42907 Chap.4 Chloride transport and corrosion initiation
Chloride transport and corrosion initiation
Week 2 builds the coordinate system for the subject’s signature failure. Concrete keeps its pore water at pH 12.5 to 13, which holds a passive iron oxide film on the steel; chloride destroys that film locally, producing pitting rather than uniform corrosion.
The chapter separates the initiation and propagation phases, distinguishes the three diffusion coefficients the lectures use, and fixes the design threshold at 0.4 per cent of cement mass on a total chloride basis. It also records why the apparent coefficient must be measured rather than predicted.
What this chapter covers
- 01
Why steel survives in concrete: low concrete tensile strength against 500 MPa steel, similar thermal expansion, chemical compatibility
- 02
Pore water at pH 12.5 to 13, and the passive iron oxide film it stabilises
- 03
Depassivation by chloride ions or by carbonation, and why the attack is pitting
- 04
The initiation phase, ended by the critical chloride threshold at the steel
- 05
The propagation phase, and the two things it needs: oxygen and moisture
- 06
The design criterion: the steel must not see threshold chloride at its own depth inside the design life
- 07
Diffusion in saturated pores against convection by capillary suction
- 08
Three coefficients: in free water, effective in pore water, and apparent in concrete
- 09
What the apparent coefficient lumps together, and why no reliable model predicts it from mix design
- 10
Chloride binding, Friedel's salt, and why the threshold is written on total chloride
- 11
The design threshold of 0.4 per cent of cement mass, and the international code spread
- 12
Measuring a coefficient: profile grinding, water soluble against acid soluble dissolution, titration and curve fitting
- 13
The near surface peak in field profiles, and why the convection zone is excluded from a fit
Deciding whether the initiation phase has ended from a laboratory report
- +1Identify which figure the criterion is written on. The design threshold is a total chloride threshold, free plus bound, and the acid soluble result is the one that dissolves both. The water soluble result gives free chloride only.
- +1Compare the acid soluble figure with the threshold: 0.61 per cent against 0.4 per cent of cement mass. The threshold has been exceeded, so the steel has depassivated and the initiation phase has ended.
- +1The wrong reading is to take the free chloride figure of 0.28 per cent, conclude it is below 0.4 and report the member as still in initiation. It is unsafe for the reason binding exists: bound chloride can be released when the pore solution chemistry changes, and carbonation of the surface layer releases it.
Key terms
- Passive film
- A thin oxide layer on the steel which, in alkaline surroundings, turns impermeable and grips the metal tightly, leaving the bar passive. The anodic half reaction has no metallic iron to work on while that layer holds.
- Depassivation
- The local destruction of the passive film, either by chloride ions reaching the bar or by carbonation dropping the pH of the surrounding concrete. It ends the initiation phase and permits corrosion to begin.
- Initiation phase
- The period during which chloride diffuses through the pore network of the cover towards the bar with nothing corroding and nothing visible. Cover is the protection throughout it, and it ends when the critical threshold is reached at the steel.
- Propagation phase
- The period of active corrosion after depassivation, during which products build up, the cover cracks, and corrosion then accelerates because the crack admits more oxygen and moisture. Its rate is not constant.
- Pitting corrosion
- Deep, local loss of steel section produced when a small depassivated anode is served by a large surrounding passive cathode. A bar can lose a structurally significant fraction of its area while most of its length still looks sound.
- Apparent diffusion coefficient
- The single coefficient used in concrete, which absorbs the porosity, the multi species interactions in the pore water, the binding capacity of the matrix and the ageing of all three. No reliable model predicts it from mix design, so it is measured.
- Chloride binding
- The matrix takes a large share of the arriving ions out of solution, some physically and some chemically, the chemically held form being a chloroaluminate. It helps, because only the fraction still loose in solution drives the diffusion.
- Chloride threshold
- The total chloride content at the steel at which the passive film fails. This subject designs to 0.4 per cent of cement mass, within a published range that varies by code and is genuinely hard to measure.
- Convection zone
- The first few millimetres of a field chloride profile, where content is lower than the model predicts because of drying, rain washout and carbonation releasing bound chloride. It is excluded when fitting a diffusion coefficient.
Chloride transport and corrosion initiation FAQ
Why is the chloride threshold written on total chloride rather than free chloride?
Because binding is not permanent. A large bound reservoir can be released when the pore solution chemistry changes, and carbonation of the surface layer in particular releases it. A criterion written on free chloride alone would pass a member that is holding that reservoir and waiting to let it go.
The practical consequence is that the acid soluble laboratory result, not the water soluble one, is the figure you compare with the threshold.
Can I estimate the diffusion coefficient from the strength grade or the permeable voids?
No, and the subject says so explicitly: there is no reliable model available to predict the apparent coefficient from the mix design or from characteristics such as permeable voids or characteristic strength. It absorbs four physically distinct things at once, including the binding capacity of the matrix, and two concretes with identical porosity and strength can bind chloride very differently.
It has to be measured, or taken from published data for a binder of the same family.
What actually destroys the passive film, and why is the damage so localised?
Chloride ions break the film over small patches rather than stripping it evenly. Each patch becomes an anode where iron dissolves, while the large surrounding area of still passive steel acts as the cathode. A small anode serving a large cathode carries a very high local current density, so the section loss is deep and narrow.
That is why the subject calls it pitting corrosion and why a visual inspection can be badly misleading.
Why do measured field profiles not look like the model curve?
The model describes diffusion only, and it draws a profile falling steadily from the exposed face inwards. Real surfaces also absorb sea water bodily by capillary suction when they dry and are rewetted, and they lose chloride to drying, rain washout and carbonation. The result is a peak a few millimetres below the surface with lower content at the face.
Standard practice is to exclude that convection zone from the curve fit and project the fitted curve back to the surface.
Assessment move
Fix the two phase model in your head first, because every later calculation is a statement about where one phase ends. Be able to name what ends initiation and what propagation needs, and use the second of those to explain the exposure ladder of the previous chapter rather than memorising it separately.
Keep the three diffusion coefficients apart by their concentration basis, since the unit changes between them and that change is examinable. Rehearse the threshold in both of its bases and practise converting between them, because mixing per cent of cement with per cent of concrete is a factor of six error that survives every other check.
Finally, learn the sentence about no reliable model predicting the coefficient, since it is the justification for both the lookup table and the performance based route.
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