Carbonation vs Chloride — The Two Root Causes of Rebar Corrosion, and How to Match the Right Protection

Reinforcing steel (rebar) in sound concrete does not corrode, because fresh concrete has a high pH of about 12.5–13.5, which forms a thin oxide film (the passive film) on the steel surface. Corrosion only begins when an "aggressor" destroys that film — and there are two principal aggressors with completely different mechanisms: carbonation and chloride attack.

Misdiagnosing the cause means protecting or repairing the wrong way, and wasting money. This article summarizes how to distinguish the two and choose the right measures.

Mechanism 1 — Carbonation (corrosion from atmospheric CO₂)

Carbon dioxide (CO₂) from the air diffuses into the concrete pores and reacts with calcium hydroxide:

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

This reaction pulls the pore-solution pH from ~13 down below ~9. When the "carbonation front" advances deep enough to reach the rebar, the passive film breaks down and the steel begins to corrode uniformly (general corrosion).

• The carbonation front advances by the law x = K√t (depth ∝ square root of time) — K depends on concrete quality, relative humidity, and CO₂ concentration.
• Worst at RH ~50–70% (enough moisture for the reaction, but a fully saturated pore network slows CO₂ diffusion) — matching ventilated urban/industrial buildings sheltered from rain.
• Common in: columns/beams in urban-industrial zones (high CO₂), old/porous concrete, thin cover.

How to test: spray a phenolphthalein solution on a freshly broken concrete surface.

• Pink/purple = pH > 9 (not yet carbonated)
• Colorless = carbonated
• Measure the "carbonation depth" against the cover depth — if the colorless front is close to the steel, it is critical.

Mechanism 2 — Chloride attack (corrosion from chloride salts)

Chloride ions (Cl⁻) from seawater, salt spray, de-icing salts, or contaminated aggregate/mixing water penetrate the concrete. When the Cl⁻ concentration at the steel reaches the "critical chloride content" (about 0.4% by mass of cement per ACI 222R — some standards are stricter at ~0.2%), the passive film is breached locally, even while the pH is still high → resulting in deep pitting corrosion.

• Penetration follows diffusion (Fick's 2nd law) — governed by the chloride diffusion coefficient Dcl and the surface concentration.
• More dangerous than carbonation because it is pitting — it eats the steel cross-section deeply at discrete points, can sever bars even with adequate cover, and occurs while pH is still high.
• Worst in Thailand: coastal structures (Gulf/Andaman), eastern-seaboard seaside estates (Map Ta Phut / Laem Chabang), and the tidal/splash zone (alternating wet-dry) which is the most aggressive zone of all.

How to test: drill out concrete powder in layers by depth, then titrate for chloride content (chloride profiling) per ASTM C1152 (acid-soluble) or C1218 (water-soluble), combined with half-cell potential per ASTM C876 to map where the steel is active.

Comparison table — diagnose before you repair

Aspect	Carbonation	Chloride attack
Aggressor	Atmospheric CO₂	Cl⁻ (seawater/spray/salt)
How it kills passivity	Drops pH < 9 across the zone	Pierces film locally (pH still high)
Corrosion pattern	Uniform (general)	Deep localized pits (pitting)
Risk environment	Urban/industrial, RH 50–70%	Coastal, splash zone, de-icing
Detected by	phenolphthalein (depth)	chloride profiling + C876
EN 206 exposure	XC1–XC4	XS1–XS3 (marine), XD1–XD3 (other)
Severity	Gradual	Fast/localized, can sever section

Tuutti Model — why "prevention" beats "repair"

Corrosion life splits into two periods per Tuutti's model (the basis of the fib Model Code for Service Life Design):

flowchart LR
  Start["Construction done"] -->|"Initiation (Ti)
CO₂/Cl⁻ reach steel
+ hit threshold"| Depass["Depassivation begins"]
  Depass -->|"Propagation (Tp)
rust expands, bursts concrete"| Crack["Cracking/spalling
= end of service life"]
  Start -. "Design/protect here
extend Ti = cheapest" .-> Depass

• Initiation period (Ti): the time for the aggressor to reach the steel and hit the critical level — good design (dense concrete + adequate cover + CP) stretches this for decades.
• Propagation period (Tp): once corrosion starts, rust expands ~2–6× in volume and bursts the concrete into cracking/spalling.
• Lesson: investing in protection at the design stage (extending Ti) is several times cheaper than repairing spalling later.

Match the protection to the cause

flowchart TD
  Q1{"Environment?"}
  Q1 -->|"Urban/industrial
inland"| Carb["Focus: anti-Carbonation"]
  Q1 -->|"Coastal/splash
/salt-exposed"| Chlo["Focus: anti-Chloride"]
  Carb --> C1["Adequate cover (EN 206 XC)
+ low w/c concrete
+ anti-carbonation coating"]
  Chlo --> C2["Corrosion-resistant rebar (epoxy/galv/stainless)
+ Cathodic Protection (zinc anode)
+ SCM to lower Dcl (fly ash/slag/silica fume)"]

Fundamentals that help both mechanisms:

• Quality concrete: low water-to-cement ratio (w/c) < 0.45, well cured, dense (low permeability)
• Adequate cover per the EN 206 / ACI 318 exposure class

Carbonation-specific:

• Increase cover + reduce porosity
• Apply an anti-carbonation coating (e.g., acrylic/elastomeric coatings that block CO₂)
• Repair: realkalisation + patch repair

Chloride-specific:

• Use corrosion-resistant rebar — epoxy-coated / galvanized / stainless (see Rebar Coating: Epoxy vs Galvanized vs Stainless)
• Install Cathodic Protection — galvanic zinc anode or ICCP per ISO 12696 (see ICCP vs Sacrificial Anode)
• Add SCMs (fly ash/slag/silica fume) to lower the chloride diffusion coefficient
• Repair: Electrochemical Chloride Extraction (ECE) + CP retrofit

Summary for engineers / project owners

1. Always diagnose the cause first — phenolphthalein reveals carbonation; chloride profiling + C876 reveals chloride. The right diagnosis saves the repair budget.
2. On the Thai coast, chloride is the main enemy — pitting is faster and more severe, so plan CP + corrosion-resistant rebar from the design stage.
3. Extending Ti beats repairing Tp — investing in quality concrete + cover + CP up front is cheaper than waiting for spalling.

Sahawatthanakit (1988) supplies zinc anodes for Cathodic Protection (per ASTM B418 / TIS 3029) and anti-corrosion coating systems for concrete and steel structures — talk to our engineering team to match the system to your site's exposure class.