SURFACE MODIFICATIONS OF PM STAINLESS STEELS
FOR ENHANCED CORROSION RESISTANCE
Chris Schade
Hoeganaes Corporation
Cinnaminson, NJ 08077
ABSTRACT
In general, PM stainless steel parts have inferior corrosion resistance to their wrought
counterparts. Furthermore, the variation in surface porosity and oxide layers exhibited by
sintered PM parts can lead to a considerable amount of uncertainty in corrosion
resistance. In the present study addition of elements, which may modify or segregate to
the surface of the powder, leading to enhanced corrosion resistance, are added prior to
sintering. The effect of these elements on mechanical properties and corrosion resistance
will be studied and compared to grades of stainless steel powder commercially available.
The effect of processing variables such as sintering atmosphere and density will also be
considered.
INTRODUCTION
The corrosion resistance of stainless steel is attributed to the formation of a passive layer
of chromium oxides that forms readily on the surface of the steel. In general, the poor
corrosion resistance of PM stainless steels (compared to wrought stainless steel) is
attributed to the porosity which gives rise to crevice corrosion.1 Thus in recent years,
focus has been on producing parts with higher density via higher compaction pressures
and sintering temperatures as well as sintering in either vacuum or 100 volume percent
hydrogen. However there exists many potential applications in which the mechanical
properties of lower density PM materials is adequate and the corrosion resistance is the
limiting factor which determines if the PM part is used.
In early work by Klar and Ro1 it was shown that water atomized powders exhibited large
differences between the bulk composition of the powder particles and the composition at
the surface. The oxidation states of various elements that segregate to the particle surface
were shown to lead to differences in corrosion resistance. In particular, the authors found
that silicon forms an oxide film on the water-atomized powder that, due to its brittle
nature can be fractured during compaction. The fractured oxide layer creates sites for
crevice corrosion. If the oxide layer remained intact it would protect the surface from
corrosion. It was also found that when sintering in a hydrogen atmosphere the silicon
oxide layer can be reduced. If the reduction of the silicon oxide layer is not complete,
leaving patches of silicon oxide, a local galvanic cell can be produced leading to poor
corrosion. It is also speculated that there is a ratio of tin to silicon at the surface that must
be met to keep the protective oxide layer intact. It is unclear at this time what is the
proper density and sintering temperature combination that leads to optimal corrosion
resistance.
Several authors1-2 have modified this surface oxide with tin. It was found that tin, like
silicon, segregated to the surface of the water-atomized powder, and formed tin oxide.
The tin helps keep the oxide layer ductile and during compaction less damage to the
oxide film occurs. In addition, due to it’s low melting point, the tin may aid in healing
any cracks in the oxide layer during the sintering process. There appears to be an
optimum ratio of tin to silicon on the surface of the powder.3 It is believed that tin
migrates from the bulk of the powder particle to the surface, while the silicon oxide
present at the surface tends to be reduced or diffuses back into the bulk particle. As a
result, the ratio of tin to silicon on the surface increases with the time and temperature of
sintering.
Additional work by Klar4 has shown that the addition of tin may affect the nitrogen
adsorption during cooling. When nitrogen is in solution in the matrix of stainless steels it
can increase the pitting and corrosion resistance. However, if the solubility is decreased,
the nitrogen will form chromium nitrides leading to the depletion of chromium in
localized areas (particularly grain boundaries) and thus poor corrosion resistance. Klar
presented data on tin bearing 316L and 304L for various cooling rates, which showed
lower nitrogen values for tin bearing grades (versus conventional grades) cooled at rates
typical of conventional sintering furnaces. The significance of this is that tin modified
grades can provide superior corrosion resistance than conventional materials when
sintered in nitrogen containing atmospheres. It also has been noted that tin acts as a
barrier to carbon diffusion in ferrous materials and may prevent chromium carbide in the
same fashion nitrides cause sensitization and hence, poor corrosion resistance.5
Although tin has been the most utilized element to improve the surface conditions other
elements are known to become surface enriched.6 Although theory does not exist for why
elements segregate to the surface of water atomized powders, it is known that elements
with a high affinity for oxygen tend to promote surface oxidation during water atomizing.
There are several characteristics listed by Klar et. al.4 that make an element a candidate
for modifying the surface oxide layer. First, the element should have high diffusivity and
low solubility in the base alloy. This will allow the element to segregate to the surface of
the water-atomized powder rather than be in solution in the base alloy. The second factor
is that the element should form an oxide film, in conjunction with the silicon-oxide that is
ductile and provides complete coverage after sintering. This allows a passive layer to be
formed on the surface of the sintered part. The element added should not adversely affect
the mechanical or corrosion properties of the material. The exact mechanism allowing
this to be achieved cannot be predicted so experimentation is necessary to determine the
best elements available.
Complicating the preceding discussion is the role of density in corrosion resistance. It
has long been seen that corrosion resistance increases with decreasing density.4 Oxygen
diffusion in low-density specimens with larger pores is facilitated and crevice corrosion is
reduced. At intermediate densities the pores are of size and shape that passivation cannot
occur due to oxygen depletion. This leads to poor corrosion resistance. At higher
densities, where a greater percentage of the porosity is closed, corrosion resistance
improves.7
The purpose of the present work is to study the effect of tin addition on the mechanical
and corrosion properties of 304L for a series of densities, sintering atmospheres and
temperatures. Using this data as a knowledge base, various oxidizable elements such
calcium, magnesium, zirconium, aluminum and lanthanum are added to 304L prior to
atomization to determine if these elements might be effective in enhancing corrosion
resistance by forming a more tenacious oxide layer on the atomized powder.
ALLOY PREPARATION AND TESTING
The powders used in this study were produced by water atomization with a typical
particle size distribution 100 w/o <150 µm (–100 mesh) and 38 to 48 w/o <45 µm (-325
mesh). All the alloying elements were pre-alloyed into the melt prior to atomization,
unless otherwise noted. The powders were of the same base composition (304L) with
only minor additions where noted. Table I list the chemistry of the individual heats along
with a letter designation indicating the change from the base 304L chemistry. Alloy
304L-Sn, which has tin and copper additions, is based on commercially available material
while the other alloys listed are experimental materials produced specifically for this
study.
Table I: Nominal Compositions of Experimental 304L Alloys (w/o)
The powders were mixed with 0.75 w/o Acrawax C lubricant. Samples for transverse
rupture (TR) and tensile testing were compacted uniaxially at compaction pressures 415
to 690 MPa (30 to 50 tsi). All the test pieces were sintered in a high temperature Abbott
continuous-belt furnace at temperatures ranging from 1120 °C (2050 °F) to 1260 °C
(2300 °F) for 30 min in either a 100% hydrogen atmosphere with a dewpoint of –40 oC (40 °F), or a mixed atmosphere of 90% nitrogen and 10% hydrogen (90/10).
Prior to mechanical testing, green and sintered density, dimensional change (DC), and
apparent hardness, were determined on the tensile and TR samples. Five tensile
specimens and five TR specimens were tested for each composition. The densities of the
green and sintered steels were determined in accordance with MPIF Standard 42. Tensile
testing followed MPIF Standard 10 and apparent hardness measurements were conducted
on tensile and TR specimens, following MPIF Standard 43.
Metallographic specimens of the test materials were examined by optical microscopy in
the polished and etched (glyceregia) conditions.
Salt spray testing on TR bars was performed in accordance with ASTM Standard B 11703. Five TR bars per alloy (prepared as previously described) were tested. The percent
area of the bars covered by red rust was recorded as a function of time. The level of
corrosion was documented photographically.
RESULTS AND DISCUSSION
Mechanical Properties of 304L with Tin
The mechanical properties of the 304L with tin additions are shown in Figures 1a - j .
The properties are shown for various sintering temperatures and compaction pressures
sintered in 100 volume percent hydrogen and 90 volume percent nitrogen/10 volume
percent hydrogen (90/10). As expected, the mechanical properties improve with
compaction pressure (i.e. density) and the denisty of the 304L with tin addition sintered
in 100% hydrogen is slightly higher than when it is sintered in 90/10. Both the ultimate
tensile strength and the yield strength are higher for the material sintered in 90/10 due to
the formation of nitrides in the microstructure. Figure 2 shows the mirostructure of 304L
with tin sintered in hydrogen and 90/10 atmospheres. The microstructure of the hydrogen
sintered materials is austenite with twins while the microstructure of the materail sintered
in nitrogen contains a large amount of nitrides. These nitrides act as second phase
particles which enhance the strength of the material.
90/10
Hydrogen
Compaction Pressure (MPa)
Compaction Pressure (MPa)
346
7
415
484
553
o
o
o
o
o
o
622
692
346
7
761
2050 F (1121 C)
484
o
o
o
o
553
622
691
761
40
45
50
55
2050 F (1121 C)
6.9
2100 F (1149 C)
2100 F (1149 C)
3
Sintered Density (g/cm )
2300 F (1260 C)
3
Sintered Density (g/cm )
6.9
415
6.8
6.7
6.6
6.5
6.4
o
o
2300 F(1260 C)
6.8
6.7
6.6
6.5
6.4
6.3
6.2
6.3
25
30
35
40
45
Compaction Pressure (tsi)
(a)
50
55
25
30
35
Compaction Pressure (tsi)
(b)
Compaction Pressure (MPa)
484
553
o
o
o
o
o
o
622
Compaction Pressure (MPa)
691
761
621
346
90
415
2050 F (1121 C)
80
80
o
o
o
o
o
70
483
60
414
50
40
30
35
345
50
345
276
40
276
40
45
50
207
30
3
UTS (x 10 psi)
414
UTS (MPa)
60
55
207
25
30
35
40
484
o
o
o
o
o
622
761
414
346
60
415
2300 F (1260 C)
55
YS (x 10 psi)
345
o
o
o
o
o
o
622
691
761
414
380
2100 F (1149 C)
50
345
45
310
40
276
40
276
35
241
35
241
30
207
30
207
172
25
25
30
35
40
45
50
3
310
YS (MPa)
45
25
55
172
25
30
35
Compaction Pressure (tsi)
415
484
553
o
o
o
o
o
o
45
50
55
(f)
Compaction Pressure (MPa)
Compaction Pressure (MPa)
346
65
40
Compaction Pressure (tsi)
(e)
622
691
346
65
761
415
484
o
o
o
o
o
o
553
622
691
761
40
45
50
55
2050 F (1121 C)
2050 F (1121 C)
60
2100 F (1149 C)
Apparent Hardness (HRB)
Apparent Hardness (HRB)
553
2300 F (1260 C)
3
YS (x 10 psi)
484
2050 F (1121 C)
380
2100 F (1149 C)
60
55
Compaction Pressure (MPa)
691
2050 F (1121 C)
50
50
(d)
553
o
45
Compaction Pressure (tsi)
Compaction Pressure (MPa)
55
552
483
(c)
415
761
621
70
Compaction Pressure (tsi)
346
60
691
2300 F (1260 C)
3
UTS (x 10 psi)
o
622
2100 F (1149 C)
2300 F (1260 C)
30
553
2050 F (1121 C)
552
2100 F (1149 C)
25
484
2300 F (1260 C)
55
50
45
2100 F (1149 C)
2300 F (1260 C)
55
50
45
40
40
35
35
25
30
35
40
45
Compaction Pressure (tsi)
(g)
UTS (MPa)
415
50
55
25
30
35
Compaction Pressure (tsi)
(h)
YS (MPa)
346
90
Compaction Pressure (MPa)
Compaction Pressure (MPa)
346
25
415
484
553
o
o
o
o
o
o
622
691
346
25
761
o
o
o
o
o
553
622
691
761
40
45
50
55
2100 F (1149 C)
2100 F (1149 C)
20
2300 F (1260 C)
Elongation (%)
Elongation (%)
484
o
2050 F (1121 C)
2050 F (1121 C)
20
415
15
10
2300 F (1260 C)
15
10
5
5
0
0
25
30
35
40
45
Compaction Pressure (tsi)
(i)
50
55
25
30
35
Compaction Pressure (tsi)
(j)
Figure 1. Properties of 304L-Sn as a function of compaction pressure. Left side of figure sintered in
hydrogen. Right side of figure sintered in 90/10.
The hardness of the material sintered in a 90/10 atmosphere is higher than the materials
sintered in hydrogen. This is likely due to the nitrides in the microstructure. The apparent
hardness of the material sintered in hydrogen at 1260 oC (2300 oF) decreases despite and
increase in density. This is most likely due to tin diffusing back into the bulk of the alloy
or the tin forming a low melting phase at the higher temperatures. The ductility of the
materials is correlated with the density and yield strength for the two different sintering
atmospheres.
(a)
(b)
Figure 2. Representative microstructures: (a) Hydrogen sintered 304L-Sn; (b) 304L-Sn sintered in 90/10.
Corrosion Properties of 304L with Tin
It has been shown in the preceding section that 304L with tin sintered in a nitrogen
bearing atmosphere has higher strength than material sintered in hydrogen. In general, if
the nitrogen is soluble in the austenite, corrosion resistance is not negatively impacted.
The nitrides that form as a second phase in the matrix, increasing the strength of the
material, also deplete the matrix of chromium by the formation of chromium nitrides.
This leaves areas depleted of chromium and generally leads to poor corrosion resistance.
Since PM stainless steels are already at a disadvantage in corrosion resistance to their
wrought counterparts, due to porosity, it is hard to justify the use of nitriding to increase
the strength of PM. However, if the corrosion resistance of austenitic PM stainless steels
sintered in nitrogen can be improved, the use of nitrogen as a strengthening agent is more
viable.
Figure 3 and 4 show results of salt spray testing for 240 hrs of TRS specimens sintered at
various compaction pressures and sintering temperatures in hydrogen and 90/10
respectively. For the TRS specimens sintered in hydrogen, the corrosion resistance
(indicated by the level of red rust) decreases with both compaction pressure and sintering
temperature (i.e. density). Conversely, the corrosion resistance of the TRS specimens
sintered in 90/10 increases with increasing compaction pressure and sintering
temperature. The results of the sintering in hydrogen support the earlier discussion in
which the corrosion resistance of PM materials decreases with increasing density. The
poor oxygen flow in smaller pores leads to increased crevice corrosion. In addition, the
hydrogen atmosphere reduces the protective silicon oxide film, leaving the surface
exposed to the salt spray solution.
2050 oF (1121 oC)
2200 oF (1200 oC)
30 tsi
(414 MPa)
40 tsi
(552 MPa)
50 tsi
(690 MPa)
Figure 3. Salt Spray testing coupons of 304L-Sn sintered in Hydrogen (240 hrs).
2300 oF (1260 oC)
2050 oF (1121 oC)
2200 oF (1200 oC)
2300 oF (1260 oC)
30 tsi
(414 MPa)
40 tsi
(552 MPa)
50 tsi
(690 MPa)
Figure 4. Salt Spray testing coupons of 304L-Sn sintered in 90/10 (240 hrs).
The reason for the corrosion behavior in the nitrogen-sintered materials is less clear. In
general, the corrosion resistance of the nitrided TRS specimens is better than the TRS
specimens sintered in hydrogen. This overall better corrosion resistance could be
attributed to the non-reducing 90/10 atmosphere. In this case, the protective silicon oxide
layer on the surface is not reduced and the tin/silicon ratio remains optimum. The
behavior of the material versus density (via compaction pressure and sintering
temperature) is opposite from most all PM materials which decrease in corrosion
resistance with increasing density. This may be explained by the fact that the nitrogen,
carbon and oxygen levels of the 90/10 sintered materials decrease with increasing
sintering temperature (see Table II). The solubility of nitrogen increases with decreasing
sintering temperature over the range of sintering temperatures used in this study.
Although the protective silicon oxide/tin layer may still exist, the increasing levels of
carbon and nitrogen can make the material more susceptible to sensitization by the
formation of chromium carbides and nitrides. In this regard, there may be an optimum
level of nitrogen for both corrosion resistance and mechanical properties.
Table II: Sintered Chemistry of 304L-Sn for various sintering temperatures and compaction pressures.
(90/10)
Compaction
Pressure
30 tsi (414 MPa)
40 tsi (552 MPa)
50 tsi (690 MPa)
Sintering
Temperature
2050°F
o
(1121 C)
2200°F
o
(1200 C)
2300°F
o
(1260 C)
2050°F
o
(1121 C)
2200°F
o
(1200 C)
2300°F
o
(1260 C)
2050°F
o
(1121 C)
2200°F
o
(1200 C)
2300°F
o
(1260 C)
Carbon
(w/o)
Sulfur
(w/o)
Oxygen
(w/o)
Nitrogen
(w/o)
.033
.001
.24
.90
.031
.001
.29
.66
.018
.001
.27
.62
.037
.001
.30
.87
.031
.001
.29
.65
.027
.001
.28
.60
.038
.001
.30
.84
.031
.001
.28
.62
.027
.001
.27
.57
Experiments with Oxides
Since the role of the silicon oxide layer (and its modification by tin) seems to play a
dominant role in the corrosion of PM alloys it was decided to explore whether other
oxides could play a similar role in corrosion protection. Taking advantage of the
oxidizing nature of water atomization several test materials were produced using high
oxygen potential elements of both high solubility (aluminum, zirconium and lanthanum)
as well as elements with low solubility (magnesium and calcium). Table I shows the
chemistry of the experimental materials. Since it was uncertain how well the oxide layer
would form during atomizing high levels of each element were used. The effectiveness
of the elements was judged based on oxygen level in the powder (Table I) and
examination of the oxide layer on sintered TRS specimens. Metallography was used to
determine if the oxide layer would exist after being exposed to a reducing atmosphere
during sintering.
The powders made with aluminum, zirconium and lanthanum all exhibited oxygen levels,
prior to sintering above 1% while the oxygen levels of the magnesium and calcium doped
304L were much closer to the typical level of stainless steels (.30%). The thickness of
the oxide layer (post sintering) correlated with the powder oxygen level in Table I.
Figure 3(a) shows a typical oxide film after sintering 100% hydrogen. A thin oxide layer
coats the inter-particle boundaries. One interesting feature of the powders is the surface
of the sintered bars for the 304L-Al and 304L-Zr sintered in a 90/10 atmosphere. Figure
3(b) shows the surface of the 304L-Zr sintered under these conditions. A thick nitride
layer exists at the surface at the particle boundaries, but the nitrides in the core of the
particles are much less than the other powders sintered in 90/10 (see 304L-Sn in Figure
2(b)). Since both zirconium and aluminum are strong nitride formers the nitrogen cannot
diffuse to the interior of the particles.
(a)
(b)
Figure 5. Representative microstructures: (a) Hydrogen sintered 304-Mg; (b) 304-Zr sintered in 90/10.
For normal PM stainless steels the silicon oxide layer on the surface of the atomized
powder is thin and does not negatively impact the green properties of the powder. This is
a key feature since the powder still needs to be compacted into parts. The green density
and green strength of the experimental powders were measured to determine if they were
suitable for compaction and these results are shown in Table III. The green density and
green strength tracked with the powder oxygen level. The 304L-Al would not be
acceptable and the 304L-Zr borderline for producing parts. Since these powders were
atomized in a nitrogen-purged chamber and they are susceptible to nitride formation, this
may also have led to the poor compaction of these powders.
Table III: Green Properties of Experimental 304L Alloys
Green
Green Strength
Density
3
(MPa) (g/cm ) (psi) (MPa)
Pressure
Pressure
Green
Density
3
Material
(tsi)
304-Sn
304L-Al
304L-Zr
304L-La
304L-Mg
304L-Ca
30
414
6.24
713
4.9
30
414
5.56
48
0.3
40
552
30
414
5.94
296
2.0
40
30
414
5.97
684
4.7
30
414
6.00
1057
30
414
6.02
1142
(tsi) (MPa) (g/cm )
40
552
6.54
Green Strength
Pressure
(tsi) (MPa)
Green
Density
3
Green Strength
(psi)
(MPa)
(MPa)
7.9
50
690
(g/cm )
6.76
(psi)
1142
1555
10.7
5.83
138
1.0
50
690
6.04
238
1.6
552
6.25
549
3.8
50
690
6.48
821
5.7
40
552
6.28
1167
8.0
50
690
6.52
1716
11.8
7.3
40
552
6.33
1653
11.4
50
690
6.58
2029
14.0
7.9
40
552
6.34
1725
11.9
50
690
6.58
2314
16.0
Since the green properties of the 304L-Mg and 304L-Ca powders were comparable to
304L-Sn and the metallography confirmed an oxide coating on the particle boundaries in
the sintered condition, these two powders were chosen for salt spray testing. Unlike the
304L-Sn these two materials exhibited better corrosion resistance with increasing density
for both the hydrogen and 90/10 atmospheres. With the 304L-Sn the tin aids the silicon
oxide layer and additional strength is gained by nitriding. However the nitriding makes
the material brittle. These new oxide layers, unlike the tin addition, allow for better
corrosion resistance at higher densities. Higher densities can provide strength without the
sacrifice in ductility, which is important for many PM stainless parts. This technique may
also allow for other strengthening methods, such as dual phase microstructures and
precipitation hardening, to be used which would be affected by sintering in a nitrogen
rich atmosphere.
2050 oF (1121 oC)
2200 oF (1200 oC)
40 tsi
(552 MPa)
50 tsi
(690 MPa)
Figure 6. Salt Spray testing coupons of 304L-Mg sintered in hydrogen (240 hrs).
2300 oF (1260 oC)
2050 oF (1121 oC)
2200 oF (1200 oC)
2300 oF (1260 oC)
40 tsi
(552 MPa)
50 tsi
(690 MPa)
Figure 7. Salt Spray testing coupons of 304L-Ca sintered in hydrogen (240 hrs).
CONCLUSIONS
•
The corrosion resistance of 304L-Sn sintered in hydrogen decreases with
increasing compaction pressure and sintering temperature (i.e. density).
•
The corrosion resistance of 304L-Sn sintered in 90/10 increases with increasing
compaction pressure and sintering temperature (i.e. density).
•
Adding oxidizable elements to the melt prior to atomization, such as calcium and
magnesium, lead to an oxide film on the powder that remains at inter-particle
boundaries after sintering in a reducing atmosphere.
•
The oxide layer formed during atomization leads to lower green density than
conventional atomized material.
•
The oxide layer formed by calcium and magnesium allowed for better corrosion
resistance with increasing compaction pressure and sintering temperatures (i.e.
density).
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compiled by J.M. Capus and R.M. German, Metal Powder Industries Federation,
Princeton, NJ, 1992, vol. 5, pp. 385-397.
3. D.H. Ro, E. Klar and C.I. Whitman, “Maximizing the Corrosion Resistance of Tin
Containing Stainless Steel Powder Compacts,” U.S. Patent No. 4,314,849. February
9, 1982.
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Stainless Steel Bodies and Article,” US Patent 4,420,336 December 13, 1983.
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by E.N. Aqua and C.I. Whitman, Metal Powder Industries Federation, Princeton, NJ,
1984, vol. 16, pp. 277-293.
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Stainless Steel Powders and Compacts Therefrom,” U.S Patent 4,240,831 December
23,1980.
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Corrosion Resistance of P/M Stainless Steels,” Advances in Powder Metallurgy and
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