Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 85

however, able to give a strength above 1200 N/mm 2 but with a lower elongation.

Other work, 146 as a further example, confirmed that values of tensile strength up

to above 1400 N/mm 2 could be obtained using a similar type of annealing cycle

but with a richer chemistry.

Zinc-coated steels

Coating with zinc (galvanizing) is one of the most widely used and cost-effective

means of protecting mild steel against atmosphere corrosion. Zinc itself has good

resistance to corrosion through the formation of protective surface films of oxides

and carbonates but zinc coatings protect steel in two ways:

1. By forming a physical barrier between the steel substrate and the environment.


2. By providing galvanic or sacrificial protection by virtue of the fact that zinc
   is more electronegative than iron in the electrochemical series. This effect is
   illustrated schematically in Figure 1.97.

Production methods

Zinc coatings are applied continuously by the hot dip galvanizing and electro-

galvanizing processes, and in order to appreciate the difference in properties

between the products, the two processes will be briefly described.

Hot dip galvanizing (HDG)

The various stages of a modem HDG line are shown schematically in Figure 1.98.

Cold-rolled strip is first welded to the trailing end of the previous coil and enters

ANODIC

Magnesium

Zinc -~

Aluminlum

Cadmium

Steel

Tin

Nickel

Copper

CATHODIC

More

active

More

noble

V///JJ'v',~'~/~'/~'l/y~//i//2/t'/i~'//.

Steel coated with zinc

it//~J-////~1/.,,~z;,~_~/////////t/.,, ~.

ii

Steel coaied With*less active metal

Figure 1.97 Galvanic protection of steel by zinc