What is corrosion?
Corrosion is the destructive and unintentional degradation of a material caused by its enviroment
What are the four key features of corrosion
- Anode
- Cathode
- Electrolyte
- Metallic Path
What are the 5 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 anode?
The metal where electrons are released at the anode, which is the more reactive metal
What is the cathode?
The metal which gains the electrons
What happens in oxidation?
The metal atoms lose electrons
What happens in reduction?
The surrounding environment gains electrons
What is the electrolyte?
The electrically conductive solution that must be present for corrosion to occur
What is the metallic path?
The two electrodes are connected by a metallic conductor
What are the four characteristics of an anodic reaction?
- Oxidation of metal to an ion with a charge
- Release of electrons
- Shift to a higher valence state
- The process of oxidation in most metals results in corrosion
What are three common corrosion cells?
- Galvanic corrosion cells
- Concentration cells
- Electrolytic cells
Name four corrosive atmospheres
- Industrial
- Marine
- Rural
- Indoor
What is the equation for the Pilling-Bedworth ratio?
P-B Ratio = AoPm/AmPo
What can be said about the oxide film for metals with a P-B ratio of < 1, < 2 and > 2
< 1 - tend to be porous and non-protective
< 2 - tend to be protective
> 2 - Compressive stresses present
What 3 factors does the corrosion rate depend on?
- Chloride content
- Oxygen Availability
- Temperature
What are the 7 common corrosion mechanisms?
- Corrosion fatigue
- Stress corrosion cracking (SCC)
- Oxygen pitting corrosion
- Galvanic attack
- Caustic attack
- Intergranular corrosion
- Hydrogen embrittlement
What are 4 protective measures?
- Protective coatings (Anodic coatings, Cathodic coatings)
- Alloying (Shifting electrode potential, protective surface film)
- Electrochemical methods (Sacrificial anode)
- Design
Do metals corrode faster at a higher or lower temperature?
Higher temperature
What 2 things should be done to avoid galvanic corrosion?
- Choose metals that are close in the galvanic series?
2. Insulate dissimilar metals
