Corrosion profile of uncoated reinforcing steel vs galvanized reinforcing steel

The adapted Tuutti schematic above showcases the performance of galvanized reinforcing steel compared to uncoated reinforcing steel in concrete. As described above, the higher chloride threshold of galvanized reinforcing steel and its immunity to the effects of carbonation delay the onset of corrosion initiation (shifts point AB to point CD) of the corrosion process, while the barrier protection offered by zinc, combined with the minimal disruption of the zinc corrosion products, serve to extend the propagation phase of the process.

Each stage of corrosion is outlined below: 

A. The initiation stage – the period in which the concrete is progressively exposed to corrosive products (chlorides, carbonation) and the uncoated reinforcing steel remains passivated (until the point A-B). The time to corrosion initiation can be quantified by deterministic chloride diffusion and carbonation modelling (only required for uncoated reinforcing steel) models based on Fick’s Second Law.

B. The propagation stage – Destruction of passivating layer on the uncoated reinforcing steel and corrosion of reinforcing steel to the acceptable limit of concrete deterioration. At the end of this time cracking and spalling of the concrete occurs

C. The life of the galvanized reinforcing steel passivating layer. The corrosion initiation stage is extended due to the increased tolerance to chloride attack and the complete avoidance of depassivation from concrete carbonation 

D. The period of protection of the galvanized reinforcing steel from rust as chlorides attack a small portion of pure zinc layer on the steel surface and corrosion products diffuse away from the reinforcing steel 

E. The period of additional protection where corrosion causes dissolution
of the Fe + Zn alloy layers 

F. By this stage all of the galvanized coating is consumed, and the corrosion rate of the reinforcing steel becomes identical to the exposed uncoated reinforcing steel in stage B – however by this stage the galvanized coating has done its job and the time to initiation of concrete cracking has been increased significantly.

Schematic model for the corrosion of galvanized reinforcing steel in concrete, after Tuutti 1982
Galvanized rebar was used extensively in the Townsville, Queensland marina.

A hot dip galvanized coating on reinforcing steel provides a significant increase to the durability of steel reinforced concrete structures

Summary of advantages

A hot dip galvanized coating on reinforcing steel provides a significant increase to the durability of steel reinforced concrete structures. The formation of a passive calcium hydroxyzincate film on the galvanized reinforcing steel surface significantly increases the critical chloride threshold of the reinforcing steel, thereby significantly delaying the time to corrosion initiation. This delay in corrosion initiation may be quantified using conventional deterministic chloride diffusion models based on Fick’s Second Law.

Time to corrosion initiation is further increased as galvanized reinforcing steel is immune to the effects of carbonation.

Should the galvanized reinforcing steel be depassivated, the resulting zinc corrosion products are much less voluminous than the iron corrosion products formed on uncoated reinforcing steel, and thus cause minimal disruption to the concrete mass. This avoids the build-up of internal pressures which lead to concrete cracking and spalling. 

The addition of the microscopic zinc corrosion products to the concrete matrix decreases its permeability by filling pores and voids, thus slowing the supply of aggressive species from the concrete surface to the reinforcing steel. The result of this process is a significant increase in time for the propagation phase of corrosion and a corresponding delay in the time to initiation of cracking of the concrete.