[Enthalpy]

Hi dear friends!

Here are interrogation and observations about bicarbonate and aluminum cleaning nickel silver.

The item is my flute. It stayed some 40 years exposed to the air in a normally dry room, the last untouched 8 years were the worst. Supposedly banal nickel (or German) silver: variable Cu-Ni18-Zn27. Definitely no silver layer on it. The black patch is adhesive tape. The clean patches are unalloyed tin hold by unsupported acrylic tape, they look like new after 40 years. The covers where I put my fingers are the less corroded ones, so fingers clean nickel silver faster than they corrode it. The cleaner end at the headjoint (second longest joint) stayed fit in the main joint all the years.

The bath is what Internet rumours recommend for true silver:

• 1 litre boiling (microwave) water (tap).
• 2 tablespoons bicarbonate ("pure" from drugstore).
• A piece of aluminium foil (supermarket, usually Fe+Si 0.5% to 1%) at the bottom.
• Plastic container (feels and looks like polypropylene, meant for cereals).

Observations:

• Aluminium fizzles in boiling bicarbonated water without the nickel silver, producing an odour known in electrochemistry.
• Bicarbonate poured in boiling water bubbles shortly, and then the alu foil doesn't fizzle. Supposedly, a decomposition cools the solution excessively. So, bicarbonate before heat.
• Action is within seconds, including from the foam. Dirtier places take longer, you guessed.
• The setup loses its efficiency quickly. The alu foil tarnishes, the bath gets less transparent, the temperature drops.
• Microwave and plastic container let re-heat easily. Wrapping paper and a lid conserve heat. An immersed resistor would help.
• Contact of nickel silver with aluminium makes no difference. For the small cleaned portion of the head joint, I avoided the contact.
• Aluminium is necessary.
• The used bath didn't etch my fingers. No strong NaOH concentration formed.
• Some places at nickel silver still needed short rubbing, which was inefficient before. A piece of old bedsheet worked with downpressure without any lotion.

Can you propose a reaction? The usual "sacrifical anode" must be ba***cks. I vaguely suspect some aluminium hydroxide scavenges the corrosion layer at nickel silver. At heat, bicarbonate might etch aluminium like NaOH does. Or the bubbles do the job. They could be strongly reducing (hydrogen??), and somehow activated.

I must still clean the main joint and the keys (removed to clean the joints, reassembled, check the contrast) so I can experiment your suggestions. No lab here, not even a thermometer, that's home chemistry - but an ohmmeter yes.

Thank you!
Marc Schaefer, aka Enthalpy

[AWK]

Sodium bicarbonate thermally decomposes at around 80°C and the solution becomes strongly alkaline (pH ~ 11.5).

[Enthalpy]

Sodium bicarbonate thermally decomposes at around 80°C and the solution becomes strongly alkaline (pH ~ 11.5).

Yes. The only argument against this explanation is that I took no precautions at all and noticed no itch.

If I had 10g of NaHCO₃ in 0.5L water, after full decomposition the pH could reach >13, but the amount of bubbles speaks against full decomposition.

I'll experiment a bit.

[AWK]

NaHCO₃ decomposes in hot water to Na₂CO₃.

[Enthalpy]

NaHCO₃ decomposes in hot water to Na₂CO₃.

I added beetroot juice to the water. It kept violet when the tap water boiled. Adding bicarbonate made little immediate difference, but after short renewed boiling the solution went yellow.

Complete conversion of 10g bicarbonate would have released 1L dioxide, so obviously the conversion to carbonate is very limited. My unprotected fingers don't itch.

[Enthalpy]

The hot solution of bicarbonate and some carbonate does nothing to the scale.

I put aluminium foil in the solution, it fizzled without any nickel silver. A small tube of aluminium foil was to release more concentrated gas. With a lighter, no explosion. Maybe the gas burns faintly with a purple flame resembling hydrogen, but the amount is too small for clear observation, and the lighter's propane could also have reignited further away. Near to the solution, in the vicinity of aluminium, something extinguishes the lighter, while this doesn't happpen clearly away from the fizzle. Vapour? Dioxide?

Heat is paramount to obtain bubbles, and the aluminium foil is inactivated once corroded. A more corrodable alloy could improve.

I let an aluminium foil react for some time in the solution, then I stirred and removed the foil. The new solution did nothing or very little to the scale.

I put a new foil at one side of the solution, and the scaled nickel silver part at the other side, and nothing happened, or very little.

The I put the foil at the bath's bottom. In the air over the bubbles, nothing happened to the scale. But the bubbles in the solution descale nickel silver instantly and very efficiently. Contact with the aluminium changes nothing, neither for the removal of scale, nor at the amount of bubbles.

The composition of the magic bubbles remains obscure to me. Aluminium may react with the bicarbonate, carbonate, or hydroxide ions, and with water. The bubbles could contain hydrogen, carbon dioxide, monoxide, or some reaction product with water, or something else. Suggestions?

Marc Schaefer, aka Enthalpy

[Enthalpy]

It works with sodium hydroxide instead of bicarbonate. So I believe the cleaning gas is hydrogen, or at least its useful component is, without any carbon compound. Opinions welcome!

The hydroxide is sold to clean drains. Starting from pH~12 to make beetroot juice yellow according to Wiki, I estimated 0.4g/L NaOH, but the beetroot juice was a bit orange. 1g/L gave true yellow as for the boiled bicarbonate solution. Heat didn't change the colour.

The cold alkali did nothing to aluminium nor nickel silver. When boiling hot, it left nickel silver nearly untouched, but it did make bubbles from an aluminium foil, and the bubbles cleaned the nickel silver. The aluminium foil corroded to brown as with bicarbonate. Here too, cooling down stops the reaction soon, and corroded aluminium slows it down.

Cleaning seemed less impressive with hydroxide, so I tried bicarbonate on the same joint end (left on the picture), which didn't clean further. The nickel silver part is more dirty there, including from rests of adhesive, which supposedly prevents cleaning.

Marc Schaefer, aka Enthalpy

[pcm81]

Hydroxide part
Few years ago i played with black iron oxide bath, which is a boiling bath of 2lb sodium nitrate ad 5lb sodium hydroxide per gallon of water. This hot solution will bubble very violently when aluminum is added.I suspect you were seeing a similar chemistry play out in your bath but on a much smaller / slower scale. Hot hydroxide can dissolve metal, so i suspect your flute got cleaned as the result of hydroxide dissolving everything, including top layer of metal.

Aluminum + baking soda part
This may be of interest: https://en.wikipedia.org/wiki/Carbon_dioxide_cleaning
I think in your case the aluminum foil and baking soda produce CO2, which is what is cleaning the flute.

[Enthalpy]

Hi pcm81, thanks for your interest!

To my understanding, CO₂ ice cleans by mechanical processes:

• The impact of the solid, where CO₂ advantageously leaves no solid nor liquid residue;
• the explosion of CO₂ ice in contact with the warmer surface.

CO₂ as bubbles or solution doesn't bring these effects. Nor do I see what cleaning reaction could occur with the corroded metal.

Consistently, warmed bicarbonate alone releases CO₂ but doesn't clean. It needs the aluminium foil, which releases hydrogen rather than CO₂.

[pcm81]

Hi pcm81, thanks for your interest!

To my understanding, CO₂ ice cleans by mechanical processes:

• The impact of the solid, where CO₂ advantageously leaves no solid nor liquid residue;
• the explosion of CO₂ ice in contact with the warmer surface.

CO₂ as bubbles or solution doesn't bring these effects. Nor do I see what cleaning reaction could occur with the corroded metal.

Consistently, warmed bicarbonate alone releases CO₂ but doesn't clean. It needs the aluminium foil, which releases hydrogen rather than CO₂.

So, what you saying is that Al + Sobium bicarb gives off H2. Which then probably oxidizes silver and other metals to make the relevant salt soluble in water and cleaning is achieved. Silver bicarb is 15 more soluble in water at 100*C than at 15*c, so that kinda makes sense that you see better result in hot bath...

Sounds to me you are just replicating results of using a weak acid to clean metal oxides.
Personally, I'd just use vinegar.

[Enthalpy]

My mistake: the outer face of my flute has a silver layer. So cleaning with aluminium foil and bicarbonate is just the usual operation. Below the silver, the body is grey, much darker than silver hence it shouldn't be the usual nickel silver, while at the bore and the tenons it's more yellow, less so than the usual brass, and there the reaction didn't clean the metal. A workshop complained the alloy is very soft.

Aluminium and bicarbonate didn't act at the keyworks, where I suspect a strong biofilm, nor against residues from adhesive tape. 7000 grit sandpaper worked there but it removed the silver layer much faster than expected. Metal tools scratched the parts. Bone had the right hardness, I shaped a scraper of it, but access in the corners was imperfect and lengthy. The best means, especially on keyworks, were toothpaste and a toothbrush. This can replace all other means I tried.

Marc Schaefer, aka Enthalpy