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Materials
Selection
A silicon
bronze impeller (Alloy 875) was improperly selected for service inside a water
pump system. The pump went prematurely offline due to failure of the
impeller. Visual examination
revealed that the exposed surface of the impeller exhibited a reddish copper
color in certain areas. It may be
noted that silicon bronze should have a uniform yellow tint. It was further observed that the impeller vanes had severely
degraded. Metallographic
studies, in conjunction with energy dispersive spectroscopy (EDS), showed that
the impeller material experienced dezincification.
The proper material for the recommended service conditions should have
been a tin bronze, which is resistant to dezincification. Dezincification is a special form of corrosion process in which the zinc is selectively leached from susceptible alloys, i.e. zinc-containing bronzes with less than 85wt% copper, thereby leaving near pure copper or copper oxide in its place. This corrosion process commonly occurs when the susceptible alloy is exposed to a water environment containing dissolved oxygen.
Liquid Metal Embrittlement A fire
occurred at a residence and caused substantial damage to the property.
Subsequent fire investigation determined that the origin of the fire was
located within the chimney where galvanized low carbon steel plates were used to
clad the inside walls. It was
evident that the steel plates had been damaged by a chimneysweeper at an
undisclosed time prior to the fire incident. Metallographic
and fractographic studies revealed that the normally ductile low carbon steel
plates were embrittled. In addition, EDS analysis of the fracture surface revealed the presence
of zinc. It was
evident that the high temperature service conditions caused the galvanized
(zinc) coating to melt and penetrate to the grain boundaries.
The steel plates
experienced liquid metal embrittlement after exposure to a high temperature.
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