Dear Amalia,
The reason why your SEM-EDS analysis will give a high value is that
it analyzing the signal from the carbon laid down on the surface by the
electron beam cracking the vacuum gas. It is possible to perform carbon
analysis in a EPMA - a specialized SEM using wavelength dispersive
spectrometers and running a better vacuum and anti-contamination systems
than in a standard SEM but even then it is difficult. I have done it but
normal would send samples off to LECO if destructive analysis was
possible. However, if you are considering conservation this method
looses a lot of information that you can obtain using metallography and
specially when combined with EPMA or LA-ICMS.
The metallography will give a fairly good estimate of the carbon
content - a standard undergraduate metallurgy practical as outline by
other than have replied and it is quick. When combined with hardness or
micro-hardness testing it will also give an indication of the mechanical
history of the metal from data such as grain shape and size (but
phosphorus will have a big effect on this so has to be considered). The
levels of phosphorus will be barely detectable in a normal SEM can be
significant, both on expression of the iron-carbon microstructures and
the hardness of the metal. Phosphorus will alter the corrosion
properties of the metal especially where there are large changes in
chemistry with respect to the adjacent metal. This effect on corrosion
can be seen in the lower part of the images in the below URLwhere
corrosion has gone in along a zone of high phosphorus material.
http://users.ox.ac.uk/~salter2/Lowbury/Sword_Section.html
I would beware of using EDX analyses from SEMs in general, the technique
has a great tendency to be misused. The results should only be treated
as semi-quantitative unless you have gone through the full
standardization procedure, which is rarely the case from my experience,
and run a secondary standard to check that there are no problems with
the analysis. The analysis has to be run under exactly the same
conditions as the standards. These conditions include obvious things
such as accelerating voltage and processing time, but also using the
correct working distance and the correct beam, sample, detector
geometry. A few degrees tilt on the sample will throw the results off
several percent. That is before thinking about problems caused by the
nature of the electron-beam sample interaction and and the X-ray sample
interaction. Therefore, unless all these variables are controlled or
fulfil the assumptions made in the correction programs the result will
be wrong. In this case, the program assumes that the carbon is even
distributed through the interaction volume, whereas the contamination is
only a thin film on the surface. However, the program corrects for the
substantial absorption of the C X-rays coming from depth hence the very
high carbon content.
Chris
On 27/05/2010 10:27, Amalia Siatou wrote:
> dear all,
>
> I'm a metals conservator from Greece doing a master in materials science. Part of my thesis is the study of archaeological iron and bronze. I came across some difficutlies in measuring the carbon content of archaeological iron (roman nails). Using SEM-EDS the percentage measured was particularly high (almost 10%) which doesn't follow the dating of the objects. Furthermore, I was anable to find adequate equipment to perform chemical analysis.
> Do you know of a different method of estimating the carbon percentage in iron?
>
> thank you in advance
>
> sincerely
> Amalia Siatou
>
--
Chris Salter
Senior Analyst,
BegbrokeNano,
Department of Materials,
Begbroke Science Park,
Sandy Lane, Kidlington,
OX5 1PF
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