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Effect of excimer laser melting on the near surface chemistry and corrosion properties of aisi 304 stainless steel
Volume 6, number 7
MATERIALS LETTERS
April 1988
EFFECT OF EXCIMER LASER ~ELTrNG ON THE NEAR SURFACE CHE~IS~Y AND CORROSION PROPERTIES OF AISI 304 STAINLESS STEEL
T.R. JERVIS Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 8754.5, USA
D.J. FRYDRYCH and D.R. BAER ~~a~erialsScience Department. Paci~c northwest Laboratotlv, Rich~a~d, WA 99352, USA Received 14 December 1987; in final form 28 January 1988
We have investigated the effect of excimer laser surface melting and resolidification on the near surface chemistry and corrosion properties of AISI 304 stainless steel. Auger electron spectroscopy sputter-depth profiles show a 30-60% enhancement of Cr at the surface. Improved passivity and an increased resistance to pitting are observed.
1. Introduction
AISI type 304 stainless steel which has been surface melted by CO7 laser radiation has been shown to exhibit increased resistance to pitting corrosion [ 11. The increase in resistance to pitting is greater even than that afforded by the inco~oration of MO as in AISI type 3 16 stainless steel. This increased resistance has been ascribed to the removal or redistribution of large scale sulfide inclusions which are sites for preferential attack [ 11. In another study, Anthony and Cline [ 21 demonstrated that CO1 laser surface melting can redistribute chromium on 304 surfaces that have been sensitized by slower heat treatment. This ~‘renormalization” renders the surface immune to intergranular corrosion attack. In both these cases, the key elements involved were: ( 1) melting of the surface with a melt duration sufficient to allow significant liquid state diffusion to occur and (2) rapid cooling of the melted region to avoid resensitization. These results were established using relatively high power CO, lasers with the beam scanned across the work surface. The use of CO* lasers for metal processing is hampered by the low absorptivity (of the order of 10e3) of metals in the infrared. Typically, average power densities of the order of 107- 10’ W cm-’ are used to treat metals. The power output and
the weak coupling between the laser energy and the workpiece determine the speed at which processing can be accomplished. Anthony and Cline, using a 350 W laser, processed material at about 5 x lo-* cm2 s- ’ [ 21, McCafferty and Moore, using 7.5 kW, were able to process about 20 times as fast [ 11. Because of the melt depth obtained, a surface roughness of several tens of micrometers resulted. This strongly limits the utility of such surfaces as remachining could well resensitize the surface to corrosion attack. In contrast to infrared absorption, metals absorb strongly in the ultraviolet with absorptivities of the order of 0.5. Since CO, and excimer lasers are comparably energy efficient, the use of excimer lasers in metals processing can yield eno~ous increases in overall energy efficiency. While the depth of melt is shallower in the case of excimer laser processing by a factor of 10, the surface properties which result from melting and rapid solidification will be similar if comparable liquid state diffusion and rapid cooling can be demonstrated.
2. Experimental and results Commercially obtained AISI 304 stainless-steel samples approximately 3 mm thick were treated with excimer radiation at 308 nm. Samples were polished
0167-577x/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-HolIand Physics Publishing Division )
225
Volume 6, number 7
MATERIALS LETTERS
to a surface finish of approximately 0.2 urn but no particular care was taken to protect the samples from air. Samples were treated by moving them under repetitive pulses of radiation. Because the output of most excimer lasers is highly non-uniform, a multielement beam homogenizer was used to create a square beam spot of 0.1 cm’ with a uniformity of approximately 95%. Pulse energy was varied from 130 to 250 mJ/pulse with a resultant fluence of 1.3 to 2.5 J cmF2. The sample was translated in front of the beam at rates from 0 to 3 mm/s at repetition rates of less than 1 Hz. Higher repetition rates could be used with consequent higher throughput. A calculation of the energy deposition indicates that melting of a minimum layer of thickness 1.0 urn occurs at a fluence of 0.7 J cm-2, so that the incident energy was more than sufficient to melt the surface. A cross sectional photograph of a sample treated with 8 shots at a fluence of 1.3 J cm-’ is shown in fig. 1. A melt depth of about 4 pm is visible. A prokilometer trace across the melted region shows a surface roughness of approximately 0.1 urn which is of the same order as observed on the polished samples. Particularly apparent in this photograph are grain boundaries crossing the melt interface. These reveal that the regrowth during excimer laser processing is largely epitaxial as might be expected since epitaxial regrowth is observed in CO2 laser processing [3]
April 1988
where the melt depths and durations are much longer. Transmission electron microscopy of foil samples showed no particular grain refinement of the surface regions, also consistent with this observed structure. The near surface chemistry of the samples was analyzed by Auger electron spectroscopy (AES) using a Physical Electronics model 545 scanning Auger microprobe and ion sputtering to determine the extent of constituent diffusion. Argon ions at 3 keV were used to sputter at a nominal rate of 5 nm/min (Al standard). Fig. 2 shows the ratio of the Cr to Fe AES signals in the near surface region for the untreated sample and those treated with 4 shots at a fluence of 2.5 J cm-’ and 8 shots at a fluence of 1.3 J cm-*. A substantial increase in the relative Cr concentration near the surface can be seen. Fig. 3 shows the oxygen signal in the unprocessed sample and in the sample processed with 8 X 1.3 J cmm2. The thickness of the oxide layer is essentially unchanged by the laser processing despite the fact that the processing was done in air. Comparison of figs. 2 and 3 shows that the bulk of the Cr redistributed occurs as an oxide on the surface. Deeper in the material (sputter times greater than 12 min), there is evidence of Cr depletion in the laser-treated samples. This depletion, which occurred in the melted zone near the surface, amounts to approximately 7% and 12% for the 4 shot and 8 shot cases respectively.
@ As received * 4 x 2.5 J/cm2 8 8x1.3Jlcm2
Depth (sputter time in minutes)
Fig. 1. Cross section of AIS1 304 stainless-steel sample treated with 8 pulses of 308 nm radiation at 1.3 J cm-‘, Grain boundaries crossing the melt interface are evidence of epitaxial regrowth.
226
Fig. 2. Sputtered Auger profile of ratio of Cr to Fe in AISI 304 stainless-steel surfaces in the as-received state and after treatment by 308 nm radiation at 2.5 J cm-’ (4 pulses) and 1.3 J cm-* (8 pulses).
MATERIALS LETTERS
Volume 6. number 7
0
....,.l..,....l...lI....
0
5
1*
15
20
25
Depth (sputter time in minutes)
Fig. 3. Sputtered Auger profile of oxygen in AISI 304 stainlesssteel surfaces in the as-received state and after treatment by 308 nm radiation at I .3 J cm- ’ (8 pulses).
In order for this substantial redistribution of Cr to occur, two conditions must obtain. There must be a strong driving force for the redistribution and there must be sufficient time available in the molten state for the redistribution to occur. Because Cr is more soluble in the melt than in the solid, there is a strong tendency for Cr to segregate in the liquid at the liquid/solid interface. This interfacial segregation provides one driving force. A second driving force is provided by the strong affinity of Cr for oxygen. The observation that redistributed Cr on the surface is an oxide reflects the strength of this driving force. Fig. 1 shows that the melt depth is much larger than the I.0 pm calculated as a minimum. Because the fl uences used were substantially higher than the minimum needed to melt, this is not surprising. It is therefore reasonable to believe that the melt duration is also much larger than the pulse length of 30 ns. In metals in the liquid state, diffusion rates of lo-’ cm’ s’ are typical. For an accumulated melt duration of a few hundred ns, diffusion distances of the order of 50 nm can be expected. The melted layer also experiences a period of time after solidi~cation at an elevated temperature. Cooling rates are such that the sample surface is below 300°C within 5 x 10-j s. Substitutional diffusion rates at elevated temperatures in fee metals are of the order of 3 x 10e9 cm’s_‘. In the available time, elements could there-
April 1988
fore diffuse approximately 10 nm after multiple pulses. The AES data are consistent with either scale of diffusion. The AES results demonstrate that laser processing creates substantial mobility of Cr in the melted layer. This mobility, coupled with the strong driving forces to accumulate Cr on the surface, result in a Cr rich surface oxide. A decrease in the Ni concentration in the oxide was also observed. Both of these observations are consistent with oxidation studies of AH1 304 stainless steel which demonstrate that chromia is the most stable oxide [ 41. Electrochemical studies have shown that altering the Cr oxide layer influences the breakdown potential [ 5 1. Potentiodynamic anodic polarization measurements were made in a commercial corrosion cell using an unstirred, non-deaerated 0.5 M NaCl solution at ambient room temperature, a saturated calomel reference electrode (SCE) and two vitreous carbon counter-electrodes. Potential scans, at a rate of 0.1 mV/s-‘, were initiated at the open circuit corrosion potential 5 min after placing the samples in the cell. Untreated areas of the specimens were masked with a stop-off lacquer. The polarization curves measured for an untreated and a laser-treated specimen, 5 pulses at 2.0 J cm-21 are shown in fig. 4. The open-circuit potential of the treated specimen measured after 5 min was - 0.128 V,,, compared to -0.2 18 VsCE for the untreated specimen. The passive current density meas-
Fig. 4. Anodic polarization curves for treated and untreated samples.
227
Volume 6, number
7
MATERIALS
ured at polarizations up to 50 mV was less than 10 nA crne2 for the treated sample and on the order of 100 nA cme2 for the untreated sample. At approximately - 0.08 Vs,,, the current density of the treated sample increased to about 100 nA cm-2. At approximately 0.0 VSCE, the current density increased to about lo4 nA cmm2 for both samples. A dramatic increase in the current density occurred at approximately $0.08 VsCE for the untreated sample, while a similar increase was delayed until about +0.20 Vs,, for the treated sample. These results show that the passive current density characteristic of the protective film on the specimens is lower for the treated compared to the untreated sample. The results also show that the ultimate breakdown potential at which the current density increased dramatically was approximately 100 mV more positive for the treated sample compared to the untreated sample. Excimer laser melting therefore produces a surface that offers improved passivity and better resistance to pitting attack compared to an untreated surface. These results are consistent with those observed by McCafferty and Moore [ 11. While we observe a Cr rich surface oxide in the processed materials, we cannot conclude that this oxide is the sole source of the improved resistance to pitting attack as we cannot rule out the possibility that inclusions have also been dispersed.
3. Summary We have demonstrated that excimer laser processing changes the near surface chemistry and corrosion properties of AISI 304 stainless steel in similar
228
LETTERS
April 1988
ways to CO2 laser processing. At repetition rates of a few tens of Hz, excimer lasers with average power output of 15 W can process material at rates of a few cm2 s-l. This represents a factor of lo3 improvement over reported results with CO, lasers. Further, a much smoother surface finish is obtained than with CO2 laser processing. Laser processing enriched the Cr content of the surface oxide. The improvement in the corrosion behavior of excimer processed surfaces is comparable to CO, processed surfaces in NaCl solutions.
Acknowledgement We would like to thank H.L. Nutter, T.G. Zocco, R. Cordi, and R.W. Springer for technical assistance and M. Nastasi and E.L. Joyce Jr. for helpful discussions. This work was supported by the United States Department of Energy under contract W-7405 ENG-36 through the Los Alamos Center for Mate-, rials Science and contract DE-AC06-76RLO- 1830 with Battelle Memorial Institute.
References [ I] E. McCafferty
and P.G. Moore, in: Laser surface treatment of metals, eds. C.W. Draper and P. Mazzoldi (Nijhoff, The Hague, 1987) p. 263: [2] T.R. Anthony and H.E. Cline, J. Appl. Phys. 49 (1978) 1248. [ 31 P.G. Moore, in: Fundamental aspects of corrosion protection by surface modification, Proceedings Vol. 84.3, eds. E. McCafferty, C.R. Clayton and J. Oudar (The Electrochemical Society, Pennington, 1984) p. 102. [4] D.R. Baer, Appl. Surface Sci. 7 (1981) 69. [ 51J. Kruger, in: Passivity and its breakdown in iron and iron based alloys, eds. R.W. Staehle and H. Okada (National Association of Corrosion Engineers, Houston, 1976) p. 9 1.
MATERIALS LETTERS
April 1988
EFFECT OF EXCIMER LASER ~ELTrNG ON THE NEAR SURFACE CHE~IS~Y AND CORROSION PROPERTIES OF AISI 304 STAINLESS STEEL
T.R. JERVIS Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 8754.5, USA
D.J. FRYDRYCH and D.R. BAER ~~a~erialsScience Department. Paci~c northwest Laboratotlv, Rich~a~d, WA 99352, USA Received 14 December 1987; in final form 28 January 1988
We have investigated the effect of excimer laser surface melting and resolidification on the near surface chemistry and corrosion properties of AISI 304 stainless steel. Auger electron spectroscopy sputter-depth profiles show a 30-60% enhancement of Cr at the surface. Improved passivity and an increased resistance to pitting are observed.
1. Introduction
AISI type 304 stainless steel which has been surface melted by CO7 laser radiation has been shown to exhibit increased resistance to pitting corrosion [ 11. The increase in resistance to pitting is greater even than that afforded by the inco~oration of MO as in AISI type 3 16 stainless steel. This increased resistance has been ascribed to the removal or redistribution of large scale sulfide inclusions which are sites for preferential attack [ 11. In another study, Anthony and Cline [ 21 demonstrated that CO1 laser surface melting can redistribute chromium on 304 surfaces that have been sensitized by slower heat treatment. This ~‘renormalization” renders the surface immune to intergranular corrosion attack. In both these cases, the key elements involved were: ( 1) melting of the surface with a melt duration sufficient to allow significant liquid state diffusion to occur and (2) rapid cooling of the melted region to avoid resensitization. These results were established using relatively high power CO, lasers with the beam scanned across the work surface. The use of CO* lasers for metal processing is hampered by the low absorptivity (of the order of 10e3) of metals in the infrared. Typically, average power densities of the order of 107- 10’ W cm-’ are used to treat metals. The power output and
the weak coupling between the laser energy and the workpiece determine the speed at which processing can be accomplished. Anthony and Cline, using a 350 W laser, processed material at about 5 x lo-* cm2 s- ’ [ 21, McCafferty and Moore, using 7.5 kW, were able to process about 20 times as fast [ 11. Because of the melt depth obtained, a surface roughness of several tens of micrometers resulted. This strongly limits the utility of such surfaces as remachining could well resensitize the surface to corrosion attack. In contrast to infrared absorption, metals absorb strongly in the ultraviolet with absorptivities of the order of 0.5. Since CO, and excimer lasers are comparably energy efficient, the use of excimer lasers in metals processing can yield eno~ous increases in overall energy efficiency. While the depth of melt is shallower in the case of excimer laser processing by a factor of 10, the surface properties which result from melting and rapid solidification will be similar if comparable liquid state diffusion and rapid cooling can be demonstrated.
2. Experimental and results Commercially obtained AISI 304 stainless-steel samples approximately 3 mm thick were treated with excimer radiation at 308 nm. Samples were polished
0167-577x/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-HolIand Physics Publishing Division )
225
Volume 6, number 7
MATERIALS LETTERS
to a surface finish of approximately 0.2 urn but no particular care was taken to protect the samples from air. Samples were treated by moving them under repetitive pulses of radiation. Because the output of most excimer lasers is highly non-uniform, a multielement beam homogenizer was used to create a square beam spot of 0.1 cm’ with a uniformity of approximately 95%. Pulse energy was varied from 130 to 250 mJ/pulse with a resultant fluence of 1.3 to 2.5 J cmF2. The sample was translated in front of the beam at rates from 0 to 3 mm/s at repetition rates of less than 1 Hz. Higher repetition rates could be used with consequent higher throughput. A calculation of the energy deposition indicates that melting of a minimum layer of thickness 1.0 urn occurs at a fluence of 0.7 J cm-2, so that the incident energy was more than sufficient to melt the surface. A cross sectional photograph of a sample treated with 8 shots at a fluence of 1.3 J cm-’ is shown in fig. 1. A melt depth of about 4 pm is visible. A prokilometer trace across the melted region shows a surface roughness of approximately 0.1 urn which is of the same order as observed on the polished samples. Particularly apparent in this photograph are grain boundaries crossing the melt interface. These reveal that the regrowth during excimer laser processing is largely epitaxial as might be expected since epitaxial regrowth is observed in CO2 laser processing [3]
April 1988
where the melt depths and durations are much longer. Transmission electron microscopy of foil samples showed no particular grain refinement of the surface regions, also consistent with this observed structure. The near surface chemistry of the samples was analyzed by Auger electron spectroscopy (AES) using a Physical Electronics model 545 scanning Auger microprobe and ion sputtering to determine the extent of constituent diffusion. Argon ions at 3 keV were used to sputter at a nominal rate of 5 nm/min (Al standard). Fig. 2 shows the ratio of the Cr to Fe AES signals in the near surface region for the untreated sample and those treated with 4 shots at a fluence of 2.5 J cm-’ and 8 shots at a fluence of 1.3 J cm-*. A substantial increase in the relative Cr concentration near the surface can be seen. Fig. 3 shows the oxygen signal in the unprocessed sample and in the sample processed with 8 X 1.3 J cmm2. The thickness of the oxide layer is essentially unchanged by the laser processing despite the fact that the processing was done in air. Comparison of figs. 2 and 3 shows that the bulk of the Cr redistributed occurs as an oxide on the surface. Deeper in the material (sputter times greater than 12 min), there is evidence of Cr depletion in the laser-treated samples. This depletion, which occurred in the melted zone near the surface, amounts to approximately 7% and 12% for the 4 shot and 8 shot cases respectively.
@ As received * 4 x 2.5 J/cm2 8 8x1.3Jlcm2
Depth (sputter time in minutes)
Fig. 1. Cross section of AIS1 304 stainless-steel sample treated with 8 pulses of 308 nm radiation at 1.3 J cm-‘, Grain boundaries crossing the melt interface are evidence of epitaxial regrowth.
226
Fig. 2. Sputtered Auger profile of ratio of Cr to Fe in AISI 304 stainless-steel surfaces in the as-received state and after treatment by 308 nm radiation at 2.5 J cm-’ (4 pulses) and 1.3 J cm-* (8 pulses).
MATERIALS LETTERS
Volume 6. number 7
0
....,.l..,....l...lI....
0
5
1*
15
20
25
Depth (sputter time in minutes)
Fig. 3. Sputtered Auger profile of oxygen in AISI 304 stainlesssteel surfaces in the as-received state and after treatment by 308 nm radiation at I .3 J cm- ’ (8 pulses).
In order for this substantial redistribution of Cr to occur, two conditions must obtain. There must be a strong driving force for the redistribution and there must be sufficient time available in the molten state for the redistribution to occur. Because Cr is more soluble in the melt than in the solid, there is a strong tendency for Cr to segregate in the liquid at the liquid/solid interface. This interfacial segregation provides one driving force. A second driving force is provided by the strong affinity of Cr for oxygen. The observation that redistributed Cr on the surface is an oxide reflects the strength of this driving force. Fig. 1 shows that the melt depth is much larger than the I.0 pm calculated as a minimum. Because the fl uences used were substantially higher than the minimum needed to melt, this is not surprising. It is therefore reasonable to believe that the melt duration is also much larger than the pulse length of 30 ns. In metals in the liquid state, diffusion rates of lo-’ cm’ s’ are typical. For an accumulated melt duration of a few hundred ns, diffusion distances of the order of 50 nm can be expected. The melted layer also experiences a period of time after solidi~cation at an elevated temperature. Cooling rates are such that the sample surface is below 300°C within 5 x 10-j s. Substitutional diffusion rates at elevated temperatures in fee metals are of the order of 3 x 10e9 cm’s_‘. In the available time, elements could there-
April 1988
fore diffuse approximately 10 nm after multiple pulses. The AES data are consistent with either scale of diffusion. The AES results demonstrate that laser processing creates substantial mobility of Cr in the melted layer. This mobility, coupled with the strong driving forces to accumulate Cr on the surface, result in a Cr rich surface oxide. A decrease in the Ni concentration in the oxide was also observed. Both of these observations are consistent with oxidation studies of AH1 304 stainless steel which demonstrate that chromia is the most stable oxide [ 41. Electrochemical studies have shown that altering the Cr oxide layer influences the breakdown potential [ 5 1. Potentiodynamic anodic polarization measurements were made in a commercial corrosion cell using an unstirred, non-deaerated 0.5 M NaCl solution at ambient room temperature, a saturated calomel reference electrode (SCE) and two vitreous carbon counter-electrodes. Potential scans, at a rate of 0.1 mV/s-‘, were initiated at the open circuit corrosion potential 5 min after placing the samples in the cell. Untreated areas of the specimens were masked with a stop-off lacquer. The polarization curves measured for an untreated and a laser-treated specimen, 5 pulses at 2.0 J cm-21 are shown in fig. 4. The open-circuit potential of the treated specimen measured after 5 min was - 0.128 V,,, compared to -0.2 18 VsCE for the untreated specimen. The passive current density meas-
Fig. 4. Anodic polarization curves for treated and untreated samples.
227
Volume 6, number
7
MATERIALS
ured at polarizations up to 50 mV was less than 10 nA crne2 for the treated sample and on the order of 100 nA cme2 for the untreated sample. At approximately - 0.08 Vs,,, the current density of the treated sample increased to about 100 nA cm-2. At approximately 0.0 VSCE, the current density increased to about lo4 nA cmm2 for both samples. A dramatic increase in the current density occurred at approximately $0.08 VsCE for the untreated sample, while a similar increase was delayed until about +0.20 Vs,, for the treated sample. These results show that the passive current density characteristic of the protective film on the specimens is lower for the treated compared to the untreated sample. The results also show that the ultimate breakdown potential at which the current density increased dramatically was approximately 100 mV more positive for the treated sample compared to the untreated sample. Excimer laser melting therefore produces a surface that offers improved passivity and better resistance to pitting attack compared to an untreated surface. These results are consistent with those observed by McCafferty and Moore [ 11. While we observe a Cr rich surface oxide in the processed materials, we cannot conclude that this oxide is the sole source of the improved resistance to pitting attack as we cannot rule out the possibility that inclusions have also been dispersed.
3. Summary We have demonstrated that excimer laser processing changes the near surface chemistry and corrosion properties of AISI 304 stainless steel in similar
228
LETTERS
April 1988
ways to CO2 laser processing. At repetition rates of a few tens of Hz, excimer lasers with average power output of 15 W can process material at rates of a few cm2 s-l. This represents a factor of lo3 improvement over reported results with CO, lasers. Further, a much smoother surface finish is obtained than with CO2 laser processing. Laser processing enriched the Cr content of the surface oxide. The improvement in the corrosion behavior of excimer processed surfaces is comparable to CO, processed surfaces in NaCl solutions.
Acknowledgement We would like to thank H.L. Nutter, T.G. Zocco, R. Cordi, and R.W. Springer for technical assistance and M. Nastasi and E.L. Joyce Jr. for helpful discussions. This work was supported by the United States Department of Energy under contract W-7405 ENG-36 through the Los Alamos Center for Mate-, rials Science and contract DE-AC06-76RLO- 1830 with Battelle Memorial Institute.
References [ I] E. McCafferty
and P.G. Moore, in: Laser surface treatment of metals, eds. C.W. Draper and P. Mazzoldi (Nijhoff, The Hague, 1987) p. 263: [2] T.R. Anthony and H.E. Cline, J. Appl. Phys. 49 (1978) 1248. [ 31 P.G. Moore, in: Fundamental aspects of corrosion protection by surface modification, Proceedings Vol. 84.3, eds. E. McCafferty, C.R. Clayton and J. Oudar (The Electrochemical Society, Pennington, 1984) p. 102. [4] D.R. Baer, Appl. Surface Sci. 7 (1981) 69. [ 51J. Kruger, in: Passivity and its breakdown in iron and iron based alloys, eds. R.W. Staehle and H. Okada (National Association of Corrosion Engineers, Houston, 1976) p. 9 1.