This document analyzes the hot corrosion behavior of laser melted GP1 stainless steel powder when exposed to a molten chloride and sulphide environment. The study found that the corrosion followed a parabolic rate law, with higher corrosion rates initially. Surface examination revealed pitting and scaling. Phase analysis identified oxides like Fe2O3 and Cr2O3 as well as other reaction products like NiFe2O4 and NiCr2O4, indicating the material underwent oxidation, chloridation and sulphidation during exposure. The results provide insight into corrosion mechanisms during additive manufacturing processes involving laser powder melting.

1. Lasers in Eng., Vol. 0, pp. 1–7
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Hot Corrosion Behaviour of Laser Melted GP1
Stainless Steel Powder in a Molten Chloride
and Sulphide Environment
A. Kurian
1,
*, M. Siva Prasad
2
, A. Ajith
1
, T.J. Cijesh
1
,
M.G. Deepu
1
, B. George
1
1
Department of Mechanical Engineering, Amal Jyothi College of Engineering,
Kanjirappally-686518, Kerala, India
2
Department of Metallurgy, Amal Jyothi College of Engineering,
Kanjirappally-686518, Kerala, India
This paper focuses mainly on the hot corrosion behaviour of laser melted
GP1 stainless steel powder using pulsed Nd:YAG laser. The objectives of
this study were to analyse the morphology, phase composition and the
oxidation kinetics of the laser melted GP1 stainless steel specimen
exposed to molten salt environment at high temperature. The surface
characteristics of hot corroded specimens were methodically analysed
using tools like microscopic inspection and X-ray diffraction (XRD).
Parabolic law based oxidation kinetics was revealed by this study and
these results are helpful to understand the hot corrosion mechanism in
additive manufacturing processes involving powder melting by laser
beam.
Keywords: Nd:YAG laser, GP1 stainless steel, powder, chloride, sulphide, laser
melting, high temperature corrosion, molten salt environment
1 INTRODUCTION
Laser melting is an additive manufacturing process in which the components
with complex shapes are formed directly from metallic powder [1]. In laser
melting process, parts are usually fabricated by melting a thin layer of metal
powder on a substrate or previously formed layer using one or more laser
beams [2].

2. 2 A. Kurian et al.
Hot corrosion is the most aggressive method of degradation of metals,
alloys and ceramics, especially when they operate in environments of
molten salts like chlorides and sulphides. Rapid degradation occurs due to
lack of a continuous solid passive layer found on the substrate [3]. Compo-
nents that are affected by the hot corrosion attack include gas turbines, indus-
trial plants and jet engines. This can also be defined as the accelerated
oxidation of materials that is induced by a film deposit of salt at elevated
temperatures [4].
There are two types of hot corrosion: Type I and Type II. A type I hot
corrosion attack happens at a temperature range of 800 to 900°C, with the
minimum threshold being the melting point of the salt deposit. The upper
temperature is taken to be the salt dew point. This happens in two stages.
An incubation period where the rate of attack is slow as the oxide layer
forms and a subsequent stage where the rate of oxidation accelerates quickly.
The incubation period is related to the formation of a protective oxide scale.
Initiation of accelerated corrosion attack is believed to be related to the
breakdown of the protective oxide scale. Many mechanisms have been pro-
posed to explain the accelerated corrosion attack, of which the salt fluxing
model is probably the most widely accepted. Oxides can dissolve in Na2SO4
as anionic species (basic fluxing) or cationic species (acid fluxing), depend-
ing on the salt composition. Salt is acidic when it is high in SO3, and basic
when low in SO3 [5–7].
The selection of this powder material is motivated by the fact that among
many stainless steel alloys, GP1 stainless steel is a versatile alloy with
properties such as high corrosion resistance, formability, high strength, weld-
ability and biocompatibility which are suitable for orthopaedic applications
such as implants and prosthesis, pharmaceuticals, architectural applications,
fasteners, airframe and aircraft engine parts, marine chemical parts, con-
denser tubing, and heat exchanger [8–10]. The main scope of this work is
to evaluate the hot corrosion behaviour of laser melted GP1 stainless steel in
an environment of molten salt.
2 EXPERIMENTAL APPARATUS AND PROCEDURES
To facilitate the hot corrosion test a Nd:YAG laser (JK150PS; JK Lasers,
Ltd.) emitting a Gaussian beam at 1064 nm was used to melt specimens with
the following process parameters: a laser power of 150 W; laser speed of
12 m/min; and a laser beam size of 250 µm. Specimens were then cut into
rectangular pieces of 20 × 15 × 5 mm3
and then mirror polished. To remove
the moisture from the specimens and also for the salt to adhere to the surface
uniformly, the specimens were preheated to 250°C in an oven.
Immediately after a coating of uniform thickness with 5 to 10 mg/cm2
of salt mixture (K2SO4-60% NaCl) was applied with a flat brush on the

3. Hot Corrosion Behaviour of Laser Melted GP1 Stainless Steel Powder 3
preheated specimen. On this specimen, cyclic hot corrosion studies were
performed for exactly 10 cycles. The duration of each cycle was 24 hours in
which heating was for 5 hours at 550 °C in a furnace followed by 19 hours of
cooling at room temperature. During the corrosion tests the weight change
measurements were taken at the end of each cycle. The spalled scale was also
retained during the measurement of the weight change to determine the total
rate of corrosion.
The specimen after corrosion tests was subjected to characterization
studies using an optical microscope (OLMBD-40/R; Olympus, Ltd.) for
surface morphology and X-ray diffraction (XRD) (SmartLab; Rigaku
Corporation) for phase analysis. Specimens were analysed with different
etchants and better results were obtained on use of aqua-regia (4HCl +
1HNO3).
3 RESULTS AND DISCUSSION
3.1 Corrosion kinetics
Figure 1 shows the weight gain per unit area versus number of cycles
from which it is evident that, the oxidation kinetics under the molten salt
environment oxidation shows a parabolic rate law of
W2
= Kpt (1)
where W is weight gain per unit area, t is number of cycles (which is a func-
tion of time) and Kp is the parabolic rate constant. Also metals exhibiting a
FIGURE 1
Plot showing the weight gain per unit area versus number of cycles.

4. 4 A. Kurian et al.
parabolic oxidation rate yield a straight line when the data are plotted W2
versus time (number of cycles), t which is shown in Figure 2. The form
of parabolic oxidation kinetics is typical of the non-steady state diffusion
controlled reactions; moreover, higher corrosion rate is observed during
initial hours, up to 6 cycles exactly, and is attributed to the rapid oxygen
pick up by diffusion of oxygen through the molten salt layer and is found
to be identical to the results reported by many researchers during their hot
corrosion studies [11, 12].
3.2 Surface morphology
The surface morphological studies have been carried out and found that the
metals and alloys undergo a severe oxidation when exposed to molten salts
at a temperature of 550°C. The accelerated corrosion rate was due to the
combined effects of oxidation, chloridation and sulphidation. This led to
the formation of dark pits as shown in Figure 3(a). After the first cycle, the
formation of dark pits was noticed.Also, after second cycle, (see Figure 3(c)),
de-scaling is clearly visible from the specimen surface. Whereas in the third
cycle, (see Figure 3(d)) the corrosion morphology of the material exposed
to K2SO4-60% NaCl shows that the surface interface is more prone to forma-
tion of fragile scale. It is observed that, the corrosion rate in K2SO4-60%
NaCl environment is higher in magnitude than in air [11, 13].
3.3 Phase composition
As revealed by XRD pattern in Figure 4, different phases were formed on
the surface after corrosion cycles as the results of various reactions. Hot
FIGURE 2
Plot showing the square of the weight gain per unit area versus number of cycles.

5. Hot Corrosion Behaviour of Laser Melted GP1 Stainless Steel Powder 5
FIGURE 3
Optical surface micrographs of the specimens after (a) 1 cycle, (b) 1 cycle (different location),
(c) 2 cycles and (d) 3 cycles of hot corrosion.
FIGURE 4
XRD pattern of the hot corrosion specimen (a) before corrosion and (b) after corrosion of 10
cycles.

6. 6 A. Kurian et al.
corrosion under molten salt (K2SO4-60% NaCl) environment at 550°C
showed that Fe2O3 and Cr2O3 as the predominant phases whereas NiFe2O4,
NiCr2O4, (Cr,Fe)2O3, FeNi and FeS were observed with low intensity. Many
researchers have pointed out that the formation of sodium chromate (K2CrO4)
could result from oxy-chlorination even the temperature is lower than the
melting point of salt deposits [11, 13]. As K2CrO4 is formed, the salt wets the
specimen surface which eventually leads to a mechanism of hot corrosion
dominated by molten salt and is further validated by XRD analysis. This is in
confirmation with past studies on the hot corrosion studies in molten salt
environment on boiler tube steel.
4 CONCLUSIONS
The conclusions of this work are:
(i) The oxidation kinetics of laser melted GP1 stainless steel in molten
salt (K2SO4-60% NaCl) environment at a temperature of 550°C follows
a parabolic rate law of W2
=Kpt;
(ii) The rate of oxidation is observed to be high in the early cycles of the
study in the investigated environments, which may be attributed to the
fact that during the transient period of oxidation, the scales formed may
be providing protection to the underlying metals;
(iii) The surface of the laser melted GP1 stainless steel specimens undergo
high temperature pitting, de-scaling and surface porosity due to hot
corrosion; and
(iv) The outer porous corroded layer mainly consists of oxides such as
Fe2O3 and Cr2O3, and minute amounts of other reaction products such
as NiFe2O4, NiCr2O4, (Cr,Fe)2O3, K2CrO4, FeNi and FeS.
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