CORROSION AND PROTECTION OF STEEL REINFORCED CONCRETE

CORROSION AND PROTECTION OF STEEL REINFORCED CONCRETEPROVIDED BY: EMAD BEHDADLECTURER: PROF.SHAMS

CORROSION AND PROTECTION OF STEEL REINFORCED CONCRETES Yogesh [15MST1003]

OUTLINE

What is Corrosion of Steel?ASTM terminology (G 15) defines corrosion as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the original steel and none of the good mechanical properties.Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.

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Electrochemical process of steelcorrosion in concrete

Ca, Na, K hydroxides in hydrated cement raise the pH to ~13.5A dense protective ferric oxide (Fe2O3) passive film forms around the reinforcementThis passive film stops iron dissolution, and is stable at pH >10 Reinforcement Protection by Passive FilmPassive film develops on the bar surfacepH >13

CAUSES OF CORROSION IN CONCRETE

COMMON CORROSION TYPES1) Crevice CorrosionCrevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas). Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen content of liquid at the mouth of the crevice which is exposed to the air is greater, so a local cell develops in which the anode, or area being attacked, is the surface in contact with the oxygen-depleted liquid.

2) PittingTheories of passivity fall into two general categories, one based on adsorption andthe other on presence of a thin oxide film. Pitting in the former case arises asdetrimental or activator species, such as Cl-, compete with O2 or OH- at specificsurface sites. By the oxide film theory, detrimental species become incorporatedinto the passive film, leading to its local dissolution or to development ofconductive paths. Once initiated, pits propagate auto-catalytically according to thegeneralized reaction, M+n + nH2O + nCl- M(OH)n + nHCl, resulting in acidification of the active region and corrosion at an accelerated rate (M+n and M are the ionic and metallic forms of the corroding metal).

Airborne, marine, industrial, groundwater, cast-inCl can penetrate through the passive filmAt Cl- > threshold, passive film breaks down, corrosion initiates Cl- threshold value is typically 0.05% by wt of concrete (0.02% prestressed concrete)Pitting corrosionChlorides are main cause of reinforcement corrosion

Ca(OH)2 + CO2 CaCO3 + H2O

EFFECT OF CARBONATION It can cause soft surface, dusting and color change It reduces quality concrete It reduces the concrete ability to protect reinforcement from corrosion (in an exposed environment) It will result in additional shrinkage in carbonated region.

DETECTING CARBONATION Depth of carbonation can be detected using an indicator. A chemical such as Phenolphthalein sprayed on to freshly broken concrete. Areas remaining alkaline will turn in a bright purply-pink color. Carbonated areas of concrete will remain unchanged in color.

e-e-

Cl-Cl-

Rebar

e-e-RebarFe Fe+++ve Ions

+ve Ions

Degree of CorrosionTime

I

Initiation

Propagation(corrosion)

Critical chloride thresholdMigration of chlorides, H20 and O2 into the concrete, no corrosion and no damage to concreteCorrosion of the steel reinforcement and cracking and/or spalling of concrete

Reinforcing steel corrosion

Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-

...........................................................

ElectrolyteCl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Cl-Corrosion = Iron + Oxygen + MoistureEither the pH falls due to carbonation or other chemicalschlorides reach the steel above the threshold concentrationan electrical charge destroys the natural protection of the steelElectrons flow and ions migrateRust expansion causes crackingRapid deteriorationSpalling

pH >~10Anode

Cathode

Cathode

e-e-

MoistureOxygenIron

Spalling

Cracks with Rust Staining

Rebar loss

Delamination

Abandoned Electric Pole

KISH ISLAND

BANDAR ABBAS

chloride induced reinforcement corrosion in concrete exposed to seawater

Corroded rebar from cracked concrete of a parking structure exposed to deicing salts

Reinforced steel in concrete cracking

CORROSION PREVENTION METHODSREBAR COATING SCARIFIED & PATCHED DECK AWAITS ANODE MESHFLY ASHHOT-DIP GALVANIZINGWIRELESS SENSOR FOR MONITORING CHLORIDE IN CONCRETEINHIBITORS

REBAR COATING Advantage of epoxy coated rebar The coating has strong adhesive force to the steel.The coating has good chemical stability.The coating has good tenacity and wear ability.The coating has high insulation and anti-cathode decomposability.The coating has wide range temperature adaptation.The coating has high adhesive force with the concrete.Products have fine Physical Property.Used in the building with high demand and under the corrosive Environment, it can lengthen the use life of the building for more the 50 years.

EPOXY COATING PREVIEW MODEL

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EPOXYCOATED BARSAnode Reduces anode areaIncreases threshold*

CathodeReduces cathodic areaElectrical Connection Electrical path between anode and cathodeMakes ionic pathway longerIonic pathREDUCED CORROSION

thermally sprayed coatings of Zn and Al, combat corrosionFor atmospheric, buried, and marine environment corrosion protection, Zn (TSZ), Al (TSA), and their alloys have proven that they provide long term corrosion protection and outperform most all other methods.

Anodic (TSZ/TSA) metal coatings applied to steel cathodes (more noble than Zn or Al), are referred to as cathodic or sacrificial protection coating systems.

These thermal spray coatings provide corrosion protection by excluding the environment (or electrolyte) and acting as a barrier coating (like paints, polymers, and epoxies), but unlike typical barrier coatings they also provide sacrificial anodic protection.

Zinc and zinc alloys are also sprayed directly onto concrete to protect the steel rebar within

Arc spraying of zinc on a concrete bridge pier in the Florida Keys. In this case the zinc acts as sacrificial anode, although it is more frequently used in impressed-current systems. Three impressed-current zinc systems have already been installed by the Ministry of Transportation of Ontario in Toronto

Sacrificial cathodic protection of steel in concrete by thermal zinc spraying

CATHODIC PROTECTION Impressed current (active) Sacrificial anode (passive)

TITANIUM ANODE MESH TYPICALLY ATTACHED TO THE CONCRETE SURFACE ANDTHEN ENCAPSULATED IN CEMENTITIOUS MATERIALS.B- EASILY CONFORMS TO THE STRUCTURE GEOMETRY.C- MOST USED IMPRESSED CURRENT ANODE FOR CONCRETE.

Mixed Metal Oxide activated Titanium Anodes in the form of a ribbon mesh can be installed in close proximity and parallel to the reinforcement bars (rebar).

MMO Ribbon Mesh

Simple to Install.No Power Supply Needed.No Wiring or Conduit.No Long-Term Monitoring or Maintenance

Conventional Patch Repair

Embedded Zinc Anode for Patch Repair

CATHODIC PROTECTION SACRIFICIAL ANODE