Building Design and Construction Handbook - Merritt - Ventech!

4.74 SECTION FOUR
4.49 CORROSION OF IRON AND STEEL
Corrosion of ferrous metals is caused by the tendency of iron (anode) to go into
solution in water as ferrous hydroxide and displace hydrogen, which in turn combines
with dissolved oxygen to form more water. At the same time, the dissolved
ferrous hydroxide is converted by more oxygen to the insoluble ferric hydroxide,
thereby allowing more iron to go into solution. Corrosion, therefore, requires liquid
water (as in damp air) and oxygen (which is normally present dissolved in the
water).
Alloying elements can increase the resistance of steel considerably. For example,
addition of copper to structural steels A36 and A529 can about double their corrosion
resistance. Other steels, such as A242 and A588, are called weathering steels,
because they have three to four times the resistance of A36 steel (Art. 4.40.4).
Protection against corrosion takes a variety of forms:
Deaeration. If oxygen is removed from water, corrosion stops. In hot-water heating
systems, therefore, no fresh water should be added. Boiler feedwater is sometimes
deaerated to retard corrosion.
Coatings
1. Paints. Most paints are based on oxidizing oil and a variety of pigments, of
which oxides of iron, zinc sulfate, graphite, aluminum, and various hydrocarbons
are a few. No one paint is best for all applications. Other paints are coatings of
asphalt and tar. The AISC ‘‘Specification for Structural Steel Buildings’’ (ASD
and LRFD) states that, in general, steelwork to be concealed within a building
need not be painted and that steel to be encased in concrete should not be
painted. Inspections of old buildings have revealed that concealed steelwork
withstands corrosion virtually to the same degree whether or not it is painted.
2. Metallic. Zinc is applied by hot dipping (galvanizing) or powder (sherardizing),
hot tin drip, hot aluminum dip, and electrolytic plates of tin, copper, nickel,
chromium, cadmium, and zinc. A mixture of lead and tin is called terneplate.
Zinc is anodic to iron and protects, even after the coating is broken, by sacrificial
protection. Tin and copper are cathodic and protect as long as the coating is
unbroken but may hasten corrosion by pitting and other localized action once
the coating is pierced.
3. Chemical. Insoluble phosphates, such as iron or zinc phosphate, are formed on
the surface of the metal by treatment with phosphate solutions. These have some
protective action and also form good bases for paints. Black oxide coatings are
formed by treating the surface with various strong salt solutions. These coatings
are good for indoor use but have limited life outdoors. They provide a good
base for rust-inhibiting oils.
Cathodic Protection. As corrosion proceeds, electric currents are produced as the
metal at the anode goes into solution. If a sufficient countercurrent is produced, the
metal at the anode will not dissolve. This is accomplished in various ways, such
as connecting the iron to a more active metal like magnesium (rods suspended in
domestic water heaters) or connecting the part to be protected to buried scrap iron
and providing an external current source such as a battery or rectified current from
a power line (protection of buried pipe lines).