Background

The field of metallurgy utilises an ever-increasing array of steel and alloy compositions in addition to a large number of processing techniques employed in material production through to product finishing. Surface analysis has a role to play in the characterisation of these materials from their microstructure to large-scale customer complaints. The high sensitivity of techniques such as secondary ion mass spectroscopy ( SIMS ) can be invaluable in establishing the material chemistry, particularly for low level additives, impurities or contaminants.
Benefits to Customer

· Location of ppm/ppb levels of elements in microstructure

· Clear and precise determination of the cause of corrosion

· Potential detection of corrosion products before the appearance of visual evidence

· Detection of low levels of corrosion accelerators such as chlorine
Case Study: Corrosion Problems on Steel Wires

The following example illustrates the use of a combination of x-ray photoelectron spectroscopy ( XPS ) and secondary ion mass spectroscopy ( SIMS ) to fully characterise a corrosion problem on steel wires. Localised corrosion had appeared on reels of wire but investigations at the manufacturer had shown no correlation between the problem and the materials used i.e. steel, drawing lubricants or post-drawing, lanolin protective coating.
X-Ray Photoelectron Spectroscopy Findings

X-ray photoelectron spectroscopy ( XPS ) compositional analysis of corroded and uncorroded areas shows a significantly higher concentration of iron in the corroded areas coupled with lower levels of calcium and sodium which are attributed to the drawing lubricants (Ca and Na stearates).

Table 1. X-ray photoelectron spectroscopy ( XPS ) surface compositions (figures in atomic %).

Analysis Area


Carbon


Oxygen


Iron


Calcium


Chlorine


Sodium


Sulphur

Corroded


35.4 ±1.2


50.8 ±1.1


8.1 ±0.4


3.5 ±0.3


0.9 ±0.4


1.3 ±0.5


-

Uncorroded


38.9 ±0.8


46.0 ±0.7


4.3 ±0.2


6.2 ±0.2


0.5 ±0.2


3.4 ±0.4


0.7 ±0.4

X-ray photoelectron spectroscopy ( XPS ) images of a corroded area clearly show an exposed iron area (rust) covering the lower two thirds of the wire. The corrosion areas are associated with striations along the wire in the drawing direction. Surrounding areas are rich in carbon, calcium and sodium residues from the drawing lubricants which is consistent with the compositional data. Although the chlorine image is weak it does correlate with the iron image suggesting an association between the two.

AZoM - Metals, ceramics, polymers and composites : x-ray photoelectron spectroscopy (XPS) image of corroded steel wire

Figure 1. X-ray photoelectron spectroscopy ( XPS ) images of corroded wire (analysis area 1.6 x 1.2 mm, resolution 10 -15 mm)
Secondary Ion Mass Spectroscopy ( SIMS ) Findings

Secondary ion mass spectroscopy ( SIMS ) mass spectra, images and depth profiles showed a consistent pattern. Within corrosion spots the organic levels are relatively low compared with the concentrations of inorganic contamination and corrosion products observed.

Tungsten-containing particles, as tungsten oxide, are present both in and around corrosion spots as a result of the break-up of the wire drawing dies during processing.

It is apparent that the corrosion arises as a direct result of a reduced level of lubricant during wire drawing which also causes the break up of the drawing dies. The reduced level of organic material and the roughening of the surface leave the steel exposed to attack by classical aqueous corrosion possibly accelerated by the presence of chlorine.

AZoM - Metals, ceramics, polymers and composites : secondary ion mass spectroscopy (SIMS) image of uncorroded steel wireAZoM - Metals, ceramics, polymers and composites : secondary ion mass spectroscopy (SIMS) image of corroded steel wire

Figure 2. Secondary ion mass spectroscopy ( SIMS ) images of drawn wires showing uncorroded wire (left) and corroded wire (right). The field of view is ~300mm.