Brian Cherry Paper Series

Up to Date Overview of Aspects of Steel Reinforcement Corrosion in Concrete – Warren Green, Frank Collins and Maria Forsyth (2017)

SUMMARY

Professor Brian Cherry has always been a firm believer in understanding first the fundamentals of any aspects of corrosion science, then the mechanisms, before embarking on the engineering of solutions to the management of materials corrosion. So it has been with steel reinforced concrete. This paper endeavours to walk a reader through the fundamental and mechanistic aspects of the excellent protection afforded to steel reinforcement by concrete (including electrochemistry), corrosion of steel reinforcement (uniform (microcell/mini-cell) and pitting (macrocell) corrosion, corrosion products composition and development), chloride induced corrosion mechanisms (passive film breakdown/pit initiation, metastable pitting, pit growth/propagation,chemical conditions within propagating pits and reinforcing steel quality effects), carbonation induced corrosion mechanisms, leaching induced corrosion of reinforcement and reinforcing steel stray current corrosion and interference.

1. INTRODUCTION

Professor Brian Cherry in his education of students and practitioners has always ensured that fundamental aspects are conveyed. As a result, it is appropriate to begin this symposium with an overview of key fundamentals and mechanisms when it comes to the protection of reinforcement by concrete. There are several degradative processes which affect some reinforced concrete structures and amongst these, the most common cause of deterioration is the corrosion of steel reinforcement. An up-to-date overview of the key fundamentals and mechanisms of corrosion of steel reinforcement in concrete is also considered warranted.

Much is expected of some of our concrete structures and buildings, some are in extremely aggressive environments, they may be many decades of age, yet still expected to meet service life, they are of critical importance in terms of function or location, it is impossible to replace them, etc such that some require repair and protection during their service lives. Dr John Broomfield will provide an overview of repair and protection aspects relating to corroding steel in concrete in the second paper of this symposium.

The subsequent papers and speakers at the symposium then address other aspects of reinforced concrete corrosion, protection repair and durability that Professor Brian Cherry has taught, researched and provided innovative solutions to over many decades, including:

• Condition assessment of structures;
• Modelling and deterioration prediction;
• Concrete repair and protection;
• Cathodic protection;
• Corrosion inhibitors; and
• Alternate metallic reinforcement.

2. STEEL REINFORCED CONCRETE

Concrete is the most widely used material of construction in the world, with 25 gigatonnes/year consumed globally. Concrete is strong in compression but weak in tension and so much of the concrete is reinforced, usually with steel. The steel reinforcement can take the form of conventional carbon steel (black steel), prestressing steel, post-tensioned steel and steel fibres and its widespread utility is primarily due to the fact that it combines the best features of concrete and steel. The two complement one another and so by combining them together a composite that has good tensile strength, shear strength and compressive strength combined with durability and fire resistance can be formed.

When suitably designed, constructed and maintained, reinforced concrete provides service lives of numerous decades to structures and buildings. Concrete provides reinforcing steel with excellent
corrosion protection. The highly alkaline environment in concrete results in the spontaneous formation of a stable, tightly adhering, thin protective oxide film (passive film) on the steel reinforcement surface, which protects it from corrosion. In addition, well proportioned, compacted and cured concrete has a
low penetrability, thereby minimising the ingress of corrosion-inducing species via the aqueous phase. It also has a relatively high electrical resistivity, which reduces the corrosion current and hence the rate of corrosion if corrosion is initiated.

There are however, several degradative processes which affect some reinforced concrete structures leading to loss of functionality, unplanned maintenance/remediation/replacement, and in the worst cases, loss of structural integrity and resultant safety risks. Amongst these, the most common cause of deterioration is corrosion of conventional carbon steel (black steel), prestressing steel and post-tensioned steel
reinforcement.

3. PROTECTION AFFORDED TO STEEL REINFORCEMENT BY CONCRETE

3.1 Portland Cement and Blended Cement Binders

The hydraulic binder of concrete commonly consists of Portland cement or of mixtures of Portland cement and one or more of fly ash, ground granulated iron blast-furnace slag or silica fume (Standards Australia, 2010). The latter are referred to as Blended cements.

AS 3972 (2010) defines Blended cement as a hydraulic cement containing Portland cement and a quantity comprised of one or both of the following:

1. Greater
   than 7.5% of fly ash or granulated iron blast-furnace slag, or both.
2. Up
   to 10% silica fume.

Furthermore, AS 3972 (2010) designates cements as general purpose and special purpose. By definition, general purpose cements may be portland cements (Type GP) or blended cements (Type GB). Special purpose cements are also defined in AS 3972 (2010) and may be portland or blended cements with restrictions being placed on their composition.

Portland cement consists (nominally) of a mixture of oxides:-

CaO, SiO₂, l₂O₃, Fe₂O₃, MgO, Na₂O and K₂O.

These are however combined together as a series of cement compounds.These cement compounds are themselves combinations of the principle oxides (Cherry & Green, 2017).

Fly ash is the solid material extracted from the flue gases of a boiler fired with coal (Standards Australia, 2016). Fly ash for use in cement needs to comply with AS 3582.1 (2016). The fly ash content of a FA blended cement based concrete varies but is commonly in the range 20-35%. For example, some Road Authorities in Australia stipulate in their structural concrete specifications a minimum 25% FA content (Roads & Maritime Services, 2013; VicRoads, 2017) so as to achieve increased durability.

Granulated iron blast-furnace slag is the glassy non-metallic granular material, essentially consisting of silicates, aluminosilicates and calcium, resulting from the rapid chilling of molten iron blast-furnace slag (Standards Australia, 2016). Ground granulated iron blast-furnace slag (BFS) for use in cement needs to comply with AS 3582.2 (2016). The slag content of a BFS blended cement based concrete varies but is commonly in the range 50-70%. For example, some Road authorities in Australia stipulate in their structural concrete specifications a minimum 50-65% BFS content (Roads & Maritime Services, 2013; VicRoads, 2017) so as
to achieve increased durability.

Silica fume is a very fine pozzolanic material
produced by electric arc furnaces as a by-product of the production of
elemental silicon or ferro-silicon alloys. Silica fume is also known as
‘condensed silica fume’ and ‘microsilica’. The
amorphous silica content of a silica fume (SF) blended cement based concrete
typically varies between 5-15%.  For
example, some Road Authorities in Australia stipulate in their structural
concrete specifications a 10% SF proportion (VicRoads, 2017).

Triple
blend cement based concretes are also used for increased durability. For
example VicRoads in their “Structural Concrete Specification 610” (2017)
stipulate that in a triple blend concrete mix, the Portland cement shall be a
minimum of 60% and the individual contribution of Slag, Fly Ash or Amorphous
Silica shall be a maximum of 40%, 25% or 10% respectively.  For marine durability, triple blend cement
based concretes such as 52% shrinkage-limited (SL) cement, 25% FA and 23% BFS
have been used (Green et al, 2009).

3.2 Alkaline Environment in Concrete

As mentioned above, the highly alkaline
environment in concrete results in the spontaneous formation of a passive iron oxide
film on reinforcing steel which protects it from corrosion.

The reaction of the cement compounds of
Portland cement or Blended cement with water results in the setting and
hardening of the cement paste so that it binds the aggregate (coarse and fine)
of the concrete together. A product of the hydration of Portland and Blended
cements is Ca(OH)₂ together with NaOH and KOH.

As a result, the pH of the pore solution of
concrete is normally in the range of 12-14 (Page & Treadaway, 1982; Tuutti,
1982; Tinnea & Young, 2000; Tinnea, 2002; Broomfield, 2007; Abd El Haleem
et al, 2010; Ghods, et al, 2011; Cherry & Green, 2017), and maintained due
to the CaO-Ca(OH)₂ pH buffer until this is overcome by, say,
carbonation, refer later, reduced by, say, leaching, refer later, or
neutralised by chloride ion induced pitting corrosion reactions, refer later.

For iron (steel) in a pH 12-14 alkaline
environment, the Potential – pH diagram, see Figure 1, tells us that the metal
will be passive.

3.3 Physical Barrier Provided by Concrete

In addition to providing a high pH passivating
environment for reinforcing steel, concrete also provides a physical barrier
against the ingress of corrosion-inducing substances.

The quality of concrete as a physical barrier may
be assessed from penetrability data (and chemical data, i.e. blended cements
have better chloride ion binding/adsorption capacity as well as improving the
porosity of the aggregate-paste transition zone). Penetrability data is
collected from measurements of a concrete’s water absorption (various tests),
water permeability (various tests), chloride diffusion (various tests) and
gaseous diffusion/penetration (O₂ and CO₂, various tests)
(Concrete Institute of Australia, 2015).

The factors affecting concrete penetrability
include the type and quality of aggregates, type of cement (binder), cement
(binder) content, water/cement (binder) ratio and production variables such as
mixing uniformity, placement, degree of compaction and adequacy of cure.

3.4 Passivity and the Passive Film on Steel Reinforcement in Concrete

3.4.1 Thermodynamics of Passivity

Thermodynamics,
the science of energy changes, is applied to corrosion studies to determine why
a particular metal does or does not tend to corrode in a particular
environment.

Chemical and electrochemical thermodynamic data [chemical potential (µ₀) values, standard free energy change of reaction ( △G₀) values, and standard electrode potential (E₀) values] provides the means for deciding which of a set of reactions is thermodynamically favoured and for predicting the most stable reaction products under specified conditions of electrode potential and solution composition.

Marcel
Pourbaix (1904-1998) recognised that the corrosion state of a metal could be represented
by a point on a diagram the two axies of which were the potential and pH. These
diagrams are now often termed ‘Pourbaix’ diagrams.

The Pourbaix diagram plots electrochemical stability for different redox states of an element as a function of pH. The Fe-H₂O system at 25°C, shown in Figure 1, is of most relevance to reinforcing steel in concrete. Typically the concentrations of dissolved ions, other than H⁺ and OH^–, are taken to be 10^-6M and solid phases are assumed to be pure.

If the potential of iron immersed in neutral water (pH about 7) is measured using a standard hydrogen electrode (SHE), it will be seen in Figure 1 to have a value represented by, say – 0.3V. An
examination of the Pourbaix diagram at Figure 1 suggests three possible means of achieving a reduction in the corrosion. First, the metal can have its potential so changed in the negative direction that it enters the domain of immunity, i.e. it can be cathodically protected. Secondly, the potential can be changed in the positive direction so that it enters the passive domain. This is anodic protection and is only applicable to some metals such as iron but not for example to zinc. Thirdly, the pH of the electrolyte can be so adjusted that the metal enters the passivity domain, and this process is considered inhibition.

To summarise, Figure 1 shows us that there are three basic zones of behavior representing states of lowest energy for iron
(steel):

1. Immunity
   – iron metal is thermodynamically stable and is immune to corrosion.
2. Corrosion
   – Fe²⁺, Fe³⁺ and HFeO₂^– ions are
   thermodynamically stable and corrosion will occur at a rate which cannot be
   predicted thermodynamically.
3. Passivity
   – iron oxides are thermodynamically stable. These oxides give rise to a
   condition termed passivity since significant corrosion may be stifled owing to
   the formation of a protective oxide layer on the metal surface (see shaded area
   on Figure 1 in the high pH region).

Having described the information that can be
gleaned from Pourbaix diagrams with respect to deciding which of a set of reactions is thermodynamically favoured and which reaction products are most stable, it is necessary to recognise that certain limitations apply to the use of thermodynamics and Pourbaix diagrams. The kinetics (rates) of possible reactions are not considered, and therefore, it is impossible to predict whether a particular reaction, which is thermodynamically favoured, will occur at a significant rate in practice. The aqueous solution composition in the vicinity of the corroding surface also needs to be known. This presents difficulties when considering real corroding systems in which concentration gradients may be developed.

3.4.2 Kinetics of Positivity

Figure 2 shows the anodic polarisation curve
for a passive metal, such as reinforcing steel in the alkaline environment of
concrete.

At potentials more negative than the equilibrium
potential or reversible potential for Fe/Fe²⁺ (i.e. E_o Fe/Fe²⁺), iron is immune to corrosion or dissolution. Raising the potential to values more positive than E_o Fe/Fe²⁺ leads to active corrosion at a rate which initially increases with increasing overpotential (maximum corrosion current, I₁). At some potential more positive or noble than the equilibrium potential for the formation of a surface oxide film (i.e. E_o Fe/Fe₂O₃) active corrosion practically ceases (i.e. I₂) owing to surface oxide film formation which renders iron passive and allows the passage of a small ‘leakage current’ (i.e. I₂→I₃). This ‘leakage current’ results from the necessity to reform the surface oxide passive film at points of local breakdown and to replace oxide lost by dissolution to the environment.

3.4.3 Passive Film Foundation

The corrosion of steel reinforcement in
concrete is, initially at least, a very slow process. As a result of the
hydration reaction of the cement (binder) compounds as previously noted, the
pore water surrounding the steel has a high pH (in the range
12-14) and so the steel is completely covered with a dense gamma ferric oxide (γ-Fe₂O₃,
maghemite) protective passive film. This is formed by
the anodic reaction (1):

4Fe
+ 12OH^– → 2γFe₂O₃ + 6H₂O + 12e^–        (1)

Once the
film has formed, the reaction continues (at a very slow rate) at the
metal/oxide interface as a result of hydroxyl ions diffusing through the film
of iron oxide. The cathodic process is ascribed to the reaction (2):

3O₂
+ 6H₂O + 12e^– → 12OH^–            (2)

This
reaction takes place on the surface of the passive film.  The cathodic process requires that oxygen
molecules diffuse through the concrete to the passive film surface and are
there reduced to hydroxyl ions.  The
electrical circuit is completed by the ionic transport through the passive film
of the hydroxyl ions resulting from the cathodic reaction moving towards the
metal/film interface and by electronic transport through the protective film to
the film/electrolyte interface.
Corrosion is proceeding in this state corresponding to the conversion of
iron to iron oxide.

Passive (leakage) corrosion current densities
(i_corr) measured for steel reinforcement in concrete are of the
order of 0.1µA/cm² (Andrade & Gonzalez, 1978; Hansson, 1984).
This current density therefore corresponds to an insignificantly slow corrosion
(penetration) rate of 1µm/year. This corrosion rate is of the order of 1mm in
1000years and so it can be seen that the reinforced concrete structure or
building element may be regarded as unaffected by corrosion. The steel in this
state is said to be “passive” or protected by a “passive film” (Cherry &
Green, 2017).

Since the passive film maintains a constant
thickness (it dissolves from the outside as quickly as it is formed on the
inside) no great swelling stresses are applied to the concrete and so no
cracking of the concrete takes place (Cherry & Green, 2017).

3.4.4 Passive Film Composition

The precise nature of the passive film which is
formed by the reaction of iron with the highly alkaline environment of the pore
water in concrete varies. As above, it can be considered to be generally a
dense gamma ferric oxide (γ-Fe₂O₃, maghemite).

The characteristics of oxides formed on steel
under alkaline conditions and their electrochemical behavior have been examined
by, for example, Nasrazadani (1997), Cornell and Schwertmann (2000) and Freire
et al (2009). Iron-oxy-hydroxides like goethite (a-FeOOH), lepidocrocite (γ-FeOOH) and
akagonite (β-FeOOH) below which more protective forms of iron oxides including
magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃)
and haematite (a-Fe₂O₃) are
assumed to be the constituents of the passive film formed on steel electrodes
under alkaline conditions.

Cohen (1978) indicated that the composition of
the passive oxide layer is a spinel a-Fe₃O₄-γ-Fe₂O₃
solid solution. Sagoe-Crentsil and Glasser (1989) attributed the passive action
of pH>12 concrete pore solution to the formation of a surface layer of Fe₂O₃-Fe₃O₄.
Ghods et al (2011) have more recently also reported two-layer oxides in passive
films, with an inner Fe²⁺ and Fe³⁺ layer and an outer
pure Fe³⁺ layer. Al-Negheimish et al (2014) have also proposed a
two-layer oxide passive film but the top layer was composed of FeO and FeOOH
with Fe₂O₃ close to the steel surface.

More recently Ghods et al (2013) report of a
tri-layer model of oxides for the passive film on the steel when exposed to
simulated concrete pore solution namely a layer of FeO at the metal surface,
then a layer of Fe₃O₄ and then an outer layer of Fe₂O₃.

MnS inclusions are often found in steels and
their effect is to change the Fe₂O₃ in the passive film
from γ-Fe₂O₃ to a-Fe₂O₃
according to Al-Negheimish et al (2014).

3.4.5 Passive Film Thickness

Singh and Singh (2012) proposed that the
passive film formed in simulated concrete pore solution is of “ultra thin
thickness” (i.e. <10 nm).

Ghods et al (2013) reported that the thickness
of the tri-layer (FeO/Fe₃O₄/Fe₂O₃)
passive oxide film was uniform between
5 and 13 nm.

Al-Negheimish et al (2014) have determined
thicknesses of 6 to 7 nm for the passive film on steel in simulated concrete pore
solution.

3.4.6 Models and Theories of Passive Film Development on Iron

Veluchamy
et al (2017) indicate that the understanding of the passive state of metals
started with Faraday (1844). Though several theories, models and experimental
works on passivity have been published in the literature, the mechanisms
underlying the stability of the passive oxide over the metal still remains a
mystery according to Veluchamy et al (2017). They undertook a detailed review
of theoretical and experimental results for the iron/electrolyte system
invoking the high field model (HFM) (ion-migration mechanism), modified high
field model, point defect model (PDM), variants of PDM (VPDM), diffusion
Poisson coupled model (DPCM) and the density functional theory based atomistic
model. The experimental and model-predicted dependencies on applied voltage,
pH, chloride and temperature have also been presented and discussed by
Veluchamy etl al (2017).

Suffice
to say that the models and theories of passive film development on iron are complex
and still not clear, thus discussion of such is beyond the scope of this paper.

4. CORROSION OF STEEL REINFORCEMENT

4.1 General

The passivity provided to steel reinforcement
by the alkaline environment of concrete may be lost if the pH of the concrete
pore solution falls because of carbonation or if aggressive ions such as
chlorides penetrate in sufficient concentration to the steel reinforcement
surface. Carbonation of concrete occurs as a result of atmospheric CO₂
gas (and atmospheric SO_x and NO_x gases) neutralising the
concrete pore water (lowering its pH to 9) and thereby destroying the passive
film. Leaching of Ca(OH)₂ (and NaOH and KOH) from concrete also
lowers pH to cause corrosion of steel reinforcement. Stray electrical currents,
most commonly from electrified traction systems, can also breakdown the passive
film and cause of corrosion of steel reinforced and prestressed concrete
elements.

The corrosion mechanisms by chlorides, carbonation,
leaching and stray electrical currents is provided in subsequent sections but
discussion of the different forms of corrosion, composition of reinforcing
steel corrosion products (rusts) and the consequence of the corrosion products
is provided herewith.

4.2 Uniform (microcell) Corrosion and Pitting (macrocell) Corrosion

According to the different spatial location
of anodes and cathodes, corrosion of steel in concrete can occur in different
forms (Elsener, 2002):

• As microcells, where anodic and cathodic reactions are immediately
  adjacent, leading to uniform steel (iron) dissolution over the whole surface.
  This uniform (or general) corrosion is typically caused by carbonation of the
  concrete or by very high chloride content at the steel reinforcement.
• As macrocells, where a net distinction between corroding areas of the
  steel reinforcement (anodes) and
  non-corroding passive surfaces (cathodes) is found. Macrocells occur mainly in
  the case of chloride induced corrosion (pitting) where the anodes are small
  with respect to the total (passive) steel reinforcement surface.

Cherry and Green (2017) also suggest that
uniform or general corrosion of steel reinforcement in concrete where the
cathodic and anodic sites may be separated by millimetres (rather than
microns), can be termed mini-cell
corrosion.

4.3 Corrosion Products Composition – Chloride Induced Corrosion

The exact nature of reinforcing steel corrosion
products (rusts) associated with chloride induced corrosion varies markedly
depending on conditions.  Corrosion
products are of various layers and of various compositions. Generally speaking
they will be layers and combinations of ferrous and ferric hydroxides,
hydroxyl/oxides and oxides each with possible varying degrees of hydration.

Melchers and Li (2008) and Pape and Melchers
(2013) have identified goethite (a-FeOOH), akagenite (β-FeOOH),
lepidocrocite (γ-FeOOH) as corrosion products within chloride contaminated
reinforcing steel samples from concrete structures. “Green rusts” can also
occur. Melchers and Li (2008) and Pape and Melchers (2013) have identified
compounds such as iron oxide chloride (FeOCl), hibbingite (a-Fe₂(OH)₃Cl), iron
chloride hydrate (FeCl₂.4H₂O), “Green Rust I” (carbonate
variety) and “Green Rust II” (sulphate variety) within the corrosion products
(rusts) of samples taken from steel reinforced and prestressed structures in
chloride-rich environments. Discussion of chloride induced corrosion mechanisms
is provided at Section 5.

For corroding mortar specimens subject to
chloride ingress, Koleva et al (2006) identified corrosion products mainly
consisting of highly crystallised goethite (a-FeOOH), lepidocrocite (γ-FeOOH) and
akagenite (β-FeOOH). Depending on the ratio of iron and chloride ions, the iron
oxychlorides and iron oyhydroxides present different morphologies and exert
influences on steel/mortar interface microstructure and on material behavior.

The corrosion products generally detected by
Vera et al (2009) from embedded steel in concrete cylinder samples exposed to
simulated marine and industrial conditions and a natural marine atmospheric
environment were:- lepidocrocite (γ-FeOOH), goethite (a-FeOOH) and magnetite (Fe₃O₄);
but in the chloride contaminated environments the presence of akagenite
(β-FeOOH)was detected; and, in the natural marine atmospheric environment the
formation of siderite (FeCO₃) was also observed.

4.4 Corrosion Products Composition – Carbonation Induced Corrosion

Attack
of concrete by carbon dioxide is termed carbonation and the mechanisms of such
are provided at Section 6, suffice to say here that in terms of carbonation-induced corrosion
products, Kolio et al (2015) on studies performed on existing reinforced
concrete facades of 12 buildings that had been in use for 30-43 years
identified corrosion products that were mostly hydroxide type of rusts (i.e.
goethite/a-FeOOH and lepidocrocite/γ-FeOOH).

Huet et al (2005) identified corrosion products
mainly composed of magnetite (Fe₃O₄) and lepidoocrocite (γ-FeOOH)
in laboratory based studies of mild steel in carbonated concrete pore solution.

4.5 Corrosion Products Development – Cracking, Delamination and Spalling

The process of reinforcement corrosion will
lead to corrosion products which, generally speaking, will occupy a greater
volume than the iron dissolved in its production, refer Figure 4 (Jaffer &
Hansson, 2009). Furthermore, when the corrosion products become hydrated the
volume increase is even greater (Broomfield, 2007). Refer hydrated haematite (a-Fe₂O₃.3H₂O)
(red rust) in Figure 3.

The consequence of higher volume corrosion
products is then to develop tensile stresses in the concrete covering the
reinforcement. Concrete, being weak in tension, will crack as a consequence of
the corrosion. Continuing formation of corrosion product(s) will enhance the
expansion which will ultimately lead to cracked pieces of concrete cover detaching,
leading to delamination and then spalling. Rust staining of the concrete may or
may not occur together with the cracking or as a prelude to delamination and
spalling. Section loss, bond loss and anchorage loss of the reinforcement also
occurs as a result of the corrosion.

Kolio et al (2015) for example, describing work
they had carried out on carbonation initiated corrosion on concrete façade
panels, determined that the corrosion products (i.e. as previously discussed,
goethite/a-FeOOH and lepidocrocite/γ-FeOOH)
had a unit volume increase of roughly 3 times the volume of iron. Furthermore,
they identified by taking into account the relative volume of the corrosion
products that the required corrosion penetration (metal loss) to initiate
visually observable cracks in the studied building façade panels was by average
53.6 µm with a corresponding corrosion (rust) products thickness of 111.7 µm.

Hydrated haematite (a-Fe₂O₃.3H₂O)
(red rust) induced cracking, delamination and spalling of cover concrete can be
frequently seen in deteriorated reinforced concrete structures and buildings,
an example of which is provided at Figure 4 for a marine structure.

4.6 Corrosion Products Development – No Visible Damage

It is most important to note however, that not
all corrosion of reinforcement leads to rust staining, cracking, delamination
or spalling of cover concrete. Localised pitting, localised corrosion at
cracks, localised corrosion at concrete defects, etc can result in marked
section loss (loss of bond, loss of anchorage) and
ultimately structural failure without the visible consequences of corrosion on
the concrete surface, i.e. no rust staining, cracking, delamination or spalling
of cover concrete (Green et al, 2013). An example of marked localised section
loss of reinforcement due to chloride induced pitting (macrocell) corrosion is
provided at Figure 5. The definition of macrocell corrosion being as at Section
4.2 where a net distinction between corroding areas of the steel reinforcement
(anodes) and non-corroding passive surfaces (cathodes) is found.

Elsener (2002) pointed out that macrocell
(pitting) corrosion is of great concern because the local dissolution rate
(reduction in cross-section of the conventional/prestressing/post-tensioned
steel reinforcement, loss of bond, loss of anchorage) may greatly be
accelerated due to the large cathode/small anode area ratio. Indeed, values of
local corrosion (penetration) rates of up to 1 mm/year have been reported for
bridge decks, sustaining walls or other chloride contaminated steel reinforced
concrete structures according to Elsener (2002).

Broomfield (2007) also notes that ‘black rust’
or ‘green rust’ (due to the colour of the liquid when first exposed to air
after breakout) corrosion can also be found under damaged waterproof membranes
and in some underwater or water saturated structures. He states that it is
potentially dangerous as there is no indication of corrosion by cracking and
spalling of the concrete and the reinforcing steel may be severely weakened
before corrosion is detected. Reinforcement bars may be “hollowed out” in such
deoxygenated conditions.

5. CHLORIDE INDUCED CORROSION MECHANISMS

5.1 General

It is known that chloride ions in sufficient
concentration can destroy the passivity of steel reinforcement (conventional,
prestressed and post-tensioned) in concrete leading to pitting corrosion.
Pitting corrosion is localised accelerated dissolution of metal that occurs as
a result of breakdown of the otherwise protective passive film on the metal
surface (Frankel, 1998). Various mechanisms of chloride ion induced corrosion
of steel reinforcement in concrete are proposed in the literature.

Frankel (1998) comments that many engineering
alloys, such as stainless steels and aluminium alloys, are useful only because
of passive films, which are thin (nanometer scale), oxide layers. Furthermore that
such passive films however, are often susceptible to localised breakdown
resulting in accelerated dissolution of the underlying metal and if the attack
initiates on an open surface, it is called pitting corrosion and at an occluded
site it is called crevice corrosion.

Fundamental studies of pitting corrosion on
such engineering alloys appear to have typically focused on the various stages
of the pitting process, namely: passive film breakdown, the growth of
metastable pits (which grow to about the micron scale and then repassivate) and
the growth of larger, stable pits (Frankel, 1998).

Like Frankel (1998), Angst et al (2011) propose
from a fundamental viewpoint that chloride induced pitting corrosion of steel
reinforcement in concrete can be considered to occur as a sequence of distinct
stages:- pit nucleation, metastable pitting and stable pit growth.

The mechanisms of pitting corrosion of steel in
chloride contaminated concrete will therefore be considered on a stages basis as
well as discussion of some other mechanistic aspects relating to the
reinforcing steel quality itself.

5.2 Passive Film Breakdown/Pit Initiation

The American Concrete Institute (ACI) Committee
222 on ‘Corrosion of Metals in Concrete’ (1985) proposes three theories to
explain the effects of chloride ions on steel reinforcement:

1. The
   Oxide Film Theory – postulates that chloride ions penetrate the passive film
   through pores or defects in the film, thereby “colloidally dispersing” the
   film.
2. The
   Adsorption Theory – postulates that chloride ions are adsorbed preferentially
   on the metal surface in competition with dissolved oxygen or hydroxyl ions. The
   chloride ion promotes the hydration of the metal atoms, thus facilitating
   anodic dissolution.
3. The
   Transitory Complex Theory – postulates that chloride ions form a soluble
   complex with ferrous ions which then diffuses away from the anodic sites.
   Thereafter, the complex decomposes, iron hydroxide precipitates, and the
   chloride ions are released to transport more ferrous ions from anodic sites.

Frankel (1998), in a comprehensive review paper
of pitting corrosion of metals, indicates that theories for passive film
breakdown and pit initiation of pure metal systems (i.e. not alloys where
pitting may also be associated with inclusions or second-phase particles) have
been categorised in three main mechanisms that focus on passive film
penetration, film breaking, or adsorption, refer Figure 6.

Soltis (2015) on the other hand, after a review
of the above three main mechanisms that focus on passive film penetration, film
breaking, or adsorption together with mechanisms such as the ‘percolation
model’ and ‘voids at the metal-oxide interface’, proposes that despite a number
of theories for passive film breakdown/pit initiation, this aspect of localised
pitting corrosion remains still the least understood, with a generally accepted
theory yet to come. Soltis (2015) then surmises that it is entirely possible that
we may fail to develop a single theory and will need to focus on the
development of system-specific models, purely because of differences in passive
film properties.

Given the Soltis (2015) proposition that there
may not be a single theory for passive film breakdown and pit initiation and
that development of system-specific models may be necessary, in the remainder
of this sub-section, literature has been reviewed with the aim of proposing a
system-specific mechanism(s) for chloride ion induced depassivation and pit
initiation of reinforcement steel in concrete.

Bird et al (1988) in laboratory based solution
studies in the pH range 10-14 and containing NaCl in the range 10^-3
– 10⁰ M, supports the idea that the first stages of localised
passive film breakdown depend upon the competition for adsorption, at the
oxide/liquid interface, between hydroxyl ions and aggressive ions such as
chloride ion.

Leek and Poole (1990) for steel in concrete suggest
that the breakdown of passivity by chloride ions is achieved by disruption of
the passive film to substrate adhesive bond. An initial bond breakdown occurs
beyond which internal pressure within the film due to surface tension effects,
acts to disband chemically unaltered film, thereby expanding the size of the
site of depassivation. Little or no chemical dissolution of the passive film
occurs.

Sagoe-Crentsil and Glasser (1990) for steel in
chloride contaminated concrete show evidence for the ‘Transitory Complex
Theory’ above. X-ray diffraction analysis of steel from chloride contaminated
cement paste samples showed the formation of an intermediate, soluble, FeCl₂.4H₂O
complex. Visually, they advise this was evident as a greenish-blue hue.

As noted previously, Pape and Melchers (2013) identified
FeOCl, a-Fe₂(OH)₃Cl,
FeCl₂.4H₂O, “Green Rust I” (carbonate variety) and “Green
Rust II” (sulphate variety) compounds within the corrosion products of samples
from reinforced and prestressed structures in chloride environments.

It is then proposed that the presence of
“green/greenish-blue rusts” may be evidence of the ‘Transitory Complex Theory’
above.

Figures 7 and 8 at Section 5.4.2 show
schematically the reactions within a propagating chloride induced pit
(Treadaway, 1988; 1991), and as such may also be considered further evidence of
the ‘Transitory Complex Theory’.

Ghods et al (2013) have recently stated that
the underlying mechanism for the depassivation process by chloride ions for
steel in concrete is still not fully understood. They propose that one of the
reasons behind this lack of understanding is the unavailability of data on the
compositional and nanoscale morphological characteristics of the passive oxide
film before and after exposure to chlorides. Furthermore they advise, since
traditional electrochemical and most microscopic techniques do not provide such
specific data, nano-scale surface chracterisation techniques are required and
although such techniques have been widely used to characterise metal and metal
alloy oxides, their application to study carbon steel and stainless steel in
alkaline environments that are representative of concrete pore solution has
been limited.

In the study by Ghods et al (2013), nano-scale
transmission electron microscopy (TEM) samples prepared with a focused ion beam
were used to investigate the passivity and depassivation of carbon steel
reinforcement in simulated concrete pore solutions. It was found that the
addition of chlorides at concentrations lower than the depassivation thresholds
did not change the physical appearance of the passive oxide films. After
exposure to chlorides in concentrations greater than depassivation thresholds,
passive oxide films were no longer uniform, with some regions of the surface
bare and some pit initiation sites formed on the surface of the steel.

It is proposed that the study by Ghods et al
(2013) is evidence for both or either the ‘Adsorption Theory’ or ‘Transitory
Complex Theory’ for chloride ion induced passive film breakdown of steel
reinforcement in concrete.

However, based on the majority of the above, it
is proposed that the ‘Transitory Complex Theory’ is the dominant mechanism for
the chloride ion induced depassivation/pit initiation of steel reinforcement in
concrete and that the ‘Adsorption Theory’ may also be a contributing mechanism.

5.3 Metastable Pitting

Frankel
(1998) advises that metastable pits are pits that initiate and grow for a limited
period before repassivating. For metals in solutions, large pits can stop
growing for a variety of reasons, but metastable pits are typically considered
to be those from micron size at most with a lifetime of the order of seconds or
less.

Angst et
al (2011) indicate for chloride induced reinforcement corrosion that once a pit
has formed, the anodic dissolution reaction has to be sustained, otherwise the
pit might cease growing and repassivate within a short time. Nucleation/repassivation
events occur during the phase of so-called metastable pitting.

Furthermore,
Angst et al (2011) propose that it is generally accepted that many of the
nucleated pits never achieve stability and that for a pit to survive the
metastable state, it is necessary that the solution within the pit cavity
(anolyte) maintains an aggressive chemical composition. This requirement is met
by hydrolysis of the dissolving iron cations and the resulting increase in H⁺
ion concentration as well as migration of chloride and other anions into the
pit to maintain charge neutrality. As long as dilution of the aggressive pit
chemistry is prevented, pit growth is self-sustaining and likely to achieve
stability.

This
mechanism of localised acidification due to metal ions hydrolysis is attributed
to Galvele (1976). Newman (2010) then highlights that definition of the pit
propagation process by the localised acidification model that has since had an
enormous influence on the development of corrosion science.

Pit
growth and pit propagation together with the chemical conditions within
propagating pits including localised acidification is discussed specifically at
Section 5.4 and illustrated in Figures 7 and 8.

Lin et
al (2010) utilised a scanning-micro reference electrode (SMRE) technique to
monitor localised corrosion processes of reinforcing steel in NaCl-containing
solution by in-situ imaging of the corrosion potential and corrosion current.
Metastable micro-pits occurred randomly and instantaneously on the surface when
the reinforcing steel was first immersed in the chloride-containing solution.
The number and size of active micro-pits changed with immersion time; some
micro-pits lost their activity and some micro-pits gathered into groups to form
visible corrosion holes that were covered by yellow-brown corrosion products.

Furthermore,
whether metastable pitting nuclei developed into macro-corrosion pits or lost
their corrosion activity, was closely associated with the local chloride
concentration. Once stable pitting nuclei were formed, further hydrolysis
reactions of Fe ions continued to lower the local pH, which accelerated
dissolution of Fe at pits (Lin et al, 2010).

A porous
cover of corrosion products and remnants of the passive film have also been
reported for carbon steel and iron in chloride solutions (Alvarez & Galvele,
1984). Loss of this pit cover it is proposed will result in dilution of the
anolyte and might stop anodic dissolution. The pit can survive rupture of the
cover and continue to grow in a stable manner only if the pit geometry has
already developed in such a way that the pit depth sufficiently limits
transport of ionic species to and from the pit bottom, in other words: that the
pit geometry (transport path) imposes a resistance against diffusion and
migration (Angst et al, 2011).

5.4 Pit Growth/Pit Propagation

5.4.1 General

Green
(1991) has proposed that for chloride induced pit growth to be sustained for
reinforcing steel in concrete, the following conditions must be maintained:

1. A sufficient concentration of
   chloride ions at the pit;
2. The recycling of chloride ions
   during the corrosion process (i.e. from the hydrolysis of the intermediate iron
   chloride/oxy-chloride complexes);
3. Diffusion of chloride ions to
   the pit from the bulk pore solution;
4. Development of acidity within
   the pit;
5. Cathodic processes on the steel
   surface; and
6. A continuous electrolytic path
   between cathodic sites and the pit.

Angst et
al (2011) advise that studying the transition from nucleation to stable pit
growth for the case of reinforcement steel embedded in concrete is much more
difficult owing to experimental reasons. For instance, the reference electrode
cannot be placed as close to the pit as in the case of solution experiments, or
the IR drop through the concrete might disturb electrochemical measurements.
Also, visual examination of an electrode embedded in concrete is impossible
during an experiment. In addition, concrete is an inhomogeneous material and
thus local chemistry is not as well defined as in solutions. Corrosion kinetics
are also clearly different for steel embedded in concrete compared with
experiments in solution.

Angst et
al (2011) proposed however, that in principle the localised pit
growth/propagation mechanisms valid for stainless or carbon steel in solution
can be expected to apply also for reinforcement steel in concrete, namely:
acidification of the anolyte and migration of chloride ions into the pit are
required to maintain the aggressive local pit chemistry, which in turn is
required to sustain anodic dissolution. In concrete, convection as a mechanism
that promotes dilution of the anolyte can be neglected when compared with the
situation of a metal surface directly exposed to bulk solution. However,
considering the presence of soluble alkaline cement (binder) hydration products
(i.e. NaOH, KOH and Ca(OH)₂) providing a pH buffering capacity,
local acidification can be considered a critical step (Angst et al, 2011).
Particularly lime/portlandite (Ca(OH)₂), segregated at the
steel/concrete interface, and its ability to retain a fall in pH, as it has
been suggested as a major reason for the inhibitive properties of concrete (Page,
1975). As long as OH^– ions migrate preferentially into the pit
cavity to maintain charge balance, they will inhibit the pH of the anolyte from
falling too fast.

Alvarez
and Galvele (1984) reported that for iron in alkaline solutions, acidification
of the anolyte is the rate controlling step; once a low pH in the pit is
reached, the behaviour becomes equal to that found in acid solutions where
chloride accumulation in the pit is the rate limiting process.

The
chloride ions enter the passive film (via predominantly the ‘Transitory Complex
Theory’) and give rise to soluble products such that a small bare area of metal
surface is formed. Ferrous ions (Fe²⁺) diffusing away from the
anodic area will react with the high pH environment to form rust product
(Fe(OH)₂ initially) which will shield the bare metal from oxygen.
Further Fe²⁺ ions will no longer be able to be oxidised by the
dissolved oxygen and so may be hydrolysed leading to the production of H⁺
ions in the incipient pit. The production of H⁺ ions reduces the pH
in the pit. Tinnea (2002) has in fact measured reductions in pH to below 4
within pits for conventional steel reinforcement in chloride contaminated
concrete.

A charge
imbalance is also then generated by the H⁺ ions production so that
Cl^– ions will enter the pit which will now contain iron chloride
(including the corrosive FeCl₃)_, iron oxy-chloride and iron
chloro-hydroxyl compounds (Treadaway, 1991).

The rate
of pitting corrosion is very large (locally) because the portion of the metal covered
by the protective passive film is in general very much larger than the portion
of the metal where the passive film has been damaged. The area over which the
reduction of oxygen takes place is therefore very much larger than the anodic
area where metal is dissolved. Consequently as the metal loss that has to
balance the oxygen reduction must all take place in the localised area where
the passive film has been damaged, the rate of penetration of the metal in that
local area is very fast. The result is rapid pitting is observed (Cherry &
Green, 2017). As previously noted, indeed, values of pitting (penetration) rates of up to 1 mm/year have
been reported for bridge decks, sustaining walls or other chloride contaminated
steel reinforced concrete structures according to Elsener (2002).

5.5 Reinforcing Steel Quality

5.5.1 Metallurgy

Angst et
al (2011) indicate that the metallurgy (physical surface heterogeneities such
as inclusions, lattice or mill-scale defects, flaws, and grain boundaries) of
the steel reinforcement also impacts the nucleation of chloride induced
pitting.

Bhandari
et al (2015) note that for engineering alloys, pits almost always initiate due
to chemical or physical heterogeneity at the surface such as inclusions, second
phase particles, solute-segregated grain boundaries, flaws, mechanical damage,
or dislocations.

5.5.2 Reinforcing Steel Defects

Lin et
al (2010) have undertaken work to image the chloride concentration on the
surface of reinforcing steel, and to further study the effects of interfacial
chloride ions on pitting corrosion in reinforcing steel by combining electron
microprobe analyser (EMPA) ex-situ imaging of the morphology and chemical
composition of reinforcing steel at the same locations. Both the
electrochemical inhomogeneities in the reinforcing steel and the non-uniform
distribution of chloride at the surface were considered the most important
factors leading to various localised attacks. They also developed a
scanning-micro reference electrode (SMRE) technique to monitor localised
corrosion processes of reinforcing steel in NaCl containing solution by in-situ
imaging of the corrosion potential and corrosion current. Combining the data of
the SMRE technique with that of the EMPA measurements they confirmed that among
the electrochemical inhomogeneities (defects) in reinforcing steel, MnS
inclusions play a leading role in the initial corrosion because chloride
prefers to adsorb and accumulate at the MnS inclusions, resulting in pitting
corrosion.

Accordingly,
dissolution of the MnS inclusion is generally accompanied by a local drop in pH
around the MnS inclusion, as follows (Lin et al, 2010):

MnS + 3H₂O
→ Mn²⁺ + HSO₃^– + 5H⁺ + 6e^–                   (3)

Once a
local reduction in pH and increase in chloride concentration is reached,
nucleating pits can initiate and form occluded pits, such that H⁺,
Cl^– and HSO₃^– can accumulate inside pits. As
previous, the metastable pitting nuclei either developed into large corrosion
pits or lost their corrosion activity, which largely depended on whether the
local chloride concentration exceeded the critical chloride concentration. Again
as previously, if stable pitting nuclei were formed, further hydrolysis
reactions of Fe ions continued to lower local pH, which accelerated dissolution
of Fe at pits (Lin et al, 2010).

6. CARBONATION INDUCED CORROSION MECHANISMS

Attack of concrete by carbon dioxide is termed
carbonation. The passivity provided to steel reinforcement by the alkaline
environment of concrete can be lost due to a fall in the pH of the concrete
pore solution because of carbonation and thereby destroying the passive film.

Carbonation
may occur when carbon dioxide gas (and atmospheric SO_x and NO_x gases) from the atmosphere dissolves in concrete pore water and penetrates
inwards or when the concrete surface is exposed to water or soil containing
dissolved carbon dioxide.

Carbon
dioxide dissolves in the pore water to form carbonic acid by the reaction:

CO₂(g) + H₂O(l) → H₂CO₃(aq)                        (4)

Carbonic
acid can dissociate into hydrogen and bicarbonate ions. The carbonic acid
reacts with Ca(OH)₂ (portlandite)in the solution contained
within the pores of the hardened cement paste to form neutral insoluble CaCO₃.
The general reaction is as follows:

Ca(OH)₂(aq) + H₂CO₃(aq) → CaCO₃(s) + 2H₂O(l)                      (5)

and the
nett effect is to reduce the alkalinity of the pore water which is essential to
the maintenance of a passive film on any reinforcing steel that may be present.
Whilst there is a pH buffer between CaO and Ca(OH)₂ that keeps the
pH at approximately 12.5, the CO₂ keeps reacting until the buffer is
consumed and then the pH will drop to levels no longer protective of steel.

The
attack of buried concrete by carbon dioxide dissolved in the groundwater is a
two stage process. The calcium hydroxide solution that fills the pores of the
concrete, first reacts with dissolved carbon dioxide to form insoluble calcium
carbonate. However it then subsequently reacts with further carbon dioxide to
form soluble calcium bicarbonate which is leached from the concrete. The extent
to which each process takes place is a function of the calcium
carbonate/calcium bicarbonate concentration of the ground water (which in turn
is a function of the pH and the calcium content) and the amount of dissolved
carbon dioxide.

The iron
oxide passive film formed on steel in alkaline conditions is stable at pH
levels greater than 10 (Broomfield, 2007) or greater than around 9.5 (Savija
& Lukovic, 2016). Since the process of the carbonation of concrete lowers
the pH of concrete pore water to lower than 9 (as low as 8.3) (Savija &
Lukovic, 2016), the passive film is simply dissolved. The steel surface is then
exposed to a pH~9 environment and corrosion thereby propagates.

Carbonation of concrete causes general corrosion
where anodic and cathodic reactions are immediately adjacent (microcells or
minicells), leading to uniform steel (iron) dissolution over the whole surface.

Localised
carbonation induced corrosion of steel reinforcement can occur at cracks, concrete defects, low cover
areas etc of reinforced concrete elements.

7. LEACHING INDUCED CORROSION OF REINFORCEMENT

The passivity provided to steel reinforcement
by the alkaline environment of concrete may also be lost if the pH of the
concrete pore solution falls because of leaching of Ca(OH)₂ (and
NaOH and KOH). Leaching from concrete can lead to a lowering of the pH below 10
to cause corrosion of steel reinforcement.

Natural
waters may be classified as “hard” or “soft” usually
dependent upon the concentration of calcium bicarbonate that they contain. Hard
waters may contain a calcium ion content in excess of 10 mg/l (ppm) whereas a
soft water may contain less than 1 mg/l calcium. The capillary pores in a
hardened cement paste contain a saturated solution of calcium hydroxide which
is in equilibrium with the calcium silicate hydrates that form the cement gel.
If soft water can penetrate through the concrete (e.g. joint, crack, defect,
etc) then it can leach free calcium hydroxide out of hardened cement gel so
that the pore water is diluted and the pH falls. Since the stability of the
calcium silicates, aluminates and ferrites that constitute the hardened cement
gel requires a certain concentration of calcium hydroxide in the pore water,
leaching by soft water can result in decomposition of these hydration products.
The removal of the free lime in the capillary solution leads to dissolution of
the calcium silicates, aluminates and ferrites and this hydrolytic action can
continue until a large proportion of the calcium hydroxide is leached out,
leaving the concrete with negligible strength. Prior to this stage, loss of
alkalinity due to the leaching process will result in reduced corrosion protection
to reinforcement.

Calcium
hydroxide that is leached to the concrete surface reacts with atmospheric
carbon dioxide to form deposits of white calcium carbonate on the surface of
the concrete. These deposits of calcium carbonate may take the form of stalactites
or severe efflorescence.

Leaching
induced corrosion of steel reinforcement can be localised and consequently more
serious where it occurs at concrete joints, cracks, defects, etc. An example of
localised corrosion of steel reinforcement in a potable water reinforced
concrete water tank is shown at Figure 9.

8. STRAY CURRENT CORROSION AND INTERFERENCE OF REINFORCEMENT

8.1 General

Stray electrical currents, most commonly from
electrified traction systems, can also breakdown the passive film and cause
corrosion of steel reinforced and prestressed concrete elements.

Stray
electrical currents are a potent source of corrosion problems in reinforced
concrete. Currents flow through any environment in response to potential
differences in that environment. If an extended reinforced concrete structure
is immersed in such an environment then these potential differences drive a
current through the steel and this gives rise to corrosion. In the regions
where the environment has the more positive potential, current will enter the
metal by means of the reaction:

O + H₂O + 2e^– → 2OH^–                      (6)

In the
regions where the environment has the more negative potential, current will
leave the metal by means of the reaction:

Fe →
Fe⁺⁺ + 2e^–                   (7)

and
as the metal is polarised in the positive direction that part of the structure
is corroded.

8.2 Ground Currents

Ground
currents are particularly effective in causing corrosion in extended reinforced
concrete structures such as concrete pipelines, tunnels and retaining walls
because the potentials that can build up in the soil between the different
portions of the structure are correspondingly larger. Although the driving
force for the corrosion is greater, the resistance between the cathode and
anode is not proportionately larger and so the corrosion currents are greater.

The
commonest cause of stray current corrosion is electrified traction systems. In
general the DC power for an electric train or tram is supplied by a sub-station
and drawn from a positive overhead cable. The negative return to the
sub-station is either through the rails on which the train is running or
through a separate conductor rail. Although most of the current driving the
trains/trams returns to the sub-station from which it is drawn via the
conductor rail of the traction system, it is inevitable that some of the
current in the conductor rail leaks from that rail and goes back to the
sub-station through the earth. This sets up potential gradients in the earth.
If a low resistance path (such as metallic reinforcement to a concrete
structure) is present then the current may pass through the reinforcement. It
can be seen from Figure 10 that where the current enters the structure, the
reinforcement is cathodically protected. Where the current leaves the structure
near the sub-station, corrosion is intensified.

The potential loss of reinforcing steel
section due to stray current or interference corrosion can be very serious and
design measures may need to be developed to mitigate such corrosion. As an
example, suitable mitigation systems needed to be designed, implemented and
maintained for the Sydney Harbour Tunnel as calculations carried out indicated
a potential corrosion problem existed on the steel reinforcing of the immersed
tunnel units and up to 150 kg of steel could be corroded per year by stray
current (McCaffrey, 1991).

8.3 Interference Currents

Interference
corrosion currents are typically associated with cathodic protection schemes.
The application of remote anode impressed current cathodic protection to (say)
a buried pipeline involves the establishment of an anode(s) (anode groundbed)
at some distance from the pipeline with the aim of passing current from the
anode(s) (groundbed) to the pipeline. If there is a reinforced concrete structure
within this potential field then potential gradients associated with the
passage of current through the soil give rise to interference corrosion of
steel reinforcement. The effect on the buried reinforced concrete structure is
to set up cathodic areas near to the anode groundbed and anodic areas closer to
the structure which the cathodic protection system has been designed to protect.

8.4 Local Corrosion Due to Stray Currents

Dependent
on the type of reinforced or prestressed concrete structure and elements that
are near an electrified traction system or cathodic protection systems, stray
current or interference corrosion of steel reinforcement may be localised. As
such, structurally significant section loss of reinforcement may occur within short
periods of time, since current density can be converted to an equivalent mass
loss or corrosion penetration rate by Faraday’s Law.

For
example, a section of reinforcing steel receiving say 1 uA/cm² of
stray current equates to a mass loss or corrosion penetration rate of
approximately 12 um/yr because of Faraday’s Law (Fontana, 1987). If the stray
current is 1 mA/cm², then the corrosion penetration rate will be
approximately 12 mm/yr. Consequently significant section loss of reinforcement
may occur within short periods of time for reinforced and prestressed elements
suffering stray current corrosion from ground currents or interference
currents.

9. CONCLUSIONS

Professor
Cherry has always been a firm believer in understanding first the fundamentals
of any aspects of corrosion science, then the mechanisms, before embarking on
the engineering of solutions to the management of materials corrosion. So it
has been with steel reinforced concrete. This paper has endeavoured to walk a
reader through the fundamental and mechanistic aspects of the excellent
protection afforded to steel reinforcement by concrete (including
electrochemistry), corrosion of steel reinforcement (uniform
(microcell/mini-cell) and pitting (macrocell) corrosion, corrosion products
composition and development), chloride induced corrosion mechanisms (passive
film breakdown/pit initiation, metastable pitting, pit growth/propagation,
chemical conditions within propagating pits and reinforcing steel quality
effects), carbonation induced corrosion mechanisms, leaching induced corrosion
of reinforcement and reinforcing steel stray current corrosion and
interference.

Concrete
is a most wonderful material of construction. When suitably designed, constructed and
maintained, reinforced concrete provides service lives of numerous decades to
structures and buildings. Concrete provides reinforcing steel with excellent
corrosion protection. The highly alkaline environment in concrete results in
the spontaneous formation of a stable, tightly adhering, thin protective oxide passive
film on the steel reinforcement surface, which protects it from corrosion. In
addition, well proportioned, compacted and cured concrete has a low
penetrability, thereby minimising the ingress of corrosion-inducing species via
the aqueous phase, and a relatively high electrical resistivity, which reduces
the corrosion current and hence the rate of corrosion if corrosion is
initiated. As Professor Cherry has said many times “good steel in good concrete
will not corrode”.

There are however, several degradative
processes which affect some reinforced concrete structures leading to loss of functionality, unplanned maintenance/remediation/replacement, and in
the worst cases, loss of structural integrity and resultant safety risks and amongst these, the most common cause
of deterioration is corrosion of conventional carbon
steel (black steel), prestressing steel and post-tensioned steel reinforcement.

10. ACKNOWLEDGEMENTS

The
authors would like to acknowledge colleagues,
mentors, peers and industry friends for their support.

Author 1 would like to specifically thank Jack
Katen and Andrew Haberfield for their help with the paper.

It is
noted that the views expressed in this paper are those of the authors and are
not necessarily of the organisations that they represent.

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12. AUTHOR DETAILS