Corrosion protection for aluminum exposed to water

I've got a project I'm working on involving some machined aluminum that will be exposed to water during normal operation. Not positive on the alloy but confident it's nothing exotic. 50xx series or 6061 I'm guessing.

Given that this part needs to accept pipe and machine threads, would anodizing be a wise choice? I'd prefer a high polish finish, but given aluminum oxide's tendency to 'creep' underneath paint, etc. I'm guessing any sort of clear varnish or the like is going to be an exercise in futility.

Also, what exactly is the difference between 'hard' anodizing and the traditional sort? Merely the thickness? Or are changes made to the process to induce slightly different characteristics in the oxide layer formed?

Thanks.

I would think anodize would be the thing.

Exposed to water is kinda vague

Lifespan is the key

how many years are needed?

I have always understood that aluminum forms a natural oxide that is tougher than nails. Many small and large boats are fabricated from aluminum and ply fresh and salt water for years with no problems..don't they!

Stuart

Most (all ?) aluminum boat fittings are anodized, last well on salt water for years and years.

Hard anodizing is done at higher current levels and it's a lot deeper. You can't color-dye hard anodizing, comes as a sort of dark greenish black and that's what you get. Seagull-poop green, sort of. On the right material it's wear-resistant as heck.

Probably don't need that for a simple piece exposed to water. Just the normal thin process will do the job.

A decade or two with occasional use several times a year and periodic maintenance. Minimal pressure/flow. Easy to get to for cleaning/re-coating.

atomarc said:

I have always understood that aluminum forms a natural oxide that is tougher than nails. Many small and large boats are fabricated from aluminum and ply fresh and salt water for years with no problems..don't they!

Stuart

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Tough as nails when dry. Highly reactive if it gets scratched/abraded when wet in the presence of salt.


Know of any good clear barrier coatings available in small quantities that have good aesthetic qualities? A mirror polish finish would look really nice.

If it'll be anything more than clean (fresh) water, I'd go with 5052 in as hard a condition as I can get it (mechanically worked, can't HT 5052). If clean water (I'd even worry about rain), 6061.

https://www.howardprecision.com/what-aluminum-alloys-are-used-in-marine-applications/ [not data-heavy, just the first I came to]

On anodize, I'd likely leave it alone if 5052, hard if 6061 and not too big, or if it's exposed to high cyclic loading. IMO, the major downside of hard anodize is the greater risk of fatigue cracks if a highly stressed geometry is flexed too much - if the anodize is cracked it helps propagate the damage.

This link is probably better for practical advice on corrosion-tolerant Al:

Tips to avoid corrosion in marine environments

Wow...too much science for me. I'll hang in there with my plywood dinghy!

Too-Dumb-Stu

you want it shiny? crome plate it.

conversion coatings: they all result in a specific color change, as far as i know (alodine, bonderite, iridite).

anodizing: chromic, sulphuric, phosphoric. you need to match it to the alloy you use. the clear anodized material they sell in the diy-stores (e.g. "alfer") are nice. i never found out the grade or type of anodize.

Berkeley Jet aluminum impellers had a green coating, which I understand was an aluminum oxide primer. Durable as heck. Stood up to years of cavitation, no corrosion.

A nickel finish would be about what I'm looking for. I know a lot of brass pipe fittings rely on it, but I've never seen it done on aluminum. Is it a reliable finish on such a reactive metal?

Galvanic corrosion is something to carefully consider. For instance, aluminum with galvanized fittings is an excellent combination. Aluminum with pretty SS fittings is a battery generating .9V 24/7 in salt water. On commercial vessels you rarely see SS in Aluminum, but on pleasure boats it is a attractive, common, but damaging error in design. Anodized or not, the battery effect is relentless. Look up galvanic corrosion for details on prevention.

Denis

Just a Sparky said:

A nickel finish would be about what I'm looking for. I know a lot of brass pipe fittings rely on it, but I've never seen it done on aluminum.

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Yeah you have but you didn't know it. To chrome plate aluminum first it gets copper, then nickel, then chrome. I'll go dig up some photos, did q/c in an aluminum wheel plating place. Kind of interesting.

Is it a reliable finish on such a reactive metal?

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How do you mean "reliable" ? Straight anodizing is probably better, copper/nickel/chrome has a tendency to peel, but it is shinier.

About your barrier coating, umm, the anodized layer is about a bazillion on that scale they use for thin surfaces (alzheimer's moment here) so any "barrier coating" you apply on top is actually going to be less wear-resistant, ya know ? It's softer and weaker. Aluminum oxide is some hard shit.

The color is just a dye, by the way. Goes on last.

Personally, unless it's a fitting on your $100,000 antique speedboat going in shows, I'd just go with normal anodize. Works great for ocean-going sailboats, should do your job fine.

For this application I would definitely go with anodizing. You are going to have to polish your part regardless. If you EN a part that is not polished prior, it will look like shit. After the zincate (should be double zincate process), there are some copper strikes that will fill in fine scratches prior to EN or Nickel/Chrome, but then your overall plating will be very thick and may throw tolerances; depends on your part.

For the clear anodize parts I do on 6061, I get the best finish I can out of the machine, tumble with a "fine" cut synthetic mix of 1/4" pyramids and cones, then a mix of 2-6 mm sized porcelain spheres (use burnishing compound), or walnut shells with red rouge (depends on part). After the burnish and/or walnut step, parts go into an ultrasonic cleaner. Rinse in distilled water. Wipe and air dry. Do not use compressed air. After that they are dipped in Bonderite C-IC 33 aero (formerly Alumiprep 33) at a ratio of 1 part Bonderite to 4 parts distilled water for 30-60 seconds (move part/agitate while in solution). You will need to experiment with your times (part dependent). This step incredibly brightens the aluminum. Rinse in distilled water. Coat in WD-40 to protect against oxidation. Take parts to Anodizer ASAP and advise the anodizer to "skip" the caustic etch, only use a light acid or heavy duty degreaser that will remove the WD-40, etc. Ask for a 0.0002-0.0005 Type II clear anodize. Your parts will only be slightly dulled after the anodize (very hard to notice).

If you don't feel comfortable doing the brightening step on your own, some shops do what's called "bright dip" anodizing. Be forewarned there are two types of "bright dipping". One is actually a chemical polishing step (nasty chemicals involved), the other is simply a brightening step similar to what I posted above. I'm not going to get into the differences between the two. If your part is polished, all you will need is the brightening step prior to anodizing.

Parts in 6061 utilizing the above steps will last above or below the waterline for many, many years.

Nice, thanks for the deep-dive.

There is a great deal of plain untreated aluminum in the marine world which just sits out in the weather and salt for essentially forever without damage.
The problem comes in when in contact with dissimilar metals and when setting against something so a cell can form or when finishes fail and corrosion sets in underneath.

One approach is to just go raw aluminum and attend to the problems of contact.
Surfaces properly bedded and fittings isolated with gaskets or sealants such as LanoCote:

LanoCote® Lubricants & Coatings

atomarc said:

Wow...too much science for me. I'll hang in there with my plywood dinghy!

Too-Dumb-Stu

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Plate the outside with flattened beer cans....
"The SS Meister Brau"