Corrosion of reinforcing steel in concrete is of increasing concern among engineers. It significantly affects functionality by inducing cracking and spalling, which compromise structural integrity.

Corrosion may be defined as the destruction or deterioration of a metal or alloy substrate by direct chemical or electrochemical attack. The corrosion reaction between the metal and its environment is driven by an electro-chemical potential. The magnitude of this potential determines the tendency of the reaction to proceed.

In marine environments, chloride ions from seawater migrate into the porous concrete by diffusion and eventually reach the steel reinforcement. In regions of low resistance, chlorides attack the passive oxide film (formed as a result of the chemical reaction between the highly alkaline concrete and the pore water and surface of the steel), and develop anodic and cathodic sites on the steel. Adjacent to these sites are oxygen-rich regions that cathodically "fuel" the corrosion reaction. As active corrosion proceeds, the lower pH in and around the anodic sites reduces the passive layer in greater proportions. The expanding iron oxide (Fe2O3) that develops as a result of corrosion imparts considerable tensile forces, leading to cracking and spalling of the covering concrete.

CP discussion

There are two forms of CP available, impressed current (ICCP) and sacrificial (galvanic). Both systems have been proved to control corrosion, when properly installed, by providing an electrical current to an affected region. The impressed current system uses a permanent, external power source, such as a rectifier, to provide a current flow from the externally placed anode to the corroding reinforcing steel. Sacrificial CP systems use galvanic cells (dissimilar metals) to create an energy source supplying current to the corroding steel.

Both systems establish a current flow from an externally placed anode that provides enough energy to negatively polarise the steel reinforcement, which removes the isolated anodic and cathodic sites and creates an electrically homogeneous condition for the steel.

Although both methods provide CP, there are major differences in how they are maintained over the life of the system. A galvanic system provides its own power and regulates its current output according to the changing environmental conditions, so there is very little need for post-installation maintenance and monitoring.

With impressed current systems, maintaining rectifier currents and making adjustments to changing conditions become an ongoing burden. For the purpose of discussion, this article will only present information pertaining to sacrificial CP, primarily zinc mesh encapsulation (commercially known as the Galvashield Lifejacket system).

Sacrificial CP

Galvanic CP is achieved by creating a current flow from the sacrificial zinc anode (electrical potential of -1.10 V with respect to a copper/copper sulphate [Cu/CuSO4] half cell) to the embedded reinforcing steel. When coupled in a common electrolyte (chloride-saturated concrete), the sacrificial anode and the reinforcing steel become a circuit in which the current flows from the zinc anode to the steel cathode until it is sufficiently polarized or in electrical equilibrium. The loss of electrical energy at the anode deteriorates the metal in a current-to-time relationship.

The lifespan of the anode is regulated by the current flow of the system and the consumption rate of the anode being used. The corrosion rate of the metal is directly proportional to the potential difference between the zinc anode and the reinforcing steel cathode and inversely proportional to the circuit resistance of the electrochemical cell.

Several forms of sacrificial anode CP systems have been developed and successfully used to provide corrosion control in marine environments. For more than a century, cast anodes have been used to protect ship hulls, propellers, and other exposed metal objects from corrosion in a purely galvanic mode. Until recently, most anode systems were externally mounted and needed replacement within a few years of operation to maintain effective corrosion control.

In the case of CP for steel-reinforced concrete, typical installations required extensive concrete repair and rehabilitation prior to installing the anode system.

This typically adds more cost to the project for mobilisation, remobilisation, labour, supervision, and other associated costs. It became quite evident that a system was needed that could provide CP and concrete repair in one operation.

Zinc mesh systems

In 1994, the concept of encapsulating internally-placed zinc mesh anodes within a stay-in-place fibreglass form filled with a sand-cement mortar was first tested by the Florida Department of Transportation (FDOT) in the US. The system comprised high-purity expanded zinc mesh that had been alloyed with trace elements to improve formability and generate better anodic performance.

The end result was a zinc mesh that balanced critical anode mass with available surface area to provide the optimum configuration. Based on proven anode performance trials utilising zinc penny web, the new design consisted of a durable, stay-in-place fibreglass form that positioned the anode relative to the embedded steel and created the essential annular space for filling with an approved material.

In addition, a supplemental bulk anode, for extended lifetime, was added to protect the submerged portion of the structure and to minimize the current demand on the lower portion of the anode mesh, which was subjected to more frequent wetting by tidal action.

Summary

Zinc mesh anodes in simple configurations are capable of supplying long-term CP to steel and steel-reinforced concrete structures in marine environments. High-purity zinc mesh anodes can be designed, manufactured, and easily installed to protect a wide variety of structures in varying states of deterioration. The zinc mesh jacket systems offer the advantage of accomplishing concrete repair and CP for structural and non-structural rehabilitation. In many cases, the jacket system also is being considered for cathodic prevention as an economical alternative to other more frequent and routine maintenance measures.

The cost of sacrificial anode systems using zinc mesh favorably compares with the cost of standard pile jacketing and becomes more economical with time as compared to impressed current systems. The zinc mesh anode systems have the distinct advantage of requiring only minimal monitoring and maintenance. Since external power supplies are not required, rectifier monitoring and adjustments are not necessary. It also eliminates costly wiring and complex conduit systems for routing current to the source.

Since tidal changes affect concrete resistivity and ultimately anode current output, it becomes essential in many cases to supplement the zinc mesh systems with a submerged bulk anode.

The bulk anode polarises the submerged portion of the structure, thus preventing excess consumption of the mesh anode during high tide. The dual anode configuration provides long-term polarisation (in excess of 200 mV) on the steel reinforcement in the splash zone and submerged portion of the substructure.

The expanded zinc mesh anode cast into concrete pile jackets is an efficient method for providing sacrificial CP to reinforced concrete marine structures such as quay walls, bridge piles, jetties, etc. The system is capable of providing and maintaining CP to the reinforced steel, using the 100 mV polarisation criterion (NACE Standard RP02090-90).

Founded in 1975, as a joint venture between Easa Saleh Al Gurg and Fosroc International, Al Gurg Fosroc has a direct presence in Bahrain, Oman, Kuwait and Qatar.

The UAE firm is committed to quality and was the first construction chemical manufacturer in the Gulf to be awarded both ISO 9002 and ISO 14001 certificates. Its total manufacturing capacity exceeds 100,000 tonnes a year.

Fosroc International is one of the world's largest manufacturers of advanced technology products for the construction and refurbishment industries.