Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always from the effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish, it may tarnish with oxygen over time. It appears as a gray or black coating over metal. Tarnish is a surface phenomenon that's self- limiting, unlike rust. Only the top limited layers of the metal react.

Silver tarnish:

Silver tarnishes is a black coating of silver sulphide from air, which has the formula of Ag2S. The chemical equation for the formation of silver tarnish is reaction with hydrogen sulphide (H2S) is 

2Ag(s) + H2S(g) → Ag2S(s) + H2(g)

There’s speculation that, nowadays, tarnishing of silver actually occurs more quickly due to the increased of hydrogen sulphide released into the atmosphere by the combustion of fossil fuels. 

This silver sulphide tarnish can be quite easily removed using polishes that contain abrasives to rub off the silver sulphide, but these have the disadvantage of also removing small amounts of silver. Chemistry provides a method which sidesteps this problem.

Remedy:

This simple method uses aluminium foil, boiling water, baking soda (sodium bi carbonate) and salt (sodium chloride). Bowl with the foil, add a teaspoon of sodium bicarbonate and a sprinkling of salt, then just add boiling water. The tarnished silver object can be placed into the water, making sure there is contact between the silver and the aluminium foil, and the silver sulphide tarnish will be disappear.

Aluminium is a metal which has a stronger affinity for sulphur than silver, so in this reaction the aluminium simply replaces the silver from the silver sulphide compound, to form the aluminium sulphide compound:

Ag2S(s) + 2 Al(s) → 2 Ag(s) + Al2S(s)

This reaction is an electrochemical reaction – a tiny electric current flows between the silver and the aluminium when they are in contact, and the silver in silver sulphide is reduced (gains electrons) to form silver metal, while the aluminium is oxidised (loses electrons) to form Al3+ ions for sulfur ions.

3 Ag+ + 3 e– → 3 Ag

Al → Al3+ + 3e

Combination of  these two half equations gives redox equation for the reaction:

 Al + 3 Ag+ → Al3+ + 3 Ag

Reason:

The need for the sodium bicarbonate and the salt during the reaction is both are vital components. The sodium bicarbonate is required to remove the thin layer of aluminium hydroxide that forms on the aluminium foil; without this, the reaction would be unable to obtain a supply of aluminium ions, the reaction could not proceed. The salt acts as a ‘salt bridge’. this aids in the transfer of electrons as the reaction progresses, preventing an imbalance in charge and allowing the oxidation and reduction reactions to proceed.

You may notice a faint aroma of eggs when carrying out this procedure – this is due to the further reactions that can occur. Aluminium sulphide can react with the water as follows:

Al2S3(s) + 6 H2O(l) → 2 Al(OH)3(aq) + 3 H2S(g)

Hydrogen sulphide (H2S) is actually the same gas that rotting eggs give off.

Reference:

1) https://www.southernliving.com/home/cleaning-and-housekeeping/does-sterling-silver-tarnish

2) https://www.homedepot.com/c/ah/how-to-clean-silver-10-ways/9ba683603be9fa5395fab90d380f435

3) https://en.wikipedia.org/wiki/Tarnish

4) https://www.bhg.com.au/how-to-fix-tarnished-silver

You can see a statue or old age roof that looks a little green around that. 

That's because of patina. 

A patina finish is a surface change that has grown more beautiful with time. Scientifically speaking, patina is the green or brown film that forms naturally on the exterior of copper due to a series of chemical reactions. 

Copper forms a patina when exposed to oxygen and weathering long time. You'll see this change in colour on old roofing, copper statues. These natural changes to copper objects be over time as the metal oxidises, making copper a one- of-a-kind symbol of heritage that is hard to replicate. The chemical reaction The formation of the patina is similar to how rust forms on iron.

The chemical reaction:

molecule in the air to form copper( I) oxide. 

• 4Cu + O2 → 2Cu2O (reddish) 

The copper(I) oxide will continue reacting to 

oxygen molecule long time to form copper(II) oxide. 

• 2Cu2O + O2 → 4CuO (black) 

carbon dioxide and water molecule in the air it coats 

the exterior with that iconic blue-green patina colour. 

• 2CuO + CO2 + H2O → Cu2CO3(OH)2 (patina)

Depends  upon the availability of carbon dioxide and water molecule in the air, different chemical reactions occur to turn the copper oxide colours of blue, green, and purple. The city  where the copper present will impact its final colour. 

For example, larger cities may have more aerosol sulfates (produced from burning fossil fuel energies) which turns the copper oxide to a dark shade of green and emerald.

There is a reason that a patina surface has come to be associated with beauty. It is a visual representation of time.

A simple copper cleaner to vanish tarnish can be made by combining lemon juice with baking soda and stirring until mixed completely. Once mixed, apply to the copper surface and buff in a circular motion using a soft and clean cloth. Then Rinse and dry.

Reference:

1) https://education.seattlepi.com/causes-copper-tarnish-4611.html

2) https://en.wikipedia.org/wiki/Tarnish

3) https://www.copperh2o.com/blogs/blog/how-to-clean-copper-the-complete-guide