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DAMC > T3-9: Cathodic Protection > Flashcards

T3-9: Cathodic Protection Flashcards

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1
Q

What does cathodic protection do?

A
  • It manipulates the corrosion reaction (which requires all four elements to occur, see diagram), via electrochemical means
  • Provides the structure with an excess of free energy
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2
Q

What does cathodic protection do with respect to the cathodic reaction?

A

It promotes only the cathodic reaction on the structure being protected

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3
Q

What does cathodic protection do with respect to the anode?

A

It controls the anode where the energy and metal loss occurs

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4
Q

[NAQ] cathodic reaction diagrams, showing how the anode is controlled

A
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5
Q

Where can cathodic protection be used (x5)?

A
  • Atmospherically exposed steel reinforced concrete
  • Buried and/or immersed steel reinforced concrete
  • New build (aka cathodic prevention)
  • Old structures, that are chloride contaminated or carbonated
  • As part of a patch repair system, to prevent incipient anode formation
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6
Q

How long does cathodic protection/prevention last (new-build)?

A

> 100 years

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7
Q

Where might cathodic protection not work (x2)?

Give an example

A
  • Need electrolytes (ie. need all components) for cathodic protection to work (e.g. would have to completely soak car, which is why they are painted)
  • If concrete is cracked
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8
Q

Where in the British Standards is cathodic protection specified?

A

It is repair principle no.10 in BS EN 1504 part 9

BS EN 1504: Products and systems for the repair and protection of concrete structures

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9
Q

What does this diagram show?

A

A metallic element without cathodic protection

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10
Q

What does this diagram show?

A

A metallic element with cathodic protection

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11
Q

What are the four parts needed for a cathodic reaction to occur?

A
  • Anode
  • Cathode
  • Electrolyte
  • Metallic bridge
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12
Q

What are two limits of cathodic protection?

Give an example where it won’t work

A
  1. Cannot work across an air gap (concrete must be repaired)
  2. Can only protect the surface of steel (does not work inside post tension ducts)
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13
Q

For cathodic protection:

  1. What reaction happens on the steel surface?
  2. What is generated on the steel surface (what is the equation)?
A
  1. Only the Cathode reaction - consumption of energy
  2. There is the generation of hydroxide (environment + energy —> OH-)
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14
Q

What does this image show?

A

Partial cathodic protection
- less corrosion

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15
Q

What does this image show?

A

No cathodic protection
- corrosion

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16
Q

What does this image show?

A

Full cathodic protection
- no corrosion

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17
Q

What does this diagram show?

A

Chloride-induced pitting corrosion (of steel in concrete)

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18
Q

Describe the diagram of the cathodic protection of chloride-induced pitting corrosion
- what forms?
- how does the pH in the pit change?
- what charge does the steel become, and what happens to the chlorides?

A
  • a passive film forms inside the pit
  • the pH in the pit increases from acidic to alkaline
  • the steel (now negatively charged) repels negatively charged CI-, which electro-migrate from pit to positively charged anode
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19
Q

What does this diagram show?

A

Carbonation-induced corrosion

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20
Q

Describe the diagram of the cathodic protection of carbonation-induced corrosion
- what forms?
- how does the pH change?
- describe the movement of ions (what moves towards steel, what moves away)

A
  • passive film forms due to increased steel pH
  • there is a pH transition (steel increases to alkaline, CO2-neutralising hydroxide lowers pH in concrete)

ion movement:
- Pre-cathodic protection: H20 and O2 move towards steel, OH- away
- Post-cathodic protection: local OH- ions on surface are increased to protect the steel

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21
Q

For measuring cathodic protection, what is difficult and easy to measure?

A

Difficult - energy movement (current)

Easy - difference in energy (voltage)

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22
Q

How is an ammeter used when measuring cathodic protection?

What is predicted?

A
  • Break circuit and use ammeter to measure Amps
  • The current flow is used to calculate galvanic anode consumption, and predict residual life
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23
Q

How is current density (amount of energy) calculated?

What is the unit?

A

Using the steel area
- unit mA/m²

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24
Q

What current density range is required to:

a) stop active corrosion (cathodic protection)

b) prevent corrosion initiation on passive steel (cathodic protection)

Study These Flashcards
A

a) 2 - 20 mA/m² steel

b) 0.2 - 2 mA/m² steel

ie. higher = does more

25
How is a voltmeter used when measuring cathodic protection?
- A **reference electrode** (half cell) and voltmeter is used - Reference electrode is made from a specific metal (or metal compound) which is contained in a saturated solution of its salts - Reference electrode potential measurements help **evaluate the degree of protection**
26
What are two typical reference electrodes for concrete?
1. **Silver chloride** 0.5 Molar potassium chloride - (Ag/AgCl 0.5M KCl) 2. **Manganese dioxide** - (Mn/MnO2)
27
[NAQ] different readings for voltage/potential measurements
28
Describe an On Potential error
- On Potential includes an error when current is flowing - Known as IR or voltage error - **Measurement at surface** is **more negative than at steel** - Switch off for Instant Off and leave off for depolarisation, 4h, 24, 48h
29
What British Standards covers the cathodic protection of steel in concrete?
BS EN ISO 12696
30
What type of diagram is this? What do the different abbreviations (x4) represent?
**Evans diagram** (**polarisation** diagram) In the figure, can see three individual voltages (potentials): - **Ec**: potential of the **metal** at the **cathodic** reaction - **Ea**: potential of the **metal** at the **anodic** reaction location (prior to connection) - **Ecorr**: the **mixed potential** (aka equilibrium potential) - **Icorr**: the **corrosion current**, flowing between the cathode/anode
31
When does the mixed/equilibrium potential (Ecorr) occur?
When both the **a****node and cathode** are **connected** via **electrolyte and a metallic bridge**
32
What does an Evans diagram for As found potential and corrosion rate look like?
33
What does an Evans diagram for the effects of partial CP look like?
Shift from -0.65V to -0.85V (mild steel in soil)
34
What does an Evans diagram for the effects of full CP look like?
Shift from -0.65V to -0.85 Cu/CuSO4 Only cathodic reaction, no anodic reaction
35
For mild steel in soil, what is the minimum CP criteria?
-0.85V Cu/CuSO4
36
What are two methods of providing the energy for CP?
Galvanic, ICCP
37
Describe Galvanic CP What is it also referred to as? What is the anode material designed to do, and what is the anode mass proportionate to?
- Also referred to as **sacrificial protection** - Based on the **potential difference** between different metals - **Anode** material is designed to **corrode and provide energy** - Anode mass is proportionate to **life**
38
Describe the current/voltage in Galvanic CP What materials are used?
- **Current is limited**, driving voltage - **Zinc**, Aluminium, Magnesium, Iron
39
[NAQ] electrochemical series in aerated water
40
Describe the Galvanic Cathodic Protection Polarisation diagram (Evans) for Iron/Zinc What are the cathodic and anodic reactions?
E_iron to -0.441 E_zinc to -0.763
41
Describe the Galvanic Cathodic Protection Polarisation diagram (Evans) for Iron/Titanium
42
Name (x4) the effects of Galvanic Cathodic Protection of a full structure
- **Global corrosion control** across surface - **Stops incipient anode formation** (effective across full area of anode application) - Can be **monitored** (as per BS EN ISO 12696 requirement) - Can use **battery powered or manual monitoring** where reference electrodes are installed
43
Describe ICCP (x3)
- The anode material is based on **lightweight stable noble material** (MMO coated titanium, graphite etc) - **Steel is anodic** to these anode materials - External energy **forces steel cathodic**
44
What is required in ICCP?
A **continuous source of energy** - based on **external power** supplied from mains AC; transformed and rectified into DC - voltage and current can be adjusted up to the power supply limit
45
Describe the ICCP Polarisation diagram What is the titanium/iron connected to?
Titanium connected to DC positive (shifts more positive) Iron connected to DC negative (shifts more negative)
46
Name (x4) the effects of ICCP on a full structure
- **Global corrosion control** across surface - **Stops incipient anode formation** (effective across full area of anode application) - **Can be monitored** (as per BS EN ISO 12696 requirement) - Can use **remote or manual monitoring** where reference electrodes are installed
47
[NAQ] exposure conditions (ICCP*)
48
[NAQ] table of when cathodic protection is effective
NB. effective in all of these conditions for reinforced concrete elements
49
Where/when can cathodic protection be used (x5)?
- **Atmospherically exposed** steel reinforced concrete - **Buried and/or immersed** steel reinforced concrete - **New build** (aka. cathodic prevention); for long term protection > 100 years - **Old** structures (chloride or carbonation-contaminated) - As part of a **patch repair system**, to prevent **incipient anode formation**
50
Name three primary benefits of cathodic protection
1. **Reduces repairs** - **sustainable** - only remove and replace damaged, spalled concrete - **no need to remove chloride-contaminated sound concrete** 2. Capable of stopping and **preventing corrosion within active pits** 3. (Monitoring) can be effectively evaluated against international criteria, using **embedded reference electrodes** to **prove no corrosion** - no need to break open and inspect - deals with hard-to-reach spaces
51
Name three secondary benefits of cathodic protection
1. **Boosting of the hydroxide** levels at the bar - promotes **passivation** (natural protection) of embedded steel - provides free protection of steel at end of system life 2. **Keeps steel negative** and **repels chloride ions** - will continue to work, no matter how much salt is delivered 3. **Additional time to concrete damage** after CP has **stopped working** - 10 to 30 years
52
[NAQ] cathodic protection conclusion
53
At what pH level does the protective oxide film need to be 're-healed'
When it reaches around **pH12**
54
Why does the cathodic reaction make the structure more negative?
It '**floods**' the structure with **electrons**
55
In cathodic protection, what does the more negative material act as?
The more negative material (the material with a lower electrode potential) acts as the **anode** - because the anode is **the electrode where oxidation occurs**, and oxidation involves the loss of electrons - opposite for cathode, which is **flooded with electrons**
56
If there is no AC power available, which cathodic protection system should be used?
Galvanic cathodic protection
57
What is an 'innovative' cathodic protection solution?
Zinc layer oxide (ZLA)
58
What is the difference in design life of ICCP vs Galvanic Cathodic Protection systems? Why the difference?
ICCP - **embedded** anodes designed for **50 years** Galvanic - **immersed** anodes designed for **25 year replacement**

DAMC (25 decks)

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