Cathodic Protection of Steel in Concrete
For many years industrial companies have fought to protect steel reinforced concrete from corrosion. The prime causes of corrosion in concrete include salt, chloride and de-icing treatments. The salt seeps into the concrete and erodes the steel reinforcing bar (rebar) causing cracks and spalling in the concrete and eventually the potential for failure of the structure. One very effective, long-term solution is metal/thermal spraying the concrete with zinc or a variety of zinc alloys. This is a technology that protects or extends the life of a wide variety of products in the most hostile environments.
The images below show the rebar corrosion in concrete.
Re-bar Corrosion in Concrete
The majority of metallised zinc cathodic protection systems are operated in galvanic or sacrificial mode. However, metallised zinc cathodic protection systems can be, and are in many instances, operated in impressed current mode. The sprayed coating, a high purity zinc alloy, is connected to one pole of a DC power supply. The steel rebars are connected to the other pole of the power supply. The electrical circuit is completed between the rebar and the zinc by the presence of moisture in the concrete. The action of the corrosion cell causes the zinc to corrode in preference to the steel rebars, therefore protecting the rebars from corrosion.
Cathodic Protection with Zinc Coating
The process of spraying the zinc onto the substrate ensures that there is a good, even connection path between the coating and the rebars through the concrete. Prior to metal spraying damaged sections of concrete need to be repaired with rebars also repaired or replaced. The surface needs to be lightly blasted to remove any surface dirt and provide a good key to enable the coating to bond. The coating would then be applied with either a Metallisation Arc140, Arc701 or Arc170 system, depending on the size and accessibility of the structure. Typical bond strength is in the region of 3MPa for zinc and coating thickness would be between 300 and 500 microns. The above picture shows an Al-Zn-In metal spray coating being applied using the Metallisation Arc spray system.
Cathodic Protection of Steel in Concrete Bridges
The costs to apply metal sprayed coatings to large concrete structures are not insignificant, particularly when many structures are difficult to access, such as bridges. However, the long-term benefits can make the process extremely commercially attractive. If performed correctly and depending on the coating applied, the process can offer corrosion protection for up to 20 years before the next significant maintenance is required. The protection offered can greatly prolong the life of the structure and also prevent costly accidents from cracked sections falling from the structure. Once applied, the coating requires minimal maintenance. If required for aesthetic purposes, zinc coatings can also be painted.
A recently developed alloy of aluminium, zinc and indium has been used in a small number of applications. This material is more active than zinc and is claimed to not require an impressed current to provide adequate levels of corrosion protection.
A recent example of corrosion protection using this alloy has been trialled by Aeroports de Paris at Charles de Gaulle (Roissy) airport. Aeroports de Paris, responsible for the maintenance of most of the Roissy airport infrastructure, recognised deterioration in some of the concrete panels at the airport and sought a long-term corrosion protection solution. The precast concrete panels, which are 2.6 x 2.8m of lightly reinforced 8cm thick units, form the underside of the concrete viaducts carrying road traffic to and from a busy terminal complex. Run-off from de-icing salt has lead to an important level of chloride in the panel concrete. Although the panels are not structurally significant, spalling could present a hazard to passing traffic.
Following a stringent review and testing of the panels to establish the deteriorating condition, an anode was applied to the panels in a test area. After grit blasting the panel surface, the anode of aluminium/zinc/indium alloy was applied by a Metallisation arc spray system to an application thickness of 300 microns. The anode connection plate in the centre of each panel is clearly visible by its red anode cable, which would not normally be on show in a typical commercial application. The other cables run to connections to the rebars and to embedded reference electrodes. As this is a test site it was necessary to install monitoring equipment. This was to allow the connection between the anode and cathode to be interrupted for measurement of electro-chemical performance. After two years the system appears to be well adapted to treat corrosion of the viaduct panels and is deemed to be a successful test.
Another significant application of the Al-Zn-In alloy in the US is the San Luis Pass Bridge near Galveston, Texas. More than 30,000 m² of concrete beams and caps are protected with this alloy, installed using Metallisation ARC 700 units by Corrosion Restoration Technologies of Jupiter, Florida.
Cape Creek Bridge, Oregon (USA)
Oregon Department of Transportation (ODOT) demonstrates another success story for cathodic protection on concrete. In a bid to reduce the high costs of bridge reconstruction, ODOT has applied a system of metallised zinc anodes and impressed current cathodic protection. This process has been used to protect its Cape Creek Bridge from corrosion and subsequent reconstruction. The bridge is exposed to a coastal environment and is subject to attack by chloride from the salty air.
Prior to the cathodic protection project on the bridge, it had suffered substantial concrete spalling on its columns and under deck. By selecting to protect the bridge in this way ODOT saved over $13 million by not having to reconstruct the bridge. The cost of cathodic protection is quite expensive. This is due to the requirement of a movable work platform, which is enclosed to contain the abrasive blasting and zinc spraying residues. These measures are critical when spraying zinc to protect the environment. However, when compared to the cost of reconstructing a bridge the size of Cape Creek Bridge – the savings are phenomenal. This is a picture of Al-Zn-In coated panels at Paris CDG Airport with monitoring.
Dave Wixson, Metallisation distributor in the US says: “Cathodic protection is a cost effective way to stop rebar corrosion in existing structurally sound structures. Rebars in dry alkaline concrete are protected by a passive ferric oxide film, however, when the rebar is hit with 250 ppm chloride solution, generally from salt, the protection breaks down. The protective ferric oxide film is converted to red rust and corrosion begins.
Concrete thickness >4cm (>1.5 in), will prevent chloride penetration. For exposed rebar and thin concrete, where there is chloride concentration in excess of about 250 ppm, rebar corrosion will be initiated with the red rust spalling adjacent concrete. Protecting the rebar with a barrier using an impressed or passive cathodic protection system, counters the corrosion.”
Many thanks to Palmer Consulting of France, TMS of the USA and Corrosion Restoration Technologies Inc (now part of Structural Group, Inc.) of the USA for information supplied.