Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2004

Coating Issues

Q. We have a fast speed boat Aluminum Hull with Sea water voyages.

After pretreatment process:

  • ash Coat - Etching primer ( Phosphoric acid )
  • 1 X 50 Microns DFT Polyamide cured Epoxy Primer
  • 1 X 125 Microns DFT Polyamine adduct cured Coal tar Epoxy Coating
  • 1 X 125 Microns DFT Polyamine cured Coal Tar Epoxy Tie coat

We used to apply a Sn ( Try Butyl Tin ) based Anti fouling fast polishing.

However as per New IMO regulations TBT is obsolete.

Therefore we have found a IMO complied Copper accrylate based Antifouling.

We have been advised that Copper ( anti fouling ) is not suitable for Aluminum Hulls. EMF ?

It is said that if the coating intact it is OK, however, if the previous coating is not intact (anti corrosive system breaches due to damages at warf) it can start pitting of Aluminum and corrosion.

Please advise

Thank you for your email. It is true that you never want metallic copper to be in direct contact with aluminum because the copper will act as an cathode and can cause severe corrosion if only a small area of aluminum (in this case the anode) is exposed to the elements. However, the anti-fouling coating is not in direct metallic contact with the aluminum, and more importantly copper acrylate is not a metal. Even if the coating is scratched through to the substrate an electrical current cannot flow between the anti-fouling paint and the aluminum substrate. Moreover, the anti-fouling coating comprises copper acrylate and not metallic copper. I doubt that the copper acrylate is electrically conductive. Therefore, I seriously doubt that the use of the copper acrylate coating can cause galvanic corrosion in areas where the aluminum substrate is exposed.

I propose that you conduct a simple experiment. Take two small pieces of aluminum and apply the complete coating system by following the same procedures as you would for the hull of the ship. However, for the one piece of aluminum apply an additional coat of the epoxy tie coat, but do not apply the anti-fouling. For the second panel use the anti-fouling exactly per your specification. Scribe a vertical scratch in the center of each panel and ensure that the aluminum substrate is exposed. The scribe should run almost down the vertical length of each panel, but should not extend to the top and bottom edges. Completely cover the back side of each panel with electrical insulation tape or aluminum duct tape, ensuring that moisture cannot penetrate through to the substrate. Immerse both pieces of aluminum into a weak salt (NaCl) solution of approximately 5% concentration. Observe corrosion in the scribe of both panels and determine whether or not the anti-fouling paint causes more corrosion.

I would be very interested to hear from you after you have conducted these experiments and to learn what you found.

Best wishes,

Ron Joseph

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