Despite not being defined in the specifications of chemical composition for the different material qualities, the need to verify
and to quantify certain trace elements in iron based materials (cast iron, low
and medium alloy steels, as well as stainless steels) is due to the influence
that these trace elements can have in the own properties of the material, or
even in the various interactions that may occur within other elements, that might
negatively affect the main characteristics of these type of materials.
Usually, it
is a residual presence (low or very low concentrations of certain
elements), which can generate an
adverse effect and consequently an undesired influence in the characteristics of such materials.
For example, to be able to quantify trace elements
levels as Ce, La and Sb allows establishing limit relations for a specific
design or thermal module of a casting, which might develop graphite nodularity
problems or certain types of graphitic malformations during solidification,
such as chunky graphite or spiky graphite. Another clear example is related to
the Pb content in stainless steels, that in mg/kg levels allows advancing
possible difficulties or problems during the lamination process.
The analytical
technique that enables this quantification is known as Inductively
Coupled Plasma Mass Spectrometry (ICP-MS), characterized by achieving an extraordinary high resolution for very low concentrations.
The mass
detector coupled with the collision cell that minimizes the interferences from double loads, oxides
or polyatomic molecules, achieve quantification limits of few ppb (parts per billion) for all
elements between Li (7 amu) and Uranium (250 amu). Its main advantage is
therefore the low detection limits obtained for most of the elements.
The complete process of sample nebulization, argon plasma atomization, electromagnetic separation of neutral molecules, interference elimination using the
collision cell, separation of ions according
to the mass ratio between the charge and
subsequent quadrupole mass detection are
perfectly described in the
following video.
This level of accuracy allows quantifying these trace elements by a prior sample preparation using acid digestion with
ultra high purity reactives, to certify afterwards levels of few ppm (mg/kg) from the different elements in the sample.
One of the main problems in confirming these results
is the absence of international certified standards contrasting the presence of
these elements to such accuracy levels and in such matrices. To overcome this important
gap and to have control
samples (QC) with certified results, IK4-Azterlan has done an intensive work putting forth
specific efforts in the
development of internal control standards from certified synthetic patterns, keeping the matrix present in the samples.
Therefore it is possible to
analyze the main elements at trace levels in iron based components and even the most critical elements for this type of materials (such
as As, Nb, Cd, Sn, Sb, Ba, La, Ce, Pr,
Nd,
Pb, Bi), that can be certified within the scope from the Standard UNE-EN ISO/IEC 17025 accreditation
from 5 down to 200 ppm.
Example
of the calibration curve for Sb121
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