T3-8: Corrosion Protection Options Flashcards
[NAQ] steel in uncontaminated concrete
What repair options and considerations (x4) exist if the chloride to cement ratio < 0.4%
- Prevent chlorides
- Coating (use coating)
- Overlays
- Waterproofing
What repair options and considerations (x4) exist if the chloride to cement ratio > 0.4%
Options 1:
- Cathodic protection
- Chloride extraction
Options 2:
- Contaminated concrete removal and reinstatement
Also concrete patch repairs
What happens if Cl- > 0.4 wt%?
The corrosion is active - chloride induced pitting
Name three problems with patch repairs of chloride contaminated concrete?
How long until additional repairs are needed?
- Incipient anode formation
- Reinforced corrosion in unrepaired concrete
- Accelerated spalling and concrete deterioration around repair
- Within 2-5 years
What effect can occur at patch repairs?
Incipient anode effect
a) what causes the incipient anode effect?
b) what happens to the material
c) when can it happen
a) caused by concentration gradients between new and old material
b) old material (with high chlorides, low pH) becomes anodic; new material (low chlorides, high pH) becomes cathodic
c) can happen at lower chloride levels than 0.4wt% Cl- to cement
What is a type of localised corrosion control at the patch repair interface?
Galvanic path repair enhancers
Describe galvanic patch repair enhancers
- It is the minimum option for repairing chloride contaminate concrete
- Galvanic anodes based on zinc
- Stops incipient anode formation (at repaired concrete interface)
- No monitoring
- Only effective close to anode; max 300mm from anode (depends on concrete resistivity and steel density)
- Ongoing corrosion remote from patch repair
- Needs traditional risk based inspection
[NAQ] Highway England specification
What do galvanic patch repair enhancers do?
- It is the minimum option for repairing chloride contaminate concrete
How much do galvanic path repair enhancers extend the life of repairs?
From 2-5 to 10, or >10 (within 300mm of the anode)
What are three things to consider with galvanic path repair enhancers?
- Corrosion remote from patch repair will continue; expect new spalls in unrepaired contaminated areas
- Needs traditional risk-based inspection (hammer survey etc)
- Anodes can affect half cell surveys
What is the best option here, and why?
Cathodic protection/ Chloride extraction
What is the ‘last resort’ option here, and why?
Contaminated concrete removal and reinstatement
What is repair principle No 10 (in BS EN 1504 p9)
Cathodic protection
What does cathodic protection do?
- 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
What does cathodic protection do with respect to the cathodic reaction?
It promotes only the cathodic reaction on the structure being protected
What does cathodic protection do with respect to the anode?
It controls the anode where the energy and metal loss occurs
[NAQ] cathodic reaction diagrams, showing how the anode is controlled
Where can cathodic protection be used (x5)?
- 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
How long does cathodic protection/prevention last (new-build)?
> 100 years
Where might cathodic protection not work?
- 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
What are two ways of restoring passivity in concrete?
- Concrete replacement
- Electrochemical control
What should be done in concrete replacement of a chloride-contaminated structure?
- Replace the whole structure
- Remove all chloride in cover concrete to less than 0.4% Cl- to cement, & replace with new concrete
What should be done in concrete replacement of a carbonated structure?
- Replace the whole structure
- Replace all concrete with less than pH10, & replace with new concrete
What are three electrochemical control methods, for restoring passivity?
- Cathodic protection
- controlling which reaction occurs on the surface
- lowering the chloride concentration at the steel below 0.4% Cl- to cement
- increase pH - Chloride extraction/ Realkalisation
- lowering the chloride concentration at the steel below 0.4% Cl- to cement
- increasing pH - Realkalisation
- increasing pH
When can electrochemical re-passivation be used?
- Atmospherically exposed steel reinforced concrete
- Old structures, that are chloride contaminated or carbonated
- As part of a patch repair system, to prevent incipient anode formation
[NAQ] typical cathodic protection installation
What are the differences (in six categories) between cathodic protection vs chloride extraction/ realkalisation
In electrochemical re-passivation, what is done to the structure to keep out CO2 and Cl- ?
Coating the structure when complete
Where does steel in concrete get ‘free protection’ from?
The high pH in pore water
[NAQ] summary of what to do if corrosion is active
NB. if less than 0.4 wt% Cl- (then passive film is ok, corrosion not active) - use coatings
What does painting/coating do?
- Keeps chlorides out
- Slows down water and oxygen getting in
- Protective oxide layer stays intact
NB. can only be done if threshold is less than 0.4 wt% Cl-
