Corrosion Flashcards
What is dry corrosion?
Direct attack on metals by oxygen at low or high temperatures, in the absence of moisture. It leads to oxidation corrosion. The resultant metal oxide is known as a corrosion product.
What is anode reaction for dry corrosion?
M = M(2+) + e(-)
What is cathode reaction for dry corrosion?
(1/2) O2 + 2e(-) = O(2-)
What is overall reaction for dry corrosion?
M + 1/2 O2 = M(+2) + O(-2) = MO
What are the types of films resulting from dry corrosion?
Stable, unstable, porous, and volatile.
What is a stable film?
Stable oxide layer adhered tightly to the metal surface. They act as a protective coating and prevent further corrosion. (Example: Pb, Cu, etc.)
What is a unstable film?
Metals decompose reversibly leading to an unstable films - continuous corrosion. (Example: Au, Ag, etc.)
What is a porous film?
Cracks in the surface of film that allow oxygen to pass through thereby allowing continuous corrosion. (Example: Li, Na, etc.)
What is a volatile film?
Film evaporates as soon as it forms exposing fresh metal surface to further attack. (Example: Mo etc.)
What is specific volume ratio?
Volume of metal oxide film formed / volume of metal ion consumed
What is the condition for stable porous film?
Specific volume ratio > 1
What is the condition for unstable porous film?
Specific volume ratio < 1
What is Pilling and Bedworth Rule.
Specific volume ratio > 1 for stable porous film and < 1 for unstable porous film.
What gases (other than oxygen) can also cause corrosion?
Cl2, SO2, H2, S, CO2, F2, and NOx
What is the formula for corrosion of silver by chlorine?
Cl2 + 2Ag = 2AgCl
What is the formula for corrosion of tin by chlorine?
Sn + 2Cl2 = SnCl4
What is electrochemical corrosion?
Two dissimilar metals are partially dipped in an electrolyte.
What is anode mechanism for electrochemical corrosion?
M = M(n+) + ne(-) (Oxidation)
What are the main mechanisms for cathode in electrochemical corrosion?
- Hydrogen evolution mechanism
2. Oxygen absorption mechanism.
What is Hydrogen evolution mechanism?
Occurs in an acidic environment to all metals
above hydrogen in electrochemical series
What is Oxygen absorption mechanism?
Occurs in the presence of neutral electrolyte
(NaCl) and atmospheric oxygen.
What is galvanic / dissimilar metal corrosion?
Refers to corrosion damage induced when two dissimilar materials are coupled in a
corrosive electrolyte.
What are the conditions for galvanic corrosion?
- Electrochemically dissimilar metals.
- Metals must be in electrical contact.
- Metals must be exposed to an electrolyte.
What is Differential Aeration Corrosion / (concentration-cell corrosion)?
Occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution or different concentrations of dissolved oxygen in the electrolyte.
