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Learning Objectives:

Define corrosion.
Understand some basic terminology related to the corrosion.

Corrosion: Corrosion is the natural process of materials deteriorating when exposed to environmental factors, such as moisture, oxygen, and chemicals. It involves the transformation of a metal into a more stable compound, often an oxide, sulfide, or carbonate.
Anode: The anode is the electrode where oxidation (corrosion) occurs in an electrochemical cell. It is the site where metal atoms lose electrons and enter the solution as ions.
Cathode: The cathode is the electrode where reduction (protection against corrosion) occurs in an electrochemical cell. It is the site where electrons are gained by substances in the solution, preventing corrosion.
Galvanic Corrosion: Galvanic corrosion, also known as bimetallic corrosion, occurs when two dissimilar metals are in direct contact with each other and exposed to an electrolyte (usually a liquid). The less noble (more reactive) metal corrodes while the more noble (less reactive) metal remains protected.
Electrolyte: An electrolyte is a substance, typically a liquid or solution, that allows the flow of electric current between the anode and cathode in a corrosion cell. Common electrolytes include water, saltwater, and acids.
Passivation: Passivation is the process of intentionally forming a protective oxide layer on the surface of a metal to prevent further corrosion. Stainless steel, for example, passivates to develop a thin, stable chromium oxide layer.
Pitting Corrosion: Pitting corrosion is a localized form of corrosion characterized by small pits or holes in the metal surface. It can be highly destructive and is often initiated by small imperfections or inclusions on the metal's surface.
Corrosion Rate: The corrosion rate is the measure of how quickly a metal corrodes, typically expressed in units like millimeters per year (mm/yr) or mils per year (mpy).
Corrosion Inhibitor: Corrosion inhibitors are chemical compounds added to an environment to reduce or prevent corrosion. They work by forming a protective film on the metal surface or altering the electrochemical reactions.
Crevice Corrosion: Crevice corrosion occurs in confined spaces or crevices, where stagnant solutions can form, leading to localized corrosion. It is common in joints, gaskets, and under deposits.
Rust: Rust is a common form of corrosion in iron and steel. It is the reddish-brown flaky coating formed on the surface due to the oxidation of iron in the presence of oxygen and moisture.
Corrosion Control: Corrosion control involves methods and techniques to prevent, mitigate, or manage corrosion, such as the use of protective coatings, cathodic protection, and proper material selection.
Corrosion-resistant Material: Corrosion-resistant materials are substances or alloys engineered to resist corrosion effectively in specific environments. Examples include stainless steel, titanium, and certain plastics.

Solved Example: 58-1-01

Indicate the false statement about corrosion.

A. Plastics and ceramics are immune to many forms of corrosion because they are not good conduction of electricity.

B. The corroded member in a corrosion cell is the cathode.

C. Passivity is a prerequisite for the corrosion protect on many metals.

D. Corrosion of metals is usually electrochemical in nature.

The corroded member in a corrosion cell is the anode.

Correct Answer: B

Solved Example: 58-1-02

Electrochemical corrosion takes place on:

A. Anodic area

B. Cathodic area

C. Near cathode

D. Near anode

Correct Answer: A

Learning Objectives:

To understand why and how corrosion occurs.

Oxidation takes place at the surface of the metal forming metal ions \[\large M^{n+}\] \[\large M^{0}\rightarrow M^{n+}+n e^{-}\] Oxygen is converted to oxide ion due to the transfer of electrons from metal. \[\large \frac{n}{2}O_{2} +2n e^{-}\rightarrow nO^{2-}\] The overall reaction is of oxide ion reacts with the metal ions to form metal oxide film. \[\large 2M + \frac{n}{2}O_{2}\rightarrow 2M^{n+}+ nO^{2-}\]

Solved Example: 34534

The pH level of a solution impacts corrosion rates. A lower pH indicates:

A. More alkaline conditions

B. More acidic conditions

C. Neutral conditions

D. None of the above

Correct Answer: B

Solved Example: 34543

Galvanic corrosion occurs when:

A. Two metals come into contact in the presence of an electrolyte.

B. A metal is exposed to moisture alone.

C. Metals are coated with non-metallic substances.

D. Temperature is increased significantly.

Correct Answer: A

Solved Example: 58-2-01

Chemical formula of Rust is:

A. $Fe_2O_3$

B. $FeO$

C. $Fe_3O_4$

D. $Fe_2O_3.XH_2O$

Correct Answer: D

Solved Example: 58-2-02

Which of the following cathodic reaction does not occur due to release of electrons at the anode?

A. Oxygen absorption

B. Hydrogen evolution

C. Electrodialysis

D. Electroplating

Oxygen absorption, hydrogen evolution and electroplating occur due to release of electrons at the anode.

Correct Answer: C

Solved Example: 58-2-03

Electrochemical corrosion can occur only if,

A. Oxygen is present in contact with metal

B. Air is present in contact with metal

C. Liquid medium is in contact with metal

D. None of the above are present

Correct Answer: C

Learning Objectives:

Given an oxidation reaction and a reduction reaction specify the overall electrochemical reaction.
Understand standard oxidation potential for corrosion reactions.
Describe the galvanic and EMF series.

EMF Series:

The EMF series is an arrangement of various metals in the order of their electrochemical activities based on their standard oxidation-reduction potentials($E_0$).
The most active metal in the series will be having a high negative standard potential while nobler metals possess relatively less negative (or more positive) standard potential ($E_0$). If we consider a couple of two metals in the EMF series, the one with higher negative $E_0$ will act as anode (and will corrode) compared to the other with a relatively less negative $E_0$ value (cathode).

EMF series lists only metals (little engineering application). Alloys not included.
Electrode potentials listed calculated from thermodynamic principles (corrosion potentials are more relevant).
Equilibrium potentials with concentrations at unit activity (Exact prediction of galvanic coupling not possible).
Predicts only tendency to corrode (Role of passive films and oxidation kinetics not predicted).
Effect of environment not predicted (Eg: Sn- Fe couple as in Tin cans)

Exceptions:

There are a few exceptions to the predicted activity of a metal (or couple) as arranged in the EMF series.

e.g.: Aluminum exhibits higher corrosion resistance due to Al$_2$O$_3$ layer present on surface.
Chromium exhibits stable Cr$_2$O$_3$ layer and is used as alloying element for corrosion resistance in stainless steels.

Galvanic Series:

Instead of standard electrode potentials, actually measured rest potentials of metals and alloys in a given environment arranged with respect to nobility and activity.
Practically measured potentials vs reference electrode.
Effect of coupling of metals and alloys on corrosion rate can be predicted. Certain anomalies e.g.: Stainless steels (active and passive) Galvanic series is generally good for stagnant conditions and not for turbulent conditions.

Difference between EMF Series and Galvanic Series:

Source: https://www.mechstuff4u.com/2017/03/difference-between-electrochemical.html

Electrochemical series is an orderly listing of standard half-cell electrode potential.
Galvanic series is an orderly listing of corrosion potential for different metals and alloys.

They both are based on thermodynamic analysis.

Electrochemical Series:

Electrode potentials are measured by dipping pure metals in their salt solution of IM concentration, without any oxide films on them.
The position of a metal in electro-chemical series is fixed.
It gives no information regarding positions of alloys.
The position of a metal in this series is permanently fixed.
This series comprises of metals and non-metals.
It predicts the relative displacement tendencies.
It is absolute.
It is quantitative.
it is a series only for pure metals.
It is used for theoretical calculations.

Galvanic Series:

This series was developed by studying corrosion of metals and alloys in unpolluted sea-water, without their oxide films, if any removed.
The position of a given metal may shift in galvanic series.
Since alloys are included in galvanic series, so their corrosion can be studied from this series.
The position of a metal, when present in the form of an alloy, is different from that of pure metal.
This series comprises of metals and alloys.
It predicts the relative corrosion tendencies.
It is relative.
It is qualitative.
It is a series for pure metals and alloys also.
It is used for practical applications.

Tugsataydin, CC BY-SA 4.0, via Wikimedia Commons

Solved Example: 4353

The electrochemical series ranks metals based on:

A. Their density

B. Their reactivity

C. Their thermal conductivity

D. Their melting points

Correct Answer: B

Solved Example: 58-3-01

When Pt and Co are electrically connected, which one gets corroded:

A. Pt

B. Co

C. None

D. Can’t decide

Correct Answer: B

Solved Example: 58-3-02

Which of the following can be used for cathodic protection:

A. Al

B. Cd

C. Cu

D. Cubic structure

Correct Answer: A

Solved Example: 58-4-01

Galvanic series gives real and useful information regarding the:

A. Electrical conductivity of metals and alloys

B. Thermal conductivity of metals and alloys

C. Thermal conductivity of metals and alloys

D. Oxidation power of metals and alloys

Galvanic series gives real and useful information regarding the corrosion behaviour of metals and alloys in a given environment, i.e. which metal will corrode at faster rate and which will not corrode at all.

Correct Answer: C

Corrosion Mechanisms and Control