Rusting, or corrosion, is a common phenomenon that we observe in our daily lives. Many objects made of iron will eventually become coated with an orange or reddish-brown layer. This is caused by a chemical process known as rusting, which is a type of corrosion.

Cossorion is a process in which refined metals are converted into more stable compounds such as metal oxides, metal sulfides, or metal hydroxides. Rusting of iron involves the formation of iron oxides through the action of atmospheric moisture and oxygen. At its core, corrosion is a spontaneous and irreversible process in which metals transform into more stable chemical compounds like oxides, sulphides, hydroxides, etc. In this lesson, we will explore the concept of corrosion in further detail, including its meaning, types, prevention, and more.

Table of Contents

Corrosion Definition

Factors Affecting Corrosion

Types of Corrosion

Corrosion Examples and Reactions

Prevention of Corrosion

Corrosion Definition

Corrosion is the process by which a material, usually a metal, breaks down due to chemical reactions with its environment.

What is Corrosion? It is essentially the process of degradation of metals due to their reaction with substances such as water or air. This reaction causes the metal to break down and disintegrate, starting from the area exposed to the environment and spreading to the entire metal structure.

Corrosion is generally considered to be an undesirable phenomenon as it can have a negative impact on the desirable properties of metals. For instance, iron is known for its good tensile strength and rigidity, however, when it is subjected to rusting, it can become brittle, flaky and structurally unsound. On the other hand, corrosion is a diffusion-controlled process and mostly takes place on exposed surfaces. Therefore, in some cases, efforts are made to reduce the activity of the exposed surface and increase the material’s corrosion resistance. Techniques such as passivation and chromate conversion are used for this purpose. However, some corrosion mechanisms are not always visible and are even less predictable.

Alternatively, corrosion can be classified as an electrochemical process since it usually involves redox reactions between the metal and certain atmospheric agents such as water, oxygen, sulphur dioxide, etc.

Does All Metals Corrode?

Metals placed higher in the reactivity series, such as iron, zinc, etc., corrode very easily due to oxidation. Metals placed lower in the reactivity series, such as gold, platinum, and palladium, do not corrode as easily because their oxidation potentials are lower.

Check out: Oxidation and Reduction

Interestingly, aluminium doesn’t corrode unlike other metals even though it is reactive. This is because aluminium is already covered by a layer of aluminium oxide, which acts as a protective barrier and prevents further corrosion.

Factors Affecting Corrosion

1. Exposure of the metals to air containing gases such as CO2, SO2, and SO3.

2. Exposure of metals to moisture, particularly salt water, which increases the rate of corrosion.

3. Presence of impurities such as salt (e.g. NaCl).

4. Temperature: An increase in temperature can lead to an increase in corrosion.

5. Nature of the first layer of oxide formed:

Some oxides, like Al2O3, form an insoluble protecting layer that can prevent further corrosion. Others, like rust, easily crumble and expose the rest of the metal.

6. Presence of Acid in the Atmosphere: Acids can easily accelerate the process of corrosion.

Rate of Corrosion

The Deal–Grove model is often used to describe the formation of an oxide layer, and helps in predicting and controlling oxide layer formation in a variety of diverse situations. An alternative method, the weight loss method, is used to measure corrosion. This method involves exposing a clean, weighed piece of metal or alloy to a corrosive environment for a certain duration, followed by a cleaning process that removes the corrosion products. The piece is then weighed again to determine the loss of weight.

The rate of corrosion (R) is calculated as:

‘(\begin{array}{l}R=\frac {kW}{\rho A \cdot t}\end{array} )’ he said

He asked, “Where?”

K = Constant

W = weight loss of the metal in time t

A = Surface area of the metal exposed

The density of the metal is ρ (in g/cm³).

#Types of Corrosion

Some of the corrosion types include:

(i) Crevice Corrosion

Whenever there is a difference in ionic concentration between any two local areas of a metal, a localized form of corrosion known as crevice corrosion can occur. This type of corrosion usually takes place in confined spaces, such as gaskets, the undersurface of washers, and bolt heads. All grades of aluminium alloys and stainless steels are also susceptible to crevice corrosion due to the formation of a differential aeration cell, which leads to the formation of corrosion inside the crevices.

(ii) Stress Corrosion Cracking

Stress Corrosion Cracking (abbreviated as SCC) is a type of corrosion that occurs when tensile stress is applied to a metal in a corrosive environment, resulting in cracking of the metal. It is often seen at high temperatures.

Stress Corrosion Cracking of Austenitic Stainless Steel in Chloride Solution

(iii) Intergranular Corrosion

Intergranular corrosion is caused by impurities present in the grain boundaries that separate the grains formed during the solidification of the metal alloy. Additionally, it can result from the depletion or enrichment of the alloy at the grain boundaries.

Aluminum-base alloys are susceptible to Intergranular Corrosion (IGC).

(iv) Galvanic Corrosion

When two metals that are electrochemically dissimilar and in an electrolytic environment have an electrical contact, galvanic corrosion can occur. This type of corrosion is the degradation of one of the metals at a joint or junction. A common example of this is when copper in a salt-water environment comes in contact with steel, which can lead to degradation.

When aluminium and carbon steel are connected and immersed in seawater, aluminium corrodes quicker while steel is shielded.

(iv) Pitting Corrosion

Pitting Corrosion is an extremely unpredictable and hazardous form of corrosion. It is initiated at a single point, forming a corrosion cell surrounded by the normal metallic surface. The pit then expands in a vertical direction, penetrating the metal and leading to structural failure if not addressed. As it is difficult to detect in its early stages, it is considered one of the most dangerous types of corrosion.

Consider a droplet of water on a steel surface; pitting will initiate at the center of the water droplet (the anodic site).

Uniform Corrosion

This type of corrosion, commonly known as atmospheric corrosion, is easily identifiable due to its surface-level attack on the metal. Although its effects are relatively low, it can still have an impact on the material’s performance.

A piece of zinc and steel immersed in diluted sulphuric acid would usually dissolve over its entire surface at a constant rate.

(vi) Hydrogen Grooving

The corrosion of the piping is caused by grooves that are formed when a corrosive agent, corroded pipe constituents, and hydrogen gas bubbles interact. When the protective coating comes into contact with the bubbles, it is usually removed.

(vii) Metal Dusting

Metal dusting is a form of corrosion that results from the exposure of vulnerable materials to environments with high carbon activities, such as synthesis gas. This leads to the break-up of bulk metal into metal powder, as a graphite layer is deposited on the metal surface from the carbon monoxide (CO) in the vapour phase. The graphite layer forms meta-stable M3C species (where M is the metal), which usually moves away from the metal surface. In some cases, however, no M3C species may be observed, indicating that the metal atoms have been directly transferred into the graphite layer.

(VIII) Microbial Corrosion

Microbial corrosion, also known as microbiologically influenced corrosion (MIC), is a form of corrosion caused by microorganisms, the most common being chemoautotrophs. Metallic and non-metallic materials, in the presence or absence of oxygen, can be affected by this corrosion.

(VIII) High-Temperature Corrosion

High-temperature corrosion, as the name implies, is a type of corrosion of materials (primarily metals) caused by exposure to high temperatures. Chemical degradation of metal can result from a hot atmosphere containing gases such as oxygen, sulfur, or other compounds. These compounds can easily oxidize the materials (metals in this case). For instance, materials used in car engines must be able to withstand prolonged periods of high temperature, during which they can be affected by a corrosive atmosphere of combustion products.

Corrosion: Examples, Reactions and Effects

Here are some common examples of corrosion seen in metals:

1. Corrosion of Copper

When copper metal is exposed to the environment, it reacts with the oxygen in the atmosphere to form copper (I) oxide, which is a red color.

2Cu(s) + ½ O₂(g) → Cu₂O(s)

Cu2O further gets oxidized to form CuO, which is black in color.

Cu2O(s) + O2(g) $\rightarrow$ 2CuO(s)

CuO reacts with CO₂, SO₃, and H₂O (present in the atmosphere) to form Cu₂(OH)₂(s) (Malachite), which is blue in colour, and Cu₄SO₄(OH)₆(s) (Brochantite), which is green in colour.

This is why copper turns a bluish-green color when observed.

The colour of the Statue of Liberty is an example of this, as its copper coating has turned to a blue-green hue.

2. Silver Tarnishing

Silver reacts with sulphur and sulphur compounds in the air, forming silver sulphide (Ag2S) which is black in colour. When exposed to the atmosphere, silver reacts with the H2S(g) present due to certain industrial processes, creating Ag2S.

2Ag(s) + H2S(g) → Ag2S(s) + H$_2^+$(g)

3. Corrosion of Iron (Rusting)

The rusting of iron, which is the most commonly seen example, occurs when iron comes in contact with air or water. This reaction can be seen as a typical electrochemical cell reaction, as demonstrated in the diagram below:

![Corrosion of Iron (Rusting)]()")

At the anode position, metal iron (Fe) loses electrons and gets converted to Fe{aq}2+. The electrons then move to the cathode position, where they combine with H+ ions released either by H2O or by H2CO3 present in the atmosphere.

$$\ce{H2O <=> H+ + OH-}$$

(\begin{array}{l}H_2CO_3 \rightleftharpoons 2H^+ + CO_3^{2-}\end{array})

The Hydrogen formed by the reaction of H+ and electrons reacts with oxygen to form H2O.

Anode Reaction

2Fe(s) → 2Fe$^{2+}$ + 4e$^{–}$ ; (E_{\ce{Fe^{2+}/Fe}^{0}} = -0.44\ \text{V})

Cathode Reaction

$\begin{array}{l}{O_2(g)} + 4{H^+}{(aq)} + 4e^- \overset{2}{\longrightarrow} {H_2}{O{(l)}}{E^o}_{{H^+}/{O_2}/{H_2}/O,,,} = 1.23V\end{array}$

Overall Reaction:

2Fe(s) + O2(g) + 4H+(aq) → 2Fe2+(aq) + 2H2O(l) \ Eocell = 1.67V

The Fe2+ ions formed at the anode react with oxygen in the atmosphere, getting oxidised to Fe3+ and forming Fe2O3. This Fe2O3 then comes out in the hydrated form as Fe2O3.xH2O.

Fe2+ + 3O2 → 2Fe2O3

Fe2O3 + xH2O → Fe2O3*xH2O (rust)

Check Out: Rusting of Iron and Prevention

Other examples include:

The reaction of Zinc with oxygen and HCl to form white coloured ZnCl2 causes corrosion of Zinc.

The Corrosion of Tin forms black coloured Na2[Sn(OH)2].

Effects

Corrosion can have a wide range of impacts on many things. Most notably, it leads to the waste of natural resources. Furthermore, it can create potentially hazardous situations, such as weakened and unstable structures, accidents due to corroded parts, and other unwanted failures such as cracked pipelines, collapsing bridges, transport vehicle crashes, and other disasters. Therefore, it is imperative to check for and prevent corrosion at all costs.

Prevention of Corrosion

Preventing corrosion is of utmost importance in order to avoid huge losses, as many of the structures we see and use are made out of metal, such as bridges, automobiles, machinery, and household goods like window grills, doors, and railway lines. To protect these materials from corrosion damage due to exposure to the weather, saltwater, acids, or other hostile environments, several treatments can be used to slow or prevent corrosion. Some of the popular methods to prevent corrosion include:

Electroplating

Galvanization

Anodization

Passivation

Biofilm Coatings

Anti-Corrosion Protective Coatings

Painting and Greasing

Use of Corrosion Inhibitor or Drying Agents

Periodic Cleaning of Metal Surface

Learn More Here: How Can Corrosion Be Prevented?

#Frequently Asked Questions on Corrosion

What is Corrosion?

Corrosion is a natural process that occurs when metals react with their environment, leading to the formation of an oxide layer on the surface of the metal. This process can cause damage to the metal, leading to decreased structural integrity and reduced performance.

No, not all metals corrode. Some metals, such as gold and silver, are naturally stable and do not corrode.

No, all metals do not corrode. The metals which are higher in the reactivity series of metals corrode easily as their oxidation potential is high. For example, iron. What affects corrosion?

The effect of temperature on corrosion depends on the type of metal and the environment. Generally, higher temperatures can accelerate corrosion, while lower temperatures can slow it down.

High-temperature corrosion is a type of corrosion caused by high temperatures, typically above 400°C. It is accelerated by oxidation, carburization, sulfidation, nitridation, and other forms of atmospheric corrosion.

High-temperature corrosion is the corrosion of metals due to heating.— title: “Corrosion” link: “/corrosion” draft: false

Rusting, or corrosion, is a common phenomenon that we observe in our daily lives. Objects made of iron often form an orange or reddish-brown layer over time due to a chemical process. This layer is a result of the rusting process, which is a form of corrosion.

Corrosion

In general, corrosion is a process through which refined metals are converted into more stable compounds such as metal oxides, metal sulfides, or metal hydroxides. The rusting of iron involves the formation of iron oxides via the action of atmospheric moisture and oxygen. From a scientific standpoint, corrosion is a spontaneous and irreversible process wherein the metals turn into a more stable chemical compound such as oxides, sulphides, hydroxides, etc. In this lesson, we will explore the concept of corrosion in greater detail, including its meaning, types, prevention, and more.

Table of Contents

Corrosion Definition

Factors Affecting Corrosion

Types of Corrosion

Corrosion Examples and Reactions

Prevention of Corrosion

Corrosion Definition: The deterioration of a material, usually a metal, due to a reaction with its environment.

What is Corrosion? It is generally defined as a natural process in which pure metals react with substances like water or air, resulting in the transformation of the metal into undesirable substances. This reaction causes damage and disintegration of the metal, starting from the portion of the metal exposed to the environment and spreading to the entire bulk of the metal.

Corrosion is usually seen as an undesirable process as it can have a negative impact on the desirable properties of a metal. For example, iron is well known for its tensile strength and rigidity, however when exposed to rusting, it can become brittle, flaky and structurally unsound. On the other hand, corrosion is a diffusion-controlled process and mostly occurs on exposed surfaces. Therefore, in some cases, processes such as passivation and chromate conversion are used to reduce the activity of the exposed surface and increase a material’s corrosion resistance. However, it is important to note that some corrosion mechanisms are not always visible and can be quite unpredictable.

Alternatively, corrosion can be classified as an electrochemical process since it usually involves redox reactions between the metal and certain atmospheric agents such as water, oxygen, sulphur dioxide, etc.

Does Corrosion Affect All Metals?

Metals placed higher in the reactivity series, such as iron, zinc, etc., corrode very easily due to oxidation. On the other hand, metals placed lower in the reactivity series, like gold, platinum, and palladium, do not corrode as they have very low oxidation potentials.

Check out: Oxidation and Reduction

Interestingly, aluminium doesn’t corrode unlike other metals even though it is reactive. This is because aluminium is already covered by a layer of aluminium oxide, which protects it from further corrosion.

Factors Affecting Corrosion

1. Exposure of the metals to air containing gases such as CO2, SO2, and SO3.

2. Exposure of metals to moisture, particularly salt water, which accelerates the rate of corrosion.

3. Presence of impurities such as salt (e.g. NaCl).

4. Temperature: An increase in temperature leads to increased corrosion.

5. Nature of the first layer of oxide formed:

• Some oxides, like Al2O3, form an insoluble protective layer that can prevent further corrosion.
• Others, like rust, easily crumble and expose the rest of the metal.

6. Presence of Acid in the Atmosphere: Acids can easily accelerate the process of corrosion.

Rate of Corrosion

The Deal–Grove model is often used to describe the formation of an oxide layer, helping to predict and control oxide layer formation in a variety of situations. Alternatively, the weight loss method is also used to measure corrosion. This method involves exposing a clean, weighed piece of the metal or alloy to a corrosive environment for a given duration, followed by a cleaning process that removes the corrosion products. The piece is then weighed to determine the loss of weight.

The rate of corrosion (R) is calculated as:

(\begin{array}{l}R = \frac{kW}{\rho A t} \end{array}) he said

He asked, “Where?”

k = constant

‘W = Weight Loss of the metal in time t,’

A = Surface Area of the Metal Exposed

ρ is the density of the metal (in g/cm³).

Types of Corrosion

Some of the corrosion types include:

(i) Crevice Corrosion

Whenever there is a difference in ionic concentration between any two local areas of a metal, a localized form of corrosion known as crevice corrosion can occur. In a simple instance, this form of corrosion mostly occurs in confined spaces (crevices). Examples of areas where crevice corrosion can occur are gaskets, the undersurface of washers, and bolt heads. All grades of aluminium alloys and stainless steels also undergo crevice corrosion. This is mainly because of the formation of a differential aeration cell that leads to the formation of corrosion inside the crevices.

(ii) Stress Corrosion Cracking

Stress Corrosion Cracking (SCC) is a phenomenon that occurs when a metal is subjected to both tensile stress and a corrosive environment, resulting in cracking of the metal. It is often observed at high temperatures.

Stress Corrosion Cracking of Austenitic Stainless Steel in Chloride Solution.

(iii) Intergranular Corrosion

Intergranular corrosion occurs when impurities are present in the grain boundaries that separate the grains formed during the solidification of a metal alloy. It can also be caused by the depletion or enrichment of the alloy at these grain boundaries.

Aluminum-based alloys are susceptible to Intergranular Corrosion (IGC).

(iv) Galvanic Corrosion

When two metals that are electrochemically dissimilar and in an electrolytic environment come into contact with one another, it can cause galvanic corrosion. This is a type of degradation that occurs at the joint or junction of two materials, and is exemplified by the corrosion of copper in a salt-water environment when it is in contact with steel.

When aluminium and carbon steel are connected and immersed in seawater, aluminium corrodes quicker while steel is shielded.

(iv) Pitting Corrosion

Pitting Corrosion is a very unpredictable and dangerous type of corrosion. It forms at a single point and can take on various shapes as it grows and penetrates the metal from the surface downwards, potentially leading to structural failure if not addressed. As it is difficult to detect, it is important to take the necessary precautions to prevent it.

Consider a droplet of water on a steel surface: pitting will initiate at the centre of the water droplet (anodic site).

Uniform Corrosion

This form of corrosion, commonly referred to as atmospheric corrosion, is easily identifiable due to its surface attack on the metal. Although the corrosion is visible, it typically has a low impact on the material’s performance.

A piece of zinc and steel immersed in diluted sulphuric acid typically dissolves over its entire surface at a consistent rate.

(vi) Hydrogen Grooving

The corrosion of the piping is caused by grooves that form due to the interaction of a corrosive agent, corroded pipe constituents, and hydrogen gas bubbles. These bubbles often remove the protective coating when they come into contact with the material.

(vii) Metal Dusting

Metal dusting is a form of corrosion that occurs when vulnerable materials are exposed to environments with high carbon activities, such as synthesis gas. This corrosion leads to the break-up of bulk metal into metal powder. Corrosion occurs as a graphite layer is formed on the metal surface from carbon monoxide (CO) in the vapour phase. This layer then forms meta-stable M3C species (where M is the metal), which usually moves away from the metal surface. In some cases, no M3C species may be observed, meaning that the metal atoms have been directly transferred into the graphite layer.

(VIII) Microbial Corrosion

Microbial Corrosion, also known as Microbiologically Influenced Corrosion (MIC), is a type of corrosion caused by microorganisms, with the most common being chemoautotrophs. Both metallic and non-metallic materials can be affected by this corrosion, regardless of the presence or absence of oxygen.

(VIII) High-Temperature Corrosion

High-temperature corrosion, as the name implies, is a type of corrosion of materials (predominantly metals) caused by the heating process. Chemical degradation of metals can take place due to a hot atmosphere that contains gases such as oxygen, sulfur, or other compounds. These compounds are capable of oxidizing the materials (metals in this case) quickly. For example, materials used in car engines must be able to withstand extended periods at high temperatures during which they can be affected by an atmosphere containing corrosive products of combustion.

Corrosion: Examples, Reactions, and Effects

Here are some common examples of corrosion seen in metals:

1. Corrosion of Copper

When copper metal is exposed to the environment, it reacts with the oxygen in the atmosphere to form copper (I) oxide, which is red in color.

2Cu(s) + O2(g) → 2Cu2O(s)

Cu2O gets oxidized further to form CuO, which is black in color.

Cu2O(s) + 2 O2(g) → 2CuO(s)

CuO reacts with CO₂, SO₃ and H₂O (present in the atmosphere) to form Cu₂(OH)₂(s) (Malachite) which is blue in colour, and Cu₄SO₄(OH)₆(s) (Brochantite) which is green in colour.

This is why copper turns a bluish-green color when observed.

The statue of liberty is a typical example of a phenomenon where a copper coating turns blue-green in colour over time.

2. Silver Tarnishing

Silver reacts with sulphur and sulphur compounds in the air to form silver sulphide (Ag2S), which is black in colour. When exposed to the atmosphere, silver reacts with the H2S(g) present due to certain industrial processes, forming Ag2S.

2Ag(s) + H2S(g) → Ag2S(s) + H$_2^+$(g)

3. Rusting of Iron

Rusting of iron, which is the most commonly seen example, happens when iron comes in contact with air or water. This reaction can be seen as a typical electrochemical cell reaction, as illustrated in the diagram below:

")

At the anode position, metal iron (Fe) loses electrons and gets converted to Fe{aq}2+. The electrons then move to the cathode position, where they combine with H+ ions released by H2O or H2CO3 present in the atmosphere.

(\begin{array}{l}H_2O \longleftrightarrow H^+ + OH^-\end{array})

(\begin{array}{l}H_2CO_3 \rightleftharpoons 2H^+ + CO_3^{2-}\end{array})

The reaction of H+ and electrons produces Hydrogen, which then reacts with Oxygen to form H2O.

Anode Reaction

2Fe(s) → 2Fe²⁺ + 4e^–; (E_{\text{Fe}^{2+}/Fe}^0 = -0.44 \text{ V})

Cathode Reaction

(\begin{array}{l}{{O}_{2(g),}}+4{{H}^{+}}_{(aq)}+4{{e}^{-}} \overset{2}{\longrightarrow} {{H}_{2(l)}}{{O}_{(l)}} \\ {{E}^{o}}_{^{{{H}^{+}}}/{{O}_{2}}/{{H}_{2}}/O,,,}=1.23V \end{array})

Overall reaction:

2Fe(s) + O2(g) + 4H+(aq) → 2Fe2+(aq) + 2H2O(l) \\ E$_{o}$ cell = 1.67V

The Fe²⁺ ions formed at the anode react with oxygen in the atmosphere, thereby getting oxidised to Fe³⁺ and forming Fe₂O₃, which comes out in the hydrated form as Fe₂O₃.xH₂O.

2Fe<sup>2+</sup> + 3O<sub>2</sub> → 2Fe<sub>2</sub>O<sub>3</sub>

Fe2O3 + xH2O → Fe2O3\ xH2O (rust)

Also Check Out: Rusting of Iron and Prevention

Other examples include:

The reaction of Zinc with oxygen and HCl results in the formation of white coloured ZnCl2, which is a result of corrosion.

The corrosion of Tin forms black-coloured Na2[Sn(OH)2].

Causes and Effects

Corrosion can have a significant impact on many things. Not only does it lead to a waste of natural resources, but it can also cause hazardous situations, such as weakened and unstable structures, accidents due to corroded parts, and cracked pipelines, bridge collapses, transport vehicle crashes, and other catastrophes. It is essential to monitor and prevent corrosion at all costs.

Prevention of Corrosion

Preventing corrosion is of utmost importance in order to avoid huge losses, as most structures we see and use are made out of metals, such as bridges, automobiles, machinery, household goods like window grills, doors, railway lines, etc. To slow or prevent corrosion damage to these materials, which are often exposed to the weather, saltwater, acids, or other hostile environments, several treatments are used. Popular methods to prevent corrosion include:

Electroplating

Galvanization

Anodization

Passivation

Biofilm Coatings

Anti-Corrosion Protective Coatings

Painting and Greasing

Use of Corrosion Inhibitor or Drying Agents

Periodic Cleaning of Metal Surfaces

Learn More Here: How Can Corrosion Be Prevented?

Frequently Asked Questions on Corrosion

What is Corrosion?

Corrosion is the process of deterioration of materials due to chemical or electrochemical reactions with their environment. It is a natural process that can cause damage to metals, alloys, and other materials.

No, not all metals corrode. Some metals, such as gold and platinum, are inert and do not corrode easily.

No, all metals do not corrode. The metals which are higher in the reactivity series of metals corrode easily as their oxidation potential is high. For example, iron. What affects corrosion?

Metals on exposure to air or moisture (salt water) increase corrosion. What is the effect of temperature on corrosion? Temperature can affect corrosion in different ways depending on the environment. In general, higher temperatures can increase the rate of corrosion.

High-temperature corrosion is a type of corrosion that occurs when metals are exposed to high temperatures, leading to an increase in corrosion rate. This is due to the increased reactivity of metals at higher temperatures, which can accelerate the corrosion process.

High-temperature corrosion is a type of corrosion that occurs when metals are heated and corrode.