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Wear 255 2003 139 145 The effect of testing temperature on corrosion erosion resistance of martensitic stainless steels D H Mesaa A Torob A Sinatorac A P Tschiptschind a Mechanical Engineering Department University of Ibagu Ibagu Colombia b Tribology and Surfaces Group School of Materials Engineering National University of Colombia Medell n Colombia c Mechanical Engineering Department University of S o Paulo S o Paulo Brazil d Metallurgical and Materials Engineering Department University of S o Paulo S o Paulo Brazil Abstract ConventionalAISI420andhigh nitrogenmartensiticstainlesssteelsweretestedundercorrosion erosionconditionsinslurrycomposed 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 H2SO4solution containing chloride ions were also carried out at the same temperatures Both conventional and high nitrogenspecimensweretemperedat200and450 Cbeforethetests Thehigh nitrogenspecimenswereproducedthroughgasnitriding ofAISI410S 13 Cr 0 03 C andAISI410 13 Cr 0 15 C stainlesssteelsat1100 C Thesetreatmentsallowedobtaininginterstitial 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 intensifi cation 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 manu facturing 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 pres sure vessels cutting tools and offshore platforms for oil extraction 1 3 Although the surface properties of conventional marten sitic stainless steels are acceptable for many purposes rela tively 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 com posed by acid solution containing hard particles 6 Corresponding author Tel 55 11 30915656 fax 55 11 30915243 E mail address antschip usp br A P Tschiptschin On the other hand it has been shown 4 7 10 that ni trogen addition to conventional stainless steels can improve both mechanical and corrosion properties In particular low carbon high nitrogen stainless steels quenched and tempered at temperatures between 200 and 450 C showed lower pas sive current density values and higher hardness than conven tional martensitic stainless steels with the same chromium content Regarding wear properties the results of slurry erosion tests applied to both conventional and high nitrogen marten sitic stainless steels indicated that the measured mass losses werehigherfornormalimpactconditions 6 11 Thisbehav ior is opposed to that observed under dry erosion conditions in which case the highest wear rates of most metals and al loys 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 oxida tive wear 0043 1648 03 see front matter 2003 Elsevier Science B V All rights reserved doi 10 1016 S0043 1648 03 00096 6 140D H Mesa et al Wear 255 2003 139 145 Boundary layer effects which are responsible for energy absorption during impacts specially for low impact an gles 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 af fect 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 lead ing to material loss in the form of fl akes When the testing temperature is increased ductile mechanisms like cutting and plastic deformation are favored and the corrosion pro cesses at the surface are intensifi ed 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 corro sion 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 MaterialCCrMnSiMoP AISI 4100 1512 10 310 410 100 03 AISI 410S0 0212 00 560 740 040 02 AISI 4200 3512 30 440 420 070 02 Table 2 High temperature nitriding conditions SteelNitriding temperature C N2pressure MPa Time at the nitriding temperature s Cooling 410SN11000 3518000Oil quenching 410N11000 0510800Oil quenching Fig 1 Tempering curves for the stainless steels studied in this investiga tion conditions were defi ned after thermodynamical simula tion 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 stain less steel were austenitized at 1050 C for 1h and then oil quenched All the conventional and high nitrogen specimens were tempered at 200 and 450 C for 1h Fig 1 shows the hardness as a function of tempering temperature for the ma terials 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 5m s and the mean impact angle was 20 or 90 depending on the position of the sample relative to the slurry fl ow Substitute ocean water ASTM standard D1141 90 containing 20 quartz particles with mean particle size between 0 3 and 0 5mm composed the slurry whose pH was controlled to 8 25 0 05 in all the tests The specifi c mass loss of the samples was calculated as the quotient between the mass loss and the area exposed to the corrosive erosive action The specimens were elec trically insulated from the holders and the contact with the slurry was restricted to an area of circa 25mm2 as shown in Fig 2 2 4 Electrochemical tests 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 D H Mesa et al Wear 255 2003 139 145141 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 300mV below the corrosion potential and the potential was changed at a rate of 1mV 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 fi ve curves for each specimen Rpwas calculated as the mean slope of the polarization curve in the region Ecorr 20mV where Ecorris the corrosion potential Epwas 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 specifi c mass loss of the studied materials as a function of the tem pering 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 benefi cial effect of nitro gen in solid solution in martensite 9 10 The specifi c mass loss increases monotonically with testing temperature for all the studied materials as a consequence of the increasing signifi cance 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 val ues of specifi c mass loss after 96h Regarding the corrosion wear rate d dt degradation of the hard par ticles and strain hardening at the surface are factors com monly associated to the slope changes in the curves of Fig 3a In this work no hardening was detected at the sur face of the specimens but signifi cant changes in morphol ogy and roughness of the abrasive particles were observed after SEM examination as can be seen in Fig 4 The mean Fig 3 Variation of specifi c mass loss with testing time as a func tion of tempering and testing temperatures Conventional AISI 420 and high nitrogen 410SN and 410N martensitic stainless steels Testing tem perature 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 96h 12 of the par ticles had less than 0 1mm in mean diameter the mean particle size at the beginning of the tests was between 0 3 and 0 5mm Moreover the surface roughness of individual particles changed drastically during the tests Increasingtestingtemperatureledtohighervaluesofmea sured specifi c mass loss in all the studied steels Specifi cally in the tests performed at 70 C the specifi c mass loss var ied 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 chemi cal mechanisms of mass removal from the surface such as 142D H Mesa et al Wear 255 2003 139 145 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 5M H2SO4 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 specifi c mass loss measured after a testing period of 96h 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 Fig 6 Variation of the specifi c mass loss with testing temperature measured after a testing period of 96h 3 1 2 Effect of impact angle Fig 7 shows the time variation curves for the specifi c mass loss 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 specifi c mass loss after a testing period of 96h Testing temperature C Slurry wear resistance Material AISI 420410N410SN Tempering temperature 450 C200 C450 C200 C450 C200 C 015 313 29 812 67 82 6 2534 936 312 112 98 96 4 7045 849 027 024 320 010 5 D H Mesa et al Wear 255 2003 139 145143 Fig 7 Variation of specifi c mass loss with testing time under oblique 20 and normal 90 impact conditions Conventional AISI 420 and high nitrogen 410SN and 410N martensitic stainless steels Testing tem perature 25 C Table 4 Pitting potential Ep and polarization resistance Rp calculated from electrochemical data MaterialTesting temperature C 02570 Ep mV Rp Ohmcm2 Ep mV Rp Ohmcm2 Ep mV Rp Ohmcm2 AISI 42044080 031033 8 Ec4 1 410N670454 835080 5 Ec5 3 410SN620351 4320106 2 Ec4 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 1From 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 Epin all the specimens In the tests performed at 70 C the Epwas 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 Rpvalues consider ably 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 Rpwas very low for all the specimens This result reveals the harmful effect of testing temperature on general corrosion resistance 1 The values of Ecorrwere 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 5M H2SO4 3 5 NaCl In the tests performed at 25 C the high nitrogen steels showed higher values of Epand 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 Thebettercorrosionresistanceofthestudiedhigh nitrogen stainless steels is a consequence of the benefi cial effect of nitrogen on surface properties whether this element is in solid solution in martensite or forming fi nely 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 144D H Mesa et al Wear 255 2003 139 145 Fig 9 Comparison between polarization curves for AISI 420 410N and 410SN steels tested at 25 C Fig 10 Variation of polarization resistance Rp with testing temperature Specimens tempered at 200 C for 1h chromium depleted zone in the high nitrogen steels is much smaller 18 19 Fig 10 shows the variation of Rpas a function of the testing temperature The Rpvalues measured in the conven tional AISI 420 specimens are always lower than those ob served in the 410SN and 410N steels However increasing the testing temperature led to a more accentuated reduction in Rpin the high nitrogen steels than in the conventional AISI 420 specimens Analysis of Figs 8 10 indicates that the effect of increas ing the testing temperature was most important on local ized 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 re sistance than conventional AISI 420 martensitic stainless steel when tested in substitute ocean water containing quartz particles 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 parti cles during the tests affected signifi cantly the slurry wear rate of all the steels at 0 C This effect became less im portant 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 studiedmaterialsshowedpoorgeneralizedandpittingcor rosion resistance with pitting potentials below the corro sion 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 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 fi nancial support References 1 R M Davison ASM Handbook Vol 13 Corrosion 1992 p 547 2 H Berns J Lueg Corrosion behavior and mechanical properties of martensitic stainless steels containing nitrogen in J Foct A Hendry Eds Proceedings of the First International Conference on High Nitrogen Steels