From the picture below, iron is normally used in the environment with pH value
about 7 – 8 and the potential is in the corrosion zone(red dot ). However, the concrete has high alkalinity (pH about 12.6 - 13.8), so will be moved to the right side, the passivity zone where the passive film is formed and prevent iron from corrosion.
Answer Concrete is permeable and allows the ingress of chloride and carbon dioxide. The carbon dioxide will react with alkali and form carbonate.
Carbonation will reduce the pH value of concrete to about 8. If the carbon dioxide penetrates deeply into concrete and
reaches the reinforcing steel,
the passive film is destroyed and steel begins to corrode.
The corrosion of reinforcing steel due to ingress of chloride is more common than that of carbon dioxide. The cracking and spalling of reinforced concrete can be frequently founded in seawater, blackish water, seacoast, salty groundwater and etc. The rust is larger than steel. Thus, the internal pressure or expansion within the concrete is
the cause of cracking and spalling.
Concrete anode is a zinc anode that is encapsulated in alkali-activated mortar with pH value ≥ 14. This alkali-activated mortar will prevent zinc anode from passivation and improve current output of the zinc anode. Moreover, the mortar does not contain any acid or chemicals such as chloride or bromide which can damage the reinforcing steel in concrete. Therefore, the zinc anode of TMP can be used to control or prevent the rust of reinforcing steel in both new and old concrete structures.
Answer 4.1 The waterproof concrete or paint is used to delay moisture, carbon dioxide or chloride from the outside because of their low permeability. In fact, there is no any type of concrete or paint that is 100% waterproof. When the concrete is cracked or the paint is damaged, moisture, carbon dioxide or chloride can penetrate into the concrete and cause rust of the reinforcing steel.
4.2 Concrete anode is sacrificial anode cathodic protection (SACP) which needs moisture (electrolyte) to complete the 4 components of cathodic protection (CP). The 4 components consist of anode (zinc anode), cathode (reinforcing steel), electrical connection (steel wire) and electrolyte (alkali-activated mortar and concrete with moisture). Moisture will lower the resistivity of concrete and facilitate current flow from the concrete anode to the reinforcing steel. The current from the concrete anode will lower the potential of reinforcing steel into the immunity zone (CP zone). Therefore, moisture is very important for activation of concrete anode which is opposite method to 4.1.
Zinc anode encapsulated in alkali-activated mortar will provide current when there is moisture in concrete as stated in 4.2. The alkalinity of the mortar will not affect the strength and pH of concrete.
Zinc anode encapsulated in acid paste will provide current at all time because the 4 components are completed by acid paste, electrolyte. There is a chance that the acid will leak out to damage the reinforcing steel and affect pH value of the concrete. The strength of the mortar is also higher than the acid paste. Therefore, the overall strength of reinforced concrete with the alkali-activated mortar is better.
The Poubaix Diagram in question 1 illustrates the principle of cathodic protection. When current is applied to steel, the potential of steel will be moved from corrosion area down to immunity area. In practice, 100 mV Polarization Decay is used to determine the cathodic protection of reinforcing steel in concrete. The potential of the steel must be positively shifted at least 100 mV vs the standard reference electrode such as a saturated copper-copper sulfate reference electrode, SCE after CP current is switched off.
Water and soil are quite constant electrolyte but concrete is different. Concrete’s electrical resistivity will vary with many factors in environment such as moisture and chloride content. Therefore, CP of reinforcing steel in concrete will determine that the potential of steel is decreased enough to control corrosion.
From the figures below, the on potential, instant off potential and final off potential values obtained from the test sample according to ASTM G109 and from the reinforced concrete dam at the mouth of Chao Phraya River are different. Thus, a fixed -850 mV to SCE could not be used to determine the CP of reinforcing steel in concrete.
TMP sent a set of TMP concrete anode to Corrosion Technology Department, Thai-French Innovation Institute, King Mongkut’s University of Technology North Bangkok for testing. 100 mV Polarization Decay, NACE Standard RP0100-2000 or ISO 12696:2012, was
used to determine CP of the reinforcing steel. The result shows that concrete anode can provide protective current to control corrosion of the reinforcing steel with minimum length of 60 cm.
The installation procedure of TMP concrete anode is the following:
1. Place concrete anode at the appropriate location.
2. Tie the wire of concrete anode to reinforcing steels. The steel surface need to
be cleaned with sand paper if there is rust on the surface.
3. Make the wire tight. Use a steel hose clip or fastener if necessary.
4. Measure the resistance between the wire and reinforcing steel. The resistance
between the wires and reinforcing steels must be less than 1 ohm.
Examples of the standard are the following:
1. NACE RP0100, Cathodic protection of prestressed concrete cylinder pipelines.
2. NACE SP0408, Cathodic protection of reinforcing steel in buried or submerged
3. ISO 12696, Cathodic protection of steel in concrete.