Thread: Over zinc'd?
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Old 02-25-2018, 03:11 PM   #10
CharlieJ
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City: St. Petersburg
Country: USA
Join Date: Oct 2007
Posts: 95
C electric Post #9:
On a fiberglass hull with no underwater aluminum components, the ABYC E-2 recommendation hull potential range for protection is: -550mVDC to -1100mVDC when measured against a silver/silver-chloride reference cell. The Galvanic Series of Metals, also from ABYC E-2, shows Zinc as having a Corrosion Potential range, again when measured against a silver/silver-chloride reference cell; in sea water flowing at 8 to 13 ft/sec and within a temperature range of 50F to 80F of -980mVDC to -1030mVDC with Mil-Spec Zn anodes at the upper end of that range.

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Too high from too much zinc can bubble paint off of metal parts, props, shafts, struts, etc., any immersed painted metals.
If you review the recommend range for cathodic protection and the available protecting potential from a zinc sacrificial anode, "over zincing" is not really possible on a fiberglass boat with a properly installed underwater paint system. I have performed many, many corrosion surveys in salt and brackish water and rarely find a hull potential more negative than -1000mVDC. The norm is probably -950mVDC.

The process of paint coming off of underwater metal parts is primarily the result of poor paint system application which is generally a result of poor substrate preparation. In some cases, it can be caused by VOC entrainment or the presence of soluble salts on the substrate that have not been removed prior to painting.

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It may cause a ring of paint bloom/bubble or disappearance around immersed through hulls.
Haloing is caused by high copper content paint and its interaction with good quality bronze through hulls. The copper in the paint matrix is anodic to the bronze which is cathodic. To prevent this condition, the bronze through hull needs to be properly primer coated with a barrier coat such as the Interlux products.

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May cause direct damage to metals like aluminum.
Absolutely correct! Aluminum is amphoteric which means it can be damaged by either acids or bases. Driving the potential of an aluminum hull or underwater aluminum component to >-1200mVDC against the reference cell with an impressed current cathodic protection system or because of a fault in the on board DC system will create an alkaline environment that will damage the aluminum hull or component.

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On wooden boats it can cause actual damage to the wood surrounding through hulls or any other immersed metal parts or bolts/screw to secure them.
Absolutely correct! The range of protection for wooden boats is -550mVDC to -600mVDC; a very difficult to achieve 50mVDC range between under protected and overprotected. If the protection potential is above this range, an alkaline environment develops and the alkalis will attach the lignin in the wood, a condition known as delignification.
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