Corrosion Flashcards
What is corrosion?
Corrosion is the destructive and unintentional degredation of a metal 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 main effects of corrosion?
- Reduces strength
- Decreases operational life
- Beneficial properties of metals are lost
- Wastage of material
- Contamination of fluids in vessels and pipes
State and explain the four key features of corrosion.
- Anode - metal which loses electrons (oxidation). Negative terminal.
- Cathode - metal which gains electrons (reduction). Positive terminal
- Electrolyte - electrically conductive solution. Positive electricity passes from anode to cathode through electrolyte as cations.
- Metallic path - two electrodes connected externally by a metallic conductor. Current flows from + to - (which is the passge of electrons in the opposite direction)
What are the 4 characteristics of anodic reactions?
- Oxidation of metal to ion with a charge
- Release of electrons
- Shift to a higher valence state
- The process of oxidation in most metals results in corrosion
Name 3 common corrosion cells.
- Galvanic corrosion cells - dissimilar anodic/cathodic metals in an electrolyte
- Concentration cells - same metals in dissimilar conditions in a common electrolyte
- Electrolytic cells - formed when external current is introduced into the system. This is often created as a means of protecting the structure (anodic protection).
How are metals naturally protected from corrosion?
Most metals are naturally coated with an oxide, but the effectiveness of this layer in protecting the metals depends on:
- continuity of the film and how well it is bonded to the material
- conductivity of ions through the oxide film
What is the Pilling-Bedworth ratio?
The P-B ratio is the ability of the oxide to protect the metal from further oxidation which depends on the relative volumes of the oxide and metal.
What do the ranges of the P-B ratio represent?
P-B < 1 = oxide films tend to be porous and non-protective (e.g Mg)
1 < P-B < 2 = oxide films tend to be protective (e.g Al)
P-B > 2 = compressive stresses in the oxide film cause the coating to crack and flake off.
What 3 things does the rate of corrosion depend on?
- Chlorine content (chlorine penetrates through surface films)
- Oxygen availability (oxygen solubility increases as temp decreases)
- Temperature (mobility of ions within oxide film increases with temp)
List 6 common corrosion mechanisms.
- Corrosion fatigue
- Stress corrosion cracking (SCC)
- Pitting corrosion
- Galvanic corrosion
- Intergranular corrosion
- Hydrogen embrittlement
What is corrosion fatigue?
Corrosion fatigue is the result of local stess fluctuations owing to mechanical or thermal cycling. Dissolved oxygen is the main driving force for corrosion fatigue.
What is stress corrosion cracking?
SCC is the cracking of metals under the combined effect of stress and corrosion. The cracks branch and propagate intergranularyly.
Stress involved may be applied or residual.
Localised corrosion on the surface acts as a stress raiser which initiates cracks.
What are some solutions to avoid SCC?
- Reduce stress levels
- Heat treatment
- Atmospheric control
What are the symptoms and causes of pitting corrosion?
Symptoms:
- Aggressive localised corrosion of tube wall
- flooded ot non-brainable surfaces are more susceptible during outage periods
Causes:
- Exposure of a pipe to water with high acidic or oxygen concentrations
- Existence of close-fitting surfaces and deposits where differences in oxygen concentration can be produced
How would you prevent pitting corrosion?
- Weld instead of rivet
- Polish surfaces
- Add drains (avoid stagnant water)
- Adjust composition (e.g. add Mo to stainless steel)
