The importance of checking anode consumption
While I’ve developed many theories about why and how corrosion occurs aboard specific vessels, I’ve never been able to prove anything without testing. It is the most important aspect of corrosion analysis, as well as the surest means of establishing a corrective path, and yet it is not done frequently.
In my experience, one of the most common reasons to carry out a corrosion test is zinc consumption. Zinc is one of three metals (along with magnesium and aluminum) used in sacrificial anodes, which are consumed based on the load placed on them. Anodes are affected by several factors, including salinity, temperature, current, the amount and type of metal they are protecting, the condition of the vessel’s bonding system, and shore power isolation.
Many vessel owners complain anodes are being consumed too quickly. Others boast about how long their anodes last. That’s a concern as well, as it’s a sign they aren’t working.
How do Sacrificial Anodes Work?
Anodes utilize a process known as cathodic protection, where the anode sacrifices itself to protect the cathode, which can be any protected metal, propeller, thru-hull, strut, etc. If the two are in contact or connected by a conductor (resistance between anode and cathode cannot exceed one ohm; it’s a lofty standard that often is not achieved because of corroded or loose connections), and both are immersed in the same electrolyte, sea or freshwater, the anode will afford protection to the cathode. The goal is to equip the vessel with the right amount of cathodic protection, i.e., an adequate number of anodes.
Every metal has a resting voltage, graduated in thousandths of a volt or millivolts (mV), and DC, which in turn is measured using a silver/silver-chloride reference electrode sometimes called a reference cell. These voltages (all are negative with just a few exceptions) are detailed in a chart referred to as the Galvanic Series of metals in seawater. The resting voltage of silicon bronze, for instance, is -260mV to -290mV; aluminum alloys is from -760mV to -1,000mV, and 316 stainless steel in still water (making it more corrosion-prone) is -430mV to -550mV.
To provide the proper amount of protection, any metal must be driven more negative with an anode by a minimum of 200mV. With that in mind, protection ranges for seawater are as follows:
⊲ aluminum: -950 mV to -1,100 mV;
⊲ a fiberglass vessel with common underwater metals excluding aluminum: -750 mV to -1,100 mV (a range more conservative than that dictated by ABYC Standards);
⊲ a timber hull with common underwater metals other than aluminum: -550 mV to -600 mV, steel hulls -850 mV to -1,100 mV.
It’s worth noting that while most metals can’t be harmed by overprotection, aluminum is an exception as it’s amphoteric (it is affected by both acid and base solutions). Overprotection creates an alkaline or base solution around protected metals; therefore, aluminum protection should not exceed -1,100mV.
Overprotection can also be harmful to the hulls of timber vessels adjacent to bonded, protected underwater metals. In a process known as delignification, the alkaline solution attacks the soft material between the grain structure.
Testing assesses the protection level, determines if the right number/mass of anodes is present, and if they are properly located. Testing can also identify a problem, such as rapid consumption of anodes or corrosion of underwater metals.
Testing is conducted with the vessel afloat, in calm water, with shore power unplugged (not just turned off), and no onboard equipment running. Using the aforementioned silver/silver-chloride reference electrode attached to at least a 20-foot-long marine-grade wire, plug it into the negative terminal of a multimeter with the scale set to volts DC (millivolts if that’s an option). The positive lead is plugged into the meter and then connected to the metal that is to be measured. If the vessel is bonded, meaning all underwater metals are connected to each other and to an anode, then connect the positive lead to a clean (if it is dirty, clean it with a Scotch-Brite pad) section of the bonding system (a bolt, bus bar, or terminal).
Drop the reference electrode over the side, about two feet below the surface making sure it does not touch the hull. Take your reading and compare it to the ranges given above for various vessels and metals. If the vessel is not bonded, then each metal will need to be measured individually. This may require lengthening the wire for the reference cell.
For the most part, the propeller and shaft will require their own testing because their connection to the bonding system is tenuous at best as it passes through the transmission, which is an oil-filled medium, thus a poor conductor. Shaft brushes are most commonly available and inexpensive but incapable of achieving the required one ohm or less resistance needed for proper cathodic protection.
Ultimately, I recommend that all underwater metals be individually tested even if they are bonded, as any significant difference will be an indication of a bonding system issue. An unbonded metal that has no anode protection will read its resting voltage, which should correspond to the Galvanic Series.
An important step after the testing is complete is to connect the shore power cord while leaving the power off. If any of the readings change by more than a few mV, then the vessel lacks isolation from the dockside shore power ground, which means it needs a galvanic isolator. If it already has one, it’s either not working or is improperly installed. Without isolation from the shoreside ground, your vessel is vulnerable to issues created by other nearby vessels or the dock itself. If you plug in on a regular basis, a galvanic isolator is cheap insurance.
-by Steve D’Antonio