--- title: "Electrochemical process of corrosion in RCC" subject: "Concrete Technology and Masonry Structure" chapter: 6 key: "semester_5/concrete_technology_and_masonry_structure:corrosion-electrochemical-process" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/corrosion-electrochemical-process site: BCT Engineering --- # Electrochemical process of corrosion in RCC Concrete Technology and Masonry Structure · Chapter 6: Concrete durability - Asked in: 2082 Baisakh, 2 marks; 2069 Chaitra, 6 marks; 2068 Baisakh (old course), 5 marks; 2066 Bhadra (old course), 2 marks; 2064 Jestha (old course) (asked 5 times) - Video: [Corrosion of Steel reinforcement || Corrosion of Steel in concrete](https://www.youtube.com/watch?v=pY1mc2CKH1I) — chemical ki Duniya. - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/corrosion-electrochemical-process - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6 ## Question Explain the electrochemical process of corrosion (rusting) of steel in reinforced concrete elements with sketches. How does the corrosion affect the concrete element? ## Short answer Corrosion occurs when the passive oxide film on steel is destroyed by carbonation (pH <9) or chlorides (>0.4% by cement weight). Anodic reaction Fe → Fe²⁺ + 2e⁻, cathodic O₂ + 2H₂O + 4e⁻ → 4OH⁻, forming rust with 2–6 times volume. This causes cracking along bars, spalling, loss of bond and cross-section, and structural deterioration. ## Answer #### Electrochemical process of corrosion Steel in sound concrete is protected by a thin passive oxide film, formed in the highly alkaline pore solution (pH 12.5 to 13.5). If the film is destroyed (by **carbonation** reducing pH below about 9, or by **chlorides** above a threshold of about 0.4% by weight of cement), the steel can corrode when oxygen and moisture are available. The bar surface behaves like a small battery: ```text concrete (electrolyte, pore water) anode cathode Fe -> Fe2+ + 2e- O2 + 2H2O + 4e- -> 4OH- ===[ steel bar ]==============[ steel bar ]=== \-- e- flow in the steel -->--/ Fe2+ ------- ion flow ------- OH- ``` 1. **Anode** (corroding area): $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$ 2. **Cathode** (oxygen-rich area): $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$ 3. Ions react: $\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2$, then it oxidises to $\text{Fe(OH)}_3$ and finally hydrated oxide $\text{Fe}_2\text{O}_3\cdot n\text{H}_2\text{O}$ (red rust). The rust has **2 to 6 times the volume** of the steel it replaces. Moisture, oxygen and a conductive path are needed; hence corrosion is high in wetting-drying and coastal zones. #### Effect of corrosion on the concrete element 1. Expansive rust creates tensile stress; **cracks run along the bars**, followed by **spalling** of the cover and **delamination**. 2. Loss of bar cross-section (pitting) reduces the tensile capacity, ductility and fatigue life of the member. 3. **Loss of bond** between steel and concrete, so anchorage fails. 4. Rust stains, exposure of bars, more ingress of water and chlorides (accelerating the process). 5. Loss of serviceability (deflection, cracks) and finally structural collapse in severe cases. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What destroys the passive oxide film on steel in concrete so that corrosion can start? A. Very high alkalinity (pH above 13) B. Carbonation lowering pH below about 9, or chlorides above a threshold C. Dry concrete with no moisture D. Excess calcium hydroxide in the paste Answer: B. The film exists in highly alkaline pore solution; carbonation (pH drop) or chlorides destroy it. (Key point: Passive film breakdown) 2. In the corrosion cell of steel in concrete, what happens at the anode and cathode? A. Anode: O2 is reduced to OH-; cathode: Fe is oxidised B. Both reactions dissolve iron as Fe2+ C. Anode: Fe -> Fe2+ + 2e-; cathode: O2 + 2H2O + 4e- -> 4OH- D. Anode: Fe2+ is reduced to Fe; cathode: water is oxidised to O2 Answer: C. Iron dissolves at the anode, giving electrons which are consumed with oxygen and water at the cathode. (Key point: Anode and cathode reactions) 3. Why does rusting crack the concrete cover? A. Rust occupies about 2 to 6 times the volume of the steel it replaces, creating tensile stress B. Rust is lighter than steel and shrinks the bar C. Rust has the same volume but is heavier D. Rust dissolves in pore water and leaves voids Answer: A. Expansive rust pushes on the cover; concrete is weak in tension so cracks run along the bars. (Key point: Rust volume expansion) 4. Which is the correct sequence/set of effects of corrosion on an RC member? A. Increase in bar area and bond, so strength increases B. Cracks along bars, spalling and delamination, loss of bar area and bond, loss of serviceability C. Only surface staining with no structural effect D. Uniform shrinkage of the whole member Answer: B. Rust expansion cracks and spalls cover; pitting reduces steel area, bond is lost and serviceability fails. (Key point: Effects of corrosion) ## Related in Concrete Technology and Masonry Structure - [Causes of deterioration; effects of steel corrosion](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acauses-deterioration-corrosion-effects) — Concrete Technology and Masonry Structure, chapter 6, asked 2× - [Preventive measures against corrosion](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acorrosion-preventive-measures) — Concrete Technology and Masonry Structure, chapter 6, asked 3× - [Concrete ingredients; concrete versus steel](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aconcrete-ingredients-vs-steel) — Concrete Technology and Masonry Structure, chapter 1 - [Rationale for reinforcing concrete with steel](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Arationale-steel-reinforcement) — Concrete Technology and Masonry Structure, chapter 1 - [Bond between steel and concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Abond-steel-concrete) — Concrete Technology and Masonry Structure, chapter 5 - [Porosity, permeability, corrosion, carbonation](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aporosity-permeability-corrosion-carbonation) — Concrete Technology and Masonry Structure, chapter 6 ## Related in other subjects - [Reinforced concrete column stresses in steel and concrete](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Arc-column-8-bars) — Strength of Materials, chapter 3 - [RCC column with footing: concrete and steel](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Arcc-column-footing-h) — Estimating and Costing, chapter 6 - [Steel requirement and doubly reinforced moment](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Asteel-requirement-and-doubly-reinforced-moment) — Design of RCC Structure, chapter 2 - [Vertical circulation elements; RCC dog-legged stair](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-circulation-068ba) — Building Technology, chapter 5