chemistry / metallurgy - stainless meets aluminium

[Birdseye]

does anyone on here know what is the chemical composition of the white powder that is formed when stainless meets aluminium in a marine environment. Knowing what the compound is, is the first step to finding a reagent that will dissolve it.

Obvious candidates are caustic soda - I once removed the faded black anodising on an ally mast with a hot string solution of this, and a lot of PE kits.

 

[vyv_cox]

If you are discussing the hard white limescale compound that forms over some time between these two in seawater it is easily dissolved by dilute hydrochloric acid. Some grades of aluminium are attacked by acid but only slowly. Slowly drip acid onto the compound. The fizzing will start immediately. Once the two parts can be separated wash copiously with water.

 

[vyv_cox]

See the pic and description at the bottom of this page Corrosion

 

[Birdseye]

I am thinking of what forms between an ally fitting, say a roller reefing drum, and a stainless bolt. The white powder that tightens up the fitting and makes the bolt often impossible to remove. Are you sure that its limescale.? I had it down as aluminium oxide or some aluminium / chrome/ iron compound. Which is why I was thinking of caustic soda.

 

[PabloPicasso]

Try copius amounts of boiling water (fresh water).

Often works and is easy to do.

 

[Chiara’s slave]

If you are discussing the hard white limescale compound that forms over some time between these two in seawater it is easily dissolved by dilute hydrochloric acid. Some grades of aluminium are attacked by acid but only slowly. Slowly drip acid onto the compound. The fizzing will start immediately. Once the two parts can be separated wash copiously with water.

I find domestic descaler, like Kilrock, can do the trick as long as you haven’t left it til it looks like a snow globe,

 

[AntarcticPilot]

does anyone on here know what is the chemical composition of the white powder that is formed when stainless meets aluminium in a marine environment. Knowing what the compound is, is the first step to finding a reagent that will dissolve it.

Obvious candidates are caustic soda - I once removed the faded black anodising on an ally mast with a hot string solution of this, and a lot of PE kits.

Don't use caustic soda on aluminium - aluminium dissolves in caustic soda!

 

[vyv_cox]

I am thinking of what forms between an ally fitting, say a roller reefing drum, and a stainless bolt. The white powder that tightens up the fitting and makes the bolt often impossible to remove. Are you sure that its limescale.? I had it down as aluminium oxide or some aluminium / chrome/ iron compound. Which is why I was thinking of caustic soda.

Did you look at my link?

 

[rotrax]

Did you look at my link?

Obviously not Vyv!

 

[Neeves]

Its so easy to be wise after the event but applying Duralac or grease when the stainless fixtures were inserted into the aluminium solves so much heartache.

Sea change has a thread on Radar installation. Many radar are attached to a mast, aluminium, with stainless bolts - they will corrode, and the scanner can become impossible to remove easily - made worse as its half way up a mast and taking boiling water up a mast is not easy. Windlass are commonly fabricated from aluminium castings with stainless bolts or studs - when new no sign of Duralac - no wonder windlass are often removed with an angle grinder. If you leave a Fortress assembled and then change your mind - you will find the white powder happily developing between aluminium and the stainless bolts - no need for Nyloc nuts - the corrosion will hold it tightly together.

A little bit of forethought goes a long way.

Jonathan

 

[thinwater]

does anyone on here know what is the chemical composition of the white powder that is formed when stainless meets aluminium in a marine environment. Knowing what the compound is, is the first step to finding a reagent that will dissolve it.

Obvious candidates are caustic soda - I once removed the faded black anodising on an ally mast with a hot string solution of this, and a lot of PE kits.

No, caustic soda is NOT obvious with aluminum. Damaging and will not work. It is not lime scale, it is aluminum hydroxide, with some contaminants.

Lactic acid is the least damaging to aluminum per pound of scale and aluminum corrosion products removed (side-by-side testing for PS). Yes, it is slower than HCl. In the US CLR is a common product. I don't know what you have.

 

[ducked]

does anyone on here know what is the chemical composition of the white powder that is formed when stainless meets aluminium in a marine environment. Knowing what the compound is, is the first step to finding a reagent that will dissolve it.

Obvious candidates are caustic soda - I once removed the faded black anodising on an ally mast with a hot string solution of this, and a lot of PE kits.

Probably a mixture of aluminium oxides and hydroxides.

I used aluminium cans as sacrificial electrodes when electrolytically de-rusting an exhausr manifold a while ago (using dissolved dead fire extinguisher powder as the electrolyte) and this generates a lot of fluffy white powder in suspension. This is likely a similar electrochemical situation to coupled stainless/aluminium

Exhausting All Options

I'd think the bulk powder might be useful as an abrasive on steel, though I havn't tried it. Maybe on my keels in the future, if I again assemble the makings.

I havn't tried to dissolve it, but I'd think acid rather than alkali. Test to antticipate trouble.

 

[Chiara’s slave]

No, caustic soda is NOT obvious with aluminum. Damaging and will not work. It is not lime scale, it is aluminum hydroxide, with some contaminants.

Lactic acid is the least damaging to aluminum per pound of scale and aluminum corrosion products removed (side-by-side testing for PS). Yes, it is slower than HCl. In the US CLR is a common product. I don't know what you have.

The best source for lactic acid I have found is kilrock kettle descaler. Amazon sell it as does my local hardware store. This is the land of tea drinkers, descaling your kettle is a regular task in hard water areas.

 

[SvenH]

I was told the white stuff is "sapphire", I mean the same aluminium oxide.
If so you can forget about dissolving it. But you may be able to dissolve the surroundings enough to loosen it, without too much damage.

 

[Fr J Hackett]

Probably a mixture of aluminium oxides and hydroxides.

I used aluminium cans as sacrificial electrodes when electrolytically de-rusting an exhausr manifold a while ago (using dissolved dead fire extinguisher powder as the electrolyte) and this generates a lot of fluffy white powder in suspension. This is likely a similar electrochemical situation to coupled stainless/aluminium

Exhausting All Options

I'd think the bulk powder might be useful as an abrasive on steel, though I havn't tried it. Maybe on my keels in the future, if I again assemble the makings.

I havn't tried to dissolve it, but I'd think acid rather than alkali. Test to antticipate trouble.

Initially the material will be aluminium oxide but with dissolved carbon dioxide in seawater there will be aluminium carbonate formed the problem of using chemical agents to remove it is that they, certainly alkalies and acids will readily attack the aluminium as it's amphoteric. By far the best method will be mechanical removal with a wire brush.