T3-6: Basic Principles of Corrosion

Question 1

Name three global implications of corrosion

Answer 1

• More CO2 emissions (new steel, cement, zinc, paints; transport, removal)
• Use of more natural resources (iron ore, lime stone, coal)
• Reduced recyclability

Question 2

Name six local implications of corrosion

Answer 2

• Reduced operational life
• Safety risk
• Costs (both environmental and financial)
• Maintenance down time
• Impact on traffic flow; pollution
• Higher whole life cost

Question 3

Define corrosion

Answer 3

A reaction between a refined metal and the environment

Question 4

What are the causes (x2) of corrosion?

Answer 4

• Metal loss/degradation
• Modification of the environment (pH, ions, etc)

Question 5

[NAQ] simple energy diagram for corrosion

Answer 5

Metal has highest energy after refining energy is applied, and before corrosion causes energy out

Question 6

Describe the process from metals as ores to refined metals

Answer 6

• Engineering metal exist as ore (low energy, very poor mechanical properties)
• The ore is refined to improve mechanical properties; adding energy
• The refined metal is a loaded energy source
• The “right conditions energy” is released

Question 7

What are the four elements needed for corrosion to occur?

Answer 7

• Anode
• Cathode
• Metallic bridge
• Electrolyte

Question 8

What is the role of the anode in a corrosion cell?

What occurs at the anode?

Name the equation

Answer 8

• The anode is the site where oxidation (loss of electrons) occurs, leading to metal dissolution and the formation of metal ions, which then migrate to the cathode
• Energy production (metal lost to environment)
• Anode = high electrode potential site; ionisation occurs

Metal → Energy + Metal with less energy (m → e- + m2+)

Question 9

What is the role of the cathode in a corrosion cell?

What occurs at the cathode?

What is developed and how does the pH change?

Name the equation

Answer 9

• The cathode is the site where reduction occurs, receiving electrons from the anode and preventing corrosion at that location
• Consumption of energy
• Energy generated by anode accumulates on the surface - develops hydroxide - pH becomes more alkaline
• Cathode = lower electrode potential site; electrons flow through electrolyte to cathode (combine with water and oxygen)

Environment + Energy → Environment uses up energy and produces new product

Question 10

What is the role of the metallic bridge in a corrosion cell?

Answer 10

• Energy transport
• Facilitates the flow of electrons between the anode (where oxidation occurs) and the cathode (where reduction occurs), thus completing the electrical circuit and enabling the corrosion process

Question 11

What is the role of the electrolyte in a corrosion cell?

What does it act as?

Answer 11

• Transport of reactants to the surface
• Removal of products from the surface
• Acts as a conductive medium, enabling the flow of ions (charge carriers) between the anode and cathode, facilitating the electrochemical reactions that drive corrosion

Question 12

How can you prevent electrolytes from reaching the surface?

Answer 12

Paint the structure

Question 13

How is steel normally protected against corrosion in (uncontaminated) concrete?

What is formed?

Answer 13

• Cement hydration produces large quantities of calcium, sodium and potassium hydroxides, leading to high pH
• This promotes the formation of passive oxide layers, which prevent further corrosion

Question 14

What pH promotes the formation of passive oxide layers, for protection against corrosion?

Is this a low or high pH?

Answer 14

pH 12.5 to 13.5

High pH

Question 15

How do the following uncontaminated concrete elements protect against corrosion?

a) Uncracked concrete (what name is given?)
b) Steel reinforcement

Answer 15

a) Restricts movement and access of chlorides and CO2 - diffusion controlled migration

b) The passive oxide layer (if intact) prevents further corrosion of reinforcement

Question 16

What are the metal states at the different parts (A-D) of the potential-pH diagram for iron in water?

Answer 16

A - Corrosion
B - Immunity (metal is thermodynamically stable and doesn’t corrode)
C - Passivity (metal forms a protective layer on its surface that inhibits further corrosion)
D - Corrosion

Question 17

What are the two main concrete deterioration processes?

Answer 17

• Chloride-induced corrosion
• Carbonation

Question 18

Describe the main steps (x3) of chloride-induced corrosion of steel in concrete?

Answer 18

• Chloride contamination leads to a loss of the protective oxide layers
• Corrosion initiates once a critical chloride level occurs at the steel surface
• Very localised pitting corrosion and section loss can occur

Question 19

[NAQ] chloride-induced pitting corrosion of steel in concrete

What is the active pit zone pH?

Answer 19

Concrete pH>12.15
Active pit zone pH<5

Question 20

What is the impact of chloride-induced pitting corrosion of steel in concrete?

Answer 20

• Presence of chloride-ions leads to localised ‘pitting’ corrosion
• Common to have significant section loss of rebar
• This impacts tensile capacity

Question 21

Describe the main steps (x3) of carbonation-induced corrosion of steel in concrete

What does the CO2 from the atmoshphere do?

Answer 21

• The passive films rely entirely on the pH; any drop in pH will lead to the breakdown of this film and initiation of corrosion
• CO2 from the atmosphere can react with the free OH- (hydroxide) within the concrete; lowering the pH of the pore water
• General corrosion occurs; leading to uniform loss of metal (typical of general atmospheric corrosion)

Question 22

[NAQ] diagram of carbonation-induced corrosion

Answer 22

Concrete pH>12.5

Question 23

[NAQ] potential-pH diagram for iron in water, showing movement from passivity to corrosion state

Answer 23

Question 24

What is the impact of carbonation-induced corrosion of steel in concrete?

What can happen in ‘extreme cases’?

Answer 24

• The loss of pH leads to general corrosion
• Section loss in extreme cases is tapered, due to general section loss following loss of protective concrete cover

Question 25

What is the impact of corrosion of steel in concrete with O2? What forms as a result, and why?

Answer 25

- O2 available (concrete - wet and dry) - **Expansive forces** and **expansive corrosion product** occur - **Cover concrete cracks form** to **relieve pressure build up** from the expansive corrosion product

Question 26

What causes a loss of tensile capacity and what causes a loss of compressive strength, due to corrosion?

Answer 26

Loss of **tensile** capacity from **reinforcing steel section loss** Loss of **compressive** strength from **cover concrete cracks**

Question 27

What is the impact of corrosion of steel in concrete without O2? What forms as a result?

Answer 27

- **No** **O2 in water saturated** concrete - **Non-expansive corrosion product forms** on top of chloride-induced pitting corrosion - The corrosion product **leaches out of concrete as a soluble by-product** - The cover concrete is **stained but not cracked**; hammer survey does not indicate damaged concrete

Question 28

What provides corrosion protection to steel in concrete?

Answer 28

The high pH (hydroxide content)

Question 29

Describe the difference between chloride-induced corrosion and carbonation

Answer 29

- **Chlorides don't change the pH**, but compete with the hydroxide and **perforate the passive oxide layer**; causing **pitting** - In carbonation, **carbon dioxide neutralises the hydroxide and lowers the pH** to levels where the **passive oxide is no longer stable**; causing generalised corrosion

Question 30

What do expansive corrosion products cause?

Answer 30

Expansive corrosion products (rust) **crack** the cover concrete, then normal aqueous corrosion continues

Question 31

Name three design factors

Answer 31

1. **Asset life** - **predicted** design life, based on **concrete cover** and **diffusion ratio** - **actual** design life based on build quality, exposure conditions, etc 2. **Buildability** - **concrete cover required** - even with PFA and GGBS it only works if it is crack-free 3. **Corrosion risk** - corrosion **control options** needed at design stage, based on **exposure conditions** - anticipate construction faults, life extension requirement, changes to purpose of use

Question 32

Why is concrete cover required?

Answer 32

For **pH and diffusion control** of **chlorides and CO2**

Question 33

Why can pitting lead to a rapid loss of tensile capacity?

Answer 33

- Once pitting corrosion starts it is **difficult to stop** - **Pit zone** continues to **attract chlorides** - Pitting corrosion is very localised and leads to **tensile capacity being lost** very quickly