Lecture 3.101- Introduction to Corrosion

“It is better to control rather than to prevent corrosion, since it is impossible to eliminate corrosion”

 Corrosion is a general term applied to the deterioration of a metal (loss of metal), as a result of its reaction with the gaseous or liquid surrounding.

Metal ores   Metallurgical processes or extraction of metals   —->  Metal                                                                                       Metal ores

Electrochemical theory of corrosion

According to this theory, corrosion of theory in aqueous solution in a two step process

  • Oxidation
  • Reduction

At the surface of metal we can consider it as a series of small voltaic or galvanic cells. Now here onwards we have this theory as a simple example of mechanism of a galvanic cell working as we studied earlier in (1.102 Mechanism of a working cell).

Now depending upon the pH of the medium the mechanism is classified into two categories hydrogen evolving or oxygen absorbing but there is much of a difference so we will here study one in detail and relate in to the other one

  • Absorption of oxygen type

 At Anode: Since this is the oxidation part the metal (say Fe) oxidizes and leaves free electrons

Fe       ——->           Fe+2 + 2e–   (oxidation of metal at anode)

The metal ion is thus dissolved into the electrolyte or whatever medium it has where are metal being a good conductor the electrons travels to the cathode part, so here we simply have a current flowing from one part of metal to the other.

At Cathode: The electron comes in contact with the moisture and oxygen in the surrounding proceeds to a reaction forming hydroxide ions

                    ½ O2 + 2e +H2O      ———–>         2OH  (reduction of oxygen gas)

 When these are formed the diffuse and form ferrous hydroxide which is precipitated

Fe+2  + 2OH  ——–>   Fe(OH)2 (s)

If sufficient oxygen is supplied the reaction proceeds as

4Fe (OH)2 + O2 + 2H2O    ——–>   Fe(OH)3 (ferric hydroxide)

The product called yellow rust actually corresponding to (Fe2O3.H2O)

In limited supply of oxygen we have black anhydrous magnetite (Fe3O4).

  • Why does rust always form at cathode?

Concluding from the reactions only the both ions Fe+2 , OH are formed by anode only but since the ferrous ions are moving freely inside the metal lattice and electrolyte as soon as the hydroxide ions is formed the reaction proceed due to the faster moment of smaller metal ions rust majorly occur at cathode.

c1c2

Figure 1 – The first picture is of the process of corrosion by absorption of oxygen methods whereas the second is by hydrogen gas evolving process note that the ratio of anodic to cathode are inverse in both the diagram.

  • Evolving Hydrogen gas

This is corrosion type happening in acidic medium where we have sufficient amount of H+ ions,

The process at anode is same just at the cathode reactions are different

Due to acidic surrounding, lack of sufficient hydroxide ions,

2H+  +  2e ——>   H2 (g)

So there the overall reaction     :

Fe + 2H+ ——->   Fe+2  +  H2 (g)   (Displacement reaction)

Since the above is a displacement reaction we can conclude that only those metals can be participating in such reaction which are more reactive or placed higher that H in electrochemical series.

NOTE: – The anodes are usually very large area, where as cathode are smaller in this type of corrosion. 

  • Are Galvanic and electrochemical series same?

Electrochemical series does not account for the corrosion of all metal and alloys. Consequently, a more practical series, called galvanic series have been prepared by studying the corrosion of metals and alloys in a given environment e.g. sea water, thus galvanic series give real and useful information for the corrosion of metal and alloy.

It’s from top as most active (or anodic) to bottom as noble (or cathode) in galvanic series.

In simple language the electrochemical series predicts the relative displacement tendencies whereas galvanic series predicts the relative corrosion tendencies.

Abhishek kumar jha

(Chemistry at Utkarshini)

                                       //  notes.doc to save file3-101introduction-to-corrosion

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