Thank you for your question!
The answer is actually a little involved.
When designing a cathodic protection (CP) system for a new asset, or retrofitting an existing asset, an engineer will choose either sacrificial anodes or an impressed current system, but usually not both.
However, there are instances where the impressed-current system will not be immediately available, such as during the early stages of construction. At this time, a sacrificial anode system may be installed until the impressed-current system is ready to come online. Once the impressed-current system is online, it will override the sacrificial anodes, which will become cathodes as long as the impressed-current system is operating.
The same effect can be seen if a CP system has two dissimilar metals used as sacrificial anodes (for instance, magnesium and aluminum protecting a steel structure). The less noble metal (magnesium) will protect the more noble metal (aluminum), which will reduce its ability to protect the structure and the lifetime of the anode.
So in summary, it is sometimes practical to use a hybrid system, but it is not “business as usual”.
Matthew D. Taylor, PhD.