Preventing aluminium corrosion in saltwater environment?

MystyBlue2

MystyBlue2

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Im going to be buying an ali sheet to fabricate an external pad for my transom and im wondering if there is any need to coat the aluminium with anything specific before the final install?

The boat is only used at sea so i can only imagine there will be some corrosion to some extent?

Any advice on what i can coat/paint on to protect from the salt etc..?

Cheers
 
Dan208 said:
Im going to be buying an ali sheet to fabricate an external pad for my transom and im wondering if there is any need to coat the aluminium with anything specific before the final install?

The boat is only used at sea so i can only imagine there will be some corrosion to some extent?

Any advice on what i can coat/paint on to protect from the salt etc..?

Cheers
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Clean, zinc chromate, Awlgrip 345 primer, base coat, top coat. That’s a top end treatment normally used on masts. You can anodise too or do nothing, especially if you use marine aluminium. Don’t let water seep in and sit under it.
 
The grade will be 5251...if i google that grade it does say it is strong and used in the marine industry but still unsure if i need to coat or if it will be fine on its own with it being used in salt water
 
Have you priced a piece of polished stainless steel in a lesser gauge to do the same job? If you are concerned about future corrosion and the cost of protective paints, it might be worth investigating?
 
Yeah i didnt think of stainless, i just assumed it would be similar to the price of gold haha

What thickness do you think i should be looking at for a transom pad?

Stainless would be better i think but the cost and weight would offset the pros...

Having said that aluminium is weaker and can corrode but cheaper so...

Stainless - mm?
Aluminium - mm?

My boat is already on the heavy side and it seems bare, i really need to try to keep weight to a minimum if possible.

What are your thoughts so i could possibly do more research and get it right the first time

Cheers
 
5251 is fine with prolonged seawater exposure when uncoated. However, you do need to electrically isolate any dissimilar metals such as stainless steel that are in contact with the aluminium. This is particularly important if you have a small amount of aluminium attached to large amount of a dissimilar metal.

You also need to be careful that any moisture cannot be trapped on the inside surface that is in contact with the hull. Moisture here can create crevice corrosion.
 
Brilliant, my intensions are to clag the plate on with ALOT of puraflex40, and use stainless hex bolts, large washers and nylock nuts at other side with a 100x600x5 sheet running along inside of transom to spread the load and to increase rigidity inside aswell,

So if the 2 metals cannot be in contact with eachother would i use some more puraflex inbetween aluminium and the stainless as much as i can to eliminate any face to face contact between these 2 different metals?

If not how can i isolate as best as i can?

Cheers
 
Dan208 said:
Yeah i didnt think of stainless, i just assumed it would be similar to the price of gold haha

What thickness do you think i should be looking at for a transom pad?

Stainless would be better i think but the cost and weight would offset the pros...

Having said that aluminium is weaker and can corrode but cheaper so...

Stainless - mm?
Aluminium - mm?

My boat is already on the heavy side and it seems bare, i really need to try to keep weight to a minimum if possible.

What are your thoughts so i could possibly do more research and get it right the first time

Cheers
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You mentioned on another post a 12’ dinghy, so assume a small outboard. Is the proposed transom pad sitting flush on the outside, or folded over the top on down the inside to accomodate the clamp mechanism?
Maybe a 2.5-3mm sheet of SS, or another option could be a plastic if just on the outside
 
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Yeah its 12ft version of a cjr or seahog trio, rated for a 30 4stroke but ive had the go ahead from a friend that runs boating and radio courses that my transom is in great shape and a light weight 40 2 stroke would suit this heavy boat brilliantly as long as im sensible and respect the throttle and power it will have,
Its me that want to be safer than safe and make sure that i have done the necessary things to be able to mount this motor and have piece of mind and not worry about transom being ripped apart.

It will only be a sheet on the back as i have a thick 5mm aluminium channel on top of transom already so this sheet will just butt up against the channel and sealed with more puraflex 40.
 
Dan208 said:
If not how can i isolate as best as i can?
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Isolation can be a challenge for some applications. Normally products such as Duralac or Tefgel when applied to bolts (these products work very differently, but are generally equally effective) combined with plastic/fibreglass washers are enough, but you do need to be careful with aluminium reinforcing plates where a large quantity of dissimilar metal is attached to a relatively small plate. Shoulder insulating plastic washers, rather than regular plastic washers can be better in some applications.
 
from 18 years of owning an aluminium boat i would say no matter what you cover it with (paint or sealant or plastic) it can oxidise underneath the coating.
All the bare parts of my boat were in good condition but any painted or covered areas even on deck level treated you to that lovely aluminium jelly ..
 
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noelex said:
Isolation can be a challenge for some applications. Normally products such as Duralac or Tefgel when applied to bolts (these products work very differently, but are generally equally effective) combined with plastic/fibreglass washers are enough, but you do need to be careful with aluminium reinforcing plates where a large quantity of dissimilar metal is attached to a relatively small plate. Shoulder insulating plastic washers, rather than regular plastic washers can be better in some applications.
Click to expand...

So would these shoulder insulating plastic washers replace or to be used along side stainless ones?

I would go with stainless plate as it seem alot less hassle and headache but with my boat already being on the small and heavy side every gram seems to count as far as weight is concerned
 
Dan208 said:
So would these shoulder insulating plastic washers replace or to be used along side stainless ones?
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For a typical application where a stainless steel bolt is used to secure an aluminium backing plate, the plastic or fibreglass insulating washer is used with an identical diameter stainless steel washer on top. The plastic is between the aluminium and stainless washer.

Shoulder insulating plastic washers are used in the same way, but the plastic shoulder helps isolate the bolt as it passes through the aluminium plate.

Duralac or Tefgel is also used as the final barrier.
 
Brilliant thank you, wish i had your knowledge man!!

To save on all that do you think a 18mm marine grade ply board, fibreglassed and flowcoated do the same job and be as strong?

Last thing i want is to ruin the stern of my boat with an ally sheet that has got holes all over it and discoloured because i cocked up isolating the metals properly and have to hack it off and ruin the gel coat to replace it

Thats why im thinking wood cos it wont do that im just unsure if it would be strong enough and waterproof enough?

Thank you for all your knowledge and speedy replies
 
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