Corrosion 1 &2 Flashcards
Magnesium (Mg)
Magnesium produces a porous oxide film, denoted by MgO. This type of oxide film is not very protective.
Aluminum (Al)
Aluminum forms a protective, adherent, nonporous oxide film, denoted by Al2O3. This type of oxide film is very protective.
Iron (Fe)
Iron forms an oxide film that spills off the surface and provides poor protection. This type of oxide film is denoted by FeO.
What is liquid metal corrosion?
Liquid metal corrosion is the deterioration of a solid metal or alloy caused by the chemical action of flowing liquid metal at high temperatures.
What are the two mechanisms of liquid metal corrosion?
The two mechanisms of liquid metal corrosion are:
Dissolution of the solid metal by the liquid metal
Internal penetration of the liquid metal into the solid metal, weakening it.
Types of Dry or chemical corrosion
Oxidation corrosion
Corrosion by other gases corrosion
Liquid metal corrosion
Type of Wet or Electrochemical corrosion
Galvanic
Concentration cell
Electrochemical corrosion by evolution of hydrogen and absorption of oxygen
What is the Pilling-Bedworth ratio (P-B ratio)?
The P-B ratio is a concept used in corrosion of metals. It’s the ratio of the volume occupied by one unit cell of the metal oxide formed during corrosion to the volume occupied by one unit cell of the original metal.
Formula for Pilling-Bedworth Ratio
PB = VM(oxide) / VM(metal)
RPB: Pilling-Bedworth ratio
VM: Molar volume (V = M / ρ)
M: Atomic/molecular mass
ρ: Density
n: Number of metal atoms per molecule of oxide
How does the P-B ratio predict corrosion behavior?
The ratio influences the characteristics of the oxide film:
Ratio > 1: The oxide layer provides protective effect
Ratio < 1: The oxide layer is thin, likely broken, and provides no protection.
Ratio between 1 and 2: The outcome is less clear-cut and depends on other factors.
Examples of Pilling-Bedworth Ratio and Oxide Formation
Magnesium (Mg): P-B ratio < 1, forms a porous oxide film (MgO) that offers little protection.
Aluminum (Al): P-B ratio ≈ 1, forms a thin, protective, and adherent oxide film (Al2O3).
Iron (Fe): P-B ratio > 1, forms a thick oxide film (FeO) that cracks and flakes off, providing poor protection.
Essential Conditions for Electrochemical Corrosion
Formation of Anodic and Cathodic Areas: The metal surface develops regions where oxidation (anode) and reduction (cathode) reactions take place.
Presence of a Conducting Medium (Electrolyte): An electrolyte, like saltwater or a conducting solution, allows ions to flow and complete the electrical circuit.
Corrosion of Anodic Areas Only: Metal atoms at the anode lose electrons and are oxidized, leading to corrosion. The cathode itself doesn’t corrode.
Formation of Corrosion Products: The products of the anodic and cathodic reactions can combine to form corrosion products, which may accumulate near either area.
What is the Electrochemical Theory of Corrosion?
This theory explains how metals corrode due to electrochemical reactions with moisture and oxygen in the atmosphere.
Reactions in Electrochemical Corrosion
Anodic Reaction (Oxidation): Metal atoms at the anode lose electrons and become ions (M → M+ + e-). This is the corrosion process itself.
Cathodic Reaction (Reduction): Electrons released at the anode travel through the metal and are consumed at the cathode, where they react with available species (e- + X → Reduced Product).
Dry Corrosion (Chemical Corrosion)
Occurs in dry environments (no liquid water)
Corrosion is typically uniform across the metal surface
A slow process compared to wet corrosion
Involves direct chemical attack of the metal by gases (e.g., oxygen)
Explained by the absorption mechanism: gases are absorbed onto the metal surface and react
