New EPA Regulations: US Vessel General Permit
In December, 2013 the US Environmental Protection Agency (EPA) issued the new National Pollutant Discharge Elimination System (NPDES) Vessel General Permit (VGP). Affected are vessels over 79 feet in length.
Included in this document under Cathodic Protection (VGP 2.2.7) are documentation requirements for a choice of anodes. The EPA’s preferred choice of anode is Magnesium, then Aluminum, followed by Zinc.
“Vessel operators should note that magnesium is less toxic than aluminum and aluminum is less toxic than zinc. If vessel operators use sacrificial electrodes, they must select electrode devices with metals that are less toxic to the extent technologically feasible and economically practicable and achievable. If a vessel selects aluminum, they must document in their recordkeeping documentation why they made this selection, and why use of magnesium is not appropriate. Likewise, if a vessel selects zinc, they must document why they did not select magnesium or aluminum.”
The choice of anodes above, (Mg, Al, Zn) is made strictly through “environmental” issues and unfortunately does not address the protection parameters of each type of anode. Each anode type possesses both positive and negative attributes, depending on the criteria the anode is placed in, along with the different types of metal (cathode) to recieve the anode’s electrical current for proper protection.
Many variables exist to help determine the best type of protection to be used:
- Water Type: fresh – brackish – salt
- Water Temperature
- Water Current
- Anode’s designed lifespan
- Current output
- Metals to be protected
In personal discussions with the EPA, they are saying it’s not mandatory to use Magnesium 1st, Aluminum 2nd and Zinc 3rd, but to educate the public on the issues. However, documentation of anode selection is mandatory.
Following is a brief scenario for the selection of the type of anode:
Pros: Preferred for freshwater. The potential of -1450 mv performs well in high resistant fresh water. Light weight.
Cons: High voltage in salt water can result in over-voltage (release of hydrogen) from the protected structure, creating excessive corrosion. Expensive. Shortest lifespan. Efficiency is 55%.
Pros: Performs well in salt and upper levels of brackish waters. Potential of -1150mv is under the overvoltage point of steel. Longer life than zinc. Light weight. Efficiency is 87%.
Cons: Maintenance may be required for continued current output, especially in lower levels of brackish and fresh waters. This alloy doesn’t slough it’s consumed metal well which can slow it’s current output and eventually passivate in certain conditions
Pros: Performs well in salt and upper levels of brackish waters. Potential of -1100mv is high enough to protect steel and aluminum. Zinc sloughs it’s consumed metal well and retains constant current throughout its life. Efficiency is high at 95%.
Cons: Only partially protects in fresh and lower levels of brackish waters. Shorter life than aluminum.
ANODES: Zinc vs Aluminum
Some companies are pushing for a switch from zinc (Zn) to aluminum (Al) anodes. Consumers should be informed before making the decision on which anode type will properly protect their investment. Both types of anodes have pros and cons, and as manufacturers of BOTH types of anodes, it’s our position to inform our customers on whether or not a switch is recommended.
For anode protection to occur, the metal requiring protection must be attached to another metal (anode) that puts out a minimum of -200mv on a sustaining basis. Both Zn and Al anodes have at least this amount of current output and will protect steel and aluminum structures in saltwater. Even though Al is more negative than Zn by -50mv, doesn’t mean Al protects better. Once the structure stabilizes, it’s protected and Zn and Al anodes both have the potential to achieve polarization.
After the anodes are in service, the consumed metal is sloughed off, exposing new metal. This process continues throughout the anode’s life. Zn anodes slough better than Al anodes during times of low current transfer. In fresh water or lower levels of brackish water, Zn anodes occasionally film over, however it’s more common in Al anodes. This film is a hydroxide layer that forms on the anode’s surface, creating a barrier and reduces or eliminates current output from the anode, thereby reducing or eliminating protection of the structure.
When a vessel is underway, corrosion rates increase and the anodes become more active. While in moorage, there is less of a draw on the anodes, allowing current output to decrease making the anodes less active. While in this situation, over time the anodes can film over and maintenance (cleaning) must occur to “reactivate” them. This filming-over is common in AL anodes when current output is minimized, so caution is advised to boaters that don’t use their vessels on a regular basis, especially in fresh and brackish waters.
Below are the “electrochemical” properties showing a direct comparison between the two types of anodes.
Zn vs Al – Comparison Zinc Aluminum
Theoretical amp hrs / lb 375 1352
Efficiency 95% 85%
Actual amp hrs / lb 355 1150
EXAMPLE of equivalent anodes: Zn vs Al
Anode size: 1-1/4” x 3” x 12” ZSS-12 ASS-5
Gross anode weight in lbs: 11.6 5.0
Wt of welding strap (core): -1.07 -1.07
True anode material lbs: 10.53 3.93
Multiply by amp hrs x355 x1150
Total life (hours) @ 1 amp 3738 hrs 4520 hrs
Total Life (days) @ 1 amp 156 days 188 days
Note: The weight of the embedded steel core was subtracted from the anode’s weight because there’s no protection added from the steel core. In the above example, the core allows for 27% of the Al anode’s total weight, verses only 10% of the Zn anode total weight. In other anodes the weight ratio from core-to-anode varies which changes the life span. In this particular scenario, the life expectancy of Al is about 21% longer than Zn.
In conclusion, we are not saying to stay with ZINC, nor are we saying switch to ALUMINUM, but to look at each situation individually and then make the decision of which anode works best for your specific situation.