Corrosion of metals Flashcards
What is environmental degradation?
the degradation of one or
more material properties due to exposure to its
surrounding environment.
Name 2 examples of environmental degradation discussed in class.
- Degradation of steel due to exposure to water or salt water
- Degradation of a plastic due to exposure to UV light
What is corrosion?
Corrosion refers to the continued (unchecked)
electrochemical attack of a material, normally resulting in
oxidation of that metal.
What does corrosion convert?
Corrosion typically converts a
refined metal to a more chemically-stable form found in the
environment.
What is the driving force behind the corrosion of metals?
The driving force is a reduction in energy
What type of cell forms during corrosion?
An active electrochemical cell forms in which one part
of the metal surface serves as the anode and the other as the cathode
What type of force is required for oxidation to occur in corrosion?
Huge thermodynamic driving force (ΔG0 «0) for oxidation to occur, or
in essence, to return to its mineral state
Is the material destroyed in corrosion?
Material is not destroyed, but converted to a different form
What is the purpose of the galvanic series?
To rank the reactivity of metals and alloys in seawater at 25 degrees celsius.
How is the EMF series measured?
Measured relative to standard
hydrogen reference half cell)
When does galvanic corrosion occur?
Occurs when two different metals or alloys are:
- electrically coupled while
- exposed to an electrolyte
What is the key to understanding galvanic corrosion?
Galvanic series
What are the three major differences between the Galvanic Series and the EMF series?
Galvanic Series lists metals and alloys, whereas EMF series only lists metals
- Galvanic values measured experimentally, whereas EMF values calculated from
thermodynamic principles - Galvanic series accounts for effects of passive films, oxidation kinetics, and the environment
What are general methods of corrosion protection?
- Self-driven formation of a protective oxide
- Physical barriers (paint, noble coating)
- Sacrificial or cathodic protection
What are examples of metals and alloys that form a protective oxide layer to protect against corrosion?
Examples include stainless steel (contains at least 10% chromium, which
forms an oxide layer), some aluminum alloys, and titanium
How does noble coating work?
- Uses a less chemically active
metal layer than the main metal - Acts as a physical barrier to
inhibit contact with water and
oxygen
What metals are used for noble coating?
Often nickel, tin or chromium
Explain how sacrificial or cathodic coating works?
- Uses a more chemically active metal
layer than the main metal - The sacrificial coating acts as the
anode, and corrodes while the main
metal stays intact - This applies Galvanic Corrosion to
protect the metal used as the cathode
What is an example of sacrificial or cathodic coating?
- Ex. Zinc on Steel is known as
galvanized steel, with the steel dipped
in molten zinc.
What is the metal coupled to in sacrificial or cathodic protection?
Metal to be protected is electrically coupled to a sacrificial anode
What can the anode be made from in sacrificial or cathodic protection?
The anode can be made from a more active metal on the galvanic scale for passive protection
What is the anode driven by in sacrificial or cathodic protection?
Driven by an applied external power source to electrically bias the
electrodes correctly for protection
What are other means of reducing corrosion relating to tackling environmental factors?
-Reduce Temperature
-Remove reactants
-Add inhibitors to fluids
How does reducing temperature work to reduce corrosion?
Reducing the temperature slows the kinetics of oxidation and reduction.
