Contrary to common belief, concrete itself is a complex composite material. It has low strength when loaded in tension and hence it is common practice to reinforce concrete with steel for improved tensile mechanical properties. The key function of tor steel is to provide anchoring for cement bond by making available rough surface, (which also increases the surface area) to achieve a higher degree of bonding with the cement slurry while it undergoes curing or setting. Steel corrodes in many media including most outdoor atmospheres. Usually it is not selected for its corrosion resistance but for such properties as strength, ease of fabrication, and cost. The principal cause of degradation of steel reinforced structures is corrosion damage to the rebar embedded in the concrete.

Reinforcement corrosion takes place due to two major reasons –
(i) Localized breakdown of the passive film on the steel by chloride ions.
(ii) General breakdown of passivity by neutralization of the concrete, predominantly by reaction with atmospheric carbon dioxide.

Sound concrete is an ideal environment for steel but the increased concentration of carbon dioxide in modern environments principally due to industrial pollution, has resulted in corrosion of the rebar becoming the primary cause of failure of this material.

In order to understand the mechanisms behind corrosion of reinforcing steel in concrete, one has to examine the chemical reactions involved. In concrete, the presence of abundant amount of calcium hydroxide and relatively small amounts of alkali elements, such as sodium and potassium, gives concrete a very high alkalinity-with pH of 12 to 13. It is widely accepted that, at the early age of the concrete, this high alkalinity results in the transformation of a surface layer of the embedded steel to a tightly adhering film, that is comprised of an inner dense spinel phase in epitaxial orientation to the steel substrate and an outer layer of ferric hydroxide. As long as this film is not disturbed, it will keep the steel passive and protected from corrosion.

When a concrete structure is often exposed to salt splashes, salt spray, or seawater, chloride ions from these will slowly penetrate into the concrete, mostly through the pores in the hydrated cement paste. The chloride ions will eventually reach the steel and then accumulate to beyond a certain concentration level, at which the protective film is destroyed and the steel begins to corrode, when oxygen and moisture are present in the steel-concrete interface.

Once corrosion sets in on the reinforcing steel bars, it proceeds in electrochemical cells formed on the surface of the metal and the electrolyte or solution surrounding the metal. Each cell is consists of a pair of electrodes (the anode and its counterpoint, the cathode) on the surface of the metal, a return circuit, and an electrolyte. Basically, on a relatively anodic spot on the metal, the metal undergoes oxidation (ionization), which is accompanied by production of electrons, and subsequent dissolution. These electrons move through a return circuit, which is a path in the metal itself to reach a relatively cathodic spot on the metal, where these electrons are consumed through reactions involving substances found in the electrolyte. In a reinforced concrete, the anode and the cathode are located on the steel bars, which also serve as the return circuits, with the surrounding concrete acting as the electrolyte.

Corrosion can also occur even in the absence of chloride ions. For example, when the concrete comes into contact with carbonic acid resulting from carbon dioxide in the atmosphere, the ensuing carbonation of the calcium hydroxide in the hydrated cement paste leads to reduction of the alkalinity, to pH as low as 8.5, thereby permitting corrosion of the embedded steel.

The rate of carbonation in concrete is directly dependent on the water/cement ratio (w/c) of the concrete, i.e., the higher the ratio the greater is the depth of carbonation in the concrete.

The most widely used process to prevent corrosion today is ‘oxide saturation’ by “bluing” which converts Fe into Fe₂O₃, but with its own limitations. Stability may be attained under normal weather conditions, but for not more then 15 days. However, this process is used, since normal rust preventive methods bring about ‘slippage’, which reduces the Pull-Draw value. The “bluing” process requires heating in open flame & air, which is an ideal condition for corrosion to occur in uneven layers. In a few cases, the Public Works Department (PWD) accepted to drop the strength by 30% with an objective to impart better life to RCC. This type of steel also has its own problems, e. g. on expansion of metal, the coating cracks exposing unprotected metal to the atmosphere especially during transportation to site. Furthermore, patchwork is not possible on this kind of a metal surface. Therefore, you can appreciate the merits, our product Z300^TM, has to offer.

It is very difficult to control the original state of corrosion. This results in loss of metal, the degree of loss is not realized when it is used. Subsequently, when the metal is subjected to procedures, such as bending and twisting, to build RCC frames, bimetallic corrosion starts. During casting of structure, wherein the cement slurry is in contact with the metal, corrosion is faster due to the exothermic reaction between the slurry and the metal surface. In the presence of water, electrolysis occurs resulting in the formation of cations and anions. Curing necessitates ‘watering’ the structure, giving rise to a highly humid environment that facilitates corrosion. Through out these stages, Fe₂O₃ as ‘blued steel’ gives way to corrosion. When RCC structures need to be constructed in extended steel bars, without knowledge of the time intervals, the problem is much more complicated.

Z300^TM process is a superior alternative to the “bluing” process, at a very nominal cost. This product works on the principle of chemisorption, wherein the primary corrosion cells form complex with our low molecular weight polymer to form a passive zone on the surface. The resulting film (which has an anchoring property that helps in forming a cohesive bond with the concrete mixture) of approximately 1-micron thickness provides protection for at least 3 months. If steel needs to be stocked for a longer duration, corrosion maybe observed in patches, which can be treated using Z300^TM The time required for curing is approximately 24 hours. This product passes the Pull Draw Test that proves the strength of the resultant cohesive bond. Z300^TM can cover approximately 175 sq. ft. of steel per litre used i.e. 16 mm bar will require 1.6 litres per MT. At prevention level, there would a substantial reduction in costs, as compared to the application during curing process.

We would recommend that steel be stored at plant or site on wooden batons or planks to control the electrolysis process and bimetallic corrosion arising due to earthing and static charge, which need to be isolated. This is an ancient method that has been reintroduced to control rate of corrosion.

We would be pleased to carry out a free Beta Testing at your site for up to 50 MT of tor steel.

‘Beta test’ form in our download section, which you can download and inform us before taking trials. You can get technical support from our team and can avoid wrong trials.