Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 127
Copper alloys
Lead and tin alloys
Cast irons
Structural steels
Cadmium
Zinc alloys
Aluminium alloys
Magnesium alloys
Galvanic effects are negligible between alloys in the same groups, but
become increasingly more pronounced with alloys that are widely separated
in the table. Galvanic corrosion may be remedied by preventing electrical
contact between the two metals by means of bushes or washers, for example.
Controlling corrosion arising from differential aeration may be more
complicated and there are various means of effecting this.
Soluble inhibitors. In closed systems (e.g. when the corroding liquid is
recirculated), the liquid may be treated with soluble inhibitors, which are
of two main categories: the first is a reagent that removes oxygen from the
solution, and the second is one that leads to the formation of a passive film
on the surface of the metal, thus stifling attack.
Cathodic protection. An example of this process is sacrificial protection
referred to earlier, whereby the metal to be protected is connected electrically
to a more reactive metal in the galvanic series. Galvanizing steel with a
layer of zinc works in this way, as do slabs of Zn, Al or Mg that are
attached at intervals to buried steel pipelines or to marine structures. An
alternative method of obtaining cathodic protection is to use an impressed
current from a suitable dc source: the steel to be protected is connected to
the negative terminal and an inert metal anode is placed nearby.
Paints and lacquers. The main aim of these surface coatings is to exclude
water and air from the metal surface. In many cases, the exclusion is not
total and a paint layer may be regarded as introducing a large ionic resistance
into the corrosion cell, thus reducing the corrosion current and hence the
rate of attack. Some pigments used (such as red lead, Pb 3 O 4 ) may act as
inhibitors, while others (such as primers containing metallic zinc powder)
are essentially sacrificial pigments.
Weathering steels are structural steels in which the resistance to atmospheric
corrosion has been improved by the addition of small amounts of elements
such as copper, phosphorus, silicon and chromium. These steels rust at a
lower rate than plain carbon steels and, under favourable climatic conditions,
they can develop a relatively stable layer of hydrated iron oxide which
retards further attack. This can provide cost savings by eliminating the