What is Corrosion?

What is Corrosion?

Definition: An electrochemical reaction occurring at numerous anodic/cathodic areas on metal surfaces, the metal ionizes into the water resulting in the formation of metal oxides.

General Corrosion: A uniform loss or general thinning of a metal at anodic/cathodic areas, however, the anodic/cathodic areas shift location resulting in an etched area over the metal surface.

Localized Corrosion: Pitting of a metal due to deposits or crevices. Oxygen under the deposit becomes depleted. Chlorides and sulfates concentrate under the deposit resulting in increased corrosion.

Stress Corrosion: Occurs either between the grains of a metal (intergranular) or across the grains (transgranular) due to high loads, welding, etc. Examples are corrosion of stainless steel by chlorides, corrosion of copper by ammonia, and corrosion of boiler steel by caustic.

Galvanic Corrosion: The sacrifice of one metal to protect the other. Zinc plated on steel (galvanizing) results in the loss of zinc to protect the steel. The corrosion rate is governed by what metals are connected and what are their relative surface areas.

Selective Corrosion: Copper alloys with high zinc concentrations (13%) are subject to dezincification unless other metals are added to the alloy. Zinc is preferentially removed leaving a soft, porous copper. Admirality is a high zinc/copper alloy but it is inhibited by adding arsenic. Aluminum alloys are subject to dealuminification. Cast iron can be corroded to graphite which will crumble and is as soft as the lead in a pencil.

Fretting Corrosion: Occurs where metals are clamped or bolted together and subjected to vibration. The normal metal oxide coating becomes damaged and results in crevice corrosion.


Five Steps to a Good Water Treatment Program

Cleaning: Rust, oil, grease, and dirt must be removed from piping/hoses and heat exchangers or corrosion, scaling, fouling, and biological control will not be achieved. Paint a dirty piece of metal with the best paint in the world and it will not stay on the metal surface.

Passivation: Filming a clean surface of the metals in a system with high concentrations of corrosion inhibitor (5 to 15 times normal) will result in immediate stifling of corrosion.

Scaling: Prevent the normal minerals in solution in the water from plating out on heat transfer equipment. Can be prevented by removing the scale formers (softening or deionizing) or putting them into solution (acid) or using special inhibitors.

Fouling: Aqueous suspended particulate matter has a negative charge. When a metal corrodes it must ionize and the surface becomes a positive charge. Fouling then occurs. This can be controlled with polymers.

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