To avoid corrosion in large metallic structures, passive cathodic
protection is widely employed. Such
structures include steel used as reinforcement in concrete buildings, bridges,
piers, pipelines, offshore platforms and ships.
Basically the steel in the structure is made the “cathode”
and a more easily corrodible “sacrificial” metal is connected to it, acting as
the “anode”. The chemical reaction
between the two metals generates an electrical current. The sacrificial metal then corrodes,
protecting the original structure.
Eventually that metal part has to be replaced, like the rod in most
domestic water heaters.
Below is an example of a passive system.
For both environmental and operating cost reasons, the
traditional passive protection is being replaced by active corrosion
control. In the ‘active’ method, a sophisticated
electronic current control system is used to inject a reverse current to that
generated from corrosion to protect the structure. Since current flow is
closely related to the flow of charge over time (I = dQ/dt), having constant
current control allows the user to accurately control the process. This is also known as impressed current
cathodic protection (ICCP)
Active corrosion control was initially discovered in the
early 1800s, but was unsuccessful due to the lack of suitable materials and current
source.
In larger systems like pipelines, the passive anodes cannot
deliver enough current to provide protection, and sophisticated monitoring and
control is often needed.
The initial cost of an active system is higher, but in the
long term, the environmental & maintenance benefits outweigh this.
TDK-Lambda’s new Z+ series
of 200 to 800W programmable power supplies offer a wide range of models and
options suitable for active corrosion systems. The series can operate in
constant current mode with currents ranging from 2A to 72A. In addition, the units can be remotely
programmed and monitored using a variety of isolated analog and digital
interfaces, including RS232/485, IEEE488 and LAN. Up to six units can be paralleled to supply
additional current.
