Cleaning and Polishing Brass (II)

Oxidation | Cleaning | Polishing | Ammonia | Cyanide
Effects of Quality | Stress Corrosion Cracking
Commercial Products | Electrolysis

Clock brass is a copper alloy consisting primarily of copper (about 65%) and Zinc (about 35%). Brass in some modern clocks also contains some lead (1-3%), which acts as a lubricant for easier machining. Copper alloys played an important role before the advent of steel. Copper is softer than its alloys, making the alloy more suited for making products that required a stronger metal, such as clocks. Another property of copper and its alloys that makes brass particularly well suited for clocks is its resistance to oxidation: copper oxide forms a protective layer on the surface that protects the metal underneath. The oxides of iron do not form a protective layer, enabling oxidation to continue.

Manufactured metals, like brass and steel, contain impurities that are not perfectly evenly distributed throughout the metals. Therefore some areas have more impurities than others, and an electrical potential exists between the atoms of the metals and the atoms of the impurities. In moist air, a film of water will condense onto the surface of the metal, into which carbon dioxide from the air can dissolve to form an electrolyte of weak carbonic acid, through which ions can flow.

The nonuniformities in the metal create areas where the metal is more easily oxidized (cathodic regions) than it is at others (anodic regions). The water acts as a salt bridge, creating a electrochemical reaction on the same principle as the galvanic cell (battery).
In the case of iron, each atom loses two electrons to become a positively charged ion,

which reacts with oxygen dissolved in the water to become hydrated iron (III) oxide precipitate.

The formula for this compound is represented in various ways, usually Fe2O3.xH2O , most commonly recognized when it has the familiar reddish-brown colour. (Iron ore occurs naturally as hydrated iron (III) oxide, which is called haematite. Iron ore also occurs naturally as iron (II) carbonate, FeCO3 , which is called siderite). When iron (III) oxide is only slightly hydrated, however, it has a black colour, which we sometimes see on clock pinions and arbors.
In the case of brass, zinc is more reactive than copper, so zinc ions are formed, which react with the hydroxide ions of water to form zinc hydroxide, Zn(OH)2 ,

reacting in turn with carbon dioxide (dissolved in the water) to form insoluble zinc carbonate, Zn2(OH)2CO3 , which forms a protective film on the surface of the brass.

Some copper will also oxidize in the presence of moisture to form green copper sulfate when oxygen and sulfur dioxide from the air are dissolved in the water:

and other similar compounds. You are likely to see this on clocks that perform their service on ships at sea and also that perform their service outdoors and near the sea, where there is plenty of salty moisture in the air to act as the electrolyte.
Brass in clocks that perform their service indoors and in areas of low moisture develop a layer of copper (II) and zinc (II) oxides, which we see as the brown discolouring. The layer of oxides protects the metal underneath. However, if the clock were not maintained, the lubricants fail and the pivots are allowed to grind away at the bushing surfaces, you would find a deposit of black powder on the base (usually the piece of wood that the mechanism is mounted on). This black powder is primarily copper (II) oxide and some zinc (II) oxide.

Since the oxide layer protects the brass underneath, it is better to clean the clock without disturbing the oxide layer. Unfortunately, people think that clean brass must have that shiny yellow colour, so clock repairers polish the brass and remove the oxide layer to make the brass look beautiful and impress the customer. The best way to clean brass is to use either detergent cleaners without ammonia and without acids or to use organic solvents, such as acetone or tetrachloroethylene, to remove the dirt and oil residues (but not the oxide layer!) The only situation, in which it would make sense to remove the oxide layer, would be if you intended to apply a protective layer of paint or lacquer to the surface of the brass.

The method for polishing metals like copper, brass, and steel, is by removing the layer of oxide, which results in a small loss of metal atoms as the oxides are dissolved. The most common method is to dissolve the oxide in a weak acid, such as acetic acid (glacial vinegar), which reacts with the basic (alkaline) oxide to produce the metal acetate (a salt) and water:

The weak acid will react with the oxide but will not react significantly with the unoxidized metals in the few minutes it would take to polish the metals. If left for an extended period of time in a solution of acetic acid, the metals would react. After the oxide layer is removed, the metal is rinsed in ammonia solution to remove any remaining acid and then rinsed in water (and dried immediately!)

Since acids are oxidizing agents and bases (alkalis) are reducing agents, it would make a lot more sense if the oxide (alkaline) layer could be removed with a base (or in an alkaline medium). However, oxides do not react with bases. If you have a weak base, such as ammonium hydroxide (acqueous ammonia), and you add acid (such that there is much more base than there is acid, resulting in a net alkaline solution), such as acetic acid, you get the ammonium salt of that acid, namely ammonium acetate:

which reacts with metal oxides to form the metallic salts of that acid:

Zinc and Copper ions form complex ions with ammonia solutions.

Note, however, that the ammonium ion is not as strongly acidic as the hydrogen ion it replaces, which is why it does not remove iron oxides as readily as the hydrogen ion (in acetic acid) at room temperature. This is one of several problems I have experienced while cleaning clocks.

The Zinc Carbonate and Copper Sulphate, that are formed when clocks are in humid conditions, also form complex ions with ammonia solutions.

Sodium and Potassium Cyanides are particularly effective in removing rust and Zinc Oxide (and Carbonate). Cyanides were therefore widely used by horologists until their use was banned (they are extremely toxic). The reason why cyanides are so effective is because of the formation of complex ions. (I do not believe that cyanides form complex ions with copper ions.)

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