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Aluminium Marine Engine Painting or not

Practical Engine Building

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Discussion and questions related to the course Practical Engine Building

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Engines on Yamaha and Sea Doo Jet skies are of aluminium but both manufactures paint them. For some reason color started to crack off very soon on most parts of my Sea Doo engine so I got block, head, valve- and PTO cover vapour blasted.

I feel uncertain whether I should paint these parts now or not. On one hand, it is said in the course that there is no reason to paint an alloy block. On the other hand, manufacturers are not do any additional work for no reason.

Any Ideas why they paint the engines?

Hello after talking to a mate that is in the industry he has said it is to help keep corrosion to a minimum

Thank you for rapid reply.

As I know Aluminium corrodes immediately when subject to oxygen. A thin corrosion layer builds up protecting the metal from further corrosion. Hence, it is recommended in the course not to paint an alloy block.

Do you personally see any need to paint a Jetski alloy block?

Uh, those are VERY different applications, especially if you're going to operate the jetski in the sea where there is a salt atmosphere! This going to be a bit technical, and you may benefit from further researching on-line as I may overlook points.

While it is true that aluminium is very reactive, and forms a 'protective' oxide skin, especially if hard anodised, that's only part of the equation.

When the engine dry, or only exposed to fresh (rain) water, there is little electro-galvanic corrosion because there is little ionisation of the water - however salt water is an excellent electrolyte, or source of ions, for reacting with the metals they're in contact with. When dis-similar metals, with different electrical potentials, have a mechanical contact with each other, and an electrolyte to supply ionised oxygen (or other oxidant) then an electrical circuit is made that operates like a battery (it's actually a wet cell, to be precise) and the more reactive metal will rapidly corrode. You will have seen this affect on older alloy vehicle wheels, as they're corroded, despite the 'protective' oxide skin.

By using paint, or some other method of protecting the alloy, to protect the alloy from the the water, it blocks the corrosion from happening - you may have noticed it forming where there are scratches and exposure to water? Ttraditionally, boats will use a 'sacrificial anode" which is a block of a more reactive metal bolted to alloy parts so that corrodes rather than the aluminium. Normally they're made of zinc.

So, I would STRONGLY recommend you paint your alloy parts, especially as you may have removed some protection - vapour cleaning 'should' be harmless, but not something I can be sure of as some methods may be harsher than others. I would also consider using zinc blocks - you may find there were some originally fitted by the manufacturer but hadn't been replaced when required.

I’d definitely recommend painting any aluminium exposed to water; look at motorcycles for example they’re all pretty much painted and are at less of a risk from the elements than a ski, Triumph tends to pay more attention to this than others probably related to British bikes and British weather. Something from a marine paint specialist like TK or Owatrol should fit the bill.

Thank you all a lot for the input. I will repaint the parts.

Indeed Jet skies do have sacrificial anodes. The 300HP Sea-Doo has one on the Jet pump and one in the inter cooler which is operated open loop with outside water. The engine cooling cycle is closed loop

The 1.8 liter SVHO Yamahas have one anode in the engine cooling cycle which is operated open loop. The inter cooler is of copper and has no anode

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