Corrosion Testing


This is a flier that was selling a corrosion test meter. However, it did make some fine points concerning corrosion that made posting this item worth while. (I understand the meter is pretty good too.) I've been using a meter that Mercury put out back in the early 70's that basically does the same thing. They have simplified the meter and anode abit since.

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Over the years, we have received many questions from readers confused about the practice of electrically bonding through hulls and other underwater metals, and we have stated the cases for and against bonding. In a recent issue, we mentioned Yacht Corrosion Consultants and their Portable Corrosion Test Meter, promising a closer look in a future issue.
Bob Ajeman of Yacht Corrosion Consultants, a division of Professional Mariner, which manufactures battery chargers and other instruments, sent us a Corrosion Test Meter ($217.95), plus a Zinc Saver Galvanic Isolator ($54.95), which isolates your boat from others at the dock to maintain an AC safety ground and protect against galvanic corrosion and zinc loss. Also included was a copy of Aleman's book, Boat 6 Yacht Corrosion Control ($29.95).

"The Corrosion Test Meter," the book says, "consists of a highly accurate millivoltmeter, a probe and a silver: silver chloride half cell. Other less expensive half cells could be used but they do not have the stability or the reliable readings of silver: silver chloride."

The meter reads the voltage of various metals on a scale from 0-1200 millivolts, actually placing the metals in their relative positions on the galvanic scale. To make a metal last forever, simply raise its relative voltage 225-250 mV by means of a sacrificial metal (zinc). In addition, there are three scales for the most common boat metals, bronze, steel and aluminum, each color coded and marked to indicate if a given fitting is "Freely Eroding," "Protected," or "Over Protected."

To use, drop the probe over the side of the boat into the water (it works best in salt water; the digital model-$226.95--is designed for fresh and saltwater), then attach the alligator clip and lead wire to the fitting in question, say a bronze through-hull. The meter tells you the millivoltage of the fitting and whether it is protected or not.

Ajeman's book gives the voltages for the various metals shown below in Table 1. On our unbonded 33-footer, we tested the bronze through hulls and stainless steel underwater fittings on our boat.  Because only the prop shaft has a sacrificial zinc, it (as well as the engine) was the only fitting indicated as protected.

Ajeman told us that the rudder stock and bronze through hulls are indeed freely eroding, and will last only as long as their projected life span-five, eight years, whatever. Because we have just a few underwater fittings to worry about, he recommended not bonding, but using zinc "fish" [anodes connected to a wire that can be dropped over the side when dockside or at the mooring) to protect the rudder stock. The bronze through-hulls might have to be replaced every eight years or so, but they are not so expensive as to justify the expense of bonding and the monitoring equipment.

Of more concern is the stainless steel rudder stock, which would be expensive to replace. Worse, because it is all too common for water to migrate down the stock and attack the welded rudder, loss of steering could be catastrophic. Said Ajeman, "stainless steel should never be used underwater, but it frequently is."

The alternative, for boats with more to lose, is to bond all underwater fittings together, including a zinc anode. All voltages should then be identical. If a permanent Corrosion Controller & Monitor is installed ($239.95), the voltage can be checked each time you come aboard and the control knob adjusted to increase or decrease the voltage to safe levels. Stray current corrosion also can be detected and observed.
If, in a bonded boat, not all readings are the same, you must determine the cause. There's a good chance that a connection between the suspect underwater fitting and the bonding system has failed, i.e., a wire is loose or not making good electrical contact. Other sources might include improper grounding of the AC system, stray AC current coming from the dock or stray DC current.

The bottom line for Ajeman is that a bonded boat that is monitored regularly, with voltages of underwater fittings controlled, is the safest way to go. If you're unwilling or unable to monitor and control the bonding system, then it's safer to unbond and trust that the quality of those fittings will give them a reasonably long and useful life span. The only pieces to be destroyed by a wayward stray current would be the prop and shaft; the unbonded pieces would survive. During each haul-out, those fittings should certainly be checked for signs of corrosion. [If you hit heavily corroded tinned bronze with a hammer it breaks apart; good bronze just dents. Newer silicon bronzes, however, rarely get that brittle, though you could get some idea of the condition by cutting it with a knife.)

Ajeman said one should think of a boat as a battery with enough juice flying around to light a flashlight bulb. Adding more zinc won't necessarily help. In the case of wooden boats, too much zinc destroys the wood. The scientific solution is to control the voltage.

We think Ajeman and Yacht Corrosion Consultants offer the simplest, easiest-to understand advice we've encountered. In fact, they've made a business of helping boaters diagnose and correct corrosion problems. We would urge anyone with a problem to seek their counsel. In an area too often shrouded with mystery and black magic, Yacht Corrosion Consultants is a beacon of light.

BOAT/U.S. sells the Corrosion Test Meter at a discount, $134.95, and the corrosion workbook for $19.95. The meter carries a one-year warranty and flat-rate lifetime repair plan. (Yacht Corrosion Consultants, 2970 Seaborg Ave., Ventura, CA 93003; 805/644-1886.)

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