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MT41013CORROSION AND ENVIRONMENTAL DEGRADATION OF MATERIALS Term Paper On Corrosion of steel in concrete Submitted by Piyush Verma (09MT3018) Supervisor Professor S K Roy Department of Metallurgical and Materials Engineering Indian Institute of technology Kharagpur West Bengal 721302 1
1 IntroductionReinforced concrete structures, because of the high alkalinity of the pore solution in theconcrete and the barrier provided by the cover concrete against the aggressive speciesfrom outside environment , the reinforcement has been believed to be” non corrodible”.,i.e. the corrosion rate of the steel reinforcement has been believed to be too slow to be ofconcern. However with passage of time some cover concrete would not be able toprovide good protection to the reinforcement due to the degradation of concrete and theingress of corrosive species from environment. It has been recognized that the concretecannot always be a non-corrosive medium to protect steel from corroding.The corrosion processes are closely related to the concrete and environmental factors. Forexample, the moisture content in the concrete depends not only on the relative humidityof the atmosphere but also upon the temperature cycling during day and night. Alsovariation of temperature has multiple simultaneous effects on different parameters whichmay counter-balance each other. The oxygen content and the PH value of pore solutiondecrease and the concentration of chloride ion increases when temperature rises.2 Corrosion Processes of Steel in ConcreteMicro-structural Defects in ConcreteMicro-cracking is one of the most important defects in concrete that would be responsiblefor serious corrosion attack of steel in concrete. It provides the short-cut for the ingress ofcorrosive species from environment into the concrete. The aggressive species couldchange the chemical properties of concrete seating a more aggressive environment in thevicinity of the reinforcement. Cracks can be formed due to bleeding effects, rapid dryingof exposed surface of wet concrete, temperature difference in the core, freeze cycles andexternal seasonal temperature variation.Basic corrosion processes of steel in concrete1) Depolarization reagent, i.e. O2 arrives at the surface through the medium surrounding it, dissolved in the medium.2) Electrochemical reactions at the interface of metal (In presence of only oxygen) Cathodic reaction O2+2H2O+4e = 4OH- 3
Anodic reaction Fe = Fe2+ + 2e (In presence of chloride ion) Cathodic reaction - - Fe + 2Cl = Complex (Fe2+ +2Cl-) +2H2O + 2e Anodic reaction - - - - Fe + 2Cl = Complex (Fe2+ +2Cl ) + 2e = Fe (OH) 2 + 2H+ + 2Cl Fe2+ can be further oxidized to Fe3+ under oxidizing conditions and can be accumulated at the surface of steel rebar or be dissolved into the pore solution.3) Accumulation of reaction products at the surface of metal.Thin passive film of Fe (OH) 2 or Fe (OH) 3 can be formed on the steel surface due tohydrolysis or oxidation of Fe2+. However under some conditions protective film cannotbe formed or would be broke down, this applies when due to carbonated concrete the Phvalue of the solution goes below 9, or when a certain amount of chloride ion haspenetrated into a concrete saturated with water and has reached the vicinity of the steel.Hence the steel will dissolve and the cross-section will go on decreasing. 4
3 Mechanisms of corrosion1) Pitting corrosion Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area2) Crevice corrosion becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions.2) Crevice corrosion Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles.4 Reasons for corrosion1) Lowering in alkalinity due to loss of carbonates by CO2The acidic gases react with the alkalis (usually calcium, sodium and potassiumhydroxides), neutralizing them by forming carbonates and sulphates, and at the same timereducing the pH value. If the carbonated front penetrates sufficiently deeply into theconcrete to intersect with the concrete reinforcement interface, protection is lost and,since both oxygen and moisture are available, the steel is likely to corrode. The extent of 5
the advance of the carbonation front depends, to a considerable extent, on the porosityand permeability of the concrete and on the conditions of the exposure. In the case ofcarbonation, atmospheric carbon dioxide (CO2) reacts with pore water alkali according tothe generalized reaction, Ca (OH) 2 + CO2 = CaCO3 + H2O . It consumes alkalinity andreduces pore water pH to the 8–9 range, where steel is no longer passive. -2) Lowering in alkalinity due to ClThe passivity provided by the alkaline conditions can also be destroyed by the presenceof chloride ions, even though a high level of alkalinity remains in the concrete. Thechloride ion can locally de-passivate the metal and promote active metal dissolution.Chlorides react with the calcium aluminate and calcium aluminoferrite in the concrete toform insoluble calcium chloroaluminates and calcium chloroferrites in which the chlorideis bound in non-active form; however, the reaction is never complete and some activesoluble chloride always remains in equilibrium in the aqueous phase in the concrete. It isthis chloride in solution that is free to promote corrosion of the steel. At low levels ofchloride in the aqueous phase, the rate of corrosion is very small, but higherconcentration increases the risks of corrosion.3) Cracks due to Mechanical LoadingCracks in concrete formed as a result of tensile loading, shrinkage or other factors canalso allow the ingress of the atmosphere and provide a zone from which the carbonationfront can develop. If the crack penetrates to the steel, protection can be lost. This isespecially so under tensile loading, for debonding of steel and concrete occurs to someextent on each side of the crack, thus removing the alkaline environment and sodestroying the protection in the vicinity of the debonding.4) Stray CurrentsStray currents, arising for instance from railways, cathodic protection systems, or highvoltage power lines, are known to induce corrosion on buried metal structures, leading tosevere localized attack. They may find a low resistance path by flowing through metallicstructures buried in the soil (pipelines, tanks, industrial and marine structures). a cathodicreaction (e.g., oxygen reduction or hydrogen evolution) takes place where the currententers the buried structure, while an anodic reaction (e.g., metal dissolution) occurs wherethe current returns to the original path, through the soil. Metal loss results at the anodicpoints, where the current leaves the structure; usually, the attack is extremely localizedand can have dramatic consequences especially on pipelines. 6
5) Water-Cement RatioConcrete placed with a high water-cement ratio, as seen under Freeze-thaw cycles, ismore porous due to the presence of excess water in the plastic concrete. The porosityincreases the rte of diffusion of water and electrolytes through the concrete and makes theconcrete more susceptible to cracking.6) Low Concrete Tensile StrengthConcrete with low tensile strength facilitates corrosion damage in two ways. First, theconcrete develops tension or shrinkage cracks more easily, admitting moisture andoxygen, and in some cases chlorides, to the level of the reinforcement. Second, theconcrete is more susceptible to developing cracks at the point that the reinforcementbegins to corrode.7) Electrical Contact with dissimilar metalsDissimilar metals in contact initiate a flow of electrons that promotes the corrosion of oneor the other, by a process known as galvanic corrosion. When two dissimilar metals are incontact with each other the more active metal (lower on the list) will induce corrosion onthe less active. Such corrosion may induce cracking and damage in the concrete.8) Corrosion due to difference in environmentsCorrosion occurs when two different metals, or metals in different environments, areelectrically connected in a moist or damp concrete.This will occur when:1. Steel reinforcement is in contact with an aluminium conduit.2. Concrete pore water composition varies between adjacent or along reinforcing bars.3. Where there is a variation in alloy composition between or along reinforcing bars.4. Where there is a variation in residual/applied stress along or between reinforcing bars. 7
5 PREVENTION METHODS1) Keep concrete always dry, so that there is no H2O to form rust. Also aggressiveagents cannot easily diffuse into dry concrete. If concrete is always wet, then there is nooxygen to form rust.2) A polymeric coating is applied to the concrete member to keep out aggressive agents.A polymeric coating is applied to the reinforcing bars to protect them from moisture andaggressive agents. The embedded epoxy-coating on steel bars provide a certain degree ofprotection to the steel bars and, thereby, delay the initiation of corrosion. These coatingspermit movement of moisture to the steel surface but restrict oxygen penetration such thata necessary reactant at cathodic sites is excluded.3) Stainless steel or cladded stainless steel is used in lieu of conventional black bars.4) FLY ASH: Using a Fly Ash concrete with very low permeability, which will delay thearrival of carbonation and chlorides at the level of the steel reinforcement. Fly Ash is afinely divided silica rich powder that, in itself, gives no benefit when added to a concretemixture, unless it can react with the calcium hydroxide formed in the first few days ofhydration. Together they form a calcium silica hydrate (CSH) compound that over timeeffectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chlorideions.5) A portion of the chloride ions diffusing through the concrete can be sequestered in theconcrete by combining them with the tricalcium aluminate to form a calcium chloroaluminate (Friedel’s salt). It can have a significant effect in reducing the amount ofavailable chlorides thereby reducing corrosion.6) Electrochemical injection of the organic base corrosion inhibitors, ethanolamine andguanidine, into carbonated concrete.7) The rougher the steel surface, the better it adheres to concrete. Oxidation treatment (bywater immersion and ozone exposure) of rebar increases the bond strength between steeland cement paste to a value higher than that attained by clean rebars. In addition, surfacedeformations on the rebar (such as ribs) enhance the bond due to mechanical interlockingbetween rebar and concrete.8) As the cement content of the concrete increases (for a fixed amount of chloride in theconcrete), more chloride reacts to form solid phases, so reducing the amount in solution 8
(and the risk of corrosion), and as the physical properties improve, the extent ofcarbonation declines, so preventing further liberation of chloride from the solid phase.9) Electrochemical Chloride Extraction (ECE) is a relatively new technology for whichlong-term service data are limited. This method employs a temporary anode that isoperated at current density 7 orders of magnitude higher than for cathodic protection,such that anions, including chlorides, electromigrate away from the embedded steelcathode. Repassivation can then occur, similar to what was discussed above inconjunction with cathodic protection, although this occurs in a shorter period of time (1–2weeks to several months). Not all chlorides are removed, but sufficient amounts aredisplaced from the steel-concrete interface.6 CONCLUSIONSCommon types of corrosion occurring are Pitting, Crevice and Intergrannular corrosion.The two most common causes of reinforcement corrosion are chloride ions andcarbonation by atmospheric carbon dioxide. In wet and cold climates, reinforced concretefor roads, bridges, parking structures and other structures that may be exposed to deicingsalt may benefit from use of epoxy-coated, hot dip galvanized or stainless steel rebar,although good design and a well-chosen cement mix may provide sufficient protectionfor many applications. Epoxy coated rebar can easily be identified by the light greencolor of its epoxy coating. Hot dip galvanized rebar may be bright or dull grey dependingon length of exposure, and stainless rebar exhibits a typical white metallic sheen that isreadily distinguishable from carbon steel reinforcing bar. Cathodic protection can beapplied too.7 REFERENCES1) Guangling Song, Ahmad Shayan, Corrosion of steel in concrete: causes, detection and prevention, a review report.2) J L Smith and Y P Virmani, Materials and methods for corrosion control of reinforced and prestressed concrete structures in new construction (2010)3) Wikipaedia.com 9