CORROSION AND PROTECTION OF CORROSION AND STEEL PROTECTION OF STEEL REINFORCED CONCRETE
REINFORCED CONCRETEPROVIDED BY: EMAD BEHDAD LECTURER: PROF.SHAMS
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PROVIDED BY: EMAD BEHDAD LECTURER: PROF.SHAMS
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.
Electrochemical process of steel corrosion in concrete
Volumetric change
Reinforcement Protection by Passive Film
Ca, Na, K hydroxides in hydrated cement raise the pH to ~13.5 A dense protective ferric oxide (Fe2O3) passive film forms around the reinforcement This passive film stops iron dissolution, and is stable at pH >10
Passive film develops on the bar surface
pH >13
Yes if:
Concrete is always dry, then there is no H2O to form rust. Also aggressive agents cannot easily diffuse into dry concrete. (b) Concrete is always wet, then there is no oxygen to form rust. (c)Cathodic protection is used to convert all the reinforcement into a cathode using a battery. This is not easy to implement because anodic mesh is expensive, and this technology is not easy to install and maintain
(a)
(d) A polymeric coating is applied to the concrete member to keep out aggressive agents. These are expensive and not easy to apply and maintain. (e) A polymeric coating is applied to the reinforcing bars to protect them from moisture and aggressive agents. This is expensive and there is some debate as to its long- term effectiveness. (f) Stainless steel or cladded stainless steel is used in lieu of conventional black bars. This is much more expensive than black bars.
Can we avoid corrosion? No, not entirely: Concrete is not usually under water or continuously dry. Aggressive agents such as carbon dioxide, de-icing agents and/or sea water can diffuse into the best of moist concrete, and corrosion will eventually result.
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 oxygendepleted liquid.
Crevice Corrosion of Rebar Has Some Similarities with Filliform Corrosion The head of the advancing filament becomes anodic, with a low pH and a lack of oxygen, as compared with the cathodic area immediately behind the head where oxygen is available through the semipermeable film. Corrosion proceeds as the cathode follows behind the anodic head (from Corrosion Basics NACE).
2) PittingTheories of passivity fall into two general categories, one based on adsorption and the other on presence of a thin oxide film. Pitting in the former case arises as detrimental or activator species, such as Cl-, compete with O2 or OH- at specific surface sites. By the oxide film theory, detrimental species become incorporated into the passive film, leading to its local dissolution or to development of conductive paths. Once initiated, pits propagate autocatalytically according to the
Airborne, marine, industrial, groundwater, castin Cl can penetrate through the passive film At Cl- > threshold, passive film breaks down, corrosion initiates Cl- threshold value is typically 0.05% by wt of concrete (0.02% prestressed concrete) Pitting corrosion Chlorides are main cause of reinforcement
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
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.
Cl+ve Ions ee-
Cl-
+ve Ions Fe Fe++ eeReb ar
Reb ar
Reinforcing steel corrosion
Migration of chlorides, H20 and O2 into the concrete, no corrosion and no damage to concrete
Corrosion of the steel reinforcement and cracking and/or spalling of concrete
Degree of Corrosion
Initiation
Propagation (corrosion)
Critical chloride threshold I Time
. Cl Cl . . .Cl Cl Cl . Cl Cl .- . Cl - . Cl . Cl Cl .- . -. pH >~10 . . . . . . . Cl . . Cl . Cl. Cl . . . Cl Cl Cl e - e- Cathode Anode . . . . . . . . . Electrolyte . . Cl Cl Cl . . Cl Cl Cl . . . Cl ClCl . . Cl- Cl -
Cl Cl -
Cl -
. . .
Cl - .
.
. .
. .
.
. Cathode . ClCl -
.
. .
Corrosion =Either
Iro Oxyge Moistu Iron + Oxygen + Moisture re n n
the pH falls due to carbonation or other chemicals chlorides reach the steel above the threshold concentration an electrical charge destroys the natural protection of the steel Electrons flow and ions migrate Rust expansion causes cracking Rapid deterioration Spalling
Spalling
Rebar loss
Cracks with Rust Staining
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 METHODS
REBAR COATING SCARIFIED & PATCHED DECK AWAITS ANODE MESH FLY ASH HOT-DIP GALVANIZING WIRELESS SENSOR FOR MONITORING CHLORIDE IN CONCRETE INHIBITORS
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
EPOXYCOATED BARSAnode Reduces anode area Increases threshold*Cathode Reduces cathodic area
REDUCED CORROSION
Electrical Connection Electrical path between anode and cathode
Makes ionic pathway longer
Ionic path
thermally sprayed coatings of Zn and Al, combat corrosion
For 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 withinArc 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
FLY ASH
using a Fly Ash concrete with very low permeability, which will delay the arrival of carbonation and chlorides at the level of the steel reinforcement. Fly Ash is a finely divided silica rich powder that, in itself, gives no benefit when added to a concrete mixture, unless it can react with the calcium hydroxide formed in the first few days of hydration. Together they form a calcium silica hydrate (CSH) compound that over time effectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chloride ions. By reducing ion diffusion, the electrical resistance of the concrete also increases
CATHODIC PROTECTIONImpressed current (active) Sacrificial anode (passive)
TITANIUM ANODE MESHTYPICALLY ATTACHED TO THE CONCRETE SURFACE AND THEN ENCAPSULATED IN CEMENTITIOUS MATERIALS. B- EASILY CONFORMS TO THE STRUCTURE GEOMETRY. C- MOST USED IMPRESSED CURRENT ANODE FOR CONCRETE.A.
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
1. 2. 3. 4.
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
REFRENCES
Concrete Society Technical Reports TR 36 and 37
www.corrocell.co.uk