Galvanic Anode systems utilize the natural potential difference between the anode material and the structure. Since we are generally talking about CP protecting steel or iron, the only metals that can be used are magnesium, zinc and aluminum as these are more electrically negative than steel/iron. The designer has minimal flexibility with these materials – the only driving force is the small difference in potential between the two metals to create a flow of current. The analogy is a gravity fed piping system that relies solely on gravity to induce flow. Great if it works but if it doesn’t work it doesn’t work.

From a design standpoint there are pros and cons of both types of systems, but remember that with galvanic systems, there is no power supply and no means of providing additional driving force to increase current output. With impressed current systems there is a power supply and you can turn the voltage up or down to adjust current output.