Currently, reinforcement corrosion is the most common pathology in reinforced concrete structures older than 10 years. According to technical surveys, it is responsible for approximately 70% of serious structural deteriorations in Brazil. Therefore, understanding its electrochemical mechanisms, correctly identifying its causes, and, above all, applying appropriate technical solutions is essential to preserving the service life of buildings.
Furthermore, it is a silent and progressive process. In aggressive environments, for example, a corroded steel bar can lose approximately 10% of its cross-section every 3 to 5 years. As a direct consequence, the deterioration progresses without evident signs until, unexpectedly, a sudden failure occurs, often with catastrophic consequences.
Given this context, this technical guide presents, in a structured way, the mechanisms of corrosion, the two main aggressive agents, carbonation and chlorides, as well as diagnostic methods, prevention strategies and structural recovery techniques.
The Electrochemical Mechanism: Why Steel Corrodes into Concrete
Passivating Layer: The Natural Protection
Under normal design and construction conditions, concrete naturally protects steel through a fundamental physical-chemical mechanism essential for structural durability. This process occurs as follows:
Alkaline concrete (pH 12-13) ↓ Formation of a passivating layer of Fe(OH)₂ ↓ Stable and impermeable layer ↓ Steel protected for a long period
This protective layer is mainly composed of:
- Stable iron oxide (Fe₂O₃)
- Iron hydroxide (Fe(OH)₂)
- Thickness between 0.1 and 1.0 micrometer
Specifically, this passivating film forms the basis of the durability of reinforced concrete, provided its conditions are maintained.
Depassivation: When Protection Fails
However, this protection is not permanent. When the pH of the concrete decreases or when chloride ions reach the reinforcement, a process called depassivation occurs.
Aggressive agents (CO₂ or Cl⁻) ↓ Penetration through pores and fissures ↓ Attack on the passivating layer ↓ DEPASSIVATION ↓ Steel exposed to corrosive environment
From this point on, therefore, the steel is no longer protected and the corrosive process begins to develop continuously.
Electrochemical Corrosion Process
Once depassivated, the armor undergoes electrochemical reactions that occur simultaneously in three distinct stages.
Step 1: Anodic Reaction (Oxidation of Steel)
Fe → Fe²⁺ + 2e⁻
In this initial stage, metallic iron loses electrons. Thus, the anodic region is formed, where corrosion effectively begins.
Step 2: Cathodic Reaction (Oxygen Consumption)
O₂ + 2H₂O + 4e⁻ → 4OH⁻
Meanwhile, the released electrons are consumed in nearby regions, allowing the process to continue.
Stage 3: Formation of Corrosion Products
Fe²⁺ + 2OH⁻ → Fe(OH)₂ 2Fe(OH)₂ + O₂ → 2Fe(OH)₃
At this point, corrosion products are formed. Since these products have a volume between 4 and 10 times greater than the original steel, internal stresses arise which, consequently, cause cracking, detachment of the concrete cover, and exposure of the reinforcement.
Necessary Conditions for Corrosion
In summary, corrosion only occurs when all of the following conditions are present:
1. Electrolyte (water in the pores) 2. Potential difference 3. Oxygen 4. Aggressive agents (CO₂ or Cl⁻)
Thus, the absence of any one of these conditions prevents corrosion. On the other hand, when all are simultaneously present, the process becomes inevitable.
Two Main Causes: Carbonation vs. Chlorides
Although the end result is similar, loss of reinforcement section, the mechanisms involved are distinct and require different approaches.
Cause 1: Carbonation (≈60% of cases)
Chemical Mechanism
CO₂ + H₂O → H₂CO₃ H₂CO₃ + Ca(OH)₂ → CaCO₃ + H₂O ↓ pH drops from 12-13 to <9 ↓ Destruction of the passivating layer
In other words, carbonation reduces the alkalinity of the concrete, making the steel vulnerable.
Factors that accelerate carbonation.
| Factor | Impact |
|---|---|
| Low coverage | Progress 5–10 times faster |
| Porous concrete | Rate 3–5 times higher |
| Inadequate cure | High porosity |
| Urban environment | Carbonation 2x faster |
| Cracking | Direct pathways for CO₂ |
Penetration Speed
Dense concrete: 1–2 mm/year; Porous concrete: 5–10 mm/year; Cracked concrete: >20 mm/year
Consequently, reduced concrete cover can lead to corrosion within a few years.
Cause 2: Chlorides (≈40% of severe cases)
Unlike carbonation, chlorides do not need to lower the pH to cause corrosion.
Cl⁻ → Reach the armor ↓ Destabilize the passivating layer ↓ Formation of localized pits ↓ Rapid and aggressive corrosion
Furthermore, this mechanism is autocatalytic, which makes its evolution much faster and more unpredictable.
| Source | Content | Environment |
|---|---|---|
| Sea breeze | 1–5% | Coastal |
| Road exits | 2–10% | Highways |
| Inadequate additives | 0,5–2% | Poorly supervised construction projects |
| Contaminated aggregates | 0,1–0,5% | Coastal regions |
Diagnosis: Identifying Corrosion
Visual Signs
Level 1 – Suspected: • Grayish stains • Efflorescence • Surface disintegration Level 2 – Confirmed: • Cracks parallel to the reinforcement • Detachment of the concrete cover • Visible steel
Therefore, visual cues should never be ignored.
Technical Diagnostic Methods
Carbonation Assay (Phenolphthalein)
Pink: High pH White: Low pH
Corrosion Potential (CSE)
> -200 mV: Low probability -200 to -350 mV: Uncertain probability < -350 mV: High probability
Electrical Resistivity
>10 kΩ·cm: Slow corrosion <1 kΩ·cm: Accelerated corrosion
Prevention: The Most Economical Strategy
It has been proven that prevention is up to 100 times cheaper than repair.
Measure 1: Adequate Coverage (NBR 6118)
| Environmental Class | Cover |
|---|---|
| CAA I | 25 mm |
| CAA II | 30 mm |
| CAA III | 40 mm |
| CAA IV | 50 mm |
Therefore, increasing the concrete cover can triple the lifespan of the structure.
Measure 2: Concrete Quality
Air-to-energy ratio 0.65 → Lifespan ~10 years; Air-to-energy ratio 0.55 → Lifespan ~20 years; Air-to-energy ratio 0.45 → Lifespan >40 years
Measure 3: Mineral Additions
Furthermore, activated silica densifies the matrix and drastically reduces permeability, increasing service life by up to 80%.
Measure 4: Steel Protection
- Galvanized steel: 3 to 5 times increase in durability.
- Stainless steel: use restricted to extreme cases.
Step 5: Proper Healing
Without proper curing, strength decreases and carbonation accelerates. Therefore, a minimum wet curing period of 14 days is recommended.
Recovery: Repair Techniques
Standard Steps
- Area delimitation
- Removal of deteriorated concrete
- Thorough cleaning of the armor
- Corrosion protection
- Adhesion bridge
- Filling with polymer mortar
- Surface protection
Conclusion: Corrosion is Preventable
In summary, reinforcement corrosion is a completely preventable problem when proper design, correct materials, and quality workmanship are adopted.
Therefore, saving 5% in the design phase can later result in costs 50 to 100 times higher. In structural engineering, durability is not an additional cost, it is a strategic investment.
