Corrosion protection for aluminum exposed to water
Close
Login to Your Account
Results 1 to 16 of 16
  1. #1
    Join Date
    May 2020
    Country
    UNITED STATES
    State/Province
    Minnesota
    Posts
    488
    Post Thanks / Like
    Likes (Given)
    194
    Likes (Received)
    189

    Default 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.

  2. #2
    Join Date
    Sep 2002
    Location
    People's Republic
    Posts
    6,325
    Post Thanks / Like
    Likes (Given)
    748
    Likes (Received)
    3808

    Default

    I would think anodize would be the thing.

    Exposed to water is kinda vague

    Lifespan is the key

    how many years are needed?

  3. #3
    Join Date
    Mar 2009
    Location
    Eureka, CA
    Posts
    4,429
    Post Thanks / Like
    Likes (Given)
    1049
    Likes (Received)
    1803

    Default

    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

  4. #4
    Join Date
    Apr 2018
    Country
    UNITED STATES MINOR OUTLYING ISLANDS
    Posts
    8,873
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4299

    Default

    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.

  5. #5
    Join Date
    May 2020
    Country
    UNITED STATES
    State/Province
    Minnesota
    Posts
    488
    Post Thanks / Like
    Likes (Given)
    194
    Likes (Received)
    189

    Default

    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.

    Quote Originally Posted by atomarc View Post
    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
    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.

  6. Likes mhajicek liked this post
  7. #6
    Join Date
    Jul 2006
    Location
    Hillsboro, New Hampshire
    Posts
    13,944
    Post Thanks / Like
    Likes (Given)
    3057
    Likes (Received)
    9237

    Default

    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...-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

  8. #7
    Join Date
    Mar 2009
    Location
    Eureka, CA
    Posts
    4,429
    Post Thanks / Like
    Likes (Given)
    1049
    Likes (Received)
    1803

    Default

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

    Too-Dumb-Stu

  9. Likes camscan liked this post
  10. #8
    Join Date
    Feb 2010
    Location
    ch
    Posts
    3,345
    Post Thanks / Like
    Likes (Given)
    277
    Likes (Received)
    467

    Default

    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.

  11. #9
    Join Date
    Aug 2008
    Location
    Californeeeah
    Posts
    1,580
    Post Thanks / Like
    Likes (Given)
    59
    Likes (Received)
    354

    Default

    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.

  12. #10
    Join Date
    May 2020
    Country
    UNITED STATES
    State/Province
    Minnesota
    Posts
    488
    Post Thanks / Like
    Likes (Given)
    194
    Likes (Received)
    189

    Default

    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?

  13. #11
    Join Date
    Jun 2008
    Location
    Bellingham, WA
    Posts
    4,480
    Post Thanks / Like
    Likes (Given)
    1110
    Likes (Received)
    2536

    Default

    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

  14. #12
    Join Date
    Apr 2018
    Country
    UNITED STATES MINOR OUTLYING ISLANDS
    Posts
    8,873
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4299

    Default

    Quote Originally Posted by Just a Sparky View Post
    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.
    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?
    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.

  15. #13
    Join Date
    Dec 2013
    Location
    South Charleston, WV
    Posts
    32
    Post Thanks / Like
    Likes (Given)
    44
    Likes (Received)
    14

    Default

    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.

  16. Likes Milland, Just a Sparky liked this post
  17. #14
    Join Date
    Jul 2006
    Location
    Hillsboro, New Hampshire
    Posts
    13,944
    Post Thanks / Like
    Likes (Given)
    3057
    Likes (Received)
    9237

    Default

    Nice, thanks for the deep-dive.

  18. Likes Wick Craft liked this post
  19. #15
    Join Date
    Oct 2010
    Location
    Maryland- USA
    Posts
    4,941
    Post Thanks / Like
    Likes (Given)
    2400
    Likes (Received)
    3085

    Default

    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

  20. #16
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    22,351
    Post Thanks / Like

    Default

    Quote Originally Posted by atomarc View Post
    Wow...too much science for me. I'll hang in there with my plywood dinghy!

    Too-Dumb-Stu
    Plate the outside with flattened beer cans....
    "The SS Meister Brau"

  21. Likes atomarc liked this post

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •