ECONOMICAL ADVANTAGES OF CATHODIC PROTECTION COMPARED TO OTHER METHODS OF CORROSION CONTROL AND PREVENTION.
Galvanising, anodising, painting, replacement of steel with better but costlier metals are some of the important methods of corrosion prevention. Each of these has its own individualal field and adaptability. Galvanising, anodising and painting are extremely useful when structures are small and are in air or are accessible for maintenance work. But in the case of structures of large dimensions buried in ground or in water any of the above methods is not practicable or economical. In marine and in oil industry, coating with cathodic protection is the only sure way of corrosion prevention besides it is the cheapest. In the case of heat exchangers where accessibility is very little and continuous operation essential, cathodic protection will go a long way to minimise corrosion troubles in pipes. Pipes carrying oil and gas at high pressures cannot take any chance with corrosion failures. At high pressures, it is not economical to keep a larger factor of safety for plate thickness and minimum reduction due to penetration or pits due corrosion will result in pipe failure. This will bring loss of revenue, increased maintenance cost and plant outages. In such cases, cathodic protection with coating will prove most economical.
For water pipes, it is possible to use cast iron or concrete pipes as an alternative material. But for certain diameters and pressure, these pipes cease to be economical and mild steel pipes are the only possible solution.
Mild steel pipes can also be gunnited by thick layers or concrete. In many cases coating and cathodic protection, depending on various factors is as costly as gunniting or in a few cases will be costlier still. Even if coating and cathodic protection prove 10% to 20% costlier than gunniting, advantages of the former would justify extra cost. Gunniting is efficient when applied in dry and less corrosive soils. But gunniting suffers from cracks and improper bonds due to mechanical vibrations, expansions and improper application. Concrete becomes 10 times more conducting when wet than when it is dry and thus in moist and corrosive soils, the cathodic protection would prove to be a better alternative.
With cathodic protection it is possible to adjust current output and voltages as and when required to maintain proper degree of cathodic protection, if coating deteriorates over a period. It is also possible to check whether the structure is sufficiently protected or not if applied with cathodic protection .
ANTI-CORROSIVE COATING AND CATHODIC PROTECTION.
As a saying goes, cathodic protection is complimentary and not supplementary to coating. In the case of underground and under maring structures, no corrosion mitigation program can be considered by coating alone. For economical reasons corrosion prevention by cathodic protection alone also cannot be considered. Not that cathodic protection alone cannot be applied but current requirement on a bare or a poorly coated surface is so high that for practical purposes, this cost out weights the saving in coating. It can be safely said that total cost of reasonably good coating along with cathodic protection is nearly the same as cost of cathodic protection on a bare metal surface. This is so far as initial investment is considered. But as the cathodic protection has to be applied continuously the energy bill or the running charges would increase to larger proportion for poorly coated surface. Even if coating work is carried out under most strict supervision, it is likely that a few pin holes may be left out or perhaps at the time of lowering the pipes or at the time of backfilling the earth, the coating may be damaged when this happens, an insignificantly small damaged area becomes anodic to the large areas with coating in good condition. This sets up accelerated corrosion and as the anodic area from which the metallic ions can migrate into soil or water is extremely small, for an equal weight, the penetration in shell thickness is greater than it would otherwise be. This penetration will in time develop into a leak causing the failure of the pipe or structure. Also, coating may be continuously in contact with water and over longer periods coating especially asphalt and gunniting become more conducting. This sets up higher corrosion currents and anodic areas can develop. When cathodic protection is applied the current required depends on this damaged or badly coated surfaces only and not on the total buried surface of the structure. To illustrate this point, in one case 45 amps were required to protect equivalent of 6 miles of poorly coated pipe whereas in another case only 2 amps could protect as many as 75 miles of well coated pipe.