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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:

ERODE PROTECT
Bronze 300 millivolts 500 to 700 millivolts
Steel 500 millivolts 750 to 950 millivolts
Aluminum 650 millevolts 800 to 1040 millivolts

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.

MILLIVOLTS METAL OR ALLOY
1580 Magnesium
1140 Galvanized Iron
1050 Zinc
860 Cadmium
790 Mild Steel
740 Alloy Steel
730 Aluminum (Forged alloy)
550 Active Stainless Steel
500 Tin
450 Manganese Bronze
450 Naval Brass (60% Copper 30% Zinc)
450 Yellow Brass
360 Admiralty Brass
340 Copper
330 Brass (60/40)
260 Silicone Bronze
260 Tin Bronze
240 Lead
200 Copper/Nickel (70/30 alloy)
150 Passive Stainless Steel
(302, 304, 316, 317, 321, 347)
110 Monel
100 Titanium
80 Silver
Left of 0 Platinum
Left of 0 Gold

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.

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