The effect of testing temperature on corrosion–erosion resistance of martensitic stainless steels

The effect of testing temperature on corrosion–erosion resistance of martensitic stainless steels

Wear 255 (2003) 139–145 The effect of testing temperature on corrosion–erosion resistance of martensitic stainless steels D.H. Mesa a , A. Toro b , A...

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Wear 255 (2003) 139–145

The effect of testing temperature on corrosion–erosion resistance of martensitic stainless steels D.H. Mesa a , A. Toro b , A. Sinatora c , A.P. Tschiptschin d,∗ b

a Mechanical Engineering Department, University of Ibagué, Ibagué, Colombia Tribology and Surfaces Group, School of Materials Engineering, National University of Colombia, Medell´ın, Colombia c Mechanical Engineering Department, University of São Paulo, São Paulo, Brazil d Metallurgical and Materials Engineering Department, University of São Paulo, São Paulo, Brazil

Abstract Conventional AISI 420 and high-nitrogen martensitic stainless steels were tested under corrosion–erosion conditions in slurry composed by substitute ocean water and quartz particles. The tests were performed at 0, 25, and 70 ◦ C, with mean impact angles of 20 and 90◦ . Polarization tests in H2 SO4 solution containing chloride ions were also carried out at the same temperatures. Both conventional and high-nitrogen specimens were tempered at 200 and 450 ◦ C before the tests. The high-nitrogen specimens were produced through gas nitriding of AISI 410S (13%Cr–0.03%C) and AISI 410 (13%Cr–0.15%C) stainless steels at 1100 ◦ C. These treatments allowed obtaining interstitial contents (nitrogen + carbon) at the surface of the specimens equivalent to the carbon content of conventional AISI 420 stainless steel. The best corrosion–erosion resistance was obtained in the nitrided AISI 410S samples tempered at 200 ◦ C and tested at 0 ◦ C under 20◦ -impact angle. Increasing testing temperature led to higher mass losses and wear rates due to the intensification of intergranular and pitting corrosion mechanisms, especially in the conventional AISI 420 stainless steels. In tests performed at 0 and 25 ◦ C, a reduction in the wear rate for longer testing times was observed, which was mainly associated to fragmentation and roughness changes of the abrasive particles. The mass losses under normal impact conditions were systematically higher than under oblique incidence, and some evidences of mass removal by brittle fracture were found after SEM examination of the worn surfaces. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Testing temperature; Corrosion–erosion resistance; Martensitic stainless steels

1. Introduction Martensitic stainless steels are commonly used for manufacturing components with high mechanical properties and moderate corrosion resistance, operating under conditions of either high or low temperature. As their properties can be changed by heat treatment, these steels are suitable for a wide range of applications such as steam generators, pressure vessels, cutting tools, and offshore platforms for oil extraction [1–3]. Although the surface properties of conventional martensitic stainless steels are acceptable for many purposes, relatively recent works [4,5] revealed serious limitations of these materials when tested in highly corrosive environments. In addition, commercial AISI 410 and AISI 420 stainless steels presented very high mass losses when tested in slurry composed by acid solution containing hard particles [6]. ∗

Corresponding author. Tel.: +55-11-30915656; fax: +55-11-30915243. E-mail address: [email protected] (A.P. Tschiptschin).

On the other hand, it has been shown [4,7–10] that nitrogen addition to conventional stainless steels can improve both mechanical and corrosion properties. In particular, lowcarbon high-nitrogen stainless steels quenched and tempered at temperatures between 200 and 450 ◦ C showed lower passive current density values and higher hardness than conventional martensitic stainless steels with the same chromium content. Regarding wear properties, the results of slurry erosion tests applied to both conventional and high-nitrogen martensitic stainless steels indicated that the measured mass losses were higher for normal impact conditions [6,11]. This behavior is opposed to that observed under dry erosion conditions, in which case the highest wear rates of most metals and alloys correspond to impact angles of 20–30◦ [12]. Some of the reasons for this divergence have been quoted in literature [13,14]: • Synergistic effects between corrosion and erosion, which lead to combined mechanisms such as spalling aided by intergranular corrosion or brittle fracture aided by oxidative wear;

0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00096-6

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• Boundary layer effects, which are responsible for energy absorption during impacts, specially for low impact angles. The testing temperature is becoming an important factor to be considered in wear and corrosion applications, since ductile-to-brittle transitions in mechanical behavior can affect the mechanisms of mass removal from the surface. At low temperatures, the wear mechanisms are associated to formation of micro-cracks at the surface, which grow leading to material loss in the form of flakes. When the testing temperature is increased, ductile mechanisms like cutting and plastic deformation are favored and the corrosion processes at the surface are intensified [13]. High-temperature nitriding of martensitic stainless steels is a way of introducing high-nitrogen contents in the steel’s surface without forming a compound layer or precipitating chromium nitrides, increasing the surface (wear and corrosion) properties at a relatively low cost. The aim of this work was to analyze the effect of testing temperature on electrochemical corrosion and slurry wear resistance of three martensitic stainless steels (nitrided and non-nitrided), from the point of view of the mechanisms of mass removal from the surface.

2. Experimental procedure 2.1. Materials Martensitic AISI 410 and AISI 420 and dual-phase ferritic-martensitic AISI 410S stainless steels were used in this investigation. The chemical composition of these materials is shown in Table 1. 2.2. High-temperature nitriding and heat treatments Table 2 shows the conditions used for high-temperature nitriding of AISI 410S and AISI 410 stainless steels. These Table 1 Chemical composition of the steels used in this work, measured by optical spectrometry (wt.%) Material

C

Cr

Mn

Si

Mo

P

AISI 410 AISI 410S AISI 420

0.15 0.02 0.35

12.1 12.0 12.3

0.31 0.56 0.44

0.41 0.74 0.42

0.10 0.04 0.07

0.03 0.02 0.02

Table 2 High-temperature nitriding conditions

Fig. 1. Tempering curves for the stainless steels studied in this investigation.

conditions were defined after thermodynamical simulation with Thermocalc® and previous experiments [14,15], with the purpose of obtaining surfaces with nitrogen or (nitrogen + carbon) contents similar to the carbon content of conventional AISI 420 martensitic stainless steel. The high-nitrogen materials obtained were named 410SN and 410N. The samples of conventional AISI 420 martensitic stainless steel were austenitized at 1050 ◦ C for 1 h and then oil quenched. All the conventional and high-nitrogen specimens were tempered at 200 and 450 ◦ C for 1 h. Fig. 1 shows the hardness as a function of tempering temperature for the materials studied in this work. 2.3. Slurry wear tests The slurry wear tests were performed at 0, 25, and 70 ◦ C by using an experimental setup described in a previous work [6]. The mean impact velocity was 3.5 m/s and the mean impact angle was 20◦ or 90◦ , depending on the position of the sample relative to the slurry flow. Substitute ocean water (ASTM standard D1141/90) containing 20% quartz particles with mean particle size between 0.3 and 0.5 mm composed the slurry, whose pH was controlled to 8.25 ± 0.05 in all the tests. The specific mass loss (Φ) of the samples was calculated as the quotient between the mass loss and the area exposed to the corrosive–erosive action. The specimens were electrically insulated from the holders and the contact with the slurry was restricted to an area of circa 25 mm2 , as shown in Fig. 2. 2.4. Electrochemical tests

Steel

Nitriding temperature (◦ C)

N2 pressure (MPa)

Time at the nitriding temperature (s)

Cooling

410SN 410N

1100 1100

0.35 0.05

18000 10800

Oil quenching Oil quenching

Polarization tests were carried out at 0, 25, and 70 ◦ C in a Princeton Applied Research potentiostat model 273. A saturated calomel electrode (SCE) was used as reference for the tests performed at 0 and 25 ◦ C, while an Ag/AgCl electrode was employed for the tests performed at 70 ◦ C. In

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Fig. 2. Detail of the specimen’s positioning system in the slurry wear testing device.

all cases, a platinum wire was used as counter-electrode. All tests started 300 mV below the corrosion potential, and the potential was changed at a rate of 1 mV/s. Previous to the tests, all the specimens were polished in abrasive paper up to grade ASTM 600, cleaned in ultrasonic device and dried with warm water. The polarization resistance (Rp ) and pitting potential (Ep ) parameters were computed from the polarization data of five curves for each specimen. Rp was calculated as the mean slope of the polarization curve in the region Ecorr ± 20 mV where Ecorr is the corrosion potential. Ep was determined as the potential in which the current density began to increase sharply in the passive region.

3. Results and discussion 3.1. Slurry wear tests 3.1.1. Effect of testing temperature Fig. 3 shows the time–variation curves for the specific mass loss of the studied materials as a function of the tempering and testing temperatures. Two main features can be seen in the curves of Fig. 3: • The corrosion–erosion resistance of the high-nitrogen stainless steels is higher than that of the conventional AISI 420 stainless steel for all the testing temperatures, which can be associated to the beneficial effect of nitrogen in solid solution in martensite [9,10]. • The specific mass loss increases monotonically with testing temperature for all the studied materials, as a consequence of the increasing significance of corrosion mechanisms at higher temperatures [13]. A reduced effect of corrosion mechanisms was observed in the tests performed at 0 ◦ C, which led to very low values of specific mass loss (Φ) after 96 h. Regarding the corrosion-wear rate (dΦ/dt), degradation of the hard particles and strain hardening at the surface are factors commonly associated to the slope changes in the curves of Fig. 3a. In this work, no hardening was detected at the surface of the specimens, but significant changes in morphology and roughness of the abrasive particles were observed after SEM examination, as can be seen in Fig. 4. The mean

Fig. 3. Variation of specific mass loss with testing time, as a function of tempering and testing temperatures. Conventional AISI 420 and high-nitrogen 410SN and 410N martensitic stainless steels. Testing temperature: (a) 0, (b) 25, and (c) 70 ◦ C.

size of the quartz particles varied strongly during the tests, given that after a testing period of 96 h, 12% of the particles had less than 0.1 mm in mean diameter (the mean particle size at the beginning of the tests was between 0.3 and 0.5 mm). Moreover, the surface roughness of individual particles changed drastically during the tests. Increasing testing temperature led to higher values of measured specific mass loss in all the studied steels. Specifically, in the tests performed at 70 ◦ C, the specific mass loss varied linearly with the testing time, which indicates a constant rate of material removal from the surface. Evidences from SEM examination of the worn surfaces showed that chemical mechanisms of mass removal from the surface, such as

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Fig. 4. Degradation of abrasive particles during the slurry wear tests.

Fig. 5. Evidences of pitting (a) and intergranular corrosion (b) in martensitic stainless steels tested in slurry composed by 0.5 M H2 SO4 + 3.5% NaCl.

intergranular and pitting corrosion were activated in the tests carried out at 25 and 70 ◦ C. Fig. 5 shows some examples of these evidences. Fig. 6 shows the specific mass loss (Φ) measured after a testing period of 96 h, as a function of testing temperature. The studied materials can be ranked in terms of increase of slurry wear resistance, as shown in Table 3.

3.1.2. Effect of impact angle Fig. 7 shows the time–variation curves for the specific mass loss (Φ) of the three studied steels tempered at 200 ◦ C, tested under oblique and normal impact conditions. Generally speaking, the measured mass losses were higher under normal impact conditions than under oblique incidence of the hard particles. This behavior has been frequently reported in literature from slurry wear tests of ductile materials [11,13,16,17] and several mechanisms of mass removal were proposed to explain the reduction in slurry wear resistance for normal incidence conditions.

Table 3 General results of specific mass loss (Φ) after a testing period of 96 h Testing temperature (◦ C)

Slurry wear resistance Material AISI 420

410N

410SN

Tempering temperature

Fig. 6. Variation of the specific mass loss (Φ) with testing temperature, measured after a testing period of 96 h.

0 25 70

450 ◦ C

200 ◦ C

450 ◦ C

200 ◦ C

450 ◦ C

200 ◦ C

15.3 34.9 45.8

13.2 36.3 49.0

9.8 12.1 27.0

12.6 12.9 24.3

7.8 8.9 20.0

2.6 6.4 10.5

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Fig. 7. Variation of specific mass loss (Φ) with testing time under oblique (20◦ ) and normal (90◦ ) impact conditions. Conventional AISI 420 and high-nitrogen 410SN and 410N martensitic stainless steels. Testing temperature: 25 ◦ C. Table 4 Pitting potential (Ep ) and polarization resistance (Rp ) calculated from electrochemical data Material

Testing temperature (◦ C) 0

AISI 420 410N 410SN

25

70

Ep (mV)

Rp Ep (Ohm cm2 ) (mV)

Rp Ep (Ohm cm2 ) (mV)

440 670 620

80.0 454.8 351.4

33.8 80.5 106.2

310 350 320


Rp (Ohm cm2 ) 4.1 5.3 4.8

3.2. Electrochemical tests Fig. 8 shows the polarization curves for AISI 420, 410N, and 410SN specimens tested at 0, 25, and 70 ◦ C, and some parameters calculated from these curves are presented in Table 4.1 From the results in Fig. 8 and Table 4, it can be said that: • The corrosion potential was not affected by variations in the testing temperature. • Increasing the testing temperature led to a reduction in the Ep in all the specimens. In the tests performed at 70 ◦ C, the Ep was below the corrosion potential. • No stable passive layer was formed at the surface of the specimens tested at 70 ◦ C. • The 410N and 410SN steels showed Rp values considerably higher than those of the AISI 420 samples at 0 and 25 ◦ C. For the tests performed at 70 ◦ C, on the other hand, the measured Rp was very low for all the specimens. This result reveals the harmful effect of testing temperature on general corrosion resistance. 1 The values of E corr were not included in Table 4 because they were very similar in all experiments. There was no dependence of this parameter with the tempering temperature or with the chemical composition of the specimen.

Fig. 8. Polarization curves obtained at 0, 25, and 70 ◦ C for (a) AISI 420, (b) 410N, and (c) 410SN samples. Solution: 0.5 M H2 SO4 + 3.5% NaCl.

In the tests performed at 25 ◦ C, the high-nitrogen steels showed higher values of Ep and lower values of critical and passive current densities than the conventional AISI 420, as can be seen in Fig. 9. This behavior was observed for the two tempering temperatures analyzed. The better corrosion resistance of the studied high-nitrogen stainless steels is a consequence of the beneficial effect of nitrogen on surface properties, whether this element is in solid solution in martensite or forming finely dispersed chromium nitrides. Although these nitrides can contain chromium amounts comparable to those of the carbides formed at the same tempering temperature, the size of the

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Fig. 9. Comparison between polarization curves for AISI 420, 410N, and 410SN steels tested at 25 ◦ C.

• Increasing the testing temperature led to a reduction in both slurry wear and electrochemical corrosion resistance of all the studied materials. In slurry wear tests performed at 0 ◦ C, virtually no corrosion was observed, while in tests carried out at 70 ◦ C pitting and intergranular corrosion were the most important mechanisms of mass removal from the surface of the steels. • The change in size and surface roughness of quartz particles during the tests affected significantly the slurry wear rate of all the steels at 0 ◦ C. This effect became less important when the testing temperature was increased, due to the stronger action of corrosion mechanisms, which maintained the slurry wear rate almost constant with time. • In the electrochemical tests performed at 70 ◦ C, all the studied materials showed poor generalized and pitting corrosion resistance, with pitting potentials below the corrosion potential. • The 410SN steel tempered at 200 ◦ C showed the best corrosion–erosion resistance from all the tested materials, while the AISI 420 steel tempered at 450 ◦ C presented the worst response in both slurry wear and electrochemical tests.

Acknowledgements

Fig. 10. Variation of polarization resistance (Rp ) with testing temperature. Specimens tempered at 200 ◦ C for 1 h.

A. Toro and D. Mesa thank to CAPES and CNPq for their graduate scholarships. A.P. Tschiptschin thanks to CNPq–PADCT Grant No. 62.0133/98-8 and FAPESP Grant No. 98/15758-4, for financial support. References

chromium-depleted zone in the high-nitrogen steels is much smaller [18,19]. Fig. 10 shows the variation of Rp as a function of the testing temperature. The Rp values measured in the conventional AISI 420 specimens are always lower than those observed in the 410SN and 410N steels. However, increasing the testing temperature led to a more accentuated reduction in Rp in the high-nitrogen steels than in the conventional AISI 420 specimens. Analysis of Figs. 8–10 indicates that the effect of increasing the testing temperature was most important on localized corrosion when the temperature was changed from 25 to 70 ◦ C, while the reduction on generalized corrosion was more accentuated from 0 to 25 ◦ C.

4. Conclusions • High-temperature nitrided AISI 410S and AISI 410 martensitic stainless steels showed better slurry wear resistance than conventional AISI 420 martensitic stainless steel, when tested in substitute ocean water containing quartz particles.

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