Re: Re: Re: Offbeat plating question


[ Follow Ups ] [ Post Followup ] [ TubeNet BBS ] [ FAQ ]

Posted by Rick Denney on February 09, 2004 at 14:49:20:

In Reply to: Re: Re: Offbeat plating question posted by bloke on February 09, 2004 at 13:47:01:

Okay, I just did a bit of research from one of my old-clock sources.

Zinc oxidizes more easily than copper. It reacts with the hydroxide ions in moist air to create zinc hydroxide, which reacts with carbon dioxide in air to produce zinc carbonate. This the gray film. It is insoluble in water, and provides a protective surface. The copper doesn't oxidize fast enough, and gets protected by the zinc carbonate. The yellow of the brass shining thorugh the zinc carbonate layer is what makes it brown.

If there is sulfer dioxide present in the air, or if the zinc carbonate gets removed by the action of electrolytic salts (such as perspiration), the copper will oxidize more quickly, forming green copper sulfate.

Saliva contains electrolytic salts and also acids, and both of these wash away the zinc carbonate, exposing fresh zinc underneat to oxidation. Thus, the zinc oxidizes from the regular water in saliva, and then the bad parts of the saliva wash the oxide (zince carbonate) away. The cycle repeats until there is no more zinc. This is red rot. Surface dezincification is caused when environmental pollution attacks the zinc carbonate, which leaves a film of red copper underneat. Unless there is a copper oxidizing agent available (i.e. sulfur), the copper remains red.

The oxidation is caused in the first place by impurities in the metal, which creates microscopic differences in eletrical potential (in other words, the impurities create little batteries). Electrolytes create tiny electrical currents which result from the chemical reactions. The nature and distrubution of impurities affects how the material oxidizes to some extent.

For people who restore old brass clock movements, the desire is to clean the movement without removing the protective oxide. They use organic solvents like acetone or detergents like most cleaning products.

Ammonia will remove the brown oxide if it is mixed with a weak acid. Ammonia is a base, and that will reduce the oxide, but only if it can be induced to react with it. Acid reacts with the oxide, but it also reacts with the metal underneat, which is bad. Thus, if you add a little acid (such as acetic acid) to a base like ammonia, it will react and form ammonium salts, but you add little enough so that the overall solution is still basic. The ammonium salts will start the reaction between the alkaline ammonia and the oxide, and the excess ammonia will keep it alkaline so that it doesn't attack the base metal. I suspect that substances that remove the brown oxide but are considered safe for the metal are made primarly of aqueous ammonia with a little acetic acid thrown in, plus some detergents and soaps. But clock people generally only do this for customers who refuse to believe that brass is clean if it isn't shiny.

So, there's the chemistry. Zinc oxidation is one step in the deszincification process, and salt (especially acidic salts) cause green deposits because they both dissolve the zinc oxide and promote the oxidation of copper. The brown brass oxide is mostly zinc carbonate, which is insoluble in water and which therefore protects the brass from further corrosion, as long as you keep the inside clean.

Rick "who collects old clocks" Denney


Follow Ups: