T3-6: Basic Principles of Corrosion Flashcards
Name three global implications of corrosion
- More CO2 emissions (new steel, cement, zinc, paints; transport, removal)
- Use of more natural resources (iron ore, lime stone, coal)
- Reduced recyclability
Name six local implications of corrosion
- Reduced operational life
- Safety risk
- Costs (both environmental and financial)
- Maintenance down time
- Impact on traffic flow; pollution
- Higher whole life cost
Define corrosion
A reaction between a refined metal and the environment
What are the causes (x2) of corrosion?
- Metal loss/degradation
- Modification of the environment (pH, ions, etc)
[NAQ] simple energy diagram for corrosion
Metal has highest energy after refining energy is applied, and before corrosion causes energy out
Describe the process from metals as ores to refined metals
- 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
What are the four elements needed for corrosion to occur?
- Anode
- Cathode
- Metallic bridge
- Electrolyte
What is the role of the anode in a corrosion cell?
Name the equation
- 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)
Metal → Energy + Metal with less energy (m → e- + m2+)
What is the role of the cathode in a corrosion cell?
Name the equation
- 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
Environment + Energy → Environment uses up energy and produces new product
What is the role of the metallic bridge in a corrosion cell?
- 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
What is the role of the electrolyte in a corrosion cell?
- 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
How can you prevent electrolytes from reaching the surface?
Paint the structure
How is steel normally protected against corrosion in (uncontaminated) concrete?
- 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
What pH promotes the formation of passive oxide layers, for protection against corrosion?
Is this a low or high pH?
pH 12.5 to 13.5
High pH
How do the following uncontaminated concrete elements protect against corrosion?
a) Uncracked concrete
b) Steel reinforcement
a) Restricts movement and access of chlorides and CO2 - diffusion controlled migration
b) The passive oxide layer (if intact) prevents further corrosion of reinforcement
What is the metal states at the different parts (A-D) of the potential-pH diagram for iron in water?
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
What are the two main concrete deterioration processes?
- Chloride-induced corrosion
- Carbonation
Describe the main steps (x3) of chloride-induced corrosion of steel in concrete?
- 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
[NAQ] chloride-induced pitting corrosion of steel in concrete
Concrete pH>12.15
Active pit zone pH<5
What is the impact of chloride-induced pitting corrosion of steel in concrete?
- Presence of chloride-ions leads to localised ‘pitting’ corrosion
- Common to have significant section loss of rebar
- This impacts tensile capacity
Describe the main steps (x3) of carbonation induced corrosion of steel in concrete
- 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)
[NAQ] diagram of carbonation-induced corrosion
Concrete pH>12.5
[NAQ] potential-pH diagram for iron in water, showing movement from passivity to corrosion state
What is the impact of carbonation-induced corrosion of steel in concrete?
- 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
What is the impact of corrosion of steel in concrete with O2?
What forms as a result?
- O2 available (concrete - wet and dry)
- Expansive forces and expansive corrosion product occur
- Cover concrete cracks form to relive pressure build up from the expansive corrosion product
What causes a loss of tensile capacity and what causes a loss of compressive strength, due to corrosion?
Loss of tensile capacity from reinforcing steel section loss
Loss of compressive strength from cover concrete cracks
What is the impact of corrosion of steel in concrete without O2?
What forms as a result?
- 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
What provides corrosion protection to steel in concrete?
The high pH (hydroxide content)
Describe the difference between chloride-induced corrosion and carbonation
- 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
What do expansive corrosion products cause?
Expansive corrosion products (rust) crack the cover concrete, then normal aqueous corrosion continues
Name three design factors
- Asset life
- predicted design life, based on concrete cover and diffusion ratio
- actual design life based on build quality, exposure conditions, etc - Buildability
- concrete cover required
- even with PFA and GGBS it only works if it is crack-free - Corrosion risk
- corrosion control options needed at design stage, based on exposure conditions
- anticipate construction faults, life extension requirement, changes to purpose of use
Why is concrete cover required?
For pH and diffusion control of CI- (chlorides?) and CO2
Why can pitting lead to a rapid loss of tensile capacity?
- Once pitting corrosion starts it is difficult to stop
- Pit zone continues to attracted chlorides
- Pitting corrosion is very localised and leads to tensile capacity being lost very quickly
