HVAC Waters

corrosion in general

Corrosion is the gradual action of natural agents, such as air or salt water, on metals. Corrosion of iron (rusting), results from iron combining with oxygen and water, forming hydrated iron oxide which is porous, weak and brittle.

Preventing Rusting of Iron
(1) most expensive - alloying the iron so that it will be chemically resistant to corrosion

• stainless steel, in which chromium or chromium and nickel are alloyed with iron

(2) less expensive - coating iron with a material that reacts more readily with the corroding substances than with iron, thus, while being consumed, protects iron

• galvanizing, in which iron is covered with zinc. In the presence of corrosive solutions, an electric potential is set up between the iron and the zinc, causing the zinc to dissolve but protecting the iron as long as any zinc remains

(3) least expensive - covering it with an impermeable surface coating so that air and water cannot reach it

• satisfactory as long as no crack appears in the coating because when the coating cracks, rusting proceeds at least as fast as it would have with no protection   
• most satisfactory coatings are baked enamels 
• least expensive are paints such as red lead

aluminum corrodes rapidly, and a thin, continuous, transparent layer of oxide forms on the surface of the metal, protecting it from further rapid corrosion.

lead and zinc are less active than aluminum and are protected by similar oxide films.

copper is a comparatively inactive metal. It slowly corrodes in air and water in the presence of such weak acids as carbonic acid, producing a green, porous, basic carbonate of copper. Green corrosion products, called verdigris or patina, also form on common copper alloys - brass (copper + zinc) and bronze (copper + lead).

silver, gold, and platinum (noble metals) are so inactive chemically that they do not suffer corrosion from the atmosphere. A combination of air, water, and hydrogen sulfide will act on silver, but the amount of hydrogen sulfide normally present in the atmosphere is so small that corrosion is negligible except for black discoloration (tarnish) which is silver sulfide.