Antifouling Choices for Underwater Metals
With the final ban of TBT paints, coatings makers have brought forth several products to fight the growth of barnacles and other beasts on metal surfaces underwater. Here's some background, and the start of a new experiment.
Propellers, shafts, struts, outboard and Saildrive engine housings, and through-hull fittings, composed variously of bronze, stainless, and aluminum, are notoriously difficult to keep clean underwater—not to mention the hulls of metal boats themselves. All these metals (assuming the bronze is high-quality, not a brassy bronze) are quite corrosion-resistant by themselves underwater. Unfortunately, since they don't resist fouling, they have to be coated with something to ward off growth.
If you paint them with a standard copper (cuprous oxide) antifouling paint, you commit the sin of placing dissimilar metals against each other in an electrolyte, and inviting corrosion. The effect is especially pronounced with aluminum: Copper, a relatively noble metal, essentially becomes the cathode of an electrochemical cell, and aluminum, a less noble metal, becomes the anode. Applying copper-based paint to bare aluminum structures in an electrolyte like seawater creates an especially intimate galvanic package where the paint tends to eat the boat.
For a time, we had TBT-based coatings to help out. Now, happily for the environment, we don't. Let's take a look at what happened to TBT, and how it's being replaced.
The End of TBT
The organotin compound tributyltin (TBT) has served for over 30 years as the biocide of choice in most antifouling paints used on metal bottoms and underwater gear. It is hugely effective in discouraging barnacles, algae, and mollusks from camping on these structures. At one time, TBT seemed environmentally safer than some of the concoctions it replaced—stuff like lime, compounds of arsenic or mercury, and DDT. But it turns out that TBT is far tougher on the environment that anyone expected, and it can harm the good guys—organisms such as oysters, dolphins, whales, seals, and humans.
By 1988, TBTs were implicated in the destruction of commercial oyster beds, and the International Marine Organization (IMO) took up the matter through its Marine Environment Protection Committee (MEPC). They have subsequently coordinated worldwide research programs with organizations like the International Standards Organisation (ISO), the American Society for Testing and Materials (ASTM), and the UK's Centre for Environment Fisheries and Aquaculture Science (CEFAS). Investigations conducted in the United Kingdom, the Mediterranean, Canada, Australia, New Zealand, Japan and the United States all implicated TBT in anomalies ranging from disrupted sex life in dog whelks to suppressed immune systems in dolphins. Ultimately, TBT was characterized as "one of the most toxic substances ever deliberately introduced into the marine environment."
As early as 1990, IMO's MEPC adopted a resolution calling for the elimination of TBT from paints applied to non-aluminum-hulled vessels under 25 meters (82 feet). IMO followed up in 1999 by directing that a legally binding treaty be developed mandating "a global prohibition on the application of organotin compounds which act as biocides in antifouling systems on ships by 1 January 2003, and a complete prohibition by 1 January 2008." The treaty was completed, with the participation of the EPA, in October 2001.
If you've shopped for a TBT-based solution this spring for an aluminum boat or outboard lower unit, you will have noticed that these coatings have already virtually disappeared from chandlery shelves. Although TBT is not yet banned outright in all applications for smaller vessels, it no longer makes sense for paint manufacturers to provide a separate formulation to a market that consumes only a small part of their overall volume. Interlux shipped its last TBT-based product to US retailers on the last business day of 2002, and Pettit will cease shipment at summer's end.
What replaces TBT? Do other formulations protect underwater gear as well as the old products? We had a bunch of questions for Interlux, Pettit and several other paint manufacturers. Some of the answers are here. Other answers will have to await the results of our in-water tests of several products designed to replace TBT paints.
Where We're Headed
One answer favored by several manufacturers is a particular compound of copper called cuprous thiocyanate, which doesn't produce the same levels of galvanic interaction with aluminum as does cuprous oxide.
At best, cuprous thiocyanate may be a stop-gap measure (although possibly a long-term one) because the use of any copper as a marine biocide is being called into question by many environmentalists and government agencies. Countries with especially sensitive local conditions, including Sweden and The Netherlands, have taken precautions to the point of placing restrictions on the use of copper in antifouling paints. Objective evaluation is elusive, and all parties agree on the need for careful research to identify the sources of elevated levels of copper in the marine environment (it doesn't all come from bottom paint) and the nature of its impact.
Alternative approaches currently available include non-toxic foul-release systems that limit adhesion of fouling organisms and provide for relatively easy cleaning.
Looking a little farther along, we might anticipate that viable alternatives would emerge from work being done on booster biocides of organic origin. These tend to be more "species specific" and are likely to continue to be used in combination with more general biocides such as copper.
The use of enzymes and other microorganisms could open the door to a non-toxic future for antifouling systems. Research is moving forward on using them to deprive potential fouling organisms of critical nutrients in the immediate area of a vessel's hull.
Other options might lie in the further development of foul-release systems, using non-toxic coatings to which fouling organisms find it difficult to adhere. This could hold special promise for large vessels when combined with a rigorous regimen of underwater cleaning. A variation of this approach is a flocked coating that forms a forest of individual, undulating fibers that denies organisms a firm grip.
Alternating electric currents can also be made to discourage barnacle larvae when channeled around a vessel's hull through a conductive layer sandwiched between two coats of insulation. It's worth staying tuned as work is done on the operating life, power consumption, reliability, and cost of such systems.
Other solutions today use more standard copper-based ingredients over an insulating epoxy barrier coat. Marine professionals we've spoken to tend to stay away from extensive barrier-coat (or buffer-coat) solutions because they demand very precise surface preparation and application techniques, and there are often problems with paint adhesion.
Finally, there's a host of solutions that sometimes work and sometimes don't, and that have provided fine grist for debate for many years. For example, many people say the best way to keep barnacles off a bronze prop is simply to burnish the prop mirror-smooth before putting it in the water, and thereafter occasionally hit it with a sponge or Scotch-Brite pad. We do think it helps, but not always. In recent years, we've suggested supplementing that burnishing with a coating of a spray like Interlux's Micron 33, just to prolong the time before visiting with a mask. That TBT-based coating is now off the market.
Many people have had good luck on props with the non-copper coatings from E-Paint in Massachusetts. Their No-Foul products have been on the market for nigh on a decade, although we just began testing them as alternative bottom paints a couple of years ago.
Another possible solution, discuussed before in these pages, is to isolate the propeller shaft with a synthetic disk like the Drivesaver (www.globerubberworks.com), and remove the sacrificial shaft zinc. The slight difference in nobility between the stainless shaft and bronze prop is said to help prevent barnacle growth.
Whatever works without damaging the metal or the environment is fine, but for the purposes of the next discussion, we're just going to look at some available coatings.
Manufacturer Moves Today
Many long-familiar products are gone or going. We wanted to know how manufacturers are filling the gap.
To seek a preliminary answer, we asked for samples of post-TBT coatings from several manufacturers and set up a simple test. We applied them to new aluminum surfaces and dunked them for the season into the waters of Long Island Sound. We intend to bring them up to check the results in the fall, around the time we haul our boats, and report back in a winter issue.
Preparation of our test surfaces was pretty much by the book, using specified treatments and primers, where available. When they weren't available, we made necessary exceptions, and not always with happy results. Applying an aerosol paint from one manufacturer over the primer from another, for example, resulted in a spectacularly crackled finish. The lesson is that care and compatibility count, and if you're in doubt, ask.
On the whole, though, the paints went on smoothly, whether applied by brush or aerosol.
Trilux, a copolymer ablative paint, is a post-TBT solution from Interlux that can be used on any material, including aluminum. Different versions can be applied by brush, roller, or spray. We used the aerosol type in black.
Pettit provided the brush-applied version of a white formulation labeled Alumacoat SR, still awaiting its EPA listing as of this writing. This paint uses a straightforward combination of cuprous thiocyanate and zinc pyrithione.
A can of AquaGard II aerosol in black arrived from Flexdel, interesting for its use of a low level of cuprous oxide to avoid corrosion with aluminum, rather than going to another compound.
We called Philadelphia Resins to see if they had a replacement for their TBT-based Clear Choice, the top-performing coating in test results published in the May 1995 issue of our colleague publication, Powerboat Reports. The company is working on a non-TBT antifoulant to replace Clear Choice, but is selling a wax-based non-toxic barrier, Easy-On Bottom, in the interim. "Easy-On is not a response to the TBT ban. It's been on the market for a while," said Philadelphia Resins recreational marine manager Bob Sciblo. "We're experimenting with non-pesticide coatings for aluminum, but haven't found a satisfactory replacement for Clear Choice yet."
Easy-On leaves a thin, waxy film on the metal surface, and must be brushed off periodically. One coat of Easy-On will provide full-season freshwater and intermittent saltwater protection, according to the company literature, which also says, "This is a cleanable barrier system and throughout the season if minor fuzz or marine attachment occurs we recommend a sponge or deck brush cleaning to maintain performance."
From E-Paint, we're testing two photoactive ablative paints: E-Paint SN-1, a no-copper, no-TBT paint recommended for commercial vessels and used on the Coast Guard's aluminum boats, and E-Paint ZO, an ablative paint which utilizes zinc omadine. The latter is also included in our antifouling test on fiberglass surfaces. The results of that test will be published next spring. Both the SN-1 and ZO require three coats.
Another coating taking part in both the fiberglass and aluminum testing is Dolphinite Go Fast, which uses silicones to reduce drag and biodegradable enzymes to fend off marine growth. Dolphinite also makes a spray paint specifically for outdrives and outboards. This is also being tested.
Interlux Veridian, a slick, silicone-rubbery covering, rounds out our test group. Veridian was recently dropped from the Interlux line-up for boat bottom covering. Interlux said the formulation was tricky and too expensive for the market as a bottom coating. It's only available in a clear version for outdrive protection.
We'll let you know how all these products fared over the summer when they emerge in the fall.
• Dolphinite, 978/356-9834, www.dolphinite.com
• E-Paint Company, 800/258-5998, www.epaint.com
• Flexdel Corp., 888/353-9335, www.aquagard-boatpaint.com
• Interlux Yacht Finishes, 800/468-7589, www.yachtpaint.com
• Pettit (Kop-Coat) 800/221-4466, www.kop-coat.com
• Philadelphia Resins, 215/855.8450, www.philadelphiaresins.com