Corrosion in reinforced concrete is one of the most frequent features of buildings, especially those exposed to aggressive environments such as onshore marine atmospheres, feels Arunendu Ta.
Corrosion is a process of chemical reaction in which metal converts to their stable oxides. This is basically electrochemical oxidation where oxygen acts as oxidant. The formation of iron oxide, commonly known as ´rusting´ is an example of electrochemical corrosion. Generally oxides or salts of original metals form in corrosion. Corrosion is also seen in ceramic and polymers although more appropriate term for this is degradation. In this case material looses properties like appearance, strength, permeability, etc. Many cases corrosion in structural elements is seen because of exposure condition to air or many vapours.
In electrochemical reaction, the iron goes into solution as ferrous ions at anodic sites on the surface, thus constituting the anodic reaction. In this oxidation process, iron atoms release electrons whose negative charge would quickly build up in the metal and prevent further anodic reaction or corrosion. Thus this dissolution will only continue if the electrons released can pass to a site on the metal surface where a cathodic reaction is possible. At a cathodic site the electrons react with some reducible component of the electrolyte and are themselves removed from the metal. The ´corrosion current´, flow of electrons continue to degrade the original metal in this process. The corroding piece of metal behaves as a ´mixed electrode´ since simultaneous anodic and cathodic reactions are proceeding on its surface.
The most common and important electrochemical reactions in the corrosion of iron are thus:
Anodic reaction (corrosion):
Fe ? Fe2 + + 2e ------------------(i)
Cathodic reactions (simplified):
2H+ + 2e ? H2
H 2O + ½ O2 + 2e ? 2OH- --(ii)
Reaction (i) is most common in acids and in the pH range 6.5-8.5 the most important reaction is oxygen reduction (ii). In this latter case, corrosion is usually accompanied by the formation of solid corrosion deposits from the reaction between the anodic and cathodic products.
Fe2+ + 2OH- = Fe (OH)2 ------------------ Iron (II) Hydroxide Pure iron (II) hydroxide is white but the material initially produced by corrosion is normally greenish in colour due to partial oxidation in air.
2Fe(OH)2 + H2O + ½ O2 ? 2Fe(OH)3 ---------------- hydrated iron (III) oxide.
Due to further hydration and oxidation reactions, the reddish rust that eventually forms is a complex mixture whose exact constitution will depend on other trace elements which are present. As the rust is precipitated due to secondary reactions it is porous and absorbent and tends to act as harmful substrate which encourages further corrosion. For other metals or different environments different types of anodic and cathodic reactions may occur.
If solid corrosion products are produced directly on the surface as the first result of anodic oxidation these may provide a highly protective surface film which retards further corrosion, the surface is then said to be ´passive´. An example of such a process would be the production of an oxide film on iron in water, a reaction which is encouraged by oxidising conditions or elevated temperatures.
2Fe + 3H2O ? Fe2O3 + 6H+ + 6e
Corrosion protection by construction chemicals
Corrosion can be taken into account as prevention during construction or usually we are forced to take action after corrosion is found in later stage.
Concrete is a composite material composed of cement (usually Portland cement, fly ash), fine aggregate (sand), coarse aggregate (gravel or crushed stone), chemical admixtures, and water. During this mixing, cement reacts with water & form hydrates around the aggregate to form a solid, bonded conglomerate. While concrete is a relatively inert and durable building product, there are mechanisms by which it can degrade. Perhaps the most common is not degradation of the concrete material itself, but corrosion of the embedded steel reinforcement. When the steel corrodes, it expands and damages the concrete, often resulting in delamination and spalls. Corrosion in reinforced concrete is one of the most frequent features of buildings, especially those exposed to aggressive environments such as onshore marine atmospheres. Under this condition, repairs & preventive actions required by using construction chemicals which is post treatment of any construction.
Other degradation mechanisms include sulfate attack and alkali silica reaction (ASR). Prevention of these degradation mechanisms at the time of construction is typically done by selecting proper ingredients & designing good dense concrete mix to protect the concrete from aggressive environments. Once damage has progressed, the problem becomes much more complicated—and expensive—to solve.
The best way to use epoxy coated/galvanised steel reinforcement to avoid corrosion during any construction. Always better to cover the reinforcements inside concrete elements so that there will be less possibility for migration of oxygen from moisture to facilitate corrosion.
During the last few decades the corrosion issues in reinforced concrete has been extensively investigated by many scholars and researchers. Corrosion inhibitors are one of the most efficient chemicals for reinforced concrete. Corrosion inhibiting admixtures specialised admixture category and are used to slow down corrosion of reinforcing steel in concrete. In order to protect metallic substrates against corrosion, certain inorganic and organic products, called corrosion inhibitors, are added in small concentration to the aggressive medium.
Inorganic corrosion inhibitor
The addition of corrosion inhibitors to the concrete mix, offers a viable corrosion protection measure. There are generally three groups of inhibitors: anodic, cathodic and mixed inhibitors. Anodic inhibitors reduce the corrosion rate by reacting with the corrosion products and form a protective film. Cathodic inhibitors reduce the corrosion rate by reacting with the cathode sites (as an oxygen-barrier) on the steel. Passivating inhibitors like nitrites represent special types of anodic inhibitors and they are generally very effective if present in sufficient concentrations.
Mixed inhibitors both influence the anodic and cathodic reaction sites, by forming an adsorption film on the metal surface. These adsorption type inhibitors are typically organic compounds. In recent years, results of many investigations on organic migrating corrosion inhibitor (MCI), which is mainly, composed of an amino carboxylate or amino alcohol shows very encouraging results.
Organic corrosion inhibitor
These inhibitors like ´CHRYSOCorrocrete´ generally acts through bipolar mechanism. The functional group responsible for this organic compound adsorption on metal surface through the lone pair of the atom while iron ions on metal surface act as acid by accepting electrons from a donor group. By this, organic corrosion inhibitors form a strong corrosion protecting passive film on steel surface. The adsorption of these compounds on anodic sites decreases anodic dissolution of stainless steel by the electron-rich heteroatom in the organic compound, which adsorbs on the anodic site through their lone pairs of electrons of an atom thus reduces the anodic dissolution of metal. In acidic solution, these compounds can exist as protonated species; these protonated species may adsorb on the cathodic sites of the stainless steel and decrease the evolution of hydrogen. In some researches it was found that organic corrosion inhibitors effectively delayed the onset of steel corrosion and inhibited the steel corrosion even when the passive film was compromised. According to the strong absorption of organic corrosion inhibitors onto the steel surface inhibited the cathodic reaction of steel corrosion by limiting the access of oxygen to the steel. It is necessary to check the concrete properties with & without corrosion inhibitor in fresh (workability, retention, setting time, pH of medium, etc.) as well as harden state (compressive strength).
It has been found through exhaustive studies by researchers that the bi-polar corrosion inhibitors are effective enough to protect/delay corrosion of reinforcements in concrete if used properly during production of concrete. The only concern is the optimum dosage of Corrosion inhibitor to be determined before its use for effective results.
Once concrete is cracked, the easy path for corrosion opened up & we need to think for various solutions for rectification & retrofitting of structures to protect corrosion. In this category, different anti-corrosive coating systems, grouts etc exist for application depending upon the nature of criticality, location & substrate.
In conclusion, our target should be to produce impermeable concrete to the maximum extent possible to block pores inside concrete matrix to restrict entry of water & migration of various harmful ions. By this, we can eliminate maximum risk of corrosion together with using good ingredients like aggregate, supplementary cementitious material along with corrosion inhibitor (for aggressive environment is must) during production of concrete as well as giving sufficient attention for curing (either with water or with good curing compound like CHRYSOCure AC) at early age (we often forget & start after cracking) makes the system perfect to limit corrosion at later ages. Needless to say the use of galvanised / anti corrosive coated reinforcement bar ID must to protect corrosion.
Arunendu Ta, Head, R&D and Product Development, Chryso India