Page 583 - Corrosion Engineering Principles and Practice

546 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 547

FIGURE 13.20 Impressed current cathodic protection being retrofitted on
an old bridge structure: note vent tubes and drill holes anodes. (Courtesy
of Vector Corrosion Technologies)

• Constant rebar potential mode: In this mode the current output is
adjusted continuously to provide a constant (preselected) rebar
potential. A method that may be used for immersed concrete
where potentials are relatively uniform and stable, the rebar
potential, measured continuously with reference electrodes, is
fed back to the rectifier unit. Successful operation in this mode
depends on minimizing the IR drop error in the rebar potential
measurements and on the accuracy and stability of the reference
electrodes over time.

An important issue in ICCP of reinforcing steel is how much
current should be impressed between the reinforcing steel and the
anode. Too little current will result in inadequate corrosion protection
of the rebar, while excessive current can result in problems such as
hydrogen embrittlement and concrete degradation. Hydrogen
embrittlement of high-strength steel is a particularly severe problem
for pre- and post-tensioned concrete systems. The current densities
involved in meeting commonly used protection criteria are typically
around 10 mA/m of rebar surface. Uniformity in the current
2
distribution is obviously a desirable feature.