EN
The Impressed Current Cathodic Protection (ICCP) system is the critical protection system of the underwater hull of a vessel. It is used to polarize the steel surface to a given protective potential to prevent galvanic corrosion when it is working properly. But any well-developed system can become imbalanced. Emergency call out situations at SME include more than 70% of the issues with overprotection, damage to the paint and false reference cell alarms, all of which appear repeatedly in these logs. These symptoms will typically occur individually, but in many cases, they are related. Knowing this triad is a first step in troubleshooting.
The Root Causes of Overprotection and Anode Overdrive
If the current is too high, the ICCP system will deliver too much current, causing the hull potential to exceed the -900 mV (wrt the Ag/AgCl reference) mark as is known as overprotection. This is not normally an instantaneous problem; it is more likely to be a progressive problem produced by the failure of the reference electrode to give a true reading or the failure of the controller to be calibrated with respect to the feedback loop. In many instances, the controller autopilot setpoint value is fixed at a wrong value because of a poor connection between the ground and the controller, or because of a poor cable insulation between the reference cell and the controller. Another mechanical cause is the anode itself, which can lose its surface area if it is partially passivated, triggering the system to increase the voltage which in turn can cause overprotection to take place at the entire wetted surface. At SME we always first eliminate the possibility of being overprotected by measuring the hull potential, physically, with a portable half-cell, off the panel and away from the ship.
Paint Damage and Cathodic Disbondment: The Secondary Effect
Overprotection is not only wasting current, it is actually destroying the protective coatings. A cathodic reaction occurs at the steel-coating interface if the hull potential falls below -1,000 mV. The alkaline environment will work to break the polymer bonds of epoxy and coal-tar paints, which can cause cathodic disbondment. The first signs are small blisters or a chalky white deposit around weld seams and around edges, where coating has naturally thinner thickness. When the paint flakes off, the bare metal pulls in more current which in turn causes more paint to come off and so on. We have conducted the test on the coating samples of vessels with damages for more than 5,000 m² in Nantong, and the result is that the adhesion of the coating decreases by as much as 60% in only three months in overprotected hulls. The answer is not to reduce current, it is to repair the coating correctly and adjust the system at the same time - it's not going to work if new paint is applied to an already highly polarised surface and the system is not adjusted at the same time, otherwise it will fail again in weeks.
Understanding Reference Cell Alarms: False Signals vs. Real Failures
One of the most misunderstood issues is the common reference cell alarm. High potential or cell failure alert can just mean your reference is unplugged at the panel. Electrodes made of silver/silver-chloride tend to become fouled by marine growth, silt or oil films which alter the internal electrode junction potential causing false mV readings. Just as frequently is cable leakage (when moisture enters the junction box or a cable with a bruised junction runs through the sea chest, a parallel resistance path occurs that emulates a drifting reference signal. In order to replace the hardware, our technical support engineers at SME undertake a three-step diagnostic: (1) checking the DC resistance of the cell against a known clean electrode, (2) checking the insulation resistance between the cell and earth (should be >20 MΩ), and (3) conducting a dynamic current interruption test to determine whether the alarm follows current intersections. More than 40% of the alarms that we attend are solved by cleaning the surface of the cell and re-sealing the cable gland, rather than replacing it.
Practical Field Troubleshooting and Corrective Workflow
Troubleshooting should be done on a procedure basis rather than guesswork. The first thing you should do is document the readings for both current and voltage over the past 72 hours. In most cases, the drift for current and voltage can reveal if you are experiencing a slow (controller drift) or sudden (cable or cell fault) system overprotection drift. If after these steps you continue to suspect overprotection, take your system into manual control. Turn your current down and observe a portable reference on the ship's hull: If your portable is normal and your panel shows overprotected, then your shipboard reference cell is suspect. If both displays show negative you are probably overdriving the anode circuit, check anode resistance, replace any anode that showed 8V DC drop across. Always in front of the anode or reference call look for a hole or pinhole in the coating. You can look for the holes the first way without the holiday detection as well. The other point is how to prove whether you have a hole is through a holiday detection, SME provide 12 months guarantee on all corrective maintenance works, we also inform clients of course, none of the guarantee can substitute the proactive measurement of reference cell weekly off potentials and cleaning on any dock. Always check for the increase in current as an indication of impending over protection event. Most overprotection cases are avoidable, if we use a disciplined and consistent approach of monitoring systems and based upon this analysis we will show more than 500 cases per year. Do not read too between lines; use the right basics principles, which will work forever for your vessels.