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Building your own corrosion meter

Jan 1, 2003
From Ocean Navigator #119 January/February 2002

From Ocean Navigator #119 January/February 2002

Underwater metal on boats often corrodes. When two different types of metal are mechanically joined (a bronze propeller and a stainless steel propeller shaft, for example) and submerged in an electrically conductive fluid (i.e., seawater), corrosion can be rapid and expensive.

A voltaic cell is created whenever dissimilar metals, connected in some way, are immersed in a conductive fluid. The voltage created depends on the relative positions of the two metals in the galvanic series. The resulting flow of electrons results in loss of material from the least noble metal; thus, in our example, the bronze propeller corrodes. Although we can't prevent the current flow, we can interfere with the process by offering the corrosion god a sacrifice, diverting the corrosion from the bronze prop to some less valuable piece of metal. The usual practice is to install a sacrificial anode, usually a zinc collar, on the prop shaft. Ideally, the single piece of zinc will protect all of the electrically connected underwater hardware. The problem is knowing when the anode is the right size, offering a corroding surface area that is neither too large nor too small.

Since the dissimilar metals create an electromotive force, a voltage, we can gauge the adequacy of the protective zinc by measuring the galvanic voltage. Directly measuring the voltage between the prop and its shaft would require that we separate them to allow introduction of a voltmeter, a task not likely to be performed even by the most inquisitive boat owner. However, we can make a useful measurement by introducing another piece of underwater metal and making measurements with reference to its position in the galvanic series.

The most common method is to use a "half-silver chloride cell." The resulting cell consists of a piece of pure silver, connected by a copper wire to a sensitive voltmeter and then with another copper wire to a probe that can be used to make electrical contact with various pieces of underwater metal.

Almost any digital voltmeter can be used to take the measurements. Analog voltmeters that can read voltages as low as 1/1000 of a volt (one millivolt, or mv) can also be used. To use the voltmeter to check on the adequacy of the sacrificial zinc, one lead wire is connected to the silver electrode and immersed in the water in which the boat is floating. The other wire from the voltmeter is connected to a piece of metal in the boat that is in contact with the seawater (the prop shaft, for example).

The amount of zinc required to protect other underwater metals varies with the type of metal involved. Underwater bronze, such as the propeller, is considered to be fully protected by the zinc on the prop shaft if the voltmeter reads between 500 and 700 mv. Voltages less than 500 mv indicate that the bronze is eroding, and more zinc is needed. Voltages above 700 mv indicate that the bronze is overprotected; there is an excess area of zinc available. (This condition won't necessarily harm a fiberglass boat but can cause trouble for wooden hulls.) The protected voltage range for steel is 750 to 950 mv and 800 to 1050 mv for aluminum.

You can make the silver probe by connecting an insulated, flexible copper wire to a small piece of pure silver (your Aunt Tillie's teaspoon, for example), or you can buy the necessary silver/silver chloride electrode, with an attached 10-foot wire lead, from Marinco Guest, under their part number 22435, for about $65. Make a corrosion test meter, learn how to use it, and you can be the most popular person in your marina or yacht club. Plus, you get to see all kinds of interesting boat bilges.Chuck Husick.