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
- 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.