Volume 6 Preprint 4
Corrosion Performance of Recast Layers Produced During Laser Drilling of Type 305 Stainless Steel
X. Y. Wang, G.K.L.Ng, Z. Liu, L. Li P. Skeldon and L. Bradley
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Volume 6 Paper C123
Corrosion Performance of Recast Layers
Produced During Laser Drilling of Type 305
, Z. Liu
, L. Li
, P. Skeldon
and L. Bradley
Corrosion and Protection Centre, UMIST, P.O. Box 88, Manchester M60
1QD, UK, X.Wangemail@example.com
Laser Processing Research Centre, Department of Mechanical,
Aerospace and Manufacturing Engineering, UMIST, PO Box 88,
Manchester M60 1QD, UK
3M Neotechnic Ltd, UP Brooks, Clitheroe, Lancashire BB7 1NX, UK
Laser drilling has been used for machining small orifices in various
materials, with applications seen in aerospace, automotive, electronics,
and medical industries. The drilling process involves melting and
vaporization. As a result, a layer of re-solidified material (recast layer) is
present on the walls of the laser-drilled holes. Since laser-drilled
components are frequently used in fluid media, it is important to
2understand the corrosion performance of the recast layer. In this work,
laser drilling of type 305 stainless steel was carried out using a pulsed
Nd:YAG laser with oxygen, argon and nitrogen as assist gases
respectively. Salt fog chamber tests and electrochemical polarization
tests in sodium chloride solution were employed to assess the corrosion
performance of the recast layers in the laser-drilled holes. Scanning
electron microscopy and X-ray energy dispersive spectroscopy were
used to correlate the corrosion performance with the microstructures. It
was found that the corrosion performance of the recast layers was
dependent on the assist gases used, increasing in order of oxygen,
argon, and nitrogen. This result was related to the oxide content and the
incorporation of nitrogen into solid solution in the recast layers.
Keywords: Laser drilling; Recast layer; Corrosion; Salt fog chamber test;
As a non-contact process, laser drilling does not cause any tool wear
and breakage and can be used to drill various types and shapes of
materials, some of which are difficult to drill by conventional drilling
methods [#ref1], [#ref2]. Thus laser drilling has been used for
machining small orifices in various materials with application seen in
3aerospace, automotive, electronics and medical industries. During laser
drilling, the drilled materials are often removed by a melt ejection
mechanism. In such cases, where the recoil pressure, used to eject the
laser-induced melt material, is insufficient to expel the viscous melt
material completely, a layer of re-solidified material, designated as the
recast layer, is often present on the walls of the laser-drilled holes
In many cases, as the laser-drilled components are frequently used in
fluid media, chemical and electrochemical corrosion properties of laser
drilled holes are important. Therefore, it is necessary to understand the
corrosion performance of the recast layers to ensure that the
applications of the laser-drilled components under those conditions are
suitable. Previous investigations have been reported on process
optimization and modeling to reduce the recast layers produced during
laser drilling [#ref2], [#ref3], [#ref4],[#ref5], [#ref6]. However, no
attention has been paid to the corrosion performance of the recast
layers in the fluid media. In this paper, laser drilling has been carried out
on thin sheets of type 305 stainless steel using a pulsed Nd:YAG laser
with oxygen, argon and nitrogen as assist gases respectively. Besides
salt fog chamber testing, electrochemical polarization tests in sodium
chloride solution was employed, using a special specimen preparation
4approach, to assess the corrosion performance of the recast layers
present on the walls of the laser-drilled holes. Scanning electron
microscopy observation and X-ray energy dispersive spectroscopy
analysis were also conducted to correlate the corrosion resistance with
the assist gases and microstructure of the recast layers.
2. Experimental Procedure
The material used in the study was a type 305 stainless steel with the
chemical composition given in Table 1, Laser drilling was carried out on
thin sheets of 0.25 mm in thickness using an Electrox Scorpion Nd: YAG
laser. During the laser drilling, three kinds of gases, oxygen, argon and
nitrogen, have been chosen as assist gases to protect the laser lens and
to facilitate melt removal in the produced holes. The laser operating
parameters are listed in Table 2.
Table 1 Chemical composition (wt%) of type 305 stainless steel
C Mn P S S Cr Ni
0.12 2.00 0.045 0.03 1.00 17.00-19.00 10.00-13.00
Table 2. General laser drilling parameters used in the experiments
Peak power Pulse width Pulse frequency Number of Gas pressure
5(kW) (ms) (Hz) pulses (bar)
5.4 0.4 20 2 3
In order to correlate the corrosion resistance with composition and
microstructure of the corresponding recast layer, the samples were
polished up to 4000 grade SiC emery paper, then etched in oxalic acid
and then observed and analyzed with a scanning electron microscope
(SEM) incorporating X-ray energy dispersive spectroscopy.
The salt fog chamber test was conducted at 303 K in a sealed, corrosion
resistant chamber containing a salt laden fog, generated by a fine spray
of NaCl solution. At the beginning of the test, thirty samples with holes
drilled using different assist gases were placed in the chamber. Two test
samples were taken out at regular intervals to check using an optical
microscopy whether corrosion had occurred. The sample was regarded
corroded if any yellow brown spot was observed inside the wall of the
laser-drilled hole. Prior to the examination, the samples taken out from
the chamber had been Immersed in deionized water for at least one
hour to remove the residual salt, and then dried.
The electrochemical polarization tests were performed using the
standard potentiodynamic polarization method with a conventional
three-electrode corrosion cell in a neutral electrolyte containing 3.5%
6NaCl solution at 303 K. Normally, electrochemical tests to study
localized corrosion require samples with exposed areas in the range of
square millimetres to square centimetres [#ref7]. However, the
thickness of the sample sheets was approximate 0.25 mm, and the
laser-drilled holes were smaller than 1 mm in diameter, the resulting
recast layers were in the range of 15-30m in thickness. Thus, the area
of the recast layer of one hole is too small to carry out the usual
large-scale electrochemical test. In order to solve this problem, six
drilled holes, produced using identical laser process parameters were
put in one sample to increase the exposed recast area for the
electrochemical tests. Prior to the tests, the recast layers were isolated
from other non-melted regions of the samples using epoxy resin, and
the tested surfaces were finely polished up to 4000 grade SiC emery
paper and degreased in ethanol to ensure that the surface state was
similar for all the samples. The electrode potential was measured with
respect to a saturated calomel electrode (SCE). During each test, the
electrolyte was firstly deaerated employing purified nitrogen gas for 90
min. Then the tested sample was immersed for 70 min to measure the
corrosion potential. This was followed by potentiodynamic polarization
at a scan rate of 1 mV/s.
3. Results and discussion
7The results of salt fog chamber test are shown in Table 3. The tests have
indicated that the corrosion resistance of the recast layers in the
laser-drilled holes was dependent on the assist gases. Of the three
assist gases, i. e. nitrogen, argon and oxygen, nitrogen resulted in the
most corrosion-resistant recast, and oxygen resulted in the least
corrosion-resistant recast layers.
Table 3 Result of the salt fog chamber test
Assist gas Day 4 Day 5 Day 6 Day 7 Day 8
- no corrosion and - corroded.
The potentiodynamic polarization curves of the as-received type 305
stainless steel and the recast layers in the laser-drilled holes produced
using the three assist gases are presented in Fig. 1. When compared
with the pitting potential of the as-received stainless steel 173 mV, the
pitting potential of the recast layers all shift to more noble values
namely 718 mV, 482 mV and 422 mV using nitrogen, argon and oxygen
as the assist gas. Furthermore, the passivation regions become wider
8and the passive current densities at given polarization potentials are
nearly one order of magnitude lower for the recast layers than for the
as-received stainless steel. Hence, it can be concluded that pitting
corrosion and dissolution in the passive region were suppressed in the
recast layers. The above results are in good agreement with the ranking
of the recast layers indicated by the salt fog chamber test (Table 3.).
Fig.1 The potentiodynamic polarization curves in 3.5% NaCl solution of
the as-received type 305 stainless steel and the recast layers in the
laser-drilled holes produced using the three kinds of assist gases.
SEM observation indicates that the microstructures of the recast layers
are independent of the assist gases. In the three recast layers, the
microstructures are characterized by a columnar structure of dendrites
mainly composed of austenite with delta phase in the interdendritic
regions, grown epitaxially from the base material. This typical
9microstructural morphology is resulting of rapid solidification as shown
in Fig. 2. Pits have formed during etching in the substrate region, only
consistent with a lower corrosion resistance compared with recast
material. The SEM morphologies and corresponding element
distributions reveal most oxide formed on the recast surface produced
using oxygen as the assist gas, followed by that using argon, and
nitrogen (Figs 3, 4 and 5).
Typical cross section SEM micrograph of laser-drilled type 305
stainless steel valve stem showing the microstructure of the substrate
and the recast layer when O
is used as the assist gas.
Fig.3 Elemental mapping of internal wall of laser-drilled type 305
stainless steel valve stem with 3 bar O
as assist gas.
Fig.4 Elemental mapping of internal wall of laser-drilled type 305
stainless steel valve stem with 3 bar Ar as assist gas.
11Fig. 5 Elemental mapping of internal wall of laser-drilled type 305
stainless steel valve stem with 3 bar N
as assist gas.
The suppressed pitting corrosion and active dissolution for the recast
layers can be ascribed to the microstructural and compositional changes
instigated on the as-received stainless steel. During the laser drilling,
on the one hand, the materials around the drilled holes are melted so
that some carbides, inclusions and precipitates that are present in the
stainless steel and are harmful to the pitting corrosion can be dissolved
[#ref8], [#ref9]. Solidification takes place at a very rapid cooling rate and
these carbides, inclusions and precipitates are removed or minimized
due to melt ejection, leading to a fine and homogeneous structure in
recast layers with fewer sites to initiate pits. Further the solubility of
sulphur in ˜-ferrite is higher than that in austenite [#ref10], such that
formation of ˜-ferrite grains in the recast layers reduces sulphur
segregation to the ˜-ferrite/˛ boundaries in the form of, for example
MnS, which further reduces the sites to initiate pits. Both of the above
microstructural changes during the laser drilling are consistent with Fig.
2 and would be helpful in improving the pitting resistance of the recast
12The differing pitting corrosion resistances among the three recast layers
produced using nitrogen, argon and oxygen as the assist gases is
probably related to the nitrogen contents in them which is dependant
upon the surrounding environment. When using oxygen as the assist
gas, there would be the least amount of nitrogen incorporated into the
recast layer due to the inhibition of oxygen and the formation of oxides
on the recast surface (Fig. 5). When using argon as the assist gas, there
would be more nitrogen incorporated into the recast layer due to the
reduction in the inhibition of oxygen and the formation of oxides on the
recast surface when compared to those using oxygen as the assist gas
(Fig. 4). When using nitrogen as the assist gas, the partial pressure of
the nitrogen gas is obviously the highest and thus there should be the
highest amount of nitrogen incorporated into the recast layer. The
nitrogen incorporated into the recast layers can exist in the forms of
free nitrogen and/or nitrides [#ref11], [#ref12]. If it is in the form of free
nitrogen, the enriched nitrogen dissolves during the propagation of the
pits to form ammonium ions
. The formation of these NH
would consume protons reducing the pH of the solution inside the pits,
and hence repassivation of pit surface[#ref11]. If the nitrogen is in the
form of nitrides such as CrN, the increased availability of nitrogen in the
atomic form in the recast layers could favour formation of the nitrides,
13chromium oxide and NH
from the dissolution of the CrN according to
the following reaction [#ref12]:
This reaction would help to repair the passive, and leave a trail of NH
that could act as a barrier between the material and the electrolyte.
Therefore, the best corrosion resistance derived from the recast layers
in the laser-drilled holes produced using nitrogen as assist gas would
be attributed to the presence of the greatest amount of nitrogen
incorporating in it.
By means of putting six drilled holes, produced using identical laser
process parameters, in one sample to increase the exposed recast area,
the electrochemical polarization tests of the recasts have been
successfully carried out. Results showed that:
a) The pitting corrosion and active dissolution were suppressed for
these recast layers when compared to those of the as-received type 305
b) Of the nitrogen, argon and oxygen assist gases, the results of the
electrochemical polarization tests are in good agreement with the result
14of the salt fog chamber tests. The recast layer produced using nitrogen
as the assist gas has the highest pitting corrosion resistance. The recast
layer produced using argon as the assist gas has intermediate pitting
corrosion resistance and that produced using oxygen as the assist gas
has the poorest pitting corrosion resistance.
c) The suppressed pitting corrosion and active dissolution for the recast
layers can be attributed to the microstructural changes instigated on the
as-received stainless steel, leading to a fine and homogeneous
structure in the recast layers with fewer sites to initiate pits. The
different pitting corrosion resistance among the recast layers produced
using the three kinds of assist gases can be associated with the
presence of the different amount of nitrogen incorporated in these
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