© 2026 Copyright PT. SENDANG BERLIAN SEJAHTERA, All rights reserved.
INTRODUCTION
Under normal conditions the reinforcement steel in concrete is protected by a coating of Iron-hydroxide (Fe(OH)2), impenetrable to oxygen, the so-called passivation layer, which can be only formed on steel in an alkaline environment with an alkalinity above pH ~ 9,5. Healthy concrete has a pH of 12 to 13, mainly caused by the presence of sodium, potassium and calcium-hydroxides in the pore-water solution of the concrete. Depending on the oxygen and humidity content of the concrete this passive layer can be further oxidized into magnetite (Fe3 O4 ) and at higher oxygen contents additionally hematite (Fe2O3 ), but still being insoluble layers promoting passivity of the steel surface.
We can conclude that a layer of concrete or any cement-containing layer on steel with at least a pH of 9.5 act as a protecting or passivating environment for the steel.
However chlorides can locally penetrate the concrete -due to its porous and inhomogeneous nature- attacking the passivation layer and cause corrosion. This corrosion process initiated by chlorides develops rather fast; the direction of the corrosion process often goes at tight angle to the reinforcement bars. By the time the damage becomes visible at the surface, the bars are already corroded to a great extent. Because of this “pitting” the constructive safety can already be at issue without hardly any visible evidence at the concrete surface.
Chlorides can penetrate concrete due to salt or brackish water (NaCl), salt spray effect on coastal areas, de-icing salts and mixed-in accelerators (CaCl2) to speed up its setting.
“Pitting” of post-tensioned cables in a bridge
deck due to de-icing salts.
Corrosion of steel in concrete
As shortly mentioned in the page before due its porous and inhomogeneous nature of concrete aggressive ions like chlorides are able to accumulate locally into relatively high concentrations and form so-called concentration cells. These concentration cells on its turn form very locally strong potential differences on the steel surface causing corrosion cells. These locally potential differences is the electromotive force (emf) or the “driving voltage” for any kind of corrosion.
Relation between chloride content in concrete
and potentials presented by C. Andrade (COST 521)
Also research by “Mercalli” showed high probability of corrosion of steel in concrete with potential differences up to 150-200mV within a small range (COST 521). Another possible cause of returning corrosion is traditional patching simply caused by the fact that fresh high alkaline concrete -used to locally repair the fractured concrete- will induce a reversal of polarity of the steel laying in the fractured area. This polarity change causes new fractures due to potential differences in the close vicinity of the patched area. These potential differences can be measured by use of reference cells or reference electrodes. Once used on a lab-scale nowadays developed into sophisticated measuring equipment which is used for potential mapping of big concrete surface areas like bridge decks.
Potential mapping of concrete to detect local
corrosion cells
For many years now potential mapping is being used as a corrosion-detection method of the reinforcement-bars of concrete. There are European as well as American manufacturers who are specialized in the development of equipment specific for this purpose. During the last two decades a large publicity on this regard has resulted both in the US and Europe in standards, such as “ASTM C876-91”, the German standard “Merkblatt B3”, and the Swiss standard “Merkblatt 2006”.
The following ASTM C876-91 standard is an effort to give a clear picture of to what extent the measured potentials can give information about the condition of the reinforcement bars and how to interpret this information.
Cathodic Protection of steel in concrete
Cathodic Protection is an electrochemical method of suppressing corrosion to very low rates with the objective to increase the service life of the structure. CP systems operate by electrically forcing the steel into a more passive state. This is basically done by forcing the potential of the steel below the most negative corrosion potentials in situ, overriding the potential differences causing the corrosion cells as mentioned above.
There are two, main types of Cathodic Protection systems, galvanic and impressed current (ICCP). Galvanic systems obtain their electric current from the corrosion of sacrificial anodes (such as zinc), while ICCP obtains power from a DC supply (often an AC/DC rectifier) distributed to the steel through an inert or longlife anode. In either case, the anodes may be mounted within a structure permanently or applied to its surface.
Impressed current anodes need to be fed by a distributed cable system, however galvanic anodes don’t need to be fed by a power source and an additional cable system, but slowly sacrifice themselves and producing in that way the current by there anodic reaction:
Due to the fact that sacrificial anodes slowly dissolve, will give them a finite durability.
Faraday’s law of electrolysis shows a direct relation between current and the mass of anode dissolved :
which M is mass [kg], and I is current [Amps]. To give the anodes a reasonable time span to do their work properly, we are now able to calculate the minimum amount of weight needed by using Faraday’s law. An additional challenge for the corrosion engineer is to calculate the spacing of the anodes in such a matter to get an equal spaced current distribution. The spacing depends on the electric field created between the anode and the cathode.
The electric field around the anode (say : potential at a specific point with a specific distance from the anode) is directly influenced by the current output of the anode, electrolytic resistivity and anode geometry. One can visualize the electric field by determining the equipotential lines around an anode and its effect on the steel reinforcement located within the electric field.
By placing the anodes evenly distributed over a certain space one will get a evenly distributed electric field as can be seen in the figure below. Experiments showed that in a normal situation appr. 3 to 4 anodes per m2 of concrete surface would give sufficient protection.
However the current demand by the steel depends on the steel surface area and the state of the steel (active or passive) which is effected by the anode’s electric field. This basically means that the bigger the steel surface area will be the higher my current demand will be. For a structures with high steel densities more anodes should be applied per m2 to meet lifetime requirements. Here below is a graph which shows the steel density of the structure and the required anode spacing.
Relation between steel density and anode spacing
Additional benefits of cathodic protection of steel in concrete is the fact that the forced electric field will move positive and negative charged ions from one side to the other. Chloride ions will be forced away from the steel (cathode) because they are negative charged particles and pushed away by the negative charged cathode (steel). This process is called dechlorination. And thanks to the cathodic reaction at the steel surface an increase of the concrete’s alkalinity will occur which will re-passivate corroded steel. This process is called realkalisation.
