Corrosion Flashcards
What is Corrosion?
Corrosion is the destructive and unintentional degradation of a material caused by its environment.
For metals, almost all environments can cause corrosion
to some degree, since the corroded state is more stable.
What are the effects of corrosion?
- Reduces strength
- Decreases operational life
- Metallic properties are lost
- Wastage of metal
- Contamination of fluids in vessels and pipes
What is the corrosion mechanism?
The anode oxidises and loses electrons.
The cathode reduces to gain electrons.
What is the Pilling-Bedworth ratio?
The ability of the oxide to protect the metal from further
oxidation and it depends on the relative volumes of the oxide and metal.
P-B Ratio = (A(0) x Rho(m))/(A(m) x Rho(0))
P-B < 1 - These oxide films tend to be porous and non-protective. Magnesium is an example.
1 < P-B < 2 - These oxide films tend to be protective.
One example is aluminium.
P-B > 2 - Compressive stresses in the oxide film
cause the coating to crack and fall off.
What increases the corrosion rate?
Higher temperatures
What does the effectiveness of the oxide layer in protecting the metal from oxidation depend on?
a. The continuity of the oxide film
b. How well is the oxide film bonded to the metal
c. The conductivity of ions through the oxide film
The corrosion rate depends on?
a. The temperature
b. The chloride content
c. Oxygen availability
How do you avoid galvanic corrosion?
a. Choose metals that are close in the galvanic series
b. Insulate dissimilar metals
3 Most important things?
- The Pilling-Bedworth ratio provides a measure
of the oxide’s ability to protect an alloy from further
oxidation: 1 < PB < 2 - Corrosion mechanisms: corrosion fatigue, SCC, oxygen
pitting, Intergranular corrosion, hydrogen embrittlement - There are diverse protection methods to inhibit the
corrosion process: coatings, alloying, electrochemical
methods and design
