A new study challenges the long-standing belief that chloride concentration alone triggers steel corrosion in concrete, revealing that flaws at the steel–concrete interface and declining alkalinity play a more critical role.
Study: Role of Chlorides in Corrosion of Reinforcing Steel in Concrete. Image Credit: Wirestock Creators/Shutterstock.com
Published in Corrosion and Materials Degradation, the research sheds new light on how corrosion develops in reinforced concrete exposed to chlorides, particularly in marine environments. The authors found that steel embedded in dense, low-permeability concrete often resists corrosion—even when chloride levels are high.
Instead of being driven by a strict “critical chloride concentration,” corrosion tends to begin at small voids or imperfections at the steel–concrete interface. Over time, chlorides gradually weaken the surrounding material, reducing its alkalinity and creating conditions for long-term deterioration. The study suggests that permeability, alkalinity, and physical signs of rust are more reliable indicators of corrosion risk than chloride levels alone.
Rethinking the Role of Chlorides
For decades, the Tuutti model has guided our understanding of steel reinforcement corrosion. It proposes that once chloride content surpasses a critical threshold, the protective high-alkaline environment around the steel breaks down, allowing corrosion to begin.
However, the current study highlights that corrosion is often more closely linked to the quality of the steel–concrete interface than to chloride content by itself.
Factors such as air voids, compaction quality, and surface damage play a larger role in determining where and how corrosion starts. Field evidence supports this: reinforced concrete structures with good compaction have shown minimal corrosion over decades of marine exposure, even under high chloride concentrations.
These observations led the researchers to investigate the underlying corrosion mechanisms with a sharper focus on interfacial conditions.
Methodology and Approach
The study combined thermodynamic modeling with long-term laboratory experiments to explore how corrosion initiates and progresses. Pourbaix diagrams and Gibbs free energy principles were used to identify the electrochemical conditions under which steel may shift from a passive to an active corrosion state. Special attention was given to air voids at the steel–concrete interface, as these areas are more prone to moisture and oxygen accumulation—ideal conditions for corrosion initiation.
In parallel, the researchers studied how the depletion of calcium hydroxide, a key contributor to concrete's high pH, affects the corrosion environment. Over time, a “neutralization front” advances inward as calcium hydroxide leaches out, particularly in the presence of chlorides, which increase its solubility.
By combining theoretical models with extended experimental observations, the study offers a well-rounded view of both early-stage corrosion and long-term material degradation.
Key Findings
The researchers identified two distinct stages of corrosion:
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Initial Localized Corrosion: This stage typically begins as pitting or crevice corrosion at air voids or surface imperfections along the steel–concrete interface. In dense, low-permeability concrete, these corrosion sites are often self-limiting because oxygen becomes depleted and corrosion products restrict further growth. However, in more permeable concrete or where cover is reduced, the continued presence of moisture and oxygen allows corrosion to progress.
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Progressive pH Decline and General Corrosion: Over time, the gradual leaching of calcium hydroxide lowers the pH of the surrounding concrete. Once pH levels drop toward neutrality, conditions become favorable for general corrosion. While chlorides accelerate this pH decline, they are not the direct cause of corrosion.
These findings reinforce the idea that interface quality, alkalinity, and permeability—not chloride thresholds—are the most important indicators for assessing corrosion risk.
Conclusion
The study concludes that corrosion in reinforced concrete is more strongly linked to physical imperfections and long-term chemical changes than to any single chloride threshold. While pitting corrosion can begin even in high-pH environments, its progression is limited in well-compacted, low-permeability concrete. The greater concern is the steady loss of calcium hydroxide, which lowers pH and, with the help of chlorides, enables more widespread corrosion over time.
For structural durability assessments, this means shifting focus toward evaluating concrete quality, permeability, and remaining alkali content rather than relying on critical chloride limits alone.
Journal Reference
Melchers, R. E., & Chaves, I. A. (2025). Role of Chlorides in Corrosion of Reinforcing Steel in Concrete. Corrosion and Materials Degradation, 6(3), 41 DOI: 10.3390/cmd603004. https://www.mdpi.com/2624-5558/6/3/41
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