Surface-applied corrosion inhibitors are emerging as a flexible alternative to cathodic protection for reinforced concrete structures.
Corrosion of reinforced concrete remains one of the most persistent maintenance challenges facing bridge owners and infrastructure managers. While impressed current cathodic protection (ICCP) systems have long been regarded as a proven method of corrosion control, surface-applied corrosion inhibitors (S-ACIs) are increasingly being adopted as a complementary or alternative strategy.
A corrosion inhibitor is any substance which, when introduced into an environment at relatively low concentration, reduces existing corrosion and limits the risk of future deterioration. Although inhibitors have been widely used for decades in the petrochemical and transportation sectors, their use within reinforced concrete structures is comparatively recent.
The protection provided by reinforced concrete relies heavily on the highly alkaline environment surrounding the embedded steel reinforcement. Under normal conditions, this alkalinity maintains a stable passive oxide film on the steel surface, preventing corrosion. However, when carbonation or chloride contamination reduces alkalinity, corrosion can initiate, leading to cracking, spalling and eventual loss of structural capacity.
S-ACIs operate by interfering with the electrochemical corrosion process occurring between anodic and cathodic sites on the reinforcing steel. Depending on the formulation, inhibitors may act anodically, cathodically or as mixed inhibitors. The systems evaluated in this study were mixed inhibitors, designed to suppress both anodic and cathodic reactions simultaneously through the formation of a protective film on the steel surface.
Because mixed inhibitors do not necessarily create a significant shift in corrosion potential, the study emphasised the importance of monitoring corrosion rate rather than relying solely on half-cell potential measurements. Embedded corrosion rate sensors developed by C-Probe Technologies were used alongside the AchillesIES remote monitoring system to measure steel loss over time and evaluate treatment performance.
Three bridge structures managed on behalf of the Highways Agency were monitored to assess the effectiveness of S-ACI treatments.
At Twyford Bridge, S-ACIs were trialled on suspended bridge slabs alongside an ICCP installation. Two products, Sika Ferrogard and Flexcrete MCI 2020, were spray-applied to opposite sections of the bridge soffit.
The soffit application proved difficult because of poor absorption into the concrete surface. Nevertheless, areas associated with newly grouted repairs showed significant reductions in corrosion rate, suggesting that diffusion pathways through repaired concrete improved inhibitor penetration.
Wansford Bridge formed part of a larger pilot study comparing ICCP systems and S-ACI technology. The inhibitor was applied to the deck soffit in two treatment coats.
Unlike Twyford, the inhibitor successfully diffused through the concrete cover and produced measurable reductions in corrosion activity. The project demonstrated that S-ACI treatments can perform effectively when the concrete condition allows adequate penetration to the reinforcement level.
Ranby Bridge provided the most comprehensive assessment of S-ACI performance. The bridge structure, including abutment walls and soffits, was treated over approximately 500m².
In areas undergoing concrete repair, the inhibitor was applied directly to exposed reinforcement before patch repairs were completed. Additional surface treatment was then applied to surrounding concrete to create a long-term reservoir of inhibitor and reduce the risk of incipient anode formation adjacent to repair zones.
This direct-to-steel application produced the most significant performance improvements, with corrosion rates reduced by as much as 100-fold in some monitored locations.
The study demonstrated that surface-applied corrosion inhibitors can significantly reduce reinforcement corrosion in reinforced concrete structures when correctly applied and monitored.
However, performance depends heavily on several factors:
Direct application onto exposed reinforcement during concrete repair proved particularly effective, while surface-applied treatments alone achieved more moderate but still meaningful reductions in corrosion rate.
The study also highlighted the importance of long-term corrosion monitoring. Without reliable monitoring systems, infrastructure owners cannot accurately assess treatment effectiveness, determine reapplication intervals or optimise maintenance strategies.
Surface-applied corrosion inhibitors are not a universal replacement for cathodic protection, but they offer a flexible and potentially cost-effective addition to the corrosion management toolkit for reinforced concrete structures.
Where conditions are favourable, S-ACIs can extend service life, reduce corrosion rates and enhance the durability of repair works, particularly when used alongside embedded monitoring systems and targeted repair strategies.
The projects demonstrated that successful corrosion prevention relies not only on the treatment itself, but also on understanding how inhibitors interact with the concrete environment over time. Continuous monitoring remains essential to ensuring long-term performance and informing future maintenance requirements.
