What is Corrosion?

Corrosion is defined as an activity that transforms refined metals into more chemically stable forms such as oxide, hydroxide, carbonate, or sulfide. It refers to the slow decomposition of things (typically metals); thanks to chemical and/or electrochemical reactions with their surroundings. Corrosion engineering is the science of preventing and controlling corrosion.

The science behind Corrosion

Corrosion Science is complex, and it belongs to the category of electrochemical phenomena. During corrosion, oxidation occurs at a specific location on an iron surface, which serves as an anode. The electrons released at this anodic site go through the metal to a specific point where they reduce oxygen in the presence of oxygen. The cation in water can also be accessed as a result of other acidic oxides dissolving in the atmosphere. This spot serves as a cathode.

Rust

Rust is a term that refers to a group of iron oxides and hydroxides that form when iron or iron alloys are exposed to oxygen and moisture for an extended period. The oxygen reacts with the metal over a while, producing new chemicals that are collectively known as rust.

The electrochemical process begins here. The reducing agent is iron, which gives up electrons, whereas the oxidizing agent is oxygen (gains electrons). As demonstrated by the effects of road salt on car corrosion, corrosion is influenced by water and accelerated by electrolytes. The reduction of oxygen is the most important reaction.

${\mathrm{O}}^{2+}+ 4{\mathrm{e}}^{- }+ 2{\mathrm{H}}_2\mathrm{O} \to 4{\mathrm{OH}}^{-}$

The presence of acid has a significant impact on this process because it produces hydroxide ions. At a low pH, the oxygen corrosion of most of the metals also accelerates. The oxidation of iron, which may be stated as follows, provides the electrons for the aforementioned reaction:

$\mathrm{Fe}\to {\mathrm{Fe}}^{2+ }+ 2{\mathrm{e}}^{-}$

$4{\mathrm{Fe}}^{2+}+ {\mathrm{O}}^{2-}\to 4{\mathrm{Fe}}^{3+} +2{\mathrm{O}}^{-}$

$$\begin{gathered} {\mathrm{Fe}}^{2+}+ 2{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{Fe}{\left(\mathrm{OH}\right)}_2 + 2{\mathrm{H}}^{+} \\ \mathrm{Fe}{\left(\mathrm{OH}\right)}_2\rightleftharpoons \mathrm{FeO} + {\mathrm{H}}_2\mathrm{O} \\ \mathrm{Fe}{\left(\mathrm{OH}\right)}_{3 }\rightleftharpoons \mathrm{FeO}\left(\mathrm{OH}\right) + {\mathrm{H}}_2\mathrm{O} \\ 2\mathrm{FeO}\left(\mathrm{OH}\right)\rightleftharpoons {\mathrm{Fe}}_2{\mathrm{O}}_3 + {\mathrm{H}}_2\mathrm{O} \end{gathered}$$

The availability of water and oxygen is also governed by the corrosion products, as shown by the preceding formulae. Iron(II)-containing minerals, such as FeO and black lodestone or magnetite, are preferred when dissolved oxygen is scarce (Fe3O4). Ferric materials having the nominal formula Fe3OH3-x Ox/2 are favored by high oxygen concentrations and a high-temperature environment. Rust appearance evolves, reflecting the slow rates at which solids react.

Corrosion rate

The Corrosion rate is the one at which any metal deteriorates in a given environment. It can also be described as the amount of corrosion loss in thickness each year.

Stress corrosion

When a material lives in a relatively inert environment yet corrodes owing to applied stress, it is called stress-corrosion. Externally applied or residual stress are also possibilities. Stress corrosion is a type of galvanic corrosion in which the material's strained portions are anodic to the material's unstressed areas.

• Galvanic corrosion occurs when ions are exchanged between two materials.
• Corrosion due to Stress Cracking - the formation of cracks in a caustic environment.
• Localized corrosion along the metal grain boundaries is known as intergranular corrosion.

Effects of corrosion

Embrittlement

Embrittlement is the loss of ductility of a substance, causing it to become brittle.

Corrosion prevention methods

Following are the various anti-corrosion methods:

Galvanization

Following are the main ways in which galvanizing can protect the underlying iron or steel:

• When the zinc coating is intact, it prevents corrosive chemicals from accessing the steel or iron beneath.
• Additional electroplating, such as a chromate conversion coating, may be added to the substrate material to give additional surface passivation.
• As a sacrificial metal, zinc serves as a sacrificial anode to preserve the underlying iron/steel. Protection can be maintained even if the underlying metal is exposed as long as there is zinc close enough to be electrically linked.
• It has a protective coating of Zinc over the metal

Anodizing

Anodizing is an electrolytic passivation process to thicken the natural oxide layer on the surfaces of metal components. As the item to be treated serves as the anode electrode in an electrolytic cell, the procedure is termed anodizing. Anodic films can also be utilized for a variety of aesthetic effects, such as absorbing dyes or adding reflected light wave interference effects with thick porous coatings.

Cathodic protection

Cathodic protection (CP) is a corrosion control technique that involves making a metal surface the cathode of an electrochemical cell. The metal to be protected is connected to a more readily corroded "sacrificial metal" that serves as the anode in a simple technique of protection. Instead of the shielded metal, the sacrificial metal corrodes.

Sacrificial anode protection

The potential of the steel surface is polarized more negatively for effective CP until the metal surface has a uniform potential. The driving power for the corrosion reaction is terminated when the potential is uniform. The anode material in galvanic CP systems corrodes under the effect of the steel and must be changed at some point.

Dealloying

Selective leaching, also known as dealloying is a kind of corrosion that occurs in some solid solution alloys when a component of the alloy is preferentially leached from the material under certain circumstances. A microscopic-scale galvanic corrosion mechanism removes the less noble metal from the alloy.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors of Science in Chemistry
• Bachelors of Science in Bio-Chemistry
• Masters of Science in Chemistry
• Masters in Inorganic Chemistry

Practice Problems

1. What kind of elements returns to their original oxidation states by the process of corrosion?
   a. metals
   b. non-metals
   c. metalloids
   d. alloys
   Ans: Option a

Explanation- Corrosion is a process through which treated metals return to their original oxidation states. The metal is oxidized by its surroundings, which are generally oxygen in the air, in this reduction-oxidation process.

2. Inclusion of which element in stainless steel alloys creates a passive coating that shields the underlying material from corrosion and may self-heal when exposed to oxygen?

a. Chromium

b. Stainless steel

c. Steel

d. Iron

Ans: Option a

Explanation-The inclusion of metallic chromium in stainless steel alloys creates a passive coating that shields the underlying material from corrosion and may self-heal when exposed to oxygen to make it corrosion-resistant.

3. What is the effect of anodizing on the metal surface due to the process of electrolytic passivation?

a. It deepens the natural oxide layer

b. It decreases the natural oxide layer

c. It depletes the natural oxide layer

d. None of the above

Ans: Option A

Explanation- Anodising is a process of electrolytic passivation that deepens the natural layer of oxide on the metal component surface.

4. Which of the following term is used to describe any metal deteriorates in a given environment?

a. corrosion rate

b. corrosion period

c. corrosion data

d. all of the above

Ans: Option A

Explanation-The corrosion rate is the one at which any metal deteriorates in a given environment. It can also be described as the amount of corrosion loss in thickness each year.

5. Which of the following is a corrosion control technique that involves making a metal surface the cathode of an electrochemical cell?

a. Cathodic Protection

b. Anodizing

c. Anode Protection

d. Dealloying

Ans: Option A

Explanation-Cathodic protection (CP) is a corrosion control technique that involves making a metal surface the cathode of an electrochemical cell.

Question itself

1. What is the corrosion process?

Answer: Corrosion is a process through which treated metals return to their original oxidation states. The metal is oxidized by its surroundings, which are generally oxygen in the air, in this reduction-oxidation process. Both electrochemically and spontaneously, this reaction is preferred.

2. What is a way to manage corrosion?

Answer: When corrosion is detected, the only way to ensure that it is repaired is to have it removed. Light surface corrosion can be eliminated by abrasion (the specifics of which are dependent on the metallurgy of the corroded component), followed by a corrosion inhibitor, such as zinc-chromate primer, another primer, and lastly paint.

3. How do you keep metal from corroding?

Answer: By removing one of these variables, corrosion can be prevented. A metal surface that has been painted or enameled forms a barrier between the metal and the moisture in the air. The process of covering a metal surface with another metal that is more prone to oxidization is known as the sacrificial coating. These are the various anti-corrosion methods employed.

Common mistakes

Never get confused between rust and corrosion.

Corrosion is a kind of oxidation, whereas rusting is a component of corrosion. The primary distinction between corrosion and rust is that the former is caused by chemical impact and affects a wide range of materials, whereas the latter is only enhanced by particular chemicals and generally affects iron-based materials.