Photos and case studies
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.
Ordinary low carbon steel material can withstand an impact induced by a chimneysweeper under expected cleaning operations. However, in this case, the normally ductile material had been embrittled. When the subject chimney was cleaned, the sweeper’s broom punctured the embrittled steel plate thereby exposing the building’s structure to potential ignition. Upon use of the chimney, a fire in the residence ensued
