Advanced Corrosion Flashcards
Corrosion Definition
Deterioration of a substance, usually metal, or its properties because of a reaction with its environment
What is corrosion?
Corrosion is the reverse process of steel manufacturing
What is passivation?
The formation of a protective oxide film on the surface reducing its chemical activity and its ability to corrode.
What elements are in the corrosion cell?
- Anode
- Cathode
- Return Path (Metallic Pathway)
- Electrolyte
What is an anode?
Part of the metal that corrodes (i.e. dissolves in the electrolyte)
What is the cathode?
The more noble region on the electrode where the electrons are consumed
What is the return path (metallic pathway)?
Connection of the anode and cathode and allows passage of the electrons, generated at the anode, to the cathode
What is the eletrolyte?
A medium that conducts ionic (rather than electron) current
How does oxygen affect the rate of corrosion?
Oxygen increases the rate of corrosion.
How does temperature affect the rate of corrosion?
Corrosion usually accelerated with increasing temperature.
How does chemical salts affect the rate of corrosion?
Increase the rate of corrosion by increasing the efficiency of the electrolyte.
How does humidity (or wetness) affect the rate of corrosion?
The wetter the environment, the more corrosion is likely to occur.
How does pollutants or acid gases affect the rate of corrosion?
Acid rain, chemical byproducts, and chlorides all promote corrosion.
What are the two broad types of corrosion?
General & Localized
What is general corrosion?
Relatively uniform loss of material
What is localized corrosion?
Corrosion that occurs at discrete sites on the metal surface. Areas adjacent are normally corroded to a much lesser extent, if at all.
What is pitting corrosion?
Corrosion that does not proceed uniformly but primarily at distinct spots
What is crevice corrosion?
Occurs on a metal surface that is shielded from full exposure to the environment because of the close proximity of another material that forms a narrow gas between substances
Of the two broad classifications of corrosion, what type represents the most significant in terms of potential for unplanned maintenance?
Localized
What is galvanic corrosion?
An electrochemical reaction of two dissimilar metals in the presences of an electrolyte and an electron conductive path, which occurs when dissimilar metals come into contact.
What is cathodic protection?
The reduction or elimination of corrosion by making the structure to be protected a cathode by means of an impressed current or attachment to a galvanic anode.
What are the two types of cathodic protection systems?
- Galvanic
- Impressed current
What materials are suitable for use as galvanic anodes?
- Aluminum
- Magnesium
- Zinc
What is an impressed current system?
An external source of direct current power is connected (or impressed) between the structure to be protected and the ground bed.
What are the various materials are for impressed current anodes?
- Scrap Steel
- Graphite
- Iron Oxide
- High-silicon chromium-bearing cast iron
- Platinized niobium & mixed metal oxide titanium
Name impressed current power sources
- Rectified commercial power
- Solar cells
- Generators
- Fuel cells
- Wind powered cells
- Thermoelectric cells
What is cathodic disbondment?
The separation of the coating from the surface through hydroxyl (OH-) formation due to increased (made more negative) potential.
Separation of coating due to increased current
The formation of a thin surface layer on metals, whether an oxide, carbonate, or other compound, that slows the rate of corrosion process is called?
Passivation
The principal component of mild carbon steel is __________?
Iron (Fe)
Which is the part of the corrosion cell where metal dissolves?
Anode
Which of these elements is likely to accelerate corrosion on a steel structure?
A. Salts
B. Low temperature
C. High temperatures
D. Pollutants
A. Salts
C. High temperatures
D. Pollutants
In a cathodic protection system the metal being protected is the _________?
A. Anode
B. Cathode
C. Ground bed
D. Rectifier
B. Cathode
When parts of a metal surface are shielded from full exposure to the environment because of the close proximity of another metal, corrosion at that location is identified as __________?
A. Fretting corrosion
B. Cavitation-erosion corrosion
C. Pitting corrosion
D. Crevice corrosion
D. Crevice corrosion
Cathodic protection using impressed current is most commonly used on __________?
A. Off-shore drilling platforms
B. High voltage transmission towers
C. Pipelines
D. Trucks
C. Pipelines
Impressed current ground bed anodes are likely to include ______?
A. Graphite
B. Zinc
C. High silicon cast iron
D. Aluminum
A. Graphite
C. High silicon cast iron
