long we can have protection and then what
are the current value that it can supply to the system. That information is necessary
information. So now, let us get into that part. One thing is anode material and cathode material.
Cathode, which is steel pipe; these two, there should be sufficient potential difference
between these two. So, if we have sufficient potential difference between these two, of
course, this would be negative and this would be positive, then only we can go for this
method. Second is, it should have a sufficient electrical energy contact. It means, let us
say the electrical in a sufficient electrical energy contact; it means, let us say a particular
anode material, the rating is given like this. Let us say, it means Ampere hour per Kilogram
or per Pound. It means that, if we send, let us say this is a system and this is my cathode
and this is my sacrificial anode, s a. This is s a, that means sacrificial anode. If we
supply 1 Ampere of current, then 1 Kilogram of anode material, how long it can give protection?
How long it can give protection, if we send 1 Ampere of current to the cathode material.
So, this is called, this is basically the idea of electrical energy contact hour, I
would say contact hour. So, how long it can protect this system, if we send 1 Ampere current
from sacrificial anode to the cathode material. So, this should be sufficiently high. Another
point is, which is very important is that cathodic protection by sacrificial means,
we should go for wherever we do not have external power source. That is, some remote places.
If we do not have external power source, then we should go for this sacrificial anode material,
no power source electrical. So, there we should go for this sacrificial anode method.
Now, what should be the criteria for anode material? Now, we see that the zinc is very
good anode material for steel protection. We can see that magnesium is a very good protection,
very good material for anode in case of sacrificial anode method. Aluminum is also very good,
but there are some issues with aluminum. Now, aluminum, there is a tendency for the aluminum
to go into passive state. Now, if it goes to passive state, if aluminum, let us say
in case of aluminum, if it goes to passive state, then there could be a serious problem.
Then instead of acting as cathode, acting as anode, aluminum going to passive, that
case instead of acting as anode, it will act as cathode. So that time, if we have steel with aluminum,
this is anode. Now, if we have passive layer on top of this, since this aluminum is a highly
passiviting metal, so that case, instead of anode, this will become cathode. This becomes,
initially it was cathode and later, it will become anode. We all know that anode material
in a galvanic couple corrodes. So, instead of getting protection, this will
corrode. So, what should I do? Generally, many a times or most of the cases or all the
cases, we have this sacrificial anode, and on top of that, we put a blanket. This blanket,
inside that blanket, we put some material and that is gypsum. We put gypsum or coke,
or bentonite. This kind of material is placed on top of this sacrificial anode. In case
of aluminum, we put Na Cl. So, what it does? It calls, it is called back fill. This is
called back fill. The function of back fill is, let us say in
this side, we have steel, and iron. Now, we have connected it. Now, let us say the current
is coming like this and entering like this is. If we do not have the back fill, there
could be a situation that some part of the channel of the soil material is highly conducting.
Some part is highly conducting and some part is less conducting, with very high resistance.
So, which side the current will flow? Current will take the path of highly conducting path,
which is the narrow position. So, that part and we have seen that, wherever the current
is coming out from the system of the anode material, that part will be corroded.
So, the current is preferentially going through this particular part. So, we have more corrosion
here. Now, it may happen like that, this entire part will be corroded and the lower part is
the dash. But the electrical connection of the conducting wire is connected on top of
this. So, this part only gives the protection and this part is invalid. So, that is why
we go for the back fill. What it does is, the back fill provides uniform conducting
surrounding. So, all throughout the surface, the current is leaving the anode material
and there would be uniform distribution of current over the entire anode surface. The
anode surface will be corroding uniformly and that is the very essential criteria for
this design of this sacrificial anode system. Now, another part is, if we have this sacrificial
anode, it should be used for a shorter duration. If we go for a very very long duration, we
have to see that time to time, we have to replace this anode material. We have to monitor
all those anode material part and all those things. So, generally we should go for shorter
version of protection and at the same time, the electrolyte should be less corrosive.
If it is highly corrosive, the anode material would be corroded very fast and the protection
will be lost. So, this is the part of sacrificial anode method. Now, there is one more method, which is called
ICCP, which is called impressed current cathodic protection, which exactly follows this. Now
here what I do is, we have a pipeline. Let say this is the buried pipeline or some water
tank. Let us consider a water tank here. A big water tank, which is buried or some tank
material, steel tank which is buried and this tank is to be protected cathodically.
Now, that case, we have a rectifier, which sends d c current. This is positive terminal
and this is negative terminal. We connect this steel material or steel tank to the rectifier
and negative terminal connected to this and the positive terminal is connected to
auxiliary anode. This is cathode and this
is auxiliary anode, since we have connected it to positive terminal and we send DC current.
So, the current is going like this. It is leaving this anode surface and entering into
the cathode surface. Now, if this tank is a very large tank, so this part, the current
is gradually entering. Now, if we consider away from this section, where we have this
auxiliary anode, we see that the current is travelling a larger distance. Fine? Also,
if we consider this part, the current has to move further large distance.
Now, instead of that, if the current is flowing like this, this path it will take. So, though
we can have a protection on this surface, but this surface will not be protected to
that extent. Now, we have a technique to have a uniform protection. But here if you see,
this rectifier is sending uniform current, constant current to this anode material and
from the anode material, current is coming out and entering into the steel material or
the steel tank. Now, here also, since the current is entering,
this part will be protected. Here also, I am sending negative current, i c to this system.
Fine? Here, the cathodic and anodic reactions would be different because this part is auxiliary
anode. Since we are using external power source, the auxiliary anode material should not corrode.
It should stay for a longer period. That is what we generally use platinum. Now a days,
titanium is coming up as a very good auxiliary anode material, which will not corrode and
which will act as anode. Its function would be just to supply current to this system.
Now, in order to have a complete protection, what we do? We put another auxiliary anode
here; ‘A’ means auxiliary anode. Now, what will do is, we will connect it to this
part. That is, this positive side. So, if we connect it to the positive side, the current
is also going through this and leaving this surface and entering to this surface. So,
we have complete protection. Now, when we have this thing, that time, we
should be careful that this connection, which is connecting wire, is basically the wire
is conducting wire is connected to the auxiliary anode as well as the cathode material, which
is the steel structure. We should have a proper insulation. Proper insulation means, there
should not be any local cell formation. Let us say, this is copper wire and this is steel.
So, copper would act as cathode and steel will act as anode in this local zone, if we
expose this join to the electrolyte. So, we should have a proper insulation in this portion.
So that, there should not be any other galvanic couple in the steel structure.
Now, another point is, this current value is to be chosen in such a way that most of
the time, this steel structure, in the solution or the electrolyte, which is basically the
soil electrolyte, that time there could be various number of reactions that can form
on this surface. There could be several local cell formations on this surface, which can
be cathodic, or which can be anodic. Now, we have to see that, whatever anodic reactions
are happening, this current should cover up that anodic reaction, so that, all the time,
the current will flow into the system and go out through this electrode, through this
electrode connecting rod. This is called impressed current cathodic protection.
Here of course, you see that all the time you have to spend and you have to send current.
So, the external power source is required. This also little costly because here you also
have to take care of auxiliary anode material, which must have a very good protection against
self corrosion in the system, so that, it can function for a longer period. Third thing
is, of course, here also you see the back fill is kept. Back fill always make the current,
which is coming out from the system, it makes it uniform distribution of the current.
Now, this is ICCP. Now, ICCP has various advantages. The advantages are, basically it can solve
a longer protection. It can make it uniform and it is not dependent on the on the corrosion
behavior or the corrosion rate of the anode material. In the ICC, in the sacrificial anode
case, we need to see what is basically the Ampere hour for that particular anode material.
But here, we should not bother about that because once we fix the anode material, which
is not self corroding in the system and if we keep on sending this current, we can have
a protection. But it is little costly method because you need to have external power source.
But this can have some other problems, which is related to stray current. Stray current
related problems, there could be possible. How it is possible?
Let us say, some company has gone for protection of steel tank, which is buried steel tank.
Now, some other company which has laid oil pipeline just below this entire system. This
is oil pipeline, which is laid by another company, now this is not protected by this
system. What would happen? Now, once it sees that that is one another conductor which is
passing through this system, the current will also try to move into the system. So, if current
enters into this system, the current has to leave somewhere. Here also, the current is
entering. Now, this current will leave and this material, let us say, this material is
coated. Since it is buried, there could be a possibility that in some part would be no
coating. Some part would be exposed to the electrolyte. So, the current will leave from
this surface and enter into the system. Say, if current leaves this pipeline and enters
into a system, this part it is protected because current is always entering into the system.
But what happens here? Since the current is leaving the system, this part would be corroded
and this is, you know highly potential field. So, this will also be polarized and current
will enter into the system and current will leave from this system to this metal electrode.
So, we say that the current is entering into the nearby pipeline and the pipeline, here
also the current is entering and it is leaving from the pipeline surface to this system.
So, this part and this part are vulnerable for corrosion.
Now, if it is painted. So, if some part is exposed because of the wear and tear, wear
erosion, so this metallic surface is exposed only. So, a huge amount of current will pass
through the system and the current density around these regions, see, this is a very
small area, a large current, the current density would be huge here. So, gradually there would
be a lot of corrosion and finally, there could be a possibility of leak. So, this leakage
leads to oil loss. Now, this entire thing, basically the corrosion
which is happening in the pipeline, surrounding nearby the pipeline is because of this stray
current. This basically nothing but the stray current and this is called stray current corrosion.
How to protect this? Because you never know and you will not be able to know that how
long this pipeline will stay and when it will start leaking. Now, the protection would be
simply, if we can make this pipeline a member of this tank system. So, how you can make the pipeline be the member
of this cathode? You just connect this pipeline with the steel tank, with the conducting wire
and the surrounding part of the conducting wire should be insulated properly. Now, if
we connect it, so current of course will start coming here because it is basically current
field. So, the potential field. So, the current will always enter here. Here also current
will enter. So, if current enters into the system, no problem. We have protection. But
when it leaves, that time it goes through the conductor and it goes into the steel tank.
So, no case where the current is leaving this system or the pipeline to the electrolyte,
that situation we are stopping and if we can stop this situation, we can protect this pipeline
also. Fine? The classic example of this is the oil pipeline.
There are instances that this pipeline is leaked and there is a huge oil loss because
of this stray current, because another company has set one ICCP on top of this pipeline.
Another example is, in cities where you have tramline, below that tramline, if water pipe
is going, that water pipe can also come under stray current corrosion. Because the tramline,
that top part, when it goes, that time it develops potential field and the current will
enter into the water pipeline and it leaves the pipeline and goes to another surrounding
metal object. So, there also we can have corrosion due to this stray current effect. The protection
is, in case of pipeline, you connect it with the steel tank, which is been protected with
the help of ICCP. Now, let us see what are the advantages of
impressed current cathodic protection? One advantage is, it is versatile and it can be
applied with a wide variety of situation. Let us say, difference in electrolyte conductivity
or different electrolyte, metal object can be large, which can be protected. For example,
pipeline and this tank, both can be under protection because of this ICCP, because the
current value is much larger compared to the sacrificial anode method. Second is, it is
also effective in high resistance soil, very very highly resistance soil. So, the resistance
is very large, so the current flow would be less and there would be resistance with the
current flow. But since we have supplying current from the outer source, we can control
the current value and have a protection. Now, third is, let us say, I have a steel
tank and on top of that, we have a small coating. Let us say paint, non-conducting coating.
Fine? Or, let us say, I have zinc coating. There, if we employ this ICCP, the current
value which will be needed to protect this would be very small. Now, let us say, if some
part of the coating is out, then the current value will be needed as per the exposed area.
So, we can monitor depending on the current, what we are sending via this external power
source. If we see that large current is required; that means, we will be coming to know that
there is a problem with the coating. So, the coating can be monitored or the coating quality
can be monitored. So, these are the advantages. What are the disadvantages? Of course, it
requires external power source. I would not say that it is a disadvantage; I would say
that this is a requirement. But since we are having external power source, the cost of
having all those systems together would be very large. So, that is one. So, what are the disadvantages? One is high
cost and second is, there is a possibility of stray current corrosion. Now, another point
is, let us say while you are supplying large current, now this is cathode and this is anode
and you have connected it to positive and negative terminal. So, if you are supplying
large i c to this system and if there is hydrogen ion, the hydrogen ion can take this electron
and form H 2 gas and it can also have hydrogen atom. Then there could a possibility of hydrogen
related embattlement effect in the steel. This is another problem. So, these are the
advantages and disadvantages of ICCP. We have also explained about, I will explain, we have
also seen sacrificial anode method and these are the two methods by which we can go for
cathodic protection. Now, there is one more protection mechanism,
which is also falling under electro chemical ways of corrosion protection. That is anodic
protection. Now, anodic protection, in case of cathodic protection, we make the steel
structure, or the structure which is to be protected a cathode. But in this case, the
structure will remain as anode, but still, we can have protection. How it is possible?
This is possible in case of active passive metal. Now, coming back to this figure, which
is the mixed potential base theory for cathodic protection, now let us say, since we are saying
that this is anodic protection; that means, instead of cathodic polarization we should
go towards the cathodic polarization side. Now, if it is activation controlled and if
it is active metal, then if we go towards the positive sides which is nothing but anodic
polarization, will see that, gradually we keep increasing the i a. i a will be increased
as we go towards the positive side from this E corr point. Now, if we increase i a; that
means, we are not getting any protection. Rather, we are increase the corrosion rate
of the metal. Now, instead of active, if it is passive metal or active passive metal,
then what would happen? Now, let us get into that part. We will modify this figure a bit. Now, if this is active passive, let us say.
If it is like this, we know that active passive, if we go towards the anodic polarization side,
the active passive metal after reaching i critical, the current value, the anodic
current density will go towards the left side. Finally, it will at the passive region, this
is nothing but the passive region. Let us say the situation is like this and this is
my cathodic polarization line. This is my anodic polarization line and with this, we
have also seen that if it is mixed potential theory, now the system will go to this point
and it will remain there. Now, if by forcefully, if we take the anodic
potential or the potential of this metal object towards the positive side, far that positive
side what we have to do? We have to send extra current. So, that current is decided by difference
between i a minus i c. This is my i a and this is my i c and this is i a. So, gradually
if we keep on going like this, we will see that gradually I am increasing my applied
current. Fine? Now, if we keep on increasing my applied current,
so actually, we will see that the anodic current density is increasing. The anodic current
density is falling this track and if we plot the anodic current density with i applied
upto this point, let us say this is 1 2 3 4 5 6 upto 6, so i applied and i a, i applied
if we keep on increasing, i a is also increasing like this.
So, my corrosion rate will also increase because i a means the corrosion rate. That is, corrosion
of or the anodic dissolution current for the metal object or metal. Now, we are also going
towards positive polarization direction. We are increasing the positive polarization;
that means, the daily is gradually increasing. Now somewhat, if we can take it to this point,
if we take the potential to this point and that time, what would be my i applied? My
i applied would be this much. So, from this, if we jump to this, my i applied would come
to this level. This is my starting level. Now, my i applied would be this much. So,
it would be little less. This is my i applied and what would be my i a there? i a would
be like here. So, as we go on increasing the anodic polarization, we are seeing that initially
the i a is increasing with increase in i applied. But later, once we go to this section, which
is the passive zone, my i applied would also go down. At the same time, my i a also will
go down. So, i a will be coming to this. So, I am putting
it as rate point and this is the point we have reached. If we maintain this potential,
this potential if we can maintain, what would happen? I will always experience my corrosion
rate would be in the passive region. We have also seen that, if we can take it to the passive
level, the material corrosion rate would be very very small. This is the very concept
of anodic protection. Somewhat, we have to take the potential to the passive range and
if we take it to the passive range, we will see that there is a very, very small anodic
current density, which is equivalent to the corrosion rate of the metal object. That corrosion rate would be very, very small.