Zinc has been used as a sacrificial anode to cathodically protect ship’s hulls for more than a century now. It has become a common practice to use cathodic protection either alone or in combination with coatings for buried pipelines, storage tanks and offshore structures.

It has been well established both in theory and in practice that the process achieves an immediate reduction in corrosion rate by making the reinforcing steel the cathode, inhibiting its tendency to oxidize. In addition, the cathodic reactions at the steel/concrete interface increase the alkalinity (raise the pH) by hydroxyl ion (OH-) generation and drive chloride ions (Cl-) away from the steel as a result of the negative charge on the ions being repelled by the negative polarity of the reinforcement and attracted to the positive polarity of an installed anode.

Thus Cathodic Protection (CP) of steel in concrete has always incorporated some degree of Realkalisation (increase in pH) and Chloride Extraction (redistribution of chlorides). Both these processes reduce the risk of corrosion of steel in the concrete and also achieve further protection. Recent innovations in materials and design have made CP, - the old remedy, ever more attracting and promising today.

Pressure-sensitive Zinc-Hydrogel Anode
For conventional CP systems (SA or ICCP) to function properly, it requires the presence of a continuous conductive electrolyte between the anode and the cathode.

This is to form a closed circuit such that the reinforcing steel is indeed made the cathode. In areas of electrical discontinuity, reinforcing steel would not be cathodically protected. One of the recent innovations is the self-adhesive and conductive zinc-hydrogel anode that provides a continuous electrolyte contact between the anode and the reinforcing steel embedded in the concrete structure. The pressure-sensitive zinc-hydrogel anode is essentially a sheet of zinc foil coated with an ionically conductive hydrogel pressure-sensitive adhesive (Figure 1), which serves as the electrolyte between the anode (zinc foil) and the cathode (reinforcing steel in concrete). The hydrogel is covered with a liner to help protect it from contamination. At time of installation, the protective liner is removed from the hydrogel by hand, and the zinc-hydrogel anode is adhered to the clean, bare concrete surface.

This zinc foil anode coated with hydrogel is provided in roll form for coverage of the concrete surface to be protected. The ionically conductive hydrogel performs two functions

• (1) to enable the anode to be securely adhered to the exterior surface of the concrete structure, and
• (2) to serve as a continuous conductive electrolyte between the anode and the concrete structure.

It works in the following way
The zinc foil is applied to the surface of structurally sound concrete structure (Fig. 2). The pressure sensitive adhensive (conductive hydrogel) bridges the gap between the zinc anode (foil in this case) and the concrete surface – making possible the electrolyte continuity across the anode and cathode (rebars).

The zinc foil must be electrically connected to the rebar network by wires so that electrons can flow through the wire from zinc (anode) to rebars (cathode). Conventional current flows in the opposite direction of electrons, i.e., from cathode to anode. Ionic current is carried by charged species such as Cl-, Na+, OH-, Ca2+ etc. across the electrolyte (concrete + hydrogel). The potential of rebar and the current flowing between the zinc foil and the rebar can give some indication about the degree of cathodic protection.

It works for the following reason
Corrosion of rebar is the process involving the oxidation reaction,

• Fe --> Fe2+ + 2e

A neutral iron atom lost two electrons and became a positively charged ion. The tendency of a metal to lose electrons can be considered as the tendency of metal to corrode.

When zinc foil is applied to the concrete surface but not connected by wire to the rebar network, the steel rebar continues to corrode at a normal rate. When zinc foil is electrically connected to the rebar network, electrons flow from zinc foil into steel rebars, thus reducing the steel’s tendency to lose electrons and hence reducing the rate of corrosion (oxidation). This is because zinc is a much more reactive metal than steel, it has a much greater tendency to lose electrons than that of steel. Since electrons carry negative charges, the accumulation of electrons within steel rebar makes steel more negatively charged, leading to cathodic polarisation, - the shift of rebar potential in the negative direction. In general, the greater the tendency for the anode to donate electrons to the cathode, the greater the degree of protection rendered to the “cathode” (the steel rebar network). However, one must remember that when an anode donates electrons to the cathode, the anode material “sacrifices” itself. What this means is that the useful life of the anode is both limited by the chemical/electrochemical surface reactivity and the amount of physical material available.

Potential applications of it
The nature of the zinc hydrogel anode makes it a perfect rehabilitation option for structures or certain sections of structures exposed to atmospheres. The conductive hydrogel ensures the continuity of electrolyte across the rebar-concrete-hydrogel-anode, and hence cathodically protects the rebar network.

It is a good idea to paint the zinc surface to preserve the anode material by reducing corrosion of zinc from external atmosphere.

Information from- http://www.corrosionsource.com/