Simple galvanic corrosion test

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Here’s a physics experiment to verify that underwater metal is protected by sacrificial zinc anodes.

Here’s a physics experiment to verify that underwater metal is protected by sacrificial zinc anodes.

Read the other story in this package: How to prevent galvanic corrosion

Active sacrificial zincs and more-noble metals like stainless steel and bronze immersed in seawater (an electrolyte) and connected by a bonding strap inside the hull are a voltaic wet cell. (Multiple cells in series comprise a battery). There is a chemical reaction between the submerged metals that results in the zincs being lower in potential (more negative) than the noble metals. A flow of electrons from the sacrificial zincs across the bonding copper strap to the more noble metals protects the more noble metals from corrosion. The cost is the zinc anodes that corrode away.

Silver is more noble than zinc, and a “silver:silver chloride half cell” is used as a reference to measure the electrical potential of metals immersed in the same electrolyte. In the photograph, such a reference cell is immersed in seawater and attached to the positive input of a digital multimeter. A sacrificial zinc is attached to the negative input, and the electrical potential between them is 1.063 volts.

1. Dissimilar metals immersed in seawater — here, a silver:silver chloride half cell and a zinc anode — generate a voltage.

Drop the reference cell over the side into the seawater (electrolyte). (Reference cells with long leads are available from Professional Mariner, www.pro mariner.com.) The negative probe is then touched to each piece of bonded metal inside the boat. The voltage between the reference cell and the zinc anodes, and between the reference cell and each piece of submerged metal on the bonding circuit will be the same. Using this procedure on Maramor, my Grand Banks 42, which has an impressive bonding system, I found that the bonding strap to the main engine seacock was defective, a condition I could not see since the bonding strap is painted over and appeared to be intact.

On Maramor the main engine seawater strainer is not on the bonding circuit, since it is isolated by non-conducting hoses. The voltage between the strainer and the reference cell is 0.393 volts, which would be expected for bronze.

2. This seawater strainer isn’t connected to the bonding system and has a voltage relative to the reference cell of 0.393 volts. When the strainer is connected by a jumper wire to the bonding circuit the potential of the strainer immediately rises to that of the sacrificial zincs relative to the reference cell, which on Maramor was 0.924 volts.

3. When the seawater strainer is connected to the bonding circuit with a jumper wire the voltage immediately increases to 0.924.