corrosion and control

What is corrosion?

Corrosion is the spontaneous destruction or deterioration of a metal or alloy through chemical or electrochemical interaction with its surrounding environment.

How is corrosion related to metallurgy?

Corrosion can be seen as the reverse process of extractive metallurgy.

dry corrosion

Occurs due to the direct chemical reactions between the environment and the metals / alloys

wet corrosion 

Wet corrosion occurs due to the existence of separate anodic and cathodic areas, between which current flows through the conducting solution

types of dry corrosion

Oxidation (due to reaction with oxygen)

Other gases (CO2 , H2S, SO2 , X2 , etc.)

Liquid metal

due to reaction with oxidation 

occurs when metals are attacked by dry oxygen Metal + Oxygen→ Metal oxide (corrosion product) Nature of the oxide film decides subsequent corrosion

• a stable film is formed ex. Al, Sn, Pb, and Cu

• Unstable metal film decomposes back into the metal and oxygen. ex. Ag, Au, Pt.

• Volatile film layer volatilizes as soon as it is formed, thereby accelerating the corrosion.

ex. MoO3

corrosion by other gases

Due to some gases SO2 , Cl2,CO2 , and H2S, depends mainly on the chemical affinity(ability to react with the metal) between the metal and the gas involved. Eg. dry Cl2 attacks silver metal and forms AgCl as a thin protective and non-porous layer on the metal. As a result of this protective layer on the metal surface, the intensity of corrosion decreases.

Liquid metal corrosion

this occurs when a molten liquid (like a hot, melted metal or salt) flows over a solid metal surface (or alloy), two things can happen:

1. Dissolution:
   The solid metal starts dissolving into the molten liquid — like sugar dissolving in hot tea.

2. Penetration:
   The molten liquid seeps into the solid metal’s surface — like water soaking into a sponge.

This weakens the metal.