Failure Analysis Assignment
Yvette
Griffiths
Fractured screw:
Fracture Point
This
screw is a typical industrially made and sold, with the standard specifications
of being steel with a galvanised zinc coating. It can be used in a variety of industry,
commercial and domestic settings, and can be obtained in a general hardware
store. The Zinc plating over the top of the steel prevents the steel from
corroding or rusting when exposed to the elements, this is done in accordance
with the ASTM standard B633 - Zinc. This is achieved due to the fact that the
Zinc has a higher potential to oxidise than the steel that it is in contact
with, causing it to corrode instead of the steel, causing the zinc to take on
the role of a "sacrificial anode". As the Zinc is oxidised, the steel
will have electrons flowing through it, which will force it to remain
unoxidised.
The
corrosion of Zinc, will eventually lead to the formation of a basic zinc
compound that will actually protect the underlying zinc metal from further
oxidation. However when steel is oxidised, it forms rust as its final product,
which will not form an adherent, impervious coating over the metal, and will
not protect the rest of the metal from further corrosion. The oxidation of
iron/steel will only occur in the presence of both air and water.
The
method of galvanising a steel screw is utilised during the manufacturing
process, on the hollow section, to the cleaned steel section, as it exits the
mill. It passes into a galvanising bath, which will apply a coating of zinc to
the surface in a way that the thickness of the coating can be controlled.
Once
the photographs of the screw in its original fractured state were taken, a
sample was taken from the screw, which was mounted, cleaned, polished and
etched. This was done to reveal the underlying structure of the sample, so that
it could be examined microscopically and any flaws or irregularities could be
determined.
The
following are still images taken from the microscope at a magnification of 50x.
This
showed a large amount of corrosion was present on the surface of the screw.
By
examining the untouched part of the specimen, under an electron microscope,
detailed pictures were taken, and a surface elemental analysis was undertaken
to identify both its elemental
composition, and any other elements that were present.
This
confirmed that the specimen was a steel screw, with a zinc coating. The
presence of the other elements indicated that corrosion was prominently
present.
Images
obtained from the Electron microscope of the surface of the specimen. Corrosion
product can be readily seen.
This
taken under a microscope, and shows very clearly the extent of the corrosion that
has taken place upon the exterior of the screw.
Conclusion:
The amount of corrosion apparent on this screw and
the lack of any other evidence for other causes of failure, gives grounds that
the corrosion was the cause for the failure, or contributed greatly to the
failure. This could have affected the screw by weakening the structure of the
screw, allowing it to be easily fractured under a small load of applied stress,
which would have been encountered in everyday use.
References:
http://www.azom.com/details.asp?ArticleID=1380
Brown,