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Old 03-02-2021, 17:09   #1
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Remove Aluminium Oxide, but not Aluminium ?

My guard-rail stanchions are aluminium, as are their bases. The two have corroded together over time. Unfortunately they are also held together by a stainless steel bolt. I would like to find a chemical which will dissolve the oxide without dissolving the aluminium. Is this possible, or what else should I try ? Thanks.
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Old 03-02-2021, 17:39   #2
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Re: Remove Aluminium Oxide, but not Aluminium ?

Very difficult situation as the oxide is much more inert and non-reactive than the base metal and of course your stanchions are mounted to a deck substrate which also is likely to be much less inert. Acid strong enough to dissolve the oxide will corrode everything else and is hazardous.

I suspect physical force, torque is a potential.

The two aluminum surfaces are likely to gaul together if you can break the bond, which bonding has caused by having formed oxide bridging, basically like a chemical weldment.

A stainless steel bolt and aluminum could cause galvanic corrosion.

Snipet of guidance:

It is no doubt that stainless steel screws are the best when it comes to holding together materials such as aluminum sheets so that when faced with a variety of elements they do not break open. However, the dissimilar nature of these two metals puts them at risk of getting corroded resulting in their destruction altogether. Steel screws can be used with aluminum but some precautions must be taken if they are to work well together.

Tips to consider when using stainless screws on aluminum

Do not expose to water
Salty water is an electrolyte that can easily corrode either the aluminum or stainless steel material when they come into contact with each other. Therefore, it is important to always separate the two metals as soon as possible especially if the environment has salty water or anything close to it.

Create barriers
To reduce chances of the metals getting corroded reduce the contact the two metals have by creating a non-reactive barrier. The best barrier material is plastic washers which ensure that the metals are sealed at their point of contact. Also, to reduce their contact with water, connecting points can be protected using tape or even paint depending on your preference.

Check the surface area of metals
Always ensure that the aluminum part of the material is larger than the stainless steel screws to avoid it being eaten away. Aluminum which has an anodic nature will quickly disintegrate when it comes into contact with salty water, and will do the same when its surface area is less than that of stainless steel.


On this subject purely discouragement from other forum threads:

"Well, if they are Aluminium stanchions in aluminium bases bolted to the toerail with stainless screws and bolted through the deck, just like my Moody 33, then it's practically impossible to remove the stanchion without destroying the base. New bases are supplied with a plastic liner between the post and the base. The reason they corrode solid is aluminium post in aluminium base with stainless bolts & split pins continually washed with salt water = CORROSION in a big way. The only way to remove the post is to destroy the base(probably damaging the post in the process) using a combination of drilling/hammering/disc cutter & any other method you can think of. Liquids and heat I found were not any good. The other problem is access to the nuts inside under the deck, as soon as you turn them the bolt turns as well, sometimes you are lucky and you can get them off. I found I had to drill out the countersunk screws which hold the bases to the toerail.
If you are lucky you'll get the stanchion complete with base off in one piece, if not they will both be damaged beyond repair.
I found it took anything from hr to 4 hours for each stanchion. It took two winters to complete the job.
Bases and stanchions still available from chandlers & internet & ebay. Not cheap if bought from chandlers. Negotiate to get a good deal. Use dichromate paste when reassembling to prevent corrosion between stainless & aluminium."




You are wasting time even trying wonder sprays. The products of corrosion on ally are more bulky than the ally metal so as a pipe in socket joint corrodes it gets ever tighter.
Most effective way is to remove stanchion and base, put the stanchion in the vice and leave the blowlamp playing on the base itself for 10 mins or so. Then use a stilson.
The less frustrating way is to junk the lot and replace with new.
Am about to try the same task today, for the third time on my boat. The blowlamp has finally worked before but this time...... we shall see
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Old 03-02-2021, 17:41   #3
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Re: Remove Aluminium Oxide, but not Aluminium ?

I've heard (but can't attest) that chromic acid will dissolve the oxide, but not the underlying aluminum.
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Old 03-02-2021, 18:48   #4
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Re: Remove Aluminium Oxide, but not Aluminium ?

When setting new screws, use Lanocote or Boeing isolating grease.


It is the SS screw that started your trouble.
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Old 03-02-2021, 19:41   #5
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Re: Remove Aluminium Oxide, but not Aluminium ?

I had the same question in the past and never found a definitive answer. Even asked a couple of friends that are chemists with no luck.

If I recall I encountered the same suggestion Gord posted but also comments online that it might or might not work. Never got around to trying it but would be very interested if you gave it a go to hear if it does or not.
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Old 03-02-2021, 19:43   #6
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Re: Remove Aluminium Oxide, but not Aluminium ?

When you eventually put all this back together do not use stainless fasteners. This application puts little stress on the fasteners. Use aluminum bolts/screws. You may be able to find these anodized, which is best. You can also buy bare aluminum fasteners (McMaster Carr) and have them anodized. This will eliminate the galvanic corrosion.

Your next problem with aluminum will be crevice corrosion in the joint between the stanchions and bases. filling and sealing this joint is the way to combat that.
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Old 04-02-2021, 09:30   #7
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Re: Remove Aluminium Oxide, but not Aluminium ?

Reference to guidance of deoxidizing aluminum / aluminium.

https://www.pfonline.com/articles/de...a-pretreatment


Types of Deoxidizers
The most commonly used acid solutions for deoxidizing are nitric-, sulfuric- or chromic-acid based. Nitric and sulfuric solutions are usually interchangeable or paired.

Of course, deoxidizing of aluminum can only be successful if there is aluminum oxide present on the part or there is a chemical species willing to donate or accept electrons in the solution. The key constituents in the chemical solution are ferric sulfate and nitric acid. Since ferric is in the +3 state, it can be the oxidizing agent and reduce the aluminum oxide on the surface. The nitric acid is very important because of its inherent oxidizing abilities, which help speed up the reaction and decrease the amount of time the part spends in the deoxidizer. Once the thin layer of aluminum oxide is gone, you are left with bare aluminum that has been activated, meaning it is looking to donate electrons to maintain equilibrium. This is why deoxidizing should be done right before anodizing or plating.

Nitric acid is the more common choice for non-etching deoxidizing because of its ability to slowly attack aluminum and because of its autocatalytic abilities as an oxidizer. Nitric-acid deoxidizers are usually light-duty solutions and are principally employed as desmutting agents. They will produce satin-type finishes without removing the finished-metal shine. Nitric-acid deoxidizers also are useful for salvaging parts, as nitric acid is known to open the pore structure of anodic films, allowing for easier stripping of anodize.

This is an effective deoxidizer/desmutting solution that can be used at room temperature without the need for fume exhaust. Immersion time is 1-5 minutes, and triple rinsing is the most effective way of removing the solution. Nitric-acid-based deoxidizers are typically considered more environmentally friendly, too, since there is no chromium involved. Tanks can be made of 316 stainless steel or polypropylene, or have some sort of polyvinylidene fluoride (PVFD) liner.

Chromic-acid-based non-etching deoxidizers are the gentlest of all deoxidizers. They will passivate bare aluminum and tend to confer a passivating action on any other solution to which they are added.
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Old 04-02-2021, 09:46   #8
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Re: Remove Aluminium Oxide, but not Aluminium ?

Quote:
Originally Posted by GordMay View Post
I've heard (but can't attest) that chromic acid will dissolve the oxide, but not the underlying aluminum.
I went back and checked some old notes and seems there's some ambiguity on what chromic acid is exactly. Seems like that is a somewhat generic term that could refer to a few different acids, some that use hexavalent chrome which is a really nasty carcinogen.

From Wikipedia

The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H2CrO4 of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6 (or VI). It is a strong and corrosive oxidising agent.


Think this is worth a little further research anyway. If there truly is something that will selectively attach aluminum oxide it would be really useful thing.
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Old 04-02-2021, 09:51   #9
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Re: Remove Aluminium Oxide, but not Aluminium ?

Here's a link to a discussion that includes comments from chemists and researchers. No agreement at all on the issue. Some say no way, others say this or that works no problem

https://www.researchgate.net/post/Wh...nic%20solvents.
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Old 04-02-2021, 10:02   #10
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Re: Remove Aluminium Oxide, but not Aluminium ?

I dont know anything that will get into the interface and eat the oxide. The gap is filled water tight. It's usually an impossible separation if it's very bad.
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Old 04-02-2021, 10:11   #11
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Re: Remove Aluminium Oxide, but not Aluminium ?

a. PB Blaster is perhaps the best liked ss/aluminum penetrating oil.

b. Some acids are more able to dissolve the oxide and leave the aluminum. If you think about it, this is one of the traits of the organic acids used in modern automotive antifreeze (the base of Decool is 2-ethylhexanoic acid, AKA 2EH, AKA OAT) is that they can actually inhibit aluminum corrosion.

Citric acid and lactic acid have this trait (tested, PS). They are not fast, but they are the best tools for descaling aluminum without damaging the aluminum. Vinegar, on the otherhand, is bad for aluminum. It's complicated.

Citric acid (also the best stainless passivizing agent) is available through Amazon and some grocery stores, and lactic acid is CLR (Home Depot).



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Old 04-02-2021, 11:23   #12
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Re: Remove Aluminium Oxide, but not Aluminium ?

Aluminum oxide is the aluminum degrading, not the stainless.

When I changed the main halyard winch on my mast, I first installed stainless threaded inserts, so I would be able to remove the winch at a later date. If the helix coils freeze in the aluminum of the mast, all the better.

Aluminum stanchions? Hmm. I have an extra set (6) of stainless steel stanchions with strong stainless steel bases, I would love to part with them.
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Old 04-02-2021, 11:33   #13
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Re: Remove Aluminium Oxide, but not Aluminium ?

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When I changed the main halyard winch on my mast, I first installed stainless threaded inserts, so I would be able to remove the winch at a later date. If the helix coils freeze in the aluminum of the mast, all the better.
Did the same when I installed a new antenna bracket at my masthead.

Based on my experience repairing and replacing things on several boats over the years I don't think using any kind of thread coating or protection between SS bolts and aluminum fittings is in the consciousness of any boatbuilder. Can't think of a single SS bolt in AL that I've ever found to be treated in any way.
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Old 04-02-2021, 13:59   #14
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Re: Remove Aluminium Oxide, but not Aluminium ?

I've had no significant problems of corrosion with SS screws into aluminium as long as they have been well coated in Duralac before assembly.

A couple of years ago I moved the gooseneck on the mast. It had been there for 30 years and I expected problems unscrewing the 6mm bolts but they had been well coated with Duralac at assembly and unthreaded easily. Mostly the threads into the mast were clean and shiny.

I acknowledge that there are many different alloys of aluminium and some cast alloys are rubbish. Some aluminium alloys contain sufficient copper (up to 10%) to be self-destructive in the marine environment.

( The gooseneck is a "claw" fitting and was not only bolted but glued with Sikaflex and this proved far more problematic. I had to drive in wedges and use a fine blade to progressively cut through the Sika as I exposed it. Amazingly, at the centre the Sika was till tacky. After 30 years! )
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Old 04-02-2021, 16:54   #15
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Re: Remove Aluminium Oxide, but not Aluminium ?

I replaced my alloy uprights with insulated 316 stainless. They had, after only 10 years become totally oxidised into the bases.
I used formic acid, a blowtorch, much effort and swearing to get them out.
The replacement stainless uprights have remained free if sometimes bent for the last 20 years. I occasionally rotate them through 180 degrees, to keep the side-decks passable on a beat. I did use Duralec when fitting, which has reduced oxidisation in the socket

From memory I used sections of an old milk jug - HDPE - round the stanchions - there was plenty of room
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