Corrosion erosion of nitrogen bearing martensitic stainless steels in seawater quartz slurry

Transcription

1 Wear 251 (2001) Corrosion erosion of nitrogen bearing martensitic stainless steels in seawater quartz slurry A. Toro a,, A. Sinatora b, D.K. Tanaka b, A.P. Tschiptschin a a Metallurgical and Materials Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, CEP São Paulo, Brazil b Mechanical Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, CEP São Paulo, Brazil Abstract AISI 410S stainless steel was nitrided at 1473 K in N 2 atmosphere, direct quenched and tempered at temperatures between 473 and 873 K. Martensitic cases with circa 0.52 wt.% N at the surface were obtained. Corrosion erosion tests were carried out in slurries composed by quartz particles and tap or substitute ocean water. The concentration of solids, the impact angle and the ph of solution were fixed, while the temperature, surface changes and mass losses were monitored during the tests. Quenched and tempered AISI 410 and 420 stainless steels were used as comparison materials. The results showed that the erosion resistance and the corrosion erosion resistance of the nitrided steel tempered at 473 K were higher than those of the AISI 410 and 420 steels tempered at the same temperature. This behavior was due to the higher hardness and better intergranular, pitting and generalized corrosion resistance of the nitrided alloy. The synergism between corrosion and wear was more important in the AISI 410 and 420 samples Elsevier Science B.V. All rights reserved. Keywords: Slurry erosion; Corrosion erosion resistance; Nitrogen in stainless steels 1. Introduction Martensitic stainless steels have been used in components operating under wear, corrosion and wear corrosion conditions found in distillation towers, slurry pumps and mixers of chemical products [1,2]. These materials show high mechanical properties and moderate corrosion resistance, but when under erosion action by the presence of hard particles in aqueous solutions their performance is reduced due to a synergistic effect between wear and corrosion mechanisms [3 5]. Much information concerning the relationship between surface properties and slurry erosion behavior is available in literature for a number of materials, including austenitic and martensitic stainless steels. These results have been acquired as a function of ph, solid content and temperature of solution [4,6 8], as well as hardness and morphology of abrasive particles [9,10]. Peterson et al. [11] and Zum Gahr [12] reported the erosion wear mechanisms in steels to the microstructure of the eroded surfaces, showing that the brittle or ductile nature of phases present can determine the wear behavior of the tribosystem. Wang Xu [7] classified a wide variety of alloys as a function of their mass Corresponding author. loss under erosion in silica water slurry, establishing a consistent relationship between hardness and wear resistance. On the other hand, Aiming et al. [3] reported the existence of a breakaway impingement velocity, which marks the transition from moderate to severe erosive wear conditions. Generally speaking, wear of ductile materials under slurry erosion can differ from that under dry erosion conditions [13]. Maximum wear rates of ductile metals submitted to solid particle impingement are commonly observed for impingement angles up to 30. Conversely, under slurry wear conditions, the maximum erosion rate can be reached at normal incidence, even for ductile materials [3,14]. Spalling of second phase particles and the existence of passive layers may explain this behavior in high-alloyed steels and, in particular, stainless steels [7,8]. Nitrogen can improve the surface properties of industrial steel components. Rogers et al. [15] showed that sub-critical nitriding of Mo steels increases the resistance to corrosion erosion of heat exchanger tubing for bubbling fluidized bed combustors, and Berns et al. [16] improved the slurry erosion resistance of stainless steels used in petrochemical applications by solution nitriding of stainless steels at high temperature (1423 K). In addition, a number of laboratory tests and theoretical models have been proposed to explain the beneficial effects of nitrogen in corrosion /01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

2 1258 A. Toro et al. / Wear 251 (2001) [17 19] and erosion resistance [20,21] of stainless steels in aqueous solutions. Some of the most important mechanisms used to explain the enhancement of corrosion resistance due to nitrogen alloying are: (a) formation of ammonia or nitrate ions near the surface, leading to corrosion inhibition through localized increase of surrounding environment ph and inhibition of pit growth; (b) retardation of nucleation and/or growth of Cr-rich carbides; (c) strengthening of passive film layer by nitrogen-induced segregation of Cr and Mo. On the other hand, solid solution strengthening and inhibition of second phase particles precipitation are pointed out as the main factors of erosion resistance improvement. Table 1 Chemical composition (% mass) of the studied steels analyzed by optical spectrometry Material %Cr %C %Ni %Mn %Si %N AISI 410S AISI AISI Objective The aim of this work was to study the erosion and corrosion erosion resistance of a low-carbon gas nitrided martensitic stainless steel tested in seawater quartz slurry. The results were compared to those obtained from commercial AISI 410 and 420 martensitic stainless steels. 3. Experimental 3.1. Materials Table 1 shows the chemical composition of materials used in this investigation. Ferrite and martensite stringers orientated in the rolling direction composed the initial 410S steel microstructure. Annealed AISI 410 and 420 steels showed disperse carbides in a matrix composed by equiaxial ferritic grains Gas nitriding of AISI 410S Gas nitriding of AISI 410S steel was performed in an experimental set-up described in a previous work [21]. The specimens were placed in a previously rinsed and evacuated tubular furnace at 1473 K under 0.25 MPa high purity nitrogen atmosphere 1 during 6 h. After nitriding they were oil-quenched and ultrasonically cleaned. The surface nitrogen content at the end of the nitriding treatment was 0.52%. As all surfaces were mechanically polished before and after the treatment to 0.03 ± 0.02 m R a, a layer of 0.2 mm in thickness was removed from the nitrided surface. Thus, the surface nitrogen content analyzed was 0.45% Heat treatments AISI 410 and 420 stainless steels were austenitized at 1273 K during 1 h under 0.15 MPa Ar atmosphere, oil-quenched and tempered during 1 h at 473, 673 and 873 K. The nitrided and oil-quenched AISI 410S was tempered at the same conditions. Some of the samples were 1 <1 ppm O 2 ; <0.5 ppm (CO + CO 2 ); <2 ppm H 2 O. Fig. 1. (a) Microstructure of the AISI 410S steel after gas nitriding (1473 K 0.25 MPa N 2 during 6 h) and oil-quenching; (b) nitrogen content and micro-hardness profiles; (c) variation of hardness as a function of tempering temperature for AISI 410, 420 and nitrided 410S steels.

3 A. Toro et al. / Wear 251 (2001) Fig. 2. (a) Schematic of slurry wear testing machine; (b) shape and dimensions of the test specimens; (c) morphology of quartz particles used in erosion and corrosion erosion testing (SEM). air-cooled after austenitizing, in order to produce specimens with a softer microstructure. A martensitic case of 1.1 mm in thickness followed by a duplex ferritic martensitic structure with 1 mm can be observed. On the other hand, the microstructure of the as-quenched AISI 410 and 420 steels was composed by martensite plus precipitated carbides. Fig. 1 shows the microstructure, nitrogen content and microhardness profiles of the gas nitrided AISI 410S, as well as the tempering curves for the AISI 410, 420 and nitrided 410S steels Corrosion erosion tests Corrosion erosion tests were performed with a test machine shown in Fig. 2a. The slurry was prepared with 5 kg of quartz addition to 20 kg of substitute ocean water 2 (20%). The slurry ph was controlled by NaOH addition to a value of 8.2 ± 0.1 in all experiments, and the quartz particle size 2 ASTM D standard. was between 0.3 and 0.5 mm with a morphology presented in Fig. 2c. The testing equipment comprises a stainless steel container fixed to a hydraulic jack. A high-density polypropylene disk driven by an electronically controlled electrical motor impels the slurry. The specimens were fixed to metallic holders electrically insulated. The set-up allowed the adjustment of impact angles of 45 or 90. An impeller rotation speed of 125 Hz (1200 rpm) was selected, based on Navier Stokes model for an approximate impingement velocity of 3.5 m/s [22]. Mass loss was periodically measured during 96 h of testing period. The wear results were normalized in terms of specific mass loss, Φ, as defined in Eq. (1): Φ = W S where W is the cumulative mass loss (g) and S the geometrical exposed surface (m 2 ). The temperature of the slurry was periodically measured during the experiments, and after each cycle of 96 h test, the container was emptied (1)

4 1260 A. Toro et al. / Wear 251 (2001) and refilled with fresh slurry. Non-corrosive erosion tests were carried out replacing substitute ocean water by tap water, while solids-free impingement tests were performed in quartz-free substitute ocean water. These two tests were performed only with specimens tempered at 473 K Analysis and evaluation methods The microstructure of all the specimens was analyzed by optical (OM) and scanning electron microscopy (SEM). Nitrogen and carbon contents were analyzed by fusion under inert gas analysis and glow discharge optical spectrometry/gd-0s. Mass losses were calculated by difference between the initial and final weight measured in a precision analytical scale with a sensitivity of 0.1 mg. SEM was used to examine the morphology of worn surfaces at the end of each cycle. The surface changes were also evaluated by using a Taylor Hobson Surtronic 3 + roughness tester. 4. Results and discussion 4.1. Changes in surface finishing Surface finishing parameters like R a and R q are commonly used to characterize surface deterioration. In this work, the surface evaluation before and after the tests showed that R a and R q were not able to describe properly the surface damage clearly observed through SEM examination (Fig. 3a and b). Fig. 3c shows the variation of the surface finishing parameter S m due to corrosion erosion. This parameter indicates the mean spacing between consecutive peaks of the roughness profile, and is strongly related to the surface degradation throughout the corrosion erosion tests, showing variations up to 1000% after 96 h test in some samples. The effectiveness of the S m parameter contrasts to the weakness of the average roughness parameters R a in describing the surface changes (Fig. 3d). These results suggest that the characterization of eroded surfaces by means of spacing parameters is Fig. 3. Variation of (a) S m and (b) R a surface finishing parameters during corrosion erosion tests, and typical aspect of surfaces submitted to corrosion erosion tests under (c) normal and (d) oblique incidence (SEM). Substitute ocean water (ASTM D standard) + 20% quartz ( mm particle size).

5 A. Toro et al. / Wear 251 (2001) Fig. 4. (a) Time variation of specific mass loss Φ in erosion tests for specimens tempered at 473 K. Tap water + 20% quartz ( mm particle size); (b), (c) and (d) time variation of specific mass loss Φ in corrosion erosion tests for specimens tempered at 473, 673 and 873 K respectively. Substitute ocean water (ASTM D standard) + 20% quartz ( mm particle size); (e) effect of synergism between corrosion and erosion in the specimens tempered at 473 K. Specific mass loss Φ measured after 96 h test.

6 1262 A. Toro et al. / Wear 251 (2001) effective, as a consequence of the increase of the number of wear marks on surfaces with testing time. Conversely, results obtained in this work revealed little evidence of significant increments in the depth of the wear marks with testing time Specific mass loss From the results shown in Fig. 4, one can see that the nitrided AISI 410S steel was more resistant to erosion and corrosion erosion than AISI 410 and 420 steels for almost all the studied tempering temperatures and impact angles. In addition, in the samples tempered at 473 K, synergism between corrosion and erosion mechanisms was more accentuated in the AISI 410 and 420 steels than in the nitrided AISI 410S steel. Some considerations concerning specific test conditions and mechanisms of mass removal are discussed as follows Non-corrosive erosion conditions and solids-free impingement Tests conducted in tap water with quartz particles did not reveal evidences of corrosion processes acting on specimens. Under these conditions, it was observed a clear relationship between the initial surface hardness and the specific mass loss Φ: harder surfaces showed lower mass losses, although this relationship was less evident in specimens submitted to impact angles of 45. Fig. 5 illustrates these results. The specific mass loss of the AISI 410S steel nitrided and tempered at 473 K was lower than that observed for commercial AISI 410 and 420 steels tempered at the same temperature. It could be noted also that the mass losses obtained for impact angles of 45 and 90 for the nitrided AISI 410S specimens were very close, while mass losses under normal incidence were substantially greater than under oblique incidence for the AISI 410 and 420 steels. The high mass loss observed in normalized AISI 410 and 420 specimens can be explained by the lower hardness of ferritic matrix. Solids-free impingement tests led to low mass Fig. 5. Relationship between surface hardness and specific mass loss Φ after non-corrosive erosion tests. Tap water + 20% quartz ( mm particle size). losses and little surface damage in comparison to erosive and corrosive erosive tests. The major differences between the nitrided and commercial steels were observed for normal impingement Effect of tempering temperature on corrosion erosion behavior During the corrosion erosion tests of nitrided AISI 410S steel under normal impingement, the higher mass losses were measured in samples tempered at 873 K, while the opposite was observed under oblique incidence. This behavior reveals a change in mechanism of mass removal that can be primarily associated with the microstructure: second phase particles (chromium nitrides and carbonitrides) are expected to be more abundant in samples tempered at 873 K in comparison to those tempered at 473 K. These phases can be more easily broken and removed from surface under normal impingement. Also, it is expected that kinetic energy of both particles and fluid should be intensively transferred to the alloy surface under normal angle impingement of hard particles. When the impingement angle is low, liquid boundary layer can behave as a damper and less energy is transferred to the surface. For the AISI 410 and 420 steels, under normal incidence conditions, corrosion erosion resistance increased with tempering temperature, while under 45 impact angle the mass loss showed a maximum at 673 K Mechanisms of mass removal and corrosion erosion synergism Scanning electron microscopy observations of worn surfaces showed that cutting of metallic matrix was the most important mechanism of material removal under noncorrosive erosion conditions. In accordance, the monotonic relationship between initial hardness and specific mass loss (Fig. 5) can be understood taking in account the absence of corrosion mechanisms. On the other hand, the high mass losses measured in corrosion erosion tests can be related to several combined mechanisms of mass removal, as follows. 1. Spalling of chromium carbides precipitated at prior austenite grain boundaries, that could be a consequence of exposure of these carbides due to localized corrosion of chromium depleted surrounding matrix. Previous works showed that precipitation of carbides and nitrides in martensitic stainless steels could also contribute to increase the pitting susceptibility [23,24]. Accordingly, under normal incidence of particles in seawater quartz slurry, pitting corrosion and spalling synergism can be a possible mass removal mechanism. 2. Intergranular corrosion followed by mechanical removal of the metallic matrix near to grain boundaries. Electrochemical results and the surface examination

7 A. Toro et al. / Wear 251 (2001) Fig. 6. Evidences of preferential attack at grain boundaries of quenched and 473 K tempered specimens: (a) AISI 410 and (b) AISI 420 (SEM). of specimens after corrosion erosion testing by SEM, shown in Fig. 6, suggest occurrence of intergranular attack of AISI 410 and 420, but no evidence of that mechanism was observed for the nitrided AISI 410S. 3. Corrosion of metallic matrix followed by mechanical removal of the corrosion products. This mechanism could be the most important for oblique incidence of particles. The corrosion resistance can be indirectly evaluated through the electrochemical polarization resistance, R p, which is inversely proportional to the corrosion rate of materials. Fig. 7 shows that R p is inversely proportional to specific mass loss Φ, when spalling is not the predominant wear mechanism. It is noticeable that AISI 420 specimens tempered at 873 K show greater resistance to generalized corrosion than those tempered at 673 K, which can be associated with a smaller chromium gradient in the matrix as a consequence of a high homogeniza- tion rate and the increase in the number of precipitated carbides in the microstructure. On the other hand, the nitrided AISI 410S does not show high specific mass losses in the entire range of tempering temperatures, indicating that no concentration gradients were developed. In summary, chemical dissolution and cutting of metallic matrix are the most important mechanisms of mass removal in conditions of oblique incidence in all the materials studied. In contrast, under normal impact conditions spalling of second phase particles and brittle fracture of martensite seem to be more significant. Intergranular corrosion highly enhanced the mass loss in AISI 410 and 420 steels under normal impact of quartz particles due to the synergism between corrosion and erosion. On the other hand, nitrided AISI 410S specimens, not being susceptible to intergranular corrosion, showed lower mass losses. 5. Conclusions Fig. 7. Relationship between polarization resistance R p and specific mass loss Φ for AISI 420 and nitrided AISI 410S steels tempered at 473 K, under oblique incidence of quartz particles. Substitute ocean water (ASTM D standard) + 20% quartz ( mm particle size). 1. Nitrided, direct quenched and 473 K tempered AISI 410S steel was more resistant to corrosion erosion than AISI 410 and 420 quenched and tempered at the same temperature, in slurry composed of quartz particles in substitute ocean water. 2. The main mechanisms of mass removal identified in nitrided AISI 410S specimens were cutting, spalling of second phase particles and dissolution of metallic matrix, as well as synergistic effects between them. The relative importance of these mechanisms varied as a function of the impact angle of quartz particles. 3. The main mechanisms of mass removal identified in AISI 410 and 420 specimens were cutting, spalling of second phase particles, dissolution of metallic matrix and intergranular corrosion. The observed synergism between these mechanisms was much more accentuated in these steels than in nitrided specimens.

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