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.
What is an example of reducing reactants to reduce corrosion?
Removing oxygen gas to create a more reducing environment
What is an example of how adding inhibitors to fluids reduces corrosion?
E.g. coolant additives in automotive cooling systems (radiators) provide corrosion prevention and protection against freezing.
Why does rust flake?
Two reasons for flaking: Volume change and chemical characteristics of iron differ from original iron
- Volume change (expansion)
- Poor adherence to the underlying metal
In galvanic corrosion, what happens to the more reactive metal and what does it act as?
The more reactive metal is oxidized and acts as the anode. It corrodes.
In galvanic corrosion, what happens to the less reactive metal and what does it act as?
The less reactive metal is redued and acts as the cathode. It is protected.
Why did the aluminum foil corrode
when coupled with stainless steel?
Aluminum is more reactive (anodic) than
stainless steel, as shown in Galvanic series
Why will the lasagna cell “fail” if the
aluminum foil does not contact the
ketchup and salt?
Metals have to be coupled to allow for flow
of electrons, and an electrolyte present to
conduct ions.
Which type of metals and alloys are more anodic and which ones are more cathodic?
More inert (less reactive) metals are more cathodic; more anodic metals are active & reactive.
What is the disadvantage of noble coating?
Prone to physical or chemical
damage, like paint or film
In the “lasagna cell” example, what is the electrolyte?
Ketchup and salt
What is stress corrosion?
Stress corrosion can promote crack
propagation in components that
are under stress
Where are stress corrosion cracks formed?
Wherever the tensile stresses are the greatest.
What type of substance can contribute to crack propagation in stress corrosion?
Metal oxide can form at crack tip
or crack vicinity and contribute to
crack propagation
When does a protective oxide coating form?
For some metals and alloys, a protective oxide coating forms upon initial
exposure to oxygen
How does the protective oxide layer protect against further corrosion (Written Response) ?
When the oxide adheres to the surface and is dense enough to limit
oxygen transport to the interior, it acts as a passive protective layer
What chemical reaction occurs when a protective oxide layer is formed?
Oxidation
What “ingredients” or conditions are required for corrosion to occur (Hint: There are 4 ingredients)
A. Reduction in energy: thermodynamic driving force
B. Medium(water or moisture) for ion
conduction and ionic transport
C. Source of oxidant (normally O2, sometimes sulfur)
D. Mass transport of oxidant to the metal surface
What is oxidation?
The process where an atom gives up an electron (s) to become a cation. It occurs at the anode.
What is reduction?
The process where an atom acquires an extra electron (or electrons) and becomes an anion.
What is the function of seawater in corrosion of aluminium and steel?
To provide an electrolyte for corrosion to occur.
