Anodize, or Alodine or paint Aluminum Catamaran structure?

Alodine 1201

on eBay or

Amazon

(affil links)

A. Hi Dan. Although it's possible to partially anodize and partially chromate conversion coat (Alodine), it's a bit complicated and reserved for applications where it's really needed. And anodizing the inside of a tube is hard (and difficult to prove). I think your best answer is to Alodine it both inside & out, and then paint everything you can reach.

You probably know this, but you never apply bottom paint (ablative copper) to aluminum.

Regards,

A. Hi Dan. Since you were considering anodizing as an alternative, and you were talking about a local vendor doing it, I thought you were planning on immersing these tubes -- which is really the right way to do it. And I thought your vendor would take the tubes through the proper sequence which includes non-etch alkaline cleaning, de-smutting appropriate to the alloy, and Alodine (with rinsing between each stage).

I imagine manual cleaning, followed by manual brushing of Alumiprep 33 ⇦this on eBay or Amazon [affil links] and Alodine 1201 ⇦this on eBay or Amazon [affil links] can be done, but you should probably check with Henkel.

Yes, if strength is not needed, anodized aluminum fasteners could be used if you can find them. Aircraft specialty suppliers would have high strength aluminum coated steel fasteners, but I'm not aware of aluminum coated stainless or monel fasteners. Boats have their unique dangers, so having a marine surveyor review your plan might catch a weak area.

Regards,

Hi Dan! I'm a catamaran sailor myself and can give you a couple observations about the construction of the classic Hobie Cats. The rails are anodized Al and the fittings and fasteners are a combination of Al and stainless (I'll go home and check to see what is used where). My boats are 1971 and 1984 models, and regularly see salt water. I sail with a club out of Naragansett Bay RI and a couple of us have pretty old and busted boats! Save for some discoloration on the stainless components, there has not been an issue with corrosion enough to cause strength concerns or binding of the travellers as they slide along the rails. If you can find high strength Al rivets as Ted suggested, that is great. But you should not have to worry much about things like padeyes and rails that are in direct contact with the anodized surface, if you choose to go that route.
Also as Ted said, Alodine only works when the surface is correctly prepped- Squeaky clean and devoid of any existing oxidation.
An Alodine-paint combination, or, even better, a 2-stage epoxy meant for use in UV exposed applications, would provide very good protection, and this is commonly done on components meant to be used in challenging environments.
I'd personally Alodine the inside of the tubes and then seal them off watertight for flotation reasons.
One thing I'll add here is that if you haven't thought about your rudder pins and brackets yet, my strong suggestion is to have them hardcoat anodized, because of the constant abrasion metal-on-metal and metal-on-sand. And have a couple extra sets made for spares ... Inspect and clean regularly and swap out as soon as you notice grooves forming. We had a guy get blown out on his H16 to Falmouth (lowest tip of Cape Cod) a couple years ago after snapping a pin and not being able to tack! He was lucky, because the next point of land is Cape Verde or the Canaries :)

Here's a link to a tech sheet for the brush-on version of the chromate coating we use at work, that should give you some idea of what to expect: http://www.galladechem.com/specs/macdermid/78660.pdf Iridite (Macdermid-Enthone) and Alodine (Henkel) are simply different proprietary product lines formulated to meet the same specs. Be aware that a product designed for immersion application may not work correctly as a brush-on, and vicw versa, so make sure to get the brush-on version of whichever one you go with. And I can't stress enough that the deoxidizer (in this case, Isoprep) is crucial to the chromate reaction occurring correctly.

Happy sailing!

  Thank you Ted and Rachel, I spoke with a pro today and he suggested Alodine and powdercoating within 72 hours and would not guarantee the inside.

The parts will have penetrations, so capping the ends might result in trapped saltwater or environments trying to exit some threaded area. Not good, so we will leave open.

This is a powercat and these are removable beams. No sail rigs.

The insides do not need to be pretty, so perhaps the powdercoating people could paint the insides for me after chromate conversion.

Thanks to all. I really like this forum. It is a bit hard to use, but worked!

A. Hi Greg. Type III anodizing is extremely abrasion resistant and highly insulating, so I think it would be perfect for the hinge pin slot. There are some observations on this site that acetone cleaning worked well for paint adhesion on Type III (as long as acetone cleaning is compatible with the selected paint).

If all of the aluminum parts are flawlessly anodized, it m-i-g-h-t be okay to use ablative paint on aluminum, but few things are as destructive to aluminum as copper (the active ingredient in anti-fouling paint) because of the huge electromotive potential difference.

Regards,

A. Hi again. Glad there are non copper anti-fouling paints for aluminum; the only ones I knew of, way back when, were tributyl tin (an environmental disaster which was banned).

If you search the site for "paint hardcoat" or "paint type III" you can see the original comments on that subject rather than my transcription.

Aircraft Stripper is really noxious stuff, but it specifically for removing paint from aluminum and other soft metals.

Although 5086 is known for its corrosion resistance and good strength, once you hardcoat 6061 it seems like it should be a fine material for corrosion resistance. Reportedly, 5086 is the material of choice for yachting but 6061 is often used on smaller skiffs and such.

Regards,