Difference between revisions of "Electrolysis"
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== Discussion of Electrolysis == | == Discussion of Electrolysis == | ||
| + | ::::[Return to [[Systems_Descriptions#Anti-electrolysis|Systems Descriptions]] document] | ||
Calder uses the terms "galvanic corrosion" (p.132) and "stray-current corrosion" (p.140) instead of "electrolysis". The result of both kinds of corrosion is that metal parts get acted on to become spongy, brittle, porous, or just lose metal and crumble away. The use of sacrificial zinc objects helps alleviate this hazard. | Calder uses the terms "galvanic corrosion" (p.132) and "stray-current corrosion" (p.140) instead of "electrolysis". The result of both kinds of corrosion is that metal parts get acted on to become spongy, brittle, porous, or just lose metal and crumble away. The use of sacrificial zinc objects helps alleviate this hazard. | ||
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A simplistic description of the cause of, and solution to, these corrosions (AKA electrolysis): | A simplistic description of the cause of, and solution to, these corrosions (AKA electrolysis): | ||
| − | Any two different metals immersed in salt water (an electrolyte) develop a difference in voltage between :each other. This galvanic voltage will cause a current to flow from the lower voltage metal to the higher voltage one, and to return through any metallic connection between the two metals. The result of these flows is that the lower voltage metal is consumed. The dissimilar metals can be physically found in two different boat parts immersed in the salt water, or in one boat part from an ALLOY of two (or more) metals, in which case the return path is the metal itself. | + | Any two different metals immersed in salt water (an electrolyte) develop a difference in voltage between :each other. This galvanic voltage will cause a current to flow through the salt water from the lower voltage metal to the higher voltage one, and to return through any metallic connection between the two metals. The result of these flows is that the lower voltage metal is consumed. The dissimilar metals can be physically found in two different boat parts immersed in the salt water, or in one boat part from an ALLOY of two (or more) metals, in which case the return path is the metal itself. |
| − | Voltage difference among boat parts can come about from electrical power sources aboard the boat. These can force stray current flow through the water and through underwater parts. The metal parts are susceptible to being consumed by the current. | + | Voltage difference among boat parts can also come about from electrical power sources aboard the boat. These can force stray current flow through the water and through underwater parts. The metal parts are susceptible to being consumed by the current. |
Placing a zinc object in the salt water, and connecting all other metal objects in the salt water electrically to the zinc object, ensures that any current generated by any dissimilar metals, or by stray currents, will cause the sacrificial zinc to be eaten away rather than any of the other parts. | Placing a zinc object in the salt water, and connecting all other metal objects in the salt water electrically to the zinc object, ensures that any current generated by any dissimilar metals, or by stray currents, will cause the sacrificial zinc to be eaten away rather than any of the other parts. | ||
Latest revision as of 22:30, 6 January 2009
Contents
Discussion of Electrolysis
- [Return to Systems Descriptions document]
Calder uses the terms "galvanic corrosion" (p.132) and "stray-current corrosion" (p.140) instead of "electrolysis". The result of both kinds of corrosion is that metal parts get acted on to become spongy, brittle, porous, or just lose metal and crumble away. The use of sacrificial zinc objects helps alleviate this hazard.
- TODO: make a reference to Calder "Boatowner's Mechanical and Electrical Manual" 2nd Ed;
- TODO: but first, make a "hardcopy" book reference list at upper levels of this wiki.
A simplistic description of the cause of, and solution to, these corrosions (AKA electrolysis):
Any two different metals immersed in salt water (an electrolyte) develop a difference in voltage between :each other. This galvanic voltage will cause a current to flow through the salt water from the lower voltage metal to the higher voltage one, and to return through any metallic connection between the two metals. The result of these flows is that the lower voltage metal is consumed. The dissimilar metals can be physically found in two different boat parts immersed in the salt water, or in one boat part from an ALLOY of two (or more) metals, in which case the return path is the metal itself.
Voltage difference among boat parts can also come about from electrical power sources aboard the boat. These can force stray current flow through the water and through underwater parts. The metal parts are susceptible to being consumed by the current.
Placing a zinc object in the salt water, and connecting all other metal objects in the salt water electrically to the zinc object, ensures that any current generated by any dissimilar metals, or by stray currents, will cause the sacrificial zinc to be eaten away rather than any of the other parts.
When to replace an anode (a zinc): [West Marine catalog 2007, p.361]:
- effectiveness depends on good electrical connection
- effectiveness directly proportional to surface area
- some manufacturers suggest replacement when zinc is 2/3 gone
- West recommends replacement when half-eroded or half-dissolved
- [ThomS's modification of above statement]: pencil-zincs may require replacement when only the part of them in the water has "half-eroded or dissolved", not the whole area or mass of zinc
Types of Material: [West Marine catalog 2007, p.361]: Zinc or aluminum for saltwater; magnesium for fresh water; aluminum for salt and brackish water.
sacrificial guppy on cable
sacrificial zinc on prop-shaft
P1090819-zincs-2007-144p6q shows the prop-shaft zincs Fall 2007, printed large enough to show their inhabitants and erosion of the zinc a bit.
We assume they installed two smaller ones rather than one zinc so that there'll be adequate total surface area without causing the prop to be in the wash of the zincs.
We don't know whether the erosion on these zincs is excessive; we think not, except that one seemingly knowledgeable sailor mentioned that maybe zinc can be eroded, leaving only any other metal the zincs include to make them stronger, etc.
We measured the shaft at just under 1.5 inches, inside the hull, maybe 6 to 12 inches aft of the shaft coupling to the transmission. Who knows whether it's the same diameter outside the hull?
sacrificial zincs in engine
There are two pencil-zincs protecting the engine; one is in the heat exchanger, and one is in the oil cooler.
See Engine zincs replace 2007 for a report and discussion of replacing the two engine pencil-zincs.
See pp.220-221 Westerbeke Parts List, item 33, #11885
- pencil-zinc in heat exchanger item 23
- pencil-zinc in lube oil cooler item 32
Engine manual mentions something like "zinc in heat exchanger" (p.20) but doesn't mention one in oil cooler.
Maintenance log (penned on back page of manual) says "replaced both zincs..." but implied only the heat exchanger one was rotted away (27Nov83, 454 hrs).
Also change ZN 01Jul95
Another ancient log (on 1-2 pages looseleaf notebook paper) may also mention condition and replacement of zincs [ThomS couldn't find these sheets Jan08 when looked quickly]
TODO: bring up to THIS page the more detailed findings from Engine zincs replace 2007 including: a: 2 sizes of pencil-zincs, though parts list says they're identical; b: give references to parts list pages where heat exchanger and oil cooler are pictured; c: check whether Calder's winterizing checklist indeed does NOT mention to check zincs
