- 6/9/2025
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00:00Hello everyone. Welcome to this exciting lecture series on electrochemistry.
00:07In this video, we are going to study about corrosion and its prevention.
00:16So, first of all, we see that corrosion is a natural process where metals deteriorate
00:23due to reactions with environmental elements like water, air or the acidic substances.
00:29This degradation reduces the material's usefulness and leads to structural change or damage over time.
00:38At its core, corrosion is a redoxidation. It means oxidation and reduction occurs simultaneously.
00:47The metal loses electrons and gets oxidized and then turns into the metal lines,
00:55which then combines with the elements like oxygen to form oxides.
00:59This not only leads to metal loss but also drastically reduces the mechanical strength and integrity of the structure.
01:08In chemical terms, corrosion is essentially the process of transforming a reactive pure metal into a more thermodynamically stable compound.
01:20These include oxides, sulphides or hydroxide, all of which form layers on the metal surface,
01:27but usually do not protect it well, except in some rare cases like aluminum.
01:34A classic example is the rusting of iron.
01:39When iron is exposed to both moisture and oxygen, it reacts to form hydrated iron oxide, commonly called a rust.
01:50This porous rust does not protect the underlying metal and allows corrosion to continue deeper, weakening iron-based materials over time.
02:03Corrosion is commonly regarded as a bad phenomenon since it compromises the metal's good characteristics.
02:15Over time, corrosion deteriorates the properties that make metals useful, like strength, durability and conductivity,
02:24which lead to structural failure or mechanical malfunctioning.
02:28This is especially critical in industries that rely on metal-based infrastructure.
02:35Iron, for example, is recognized for its tensile strength and stiffness,
02:41especially when alloyed with other elements to form materials like steel.
02:46These traits make iron an essential component in buildings, vehicles, tools and machinery.
02:53However, once corrosion sets, those very advantages start to diminish significantly.
03:00Rusting, on the other hand, causes iron items, becomes brittle, flaky and structurally unsound.
03:09Deformation of rust weakens the metals from surface inward.
03:16As the iron oxide layer forms and breaks off, it continuously exposes fresh metal underneath it to further corrosion,
03:26which accelerates the degradation process.
03:29Corrosion is basically an electrochemical process because it usually involves a redox reaction between the metal and the atmospheric agents,
03:38like water, oxygen and sulfur dioxide.
03:43These reactions occur naturally and can even be spontaneous under normal environmental conditions.
03:49The metal atoms lose electrons and get oxidized, while the environmental agents gain electrons and get reduced,
03:57which facilitate the complete redox cycle that leads to material breakdown.
04:10Now we will look into the some of the factors which affect the process of the corrosion.
04:16First of all, we see that metals are exposed to gases such as CO2, SO2 and SO3 in the air, which can significantly accelerate the corrosion process.
04:28These gases are often by-products of industrial pollution and when they come into contact with metal surface, especially in the presence of moisture,
04:37they can form acids like sulfuric acid or sulfuric acid, making the corrosion process more aggressive.
04:46Exposing to moisture, particularly salt water, greatly increases the rate of corrosion.
04:53Salt water acts as a powerful electrolyte which enhances the electrochemical reactions involved in the corrosion process.
05:01This is why marine environments or coastal regions pose a much higher risk for the metallic structure and vehicles.
05:13Impurities such as salt, especially NaCl, which is present in the environment or deposit on the metal surface can drastically boost corrosion rates.
05:25These impurities attract moisture from air, promoting continuous electrochemical activity and breakdown of the metal surface.
05:37Temperature plays a crucial role. As the temperature rises, so does the rate of the corrosion.
05:42At higher temperatures, it will increase the kinetic energy of the molecule which speeds up chemical reactions,
05:53including the redox processes that drive the corrosion.
05:58The nature of the first oxidant layer that forms on the metal surface, matters greatly.
06:04Some oxides like Al2O3 formed on the aluminum create an insoluble and protective coating that prevents further oxidation.
06:14In contrast, rust formed on the iron is porous and crumbly, which exposes the underlying metal to continued corrosion.
06:26Next, we see that the presence of acid in the atmosphere can speed up the corrosion process drastically.
06:33Acidic environments, weather farm, weather from acid rain, or industrial emissions, provide additional protons, which are the H plus sign,
06:44and facilitate the breakdown of metal surfaces through accelerated redox activity.
06:50Now, we will explore the different types of corrosion that affect metals in various environments.
07:01Understanding these distinctions is essential for material selection, industrial design, and implementing proper protection strategies.
07:11We will start with the common, yet often overlooked form of corrosion, which is the ceramic corrosion.
07:21It occurs when there is a difference in ionic concentrations between any two local locations of a metal,
07:28which form a small electrochemical cell on the surface that accelerates the corrosion in that confined area.
07:36This form of corrosion usually happens in places where matters are not fully exposed to the environment,
07:45such as gaskets and the underside of washers and the bolt heads.
07:51These tight spaces can trap electrolytes like water or salt, making them prime site for the localized attacks.
08:01What is particularly important to note is that Ceramic corrosion occurs in all grades of aluminum alloys and stainless steels.
08:10Even metals known for their corrosion resistance can be vulnerable in such micro-environments where oxygen or iron distributions become uneven.
08:22Let us now look into two other critical types that affect metal durability and performance.
08:33These are the stress corrosion cracking and the intergranular corrosion.
08:38These forms are not always visible on the surface but can lead to catastrophic failures in the engineering applications.
08:46First of all, we will talk about the stress corrosion cracking.
08:51This is the form of corrosion due to stress where the metal breaks as a result of the corrosive environment and the tensile stress exerted on it.
09:01It is often tricky to detect until the material suddenly fractures.
09:06It is especially common in hot weather where elevated temperatures intensify both stress and chemical reactivity.
09:17A good example of this is the stress corrosion cracking of the austenitic stainless steel in chloride solutions,
09:24where both tensile strength and chloride ions work together to cause the failure.
09:30Now we will talk about the intergranular corrosion.
09:34This occurs when contaminants are present in the grain boundaries that separate the grain formed during the solidification of the metal alloys.
09:44These boundaries are structurally weaker and more reactive than the other parts.
09:50Sometimes, it is not just the contaminants.
09:54depletion or the enrichment of the alloy at these grain boundaries can also initiate corrosion, especially in heat-treated or welded materials.
10:06A common example of intergranular corrosion is in the aluminum base alloys,
10:12where the attack follows the grain boundaries which severely compromises the mechanical integrity.
10:22Now, let us look at another important and widely observed form of corrosion, which is galvanic corrosion.
10:30This type plays a major important role, especially when different metals are joined together in structural, marine or piping systems.
10:40So, galvanic corrosion happens when an electric contact head develops between two metals that are electrochemically different,
10:49and they are placed in an electrolytic environment like salt water.
10:53One of the metals become anode and corrodes faster while the other acts as a cathode and it is protected.
11:03It describes the breakdown of one of these metals at a joint or junction.
11:09A classic example is the breakdown of one of these metals at a joint or junction,
11:15such as when copper comes into contact with steel in salt water environment.
11:20Here, steel becomes the sacrificial metal and the copper is protected.
11:27Another real well scenario involves the aluminum and carbon steel linked which are linked and they are submerged in the seawater.
11:39In this case, aluminum crudes faster while the steel is protected,
11:43which demonstrates how galvanic interaction favors one metal over the other.
11:49Now, let us take a closer look at pitting corrosion, which is one of the more challenging types of corrosion to manage.
12:04Unlike uniform corrosion, pitting is highly localized and therefore unpredictable.
12:10This unpredictability makes it especially difficult to detect in early stages because the surrounding metal surface often appears normal and intact.
12:25Pitting corrosion typically begins at a very specific point.
12:29A small imperfection or damage site and then expands to form what we call a corrosion cell.
12:36This cell is surrounded by unaffected metallic surface which unfortunately makes it harder to spot since the damage is not widespread but rather concentrated in some location.
12:51As the corrosion progresses, the initial pit can grow in size and shape, often developing vertically into the metal.
13:01This deep penetration weakens the metal significantly and potentially leading to serious structural failure if the corrosion is not identified and controlled in time.
13:14An easy way to visualize this is to think about a droplet of water sitting on a steel surface.
13:29Within that droplet, a tiny area near the center acts as an anodic site where the metals begin to corrode.
13:46The pit forms there and it continues to deepen beneath that droplet which effectively eat away the metal from the inside out.
14:01Because of its insidious nature, pitting corrosion is considered one of the most hazardous forms of corrosion in industrial application.
14:10So, early detection and preventive measures are crucial in this case.
14:15Now, moving on to the uniform corrosion, which is actually the most common type of corrosive corrosion we encounter.
14:28Unlike pitting corrosion, uniform corrosion affects the entire surface of the metal more or less evenly.
14:35This means the metal is gradually worn away across its whole exposed surface.
14:40One of the advantages of uniform corrosion is that it is relatively easy to absorb because the damage is visible and consistent.
14:53Think of the typical rusting you see on iron or steel surfaces left out in the open area.
15:00You can clearly see the rust forming and it progresses steadily.
15:06Because the metal crudes evenly, the metal tends to retain much of its original structure integrity for a longer time compared to localized corrosion rights.
15:17The loss of material happens at a constant predictable rate which often allows engineers to plant maintenance and replacement accordingly.
15:26A classic example to illustrate this is when a piece of zinc or steel is placed in diluted sulphuric acid.
15:35So, this is another example.
15:38The metal dissolves steadily across its entire surface rather than in isolated spots which shows uniform corrosion rates.
15:47This steady and predictable nature makes uniform corrosion somewhere easier to manage but it still requires monitoring to prevent long term degradation or the equipment failure.
16:00Now, it is time to explore some real world examples of corrosion by looking at copper or the other material which is very common metal used in various applications.
16:16So, first of all we see about the copper corrosion.
16:22When copper is exposed to environment, it reacts with oxygen present in the air.
16:28Initially, copper combines with oxygen to form copper oxide which appears as a reddish brown layer on the matter's surface.
16:37The chemical reaction for this process is given as this one.
16:42Two copper atoms react with a half molecule of oxygen molecule and they form copper oxide.
16:50This copper oxide does not stop there.
16:54It further oxidizes to form copper oxide which is black in color.
16:59The reaction for this transformation is given as this.
17:03Cu2O plus 1 by 2 O2.
17:07They give rise to copper oxide which is CuO.
17:12Now, this black copper oxide can react with other environmental elements like carbon dioxide, sulfur dioxide, and water.
17:25These reactions lead to the formation of complex copper compounds such as copper hydroxide,
17:32copper hydroxide carbonate which is known as the mellachite given as this one.
17:39It is a blue-green mineral and it can also form copper sulphate hydroxide which is called borochentite which is green in color.
17:53An iconic example of these corrosion products in nature is the distinctive blue-green patina on the statue of the liberty.
18:05This degrade discoloration is a direct result of the corrosion layer formed over time due to copper's interaction with the atmosphere.
18:14Understanding these reactions helps us appreciate how corrosion not only affects the matter's appearance but also influences the longevity and the structural properties.
18:27Now, let us consider another common corrosion example which is the silver tarnishing.
18:37Silver reacts with sulfur compounds present in the air to form silver sulphide which is a dark substance.
18:45This tarnishing process is very familiar because it causes silver objects to darken over time.
18:53More specifically, exposed sulfur reacts with hydrogen sulphide which is the H2S in the environment which is present due to some industrial processes and it leads to the formation of silver sulphide.
19:11The chemical reaction can be written as 2Ag plus H2S direct to form Ag2S plus H2.
19:23So, hydrogen gas evolved according to this reaction and the silver gets corroded.
19:30This silver sulphide which is the H2S, it forms a thin but visible layer on the silver surface which is responsible for the characteristic tarnished appearance.
19:43This example highlights how environmental pollutants like H2S from the industrial activities can accelerate corrosion and alter the appearance and properties of metals like silver.
19:56Moving next towards some of the examples, let us now examine the corrosion of iron which is also known as rafting and it is the most typical occurrence or type of corrosion.
20:13Rafting takes place when iron comes into contact with air or water which are both common in most natural environments.
20:23The overall reaction resembles that of a normal electrochemical cell meaning it includes an anodic and cathodic region where oxidation and reduction occurs simultaneously on the iron surface.
20:40We see that at the anode position, matter loses electrons and it is converted to Fe2 plus ions in this step.
20:50So, we can see the reaction here.
20:52Two Fe atoms, they release four electrons to form spheric ions which are Fe2 plus ions.
21:00So, this position could be considered as the anode position.
21:04From the diagram, we can see that from this side, iron gets corroded.
21:10From this side, some of the iron has been worn away or it is depleted or it is converted into the rust.
21:18So, we see that this is the anodic side.
21:21From the anode side, iron which is in a solid form, it loses two electrons to get converted to Fe2 plus ions.
21:29So, in the process of oxidation happens.
21:32So, this is the oxidation side.
21:34These Fe2 plus ions, they move from the solid surface to water which is basically acting as electrolyte.
21:43So, these ions move freely in the water or the moisture present on the iron surface.
21:50So, we see that the electrons that are lost by the iron flows through the metallic surface to the opposite side.
22:05Where they participate in the reduction reaction involving oxygen and water.
22:10So, the reaction that we see that O2 plus 2H2O, they combine with four electrons to form four hydroxide ions.
22:19An alternative reaction for this is this one.
22:22So, we can basically use any of these reactions.
22:25But the end result will be same.
22:28So, we see that this is the cathodic reaction which generates basically the hydroxide ions.
22:36Okay.
22:37Next, we see that the H plus ions are also emitted in the atmosphere by either H2O or H2CO3.
22:45So, if we see the reaction with respect to the H plus ions, then we get this reaction.
22:53This reaction basically of the H plus ions contribute to the acidity of the environment.
22:59This side could be considered as the cathodic partition.
23:03Okay.
23:04As we have already seen that, the iron and the hydroxide ions, they react to form the iron hydroxide.
23:14The reaction is given as this one, 2Fe2 plus, which is from the anode, plus four hydroxide ions, which is from this cathode side.
23:24They combine here to form two FeOH2.
23:29So, this is the water soluble component.
23:33It dissolves in this water and move around freely in this electrolytic solution.
23:39But we see that this iron oxide, which is FeOH2, it reacts with oxygen, which is present in the air.
23:50So, we see that oxygen gets combined with this overall material, which is basically the FeOH, and it reacts to form a red rust, which is called the Fe2O3 dot H2O.
24:03So, this is basically our rust.
24:07It is commonly observed as a flaky and porous rust on the iron surface.
24:12So, these are basically the reactions.
24:15Fe, iron will lose electrons to form Fe2 plus signs.
24:19Okay.
24:20Oxygen will react with water to form hydroxide ions.
24:25Hydroxide ions will combine with Fe2 plus signs.
24:28They will react further with the oxygen to form basically the final form of rust, which is the Fe2O3 dot XH2.
24:37X can be any number of molecules which can be present or physically combined with Fe2O3, which is the iron oxide.
24:47So, this was basically the general depiction of the iron rusting.
24:54Now, let us talk about corrosion prevention, which is essentially for preserving the integrity and lifespan of metals.
25:05Corrosion can be prevented in a number of ways, and today, we will go over a few of the more popular ones, starting with the electroplating.
25:14Electroplating is an electrolysis based method that is widely used in both industrial and decorative applications.
25:22In this process, we code metal 1, which can be any metal, with a thin layer of another metal 2.
25:29And this metal 2 provides protection from the corrosion to metal 1.
25:35The new metal covering protects the metal 1 from corrosion in this way.
25:42So, we see that in electroplating, metal 1, which is metal to be plated, is connected to the cathode and metal 2, which is the coating metal, is used as the anode in this procedure.
26:00So, metal 1 is used as a cathode and metal 2 used as anode in this procedure.
26:13In this cell, metal 1 will be connected with the negative terminal of the battery, and metal 2 is connected to the positive terminal of the battery.
26:25This battery provides a direct current supply.
26:29The electrolyte contains dissolving ions of the coating metal.
26:33Okay.
26:34If we want to coat silver on this spoon, so the electrolyte will also contain the electrolytic solutions of this metal also.
26:46So, we see that when electricity flows, oxidation occurs at the anode, which is the positive terminal.
26:54Okay.
26:55Leading to the dissolution of the metal 2 ions into the electrolytes.
26:59For example, if silver is to be electroplated, so silver ions will get oxidized and its ions will dissolve in this electrolytic solution.
27:10These ions then move towards the cathode, okay, which is our metal one, where they are reduced and deposited as a thin coating on the metal.
27:20This forms a uniform and corrosion resistance layer on this object.
27:25Common metals used in electroplating include copper, nickel, gold, silver, and zinc, and some of the others.
27:37These metals are often selected based on desired surface properties like appearance, conductivity, or resistance to corrosion.
27:45So, this was basically the process of electroplating.
27:54Moving on to another method of corrosion prevention, we will now talk about the cathodic production.
28:01In this process, the base metal is connected to the sacrificial metal that crows itself instead of the base metal.
28:10This is a very common and effective method for protecting pipelines, storage tanks, and other equipments.
28:17The sacrificial metal, which is chosen based on the reactivity, must be more reactive than the base metal.
28:24As a result, the sacrificial metal releases electrons and becomes oxidized, sacrificing itself to protect the more valuable metal.
28:34The ions produced from the oxidation of the sacrificial metal participate in corrosion reactions, but crucially, they do so instead of the base metal, effectively preserving it.
28:51So, this is basically a general diagram for the cathodic production.
28:56We see that anode is an electrode which provides electric current to protect metal from corrosion.
29:05Anode will protect cathode by providing current to the anode.
29:11Cathode is the metal structure that needs the protection such as the pipes or tanks.
29:17So, anode is basically the sacrificial metal and cathode is the metal which is to be protected.
29:24Here, the electric current, it provides the flow of electricity from anode to the cathode.
29:31Okay.
29:32So, using this basic structure or the diagram, we can see that cathode gets reduced and anode gets oxidized.
29:43due to the oxidation of anode or more reactivity of the anode, anode gets corroded earlier than the cathode.
29:52And in this way, our valuable metal which is basically the cathode gets protected and anode gets corroded away instead of cathode.
30:01So, this is basically the cathodic protection method of the prevention of corrosion.
30:06Another widely used corrosion prevention technique is the galvanization.
30:13This provides, this procedure includes applying a thin layer of zinc to iron which create a barrier against environmental exposures.
30:25In most cases, this is accomplished by dipping iron in molten zinc which is a process known as the hip-hot dip galvanization.
30:36This zinc layer serves two purposes.
30:39It acts as a physical barrier and it also provides sacrificial protection as zinc is more reactive than the iron.
30:47So, in the previous slide, we see that how the sacrificial metals work.
30:53Okay.
30:54So, in this case, zinc is basically the sacrificial metal.
30:59As a result, even if the zinc layer gets scratched or damaged, it continues to protect the underlying iron through preferential oxidation which ensures the iron remains safe from the corrosion.
31:14So, this method is basically the variation of our electroplating in which one metal is basically deposited on another metal or electroplated on another metal.
31:32Let us look at some of the more methods of prevention from corrosion.
31:37First of all, we will talk about painting and greasing.
31:41Applying layers of paint or grease to the metal act as a barrier which is a physical barrier
31:48which is shielded from environmental elements such as moisture, air and chemicals.
31:53This layer prevents the metal from coming into contact with the outside world and in doing so, it prevents corrosion.
32:01This method is widely used in automotive construction and machinery applications where regular maintenance through repainting or re-greasing can greatly extend the structure's lifespan.
32:16Another chemical method involves using corrosion inhibitors.
32:21Corrosion inhibitors.
32:22Corrosion inhibitors are substances that when introduced into a corrosion environment reduce the rate of corrosion.
32:29This can be added to liquids or gases that come into contact with the metal surface.
32:35They work by either forming a protective film on the metal surface or neutralizing the corrosive agents such as oxygen or acid in the environment.
32:46Corrosion inhibitors are commonly used in boilers, cooling systems, pipelines or even fuel systems.
32:55Finally, one of the most fundamental ways to prevent corrosion is to make use of the right material.
33:02As the saying goes, prevention is better than the cure.
33:06Corrosion can also be avoided by selecting the proper material during the design and manufacturing process.
33:13For example, aluminum and stainless steel are extremely corrosion resistant due to their formation of stable protective oxide film on their surface.
33:24This makes them ideal choice for marine environments, food processing equipment and other outdoor structures.
33:33So, this marks the end of our lecture.
33:40It was all about corrosion and its prevention.
33:43I hope that you have learned something new and understand the complete concept of corrosion, its types and the methods of its protection.
33:52Thank you very much.
33:53Thank you very much.
33:54Thank you very much.
34:02Thank you very much.
34:04Thank you very much.
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