Corrosion and Passivation of Steel Reinforcement
Exposed steel will corrode in moist atmospheres due to differences in the electrical potential on the steel surface forming anodic and cathodic sites. The metal oxidises at the anode where corrosion occurs according to:
Fe (metal) --> Fe2+ (aq.) + 2e-
Simultaneously, reduction occurs at cathodic sites, typical cathodic processes being:
½O2 + H20 + 2e-(metal) --> 2OH-(aq.)
2H+(aq.)+ 2e- (metal) --> H2(gas)
The electrons produced during this process are conducted through the metal whilst the ions formed are transported via the pore water which acts as the electrolyte.
Concrete as an Environment
The environment provided by good quality concrete to steel reinforcement is one of high alkalinity due to the presence of the hydroxides of sodium, potassium and calcium produced during the hydration reactions. The bulk of surrounding concrete acts as a physical barrier to many of the steel's aggressors. In such an environment steel is passive and any small breaks in its protective oxide film are soon repaired. If, however, the alkalinity of its surroundings are reduced, such as by neutralisation with atmospheric carbon dioxide, or depassivating anions such as chloride are able to reach the steel then severe corrosion of the reinforcement can occur. This in turn can result in to staining of the concrete by rust and spalling of the cover due to the increase in volume associated with the conversion of iron to iron oxide.
Factors Affecting Corrosion Rates of Steel in Concrete
The factors which determine the corrosion rate of steel in concrete are; the presence of an ionically conducting aqueous phase in contact with the steel (i.e. pore water), the existence of anodic and cathodic sites on the metal in contact with this electrolyte and the availability of oxygen to enable the reactions to proceed.
The permeability of the concrete is important in determining the extent to which aggressive external substances can attack the steel. A thick concrete cover of low permeability is more likely to prevent chloride ions from an external source from reaching the steel and causing depassivation.
Alternatives for the Reinforcing Phase
Where an adequate depth of cover is difficult to achieve due to design considerations or where aggressive environments are expected such as in marine structures or bridge decks, additional protection may be required for the embedded steel. This may take many and varied forms and commercial interest in this field is strong. The steel reinforcement itself may be made more able to maintain its passivity by providing it with a protective coating such as zinc, epoxy resin or stainless steel cladding. In extreme circumstances, solid stainless steel reinforcements may be used, although the perceived additional cost restricts its use in all but the most specialized applications.
The Ideal Situation
There can be little doubt that the most effective way of protecting steel which is embedded in concrete is to provide it with an adequate depth of cover by high strength, low permeability concrete free from depassivating ions such as chlorides. However, in the real world, concrete is laid by the tonne in all weathers and environments, exposed to industrial atmospheres, de-icing salts and seawater.
The Real Situation
Contaminated materials and poor workmanship are hard to avoid completely but by understanding the often complex chemical and electrochemical conditions that can exists it should be possible to develop ways of producing structures which will last long into the next century.
The majority of reinforced concrete around the world performs adequately and gives few problems. A minority of structures have deteriorated due to either the action of aggressive components from the external environment or incompatibility of the mix constituents. Problems can arise as a result of incomplete or inaccurate site investigation, poor design, badly specified concrete, poor workmanship and a range of other factors.
Stages of Deterioration
The mechanisms of deterioration are primarily chemico-physical in nature (i.e. a chemical reaction with the formation of products greater in volume than the reactants producing physical effects such as cracking and spalling) and occur in three discrete stages:
• Stage 1: Initiation (t0) – Concentration of aggressive species is insufficient to initiate any chemical reactions or the chemical reaction is occurring very slowly. No physical damage has occurred. The duration of t0 may vary from a few minutes to the design life of the structure.
• Stage 2: Propagation (t1) – Chemical reactions begin or are continuing, some physical damage may occur but is insufficient to cause distress. Acceleration of the deterioration process usually occurs during this stage due to increased accessibility of aggressive ions or modification of the concrete environment.
• Stage 3: Deterioration (t2) – Rapid breakdown of the fabric of the structure. The combined effects of the physical and chemical processes are of sufficient severity that the structure is no longer serviceable (failure occurs) and major remedial work or, in extreme cases, demolition is required.
Modes of Deterioration
Deterioration may occur due to a number of mechanisms on which a large body of literature already exists. These include:
• Corrosion of reinforcement, due to:
• Chloride ions.
• Change in the rebar environment (impinging cracks).
• Sulphate attack of concrete.
• Salt recrystallisation (exfoliation)
• Soft water/acid attack of concrete.
• Alkali Aggregate Reaction (AAR).
• Thermal incompatibility of concrete components (TICC).
• Frost Damage
All these factors must be considered during design and specification.
Factors Influencing Rates of Deterioration
The environment provided by good quality concrete for the embedded steel reinforcement is one of high alkalinity (generally >pH 13), produced by the hydroxides of sodium, potassium and calcium released during the various hydration reactions. In addition, the bulk of surrounding concrete acts as a physical barrier to most of the substances that may lead to degradation of the reinforcement.
Provided this environment is maintained, the steel remains passive and any small breaks in the stable protective oxide film are soon repaired. However, if the alkalinity of the surroundings is reduced, for example by reaction with atmospheric carbon dioxide (carbonation), or if depassivating chloride ions are made available at the surface of the steel then corrosion may be initiated, resulting in loss of steel section and spalling of cover.
Depth of Cover
Inadequate cover is invariably associated with areas of high corrosion risk due to both carbonation and chloride ingress. By surveying the surface of a structure with an electromagnetic covermeter and producing a cover contour plot, the high-risk areas can be easily identified. A cover survey of newly completed structures would rapidly identify likely problem areas and permit additional protective measures to be taken.
While this remains an ill-defined area, two forms of crack are of interest when evaluating the condition of a reinforced concrete structure; those present before the onset of corrosion which might assist the corrosion processes (large shrinkage and movement cracks), and those produced as a direct consequence of corrosion (expansive corrosion products leading to cracking and spalling).
It should be remembered that reinforced concrete is intrinsically a cracked material because the steel stops the structure failing in tension but the brittle concrete cracks to the depth of the reinforcement. Only those cracks above a critical width which intersect the steel are liable to assist the corrosion processes.
Presence of Chloride Ions
Chloride ions can enter concrete in two ways:
• They may be added during mixing either deliberately as an admixture or as a contaminant in the original constituents
• They may enter the set concrete from an external source such sea water.
Once chloride ions have reached the reinforcement in sufficient quantities they will depassivate the embedded steel by breaking down the protective oxide layer normally maintained by the alkaline environment.
The concentration of chloride ions required to initiate and maintain corrosion is dependant upon the alkalinity and it has been shown that there is an almost linear relationship between hydroxyl ion concentration and the respective threshold level of chloride.
Carbon dioxide present in the atmosphere combines with moisture in the concrete to form carbonic acid. This reacts with the calcium hydroxide and other alkaline hydroxides in the pore water resulting in a reduction in the alkalinity of the concrete. The rate at which this neutralisation occurs is influenced by factors such as moisture levels and concrete quality.
The depth of carbonation in a structure can be quite easily established by the use of phenolphthalein indicator on freshly exposed material. The distinctive colour change, from deep pink in unaffected concrete to clear in the carbonated region, is sufficiently accurate for most practical purposes provided a number of measurements are obtained to allow for local variations.
The microclimate to which the reinforced concrete member is exposed directly affects the likelihood and extent of reinforcement corrosion. Factors such as chloride ion levels and pH have already been discussed but the most important aspect of the local environment is the moisture level. Carbonation, chloride ion ingress, resistivity and corrosion rate are all greatly influenced by the degree of saturation.
• The majority of reinforced concrete structures show excellent durability and perform well over their design life.
• Adverse environments or poor construction practice can lead to corrosion of the reinforcing steel in concrete.
• The major mechanisms for corrosion are atmospheric carbon dioxide ingress (carbonation) and chloride attack from cast-in or diffused chlorides.
• The corrosion and deterioration mechanisms are essentially the same for both carbonation and chloride attack.
• Proper choice of materials, adequate cover to reinforcement, good quality concrete and attention to the environment during construction will enhance the durability of reinforced concrete structures.