Corrosion of metals

Question 1

What is environmental degradation?

Answer 1

the degradation of one or
more material properties due to exposure to its
surrounding environment.

Question 2

Name 2 examples of environmental degradation discussed in class.

Answer 2

• Degradation of steel due to exposure to water or salt water
• Degradation of a plastic due to exposure to UV light

Question 3

What is corrosion?

Answer 3

Corrosion refers to the continued (unchecked)
electrochemical attack of a material, normally resulting in
oxidation of that metal.

Question 4

What does corrosion convert?

Answer 4

Corrosion typically converts a
refined metal to a more chemically-stable form found in the
environment.

Question 5

What is the driving force behind the corrosion of metals?

Answer 5

The driving force is a reduction in energy

Question 6

What type of cell forms during corrosion?

Answer 6

An active electrochemical cell forms in which one part
of the metal surface serves as the anode and the other as the cathode

Question 7

What type of force is required for oxidation to occur in corrosion?

Answer 7

Huge thermodynamic driving force (ΔG0 «0) for oxidation to occur, or
in essence, to return to its mineral state

Question 8

Is the material destroyed in corrosion?

Answer 8

Material is not destroyed, but converted to a different form

Question 9

What is the purpose of the galvanic series?

Answer 9

To rank the reactivity of metals and alloys in seawater at 25 degrees celsius.

Question 10

How is the EMF series measured?

Answer 10

Measured relative to standard
hydrogen reference half cell)

Question 11

When does galvanic corrosion occur?

Answer 11

Occurs when two different metals or alloys are:

1. electrically coupled while
2. exposed to an electrolyte

Question 12

What is the key to understanding galvanic corrosion?

Answer 12

Galvanic series

Question 13

What are the three major differences between the Galvanic Series and the EMF series?

Answer 13

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

Question 14

What are general methods of corrosion protection?

Answer 14

1. Self-driven formation of a protective oxide
2. Physical barriers (paint, noble coating)
3. Sacrificial or cathodic protection

Question 15

What are examples of metals and alloys that form a protective oxide layer to protect against corrosion?

Answer 15

Examples include stainless steel (contains at least 10% chromium, which
forms an oxide layer), some aluminum alloys, and titanium

Question 16

How does noble coating work?

Answer 16

• Uses a less chemically active
  metal layer than the main metal
• Acts as a physical barrier to
  inhibit contact with water and
  oxygen

Question 17

What metals are used for noble coating?

Answer 17

Often nickel, tin or chromium

Question 18

Explain how sacrificial or cathodic coating works?

Answer 18

• 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

Question 19

What is an example of sacrificial or cathodic coating?

Answer 19

• Ex. Zinc on Steel is known as
  galvanized steel, with the steel dipped
  in molten zinc.

Question 20

What is the metal coupled to in sacrificial or cathodic protection?

Answer 20

Metal to be protected is electrically coupled to a sacrificial anode

Question 21

What can the anode be made from in sacrificial or cathodic protection?

Answer 21

The anode can be made from a more active metal on the galvanic scale for passive protection

Question 22

What is the anode driven by in sacrificial or cathodic protection?

Answer 22

Driven by an applied external power source to electrically bias the
electrodes correctly for protection

Question 23

What are other means of reducing corrosion relating to tackling environmental factors?

Answer 23

-Reduce Temperature
-Remove reactants
-Add inhibitors to fluids

Question 24

How does reducing temperature work to reduce corrosion?

Answer 24

Reducing the temperature slows the kinetics of oxidation and reduction.

Question 25

What is an example of reducing reactants to reduce corrosion?

Answer 25

Removing oxygen gas to create a more reducing environment

Question 26

What is an example of how adding inhibitors to fluids reduces corrosion?

Answer 26

E.g. coolant additives in automotive cooling systems (radiators) provide corrosion prevention and protection against freezing.

Question 27

Why does rust flake?

Answer 27

Two reasons for flaking: Volume change and chemical characteristics of iron differ from original iron

1. Volume change (expansion)
2. Poor adherence to the underlying metal

Question 28

In galvanic corrosion, what happens to the more reactive metal and what does it act as?

Answer 28

The more reactive metal is oxidized and acts as the anode. It corrodes.

Question 29

In galvanic corrosion, what happens to the less reactive metal and what does it act as?

Answer 29

The less reactive metal is redued and acts as the cathode. It is protected.

Question 30

Why did the aluminum foil corrode
when coupled with stainless steel?

Answer 30

Aluminum is more reactive (anodic) than
stainless steel, as shown in Galvanic series

Question 31

Why will the lasagna cell “fail” if the
aluminum foil does not contact the
ketchup and salt?

Answer 31

Metals have to be coupled to allow for flow
of electrons, and an electrolyte present to
conduct ions.

Question 32

Which type of metals and alloys are more anodic and which ones are more cathodic?

Answer 32

More inert (less reactive) metals are more cathodic; more anodic metals are active & reactive.

Question 33

What is the disadvantage of noble coating?

Answer 33

Prone to physical or chemical
damage, like paint or film

Question 34

In the “lasagna cell” example, what is the electrolyte?

Answer 34

Ketchup and salt

Question 35

What is stress corrosion?

Answer 35

Stress corrosion can promote crack
propagation in components that
are under stress

Question 36

Where are stress corrosion cracks formed?

Answer 36

Wherever the tensile stresses are the greatest.

Question 37

What type of substance can contribute to crack propagation in stress corrosion?

Answer 37

Metal oxide can form at crack tip
or crack vicinity and contribute to
crack propagation

Question 38

When does a protective oxide coating form?

Answer 38

For some metals and alloys, a protective oxide coating forms upon initial
exposure to oxygen

Question 39

How does the protective oxide layer protect against further corrosion (Written Response) ?

Answer 39

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

Question 40

What chemical reaction occurs when a protective oxide layer is formed?

Answer 40

Oxidation

Question 41

What “ingredients” or conditions are required for corrosion to occur (Hint: There are 4 ingredients)

Answer 41

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

Question 42

What is oxidation?

Answer 42

The process where an atom gives up an electron (s) to become a cation. It occurs at the anode.

Question 43

What is reduction?

Answer 43

The process where an atom acquires an extra electron (or electrons) and becomes an anion.

Question 44

What is the function of seawater in corrosion of aluminium and steel?

Answer 44

To provide an electrolyte for corrosion to occur.