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| Corrosion Test Meter A Corrosion Test Meter 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 reliable readings of the silver:silver chloride. The full voltage scale of the meter is 1200 millivolts (1.2 volts).
Metals freely erode at the following approximate voltages depending on their composition:
At approximately 1050 millivolts, steel and even more so aluminum, are damaged by over protection. The more Noble the metal the lower the voltage and the more resistant it will be to galvanic corrosion. Here is the Noble Scale with the approximate voltages you can expect to read on the Corrosion Test Meter at 75ºF seawater temperature. Composition of metals, water temperature, salinity, velocity and oxygen content will all vary the meter readings slightly - that is, stainless steel could read from 0 to 575 depending on composition and oxygen.
A good rule of thumb is to select metals to be connected together that are within 200 millivolts of each other to reduce galvanic corrosion. After bonding, a Sacrificial Anode is added to the bonding system. This will raise the voltage of the system up into or over the "protected" area on the meter, depending on the size of the Zinc. It sometimes takes 5 to 24 hours for a hull to polarize. The voltage will continue to rise until polarization is reached. The amount of boat metal to be protected, type of metal and size of Zinc determines polarization time. |
