Applications of Cathodic Current Measurements in Corrosion Protection
Sacrificial anode on a fishing boat hull
Corrosion protection systems are critical for marine pipelines, buried pipelines, ship hulls, and other structures in corrosive environments. No one wants their boat hull or gas pipeline rusting out prematurely, and electrical systems can be used to inject a cathodic current for corrosion protection. These systems are simple in their design, but they rely on a fundamental cathodic current measurement in a controlled experiment.
When we apply our knowledge of cathodic currents to electronic design, it largely applies to power supplies and power supply design. The first part of this blog will explore the electrochemistry behind cathodic currents, while the latter half is going to be looking at how to design and simulate power supplies to avoid any potential hazards as a result of AC/DC power supplies.
It Starts With Half-Cell Potential and Cathodic Current Measurements
These simple experimental voltammetry measurements give you an idea of the voltage required to suppress a corrosion reaction. The experimental system is similar to that used in linear sweep and cyclic voltammetry. The goal in designing these systems is to prevent corrosion at a target structure (the cathode), yet it increases the corrosion rate at a nearby sacrificial structure (the anode).
These simple electrochemical measurements yield a cathodic current measurement and a half-cell voltage. Simply change the sign of the measured cathodic current to determine the input current required to suppress corrosion at the cathode. You can then use this measurement to design an impressed current cathodic protection system to prevent corrosion.
Impressed Current Cathodic Protection Systems
A critical application of cathodic current measurements and systems is in impressed current cathodic protection. Metal structures deployed in corrosive environments are susceptible to corrosion from electrochemical reactions. A typical solution is to apply a coating to the metal structure to prevent corrosion. While this provides corrosion resistance, it is not the entire solution to preventing corrosion and ensuring the maximum lifetime of the structure.
The goal in a cathodic current protection system is to prevent surface corrosion or, in extreme cases, pitting corrosion. An example of extreme pitting corrosion is shown below. In this type of corrosion, a corrosive electrolytic solution starts to creep deep into the structure as it corrodes. This type of accelerated corrosion mechanism is common in stainless steel structures exposed to acidic solutions.
As you’ll be largely working within a DC power supply structure here, it will be invariably important to work strictly within your measured, modeled, and simulated power supply results. Determining the current and creating a well-made, well-simulated power supply will go far in ensuring that you have the structural integrity to avoid corrosion.
Pitting corrosion is preventable with an impressed cathodic current protection system.
The central idea in impressed current cathodic protection is to apply a DC current into the structure that requires protection. This type of system requires a connection between a cathode (the structure to be protected) and an anode (a sacrificial metal that is not part of the structure). This type of protection works by converting all available cathodic sites in the protected structure to anodic sites. The corrosive environment (usually seawater or soil) will act as an electrochemical solution through which the injected DC current can flow.
In this way, the injected DC current suppresses the electrochemical reaction that corrodes the protected structure. A frequent test performed as part of corrosion protection design is a potential measurement against a copper sulfate reference electrode (referenced to as CSE). The anode and cathode half cell potentials are measured and used to determine the appropriate voltage that must be applied to the structure to prevent corrosion. Typical DC voltages seen at the cathode structure are -0.9 to -1.2 V.
Cathodic current used on an underground structure for corrosion protection. Similar structures are used in seawater.
For underground structures, the DC voltage provided by the power supply needs to be larger as there is a voltage drop across the soil between the anode and cathode structures. Determining the necessary overvoltage requires a soil resistivity measurement. There are some standard gradient equations that are then used to determine the overvoltage needed to compensate for this voltage drop. This typically raises the required DC voltage to approximately -2 V. This ensures the corrosion reaction at the cathode is suppressed.
After the system is designed and installed, the rate of corrosion in the system should be quite low. During monitoring, a DC potential measurement between the anode and cathode provides an indication to the extent of corrosion. This simple DC measurement is very powerful as it simply requires a comparison of the measured voltage to the potential when the system was installed. As corrosion progresses, the potential measured between the anode and cathode will tend to 0 V.
Designing Impressed Current Cathodic Protection Systems and Power Supplies
If you’re designing an impressed current cathodic system, you’ll likely be connecting to an AC main line to access power, which is then converted to DC. The input AC current is then rectified, regulated, and subsequently fed to the protected cathodic structure and the sacrificial anodic structure. These low-frequency/DC systems simply require designing for high power and extreme temperature shifts.
Thermal circuit simulations and reliability simulations are invaluable for investigating variations in circuit system behavior across a broad frequency range. Being able to accurately determine the thermal or hardware integrity of a design before it goes live is part of the job in determining reliability, and SPICE software is paramount in these endeavors.
An alternative type of corrosion protection system is a galvanic protection system. In this system, there is no AC/DC power source, and the anode and cathode are shorted with a conductor. This passive system wears out faster than an impressed cathodic current protection system. In the impressed system, the external power source provides greater control over the applied cathodic current and allows the system to be monitored over time. Insufficient or excessive current through the system can be measured and adjusted to ensure appropriate corrosion protection.
Working with simulation software on power supplies more broadly is invaluable. Whether it’s providing first-time-right design validation, or making voltage concerns dissipate with proper planning, you can rest assured that no matter the low or high-power, magnetic, or thermal constraints to your designs, PSpice will be able to manage it in your electronic applications.
Corrosion protection systems are meant to be reliable over long periods, and you’ll need to use the best PCB design and analysis software to build your new system. The SPICE simulator and circuit design tools in PSpice Simulator for OrCAD, and the full suite of analysis tools from Cadence, are ideal for designing cathodic current systems for corrosion protection. You’ll also have access to verified models directly from manufacturers for simulating circuit behavior.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.