Environmental Engineering Reference
In-Depth Information
of the wire was encountered at the emerging point from the molten bath. The
occurrence was more in the case of steel wires with higher carbon content or
above a certain diameter of the wire. The incidence of such failures could be
minimized by lowering the slant of the emerging wire over the pulley, apparently
because of the reduction in stress. The intrinsic higher strength of the high carbon
steels made them susceptible to LME, whereas under the identical conditions the
low-carbon-steel wires did not suffer from LME.
The hot shortness of copper-containing steels during hot rolling is an example
of LME. Iron oxides form preferentially at the surface below which a copper-
enriched phase forms. This phase melts at the hot working temperatures and
penetrates along the grain boundary under the action of rolling stresses. Similar
hot shortness have been reported in lead- or tellurium-containing steels where
these elements are added for the improvement of machinability.
The combined presence of a molten metal and stress makes the processes of
welding, brazing, and soldering ideal for liquid metal embrittlement. The condi-
tion of good wellability is enhanced by the use of fluxes in these operations. In
one reported case [9], where the operating temperatures of aircraft radiations
were raised by a replacement of lead-tin solders with a higher melting point
lead-silver solder, extensive cracking of radiations followed and the practice was
discontinued. The embrittlement resulted from the improved wettability of steel
by molten lead with the addition of silver.
Zircaloy-2, used as cladding material for fissionable fuel rods in water-cooled
nuclear reactors, has been reported to fail by LME in contact with liquid cad-
mium, which appears as a fission product in the operation.
Cadmium plating on steel and titanium bolts and nuts has been reported [3]
to cause failure of the fasteners used in military jet engine compressor ducts or
in solid fuel rocket engines where aerodynamic heating melted the cadmium.
Failure of titanium jet compressor discs by LME in contact with cadmium-plated
bolts has also been reported.
The incidence of a disaster from LME failure because of a chance contamina-
tion of liquid metal took place in the cyclohexane plant at Flixborough in 1974
[10]. The unsupported length of a pressurized piping collapsed, killing 28 people
and causing widespread destruction. Many of the stainless steel pipes recovered
after the fire showed extensive cracking. Investigations revealed that zinc from
galvanized walkways, staircases, and girders had been transported to the external
surfaces of pipes, either as molten droplets or vapors, and had caused the embrit-
tlement.
7.3 CORROSION BY LIQUID METALS
Corrosion by liquid metals becomes a matter of concern when they have to remain
in contact with the solid metal over a long period. For example, due to their
