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Aluminium Questions

Q. What is a typical paint system for an aluminium vessel?

A. If painting on to new or completely sanded aluminium we recommend using Penguard HB  or Jotacote Universal as a primer. Then a topcoat or antifouling can be applied depending on whether the area to be painted is above or below the waterline. For information and advice on the most suitable paint system please contact us.

 

Q. Is it okay to use copper based antifouling on an aluminium vessel?

A. The simple answer used to be no, because the copper will react with the aluminium hull causing galvanic corrosion and so the recommendation used to be to use antifouling in only grey and white as these colours contain no copper and so will not damage the vessel. Recently however Jotun has approved the use of its commerical copper based antifoulings (like SeaQuantum Classic and SeaQueen) for use on aluminium. More information is available in Jotun's technical release and there is also a reference list of aluminium vessels currently using Jotun's copper-based antifouling.

 

Q. What is galvanic corrosion?

A. Galvanic corrosion is an electrochemical process that occurs when one metal corrodes when connected to another and immersed in an electrolyte (saltwater). It occurs because different metals and alloys have different electrical potentials, and under the right conditions one will transfer elections to another. The electrical potential of various metals and alloys can be found here.

An example of galvanic corrosion would be an aluminium hull connected to a stainless steel propeller shaft. In this case aluminium is more reactive and will act as what is called an anode and the less reactive stainless steel will act as a cathode. In a conductive solution like saltwater the electric potential will cause ion migration from the anode to the cathode causing anode to "dissolve".

This sort of reaction can be incredibly useful (for example it is used in many batteries), however on the hull of a aluminium vessels it is certainly not desirable. The easiest way to prevent galvanic corrosion is to insulate the two metals as without electrical contact there can be no ion migration. Another option is to use sacrificial anodes. These are reactive metals which will preferentially corrode and are typically made from zinc, magnesium or aluminium.

 

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