DEVELOPMENT OF POST-OXIDATION TREATMENTS TO IMPROVE WEAR AND CORROSION RESISTANCE

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1 Memorie >> Trattamenti termici DEVELOPMENT OF POST-OXIDATION TREATMENTS TO IMPROVE WEAR AND CORROSION RESISTANCE R. Sola, R. Giovanardi, P. Veronei, G. Poli, S. Maini, A. Zanotti Ga nitrocarburizing and pot-oxidation treatment were performed on teel and cat iron, to improve wear and corroion reitance. In both material the compound layer after nitrocarburizing, mainly compoed by ε-fe 2-3 (N,C) and γ - Fe 4 N phae. Firt the oxidizing tep, a ubequent impregnation with two different lubricating oil wa performed in order to further enhance corroion reitance. Wear reitance wa meaured uing a ball-on-dik tribometer with Al 2 and 100Cr6 counterpart and the corroion reitance of the ample wa tudied uing alt pray tet in accordance with ASTM B117 and electrochemical cell. Experimental reult how that the nitrocarburizing treatment improve ignificantly the wear reitance of the teel but not the wear reitance of the tudied cat iron. The application of nitrocarburizing alone doe not ignificantly improve corroion reitance and it may even promote localized corroion. The ubequent potoxidation tep lead to a light decreae of the corroion rate, becaue it partially eal the porou nitrocarburized layer. The final oil impregnation tep reulted much more effective in further decreaing the corroion rate by one order of magnitude. KEYWORDS: nitrocarburizing, pot-oxidation, oil-impregnation, wear, corroion INTRODUCTION Nitrocarburizing treatment ha been ued for many year in order to improve the wear and fatigue reitance of teel. It i a thermo-chemical treatment involving nitrogen and carbon diffuion, which lead to urface hardening of the part treated, due to the preence of compreive internal tree within the nitrogen/carbon enriched layer. In the nitrocarburized layer it i poible to identify two different zone. Superficially the nitrocarburizing treatment form the white compound layer; it i uually very thin (5-30 μm) and it i mainly contituted by the nitride and carbide of alloying element. Under the compound layer there i the diffuion zone, in which nitrogen and carbon are dipered a compound but epecially in a olid olution with the crytal lattice. The compound layer i uually porou and thi tend to decreae the corroion reitance [1]. A poible olution to improve corroion reitance conit in the pot-oxidation treatment. Thi econd treatment caue the ubequent partial decompoition of the Fe-carbonitride in oxide (mainly Fe 3 ) and the next contipation of the uperficial micro-poroity of the compound layer [2]. The pot-oxidation leave a little uperficial poroity able to R.Sola, R. Giovanardi, P. Veronei, G. Poli Univerità degli Studi di Modena e Reggio Emilia, Modena, Italy S. Maini, A. Zanotti Proterm, Calderara di Reno (BO), Italy hold a lubricant oil, which can be ued to increae the corroion and wear reitance. Aim of thi work i the characterization and optimiation of three pot-nitrocarbuzing treatment on ferrou alloy, in order to improve corroion reitance and to reduce wear and friction. In particular, pot-oxidation and pot-oxidation followed by oil impregnation (2 different oil) have been applied to teel and to a pheroidal cat iron (), after ferritic nitrocarburizing. MATERIALS AND EXPERIMENTAL PROCEDURE A ubtrate material, teel and pheroidal cat iron were ued in the experimental tudie. To verify the chemical compoition of the teel a imultaneou quantometer ARL 3580 and a glow dicharge optical emiion pectrocopy (LECO GDS-750) were ued and the reulting average chemical compoition are given in the Tab. 1. The ample for gaeou nitrocarburizing were dik 8 mm thick cut from a bright rod (40 mm in diameter) teel tock. A central hole wa drilled in the dik to facilitate upenion in the nitrocarburizing furnace. The urface of the dik wa polihed on ilicon carbide paper, with a final grid on 2000 grade paper. The dik were cleaned and degreaed prior to nitrocarburizing. The treatment conidered in thi reearch are: 1) Gaeou nitrocarburizing (ample indicated by NC ) 2) Gaeou nitrocarburizing + pot-oxidation (ample indicated by NC+O ) 3) Gaeou nitrocarburizing + pot-oxidation + impregnation in la metallurgia italiana >> ettembre

2 Trattamenti termici << Memorie C Si S P Mn Ni Cr Mo V Al Fe ret ret Mg Cu Tab. 1 Chemical compoition of and. Compoizione chimica del e della. RUST VETO TB oil (ample indicated by oil TB ) 4) Gaeou nitrocarburizing + pot-oxidation + impregnation in RUST VET77 oil (ample indicated by oil 377 ) All ample were treated in a furnace at 560 C for 8 h, and ubequently ome of them were pot-oxidied at 530 C for 3 h. Some of pot-oxidied ample were impregnated in the lubricant oil by immerion. To characterize the nitrocarburized and pot-oxidied layer, all the ample were initially analyzed with optical and canning electron microcopy, and their microhardne profile (Vicker, 0.5 kg) and X ray-diffraction (CoKα radiation). The corroion behaviour wa determined uing alt-pray (fog) tet according to ASTM B117 and ISO9227 tandard for metal and alloy, and monitoring the open circuit potential in an HCl olution (0.1M). The wear teting wa done on a ball-on-dik CSM High Temperature Tribometer without lubrication under 5 N load and at liding velocity of 0.2 m/ for a ditance of 1000 m (ending condition). During tet, the ample rotate againt tationary Al 2 and 100Cr6 ball of 6 mm in diameter. The wear propertie were evaluated by meauring wear track profile uing a CSM Concan optical profilometer and the wear track morphology i alo oberved with tereo-microcope and canning-electron microcope. RESULTS AND DISCUSSION Fig. 1 A. nitrocarburized. B. nitrocarburized and pot-oxidied. A. nitrocarburato. B. nitrocarburato e pot-oidato. Microtructure, microhardne and diffraction Microcopy analyi how that after nitrocarburizing treatment the ample maintain the ame bulk microtructure, a expected according to the low temperature treatment. In the it i evident that the nitrocarburizing proce produce a uperficially two-phae compound layer (18-20μm thickne) which XRD indicated compoed of the ε-fe2-3(c,n) and γ -Fe4N (vedi Fig. 1). Below the compound layer a diffuion layer can be een ( μm). In the the pot-oxidation treatment allowed the formation of an oxidized layer about 2 μm thick, over the compound zone, and caue a dimenional increae the compound layer and diffuion layer, becaue during thi treatment C and N atom can further diffue inide the material. In the the compound layer reulted more irregular than in the, and the diffuion layer i thicker than, probably becaue the preence of graphite inhibit the C and N diffuion inide the material. Alo in thi cae the pot-oxidation treatment increae the compound layer and diffuion layer thickne (vedi Fig. 2 e Fig. 3). Thee reult are confirmed by microhardne tet. The hardening depth i higher in the teel than in cat iron, probably becaue the graphite doen t allow to C and N atom to diffue deeply, in addiction the teel reach higher hardne. In table Tab. 2 the HV0.5 value meaured on the urface of the ample, the total hardening depth (ditance from external urface of hardened layer to the point in which the hardne value i equal to the hardne value of material before the treatment) and effective hardening depth (ditance from external urface of the hardened layer to the point in which the hardne i equal the conventional value of 100 HV more than material hardne before the treatment) are reported. The nitrocarburized ample preent a porou compound layer, but after pot-oxidation treatment the poroity i partially contipated by oxide. Thi i clearly hown in the SEM microphotograph reported below (Fig.4). From X-ray diffractogram of nitrocarburized ample it i evident that the uperficial layer of both material i mainly compoed of Fe 2,3 N and γ -Fe 4 (N,C) carbonitride. The uperficial layer of nitrocarburized + pot-oxidied ample i mainly compoed of magnetite (Fe 3 ), promoted by the NOx of the oxidiz Fig. 2 A. nitrocarburized. B. nitrocarburized and pot-oxidation. A. nitrocarburata. B. nitrocarburata e pot-oidata. Fig. 3 The uperficial oxide layer in the (A) and (B) nitrocarburized and pot-oxidied. lo trato di oido uperficiale in (A) e (B) nitrocarburati e pot-oidati. 12 ettembre 2008 << la metallurgia italiana

3 Memorie >> Trattamenti termici material treatment nitrocarbuurized nitrocarburized + pot-oxidied nitrocarburized nitrocarburized + pot-oxidied HV effective hardening depth [mm] total hardening depth [mm] Tab. 2 HV 0.5 urface microhardne meaured on the urface of the ample, and relative effective and total hardening depth. Microdurezza HV 0.5 miurata ulla uperficie dei campioni e relative profondità di indurimento totale ed efficace. Fig. 4 SEM photomicrograph. A. nitrocarburized. B. pot-oxidied. Micrografie SEM. A. Campione nitrocarburato. B. campione nitrocarburato e pot-oidato. treatment NC NC+O oiltb oil377 ample volume tatic partner (pin) 100Cr6 tatic partner volume ing atmophere and ematite (Fe 2 ) (vedi Fig. 5). The reult obtained could be explained conidering the mechanim of carbonitride formation and the phenomena that happened during the ubequent pot-oxidation proce. The firt compound formed during the nitrocarburizing treatment are γ carbonitride, then increaing the nitrogen concentration, ε carbonitride appear. When nitrogen exceed the aturation threhold of ε carbonitride, nitrogen recombination in molecular form (N 2 ) happen and thi caue the micro-hole formation in the compound layer. Therefore, γ carbonitride, due to their lower nitrogen content repect to ε carbonitride, are uually formed near the interface with the material. The oxidation of nitrocarburized layer i characterized by the ε carbonitride decompoition and the ucceive formation of Fe oxide (Fe 3 ) that contipate the uperficial micro-poroity [3,4]. In the oxidizing atmophere the firt oxide formed i Fe 3, but, with oxygen concentration increaing, Fe 2 i formed a well. Tribological analyi The liding tet allow to acquire quantitative information regarding ample and tatic partner wear reitance, by meaurement of ample and tatic partner volume removed. The ample and tatic partner volume removed are reported in the Tab. 3 and 4. In the teel all the treatment induce an exceptional reduction of the ample volume removed and thu a great increae of wear reitance. Among the treated ample, there aren t ignificant difference: all them how a great improve of the wear reitance. The TB oil impregnation lead to the lowet volume removed, and hence the bet wear reitance. Thi behaviour i almot unchange in cae of Al 2 pin, but, a expected, the volume removed increae wiht repect to 100Cr6 pin. Intead, the wear behaviour i different ignificantly from. In fact, the ample exhibit the bet wear reitance and the treatment increae the volume removed. It important underlying that, in any cae, the volume removed value are very low. In particular it very evident the difference between the ample removed volume of material in favour of cat iron which i more than two order of magnitude lower, probably due to the preence of graphite that act a olid lubricant [5]. However in the cat iron the treatment inhibit the ample volume tatic partner volume Tab. 3 and ample and tatic partner volume removed, load 5N, pin 100Cr6. Volume aportato per il e per la, carico 5N, pin 100Cr6. la metallurgia italiana >> ettembre

4 Trattamenti termici << Memorie treatment NC NC+O oiltb oil377 tatic partner (pin) Al 2 ample volume tatic partner volume ample volume tatic partner volume Tab. 4 and ample and tatic partner volume removed, load 5N, pin Al 2. volume aportato per il e per la, carico 5N, pin Al 2. effect of the grafite on the urface, hence they decreae the material wear reitance and thi phenomenon it emphaized in cae of Al 2. In general, the and ample ubjected to the uperficial treatment preent almot the ame ample removed volume, in pite of the ample removed volume of material i very different. Thi i due to the treatment that form uperficially the ame compound (alo confirmed by X-ray diffraction), and the wear reitance i determined epecially by the uperficial propertie and marginally by the ubtrate. Regarding the volume removed for tet performed with 100Cr6 pin, the ample induce a neglectible wear to the pin, epecially the (probably due to the preence of graphite). The treatment increae the tatic partner volume removed, in fact they form uperficially very hard compound (carbonitride and oxide) that contribute to the pin wear. The lubricant effect of TB oil impregnation i deciive, in fact in thi cae the pin wear i very low. Very different i the 377 oil behaviour, becaue it doen t change ignificantly the pin volume removed with repect to nitrocarburized and nitrocarburized + pot-oxidied ample, due to the phyical and chemical difference between 377 and TB oil. In cae of Al 2 pin there i a lower ample removed volume with and nitrocarburized and with impregnated into oil TB. In thi cae the lubricant effect of TB oil i greater than 377 oil, epecially whem i ued a ubtrate. In fact, the wear the Al 2 pin more than. Moreover to that it important underlying that with Al 2 pin there aren t relevant difference between tatic partner volume removed with material and tatic partner volume removed with treated ample, becaue the Al 2 pin i hard and it i carcely worn by the treated and ample inditinctly. CORROSION TESTS Fig. 5 X-ray diffractogram for the nitrocarburizing and nitrocarburizing+pot-oxidized ample. Diffrattogrammi relativi a campioni nitrocarburati e nitrocarburati e pot-oidati. The alt-pray (fog) tet, according to ASTM B117 and ISO 9227, how that all the material, with the ame treatment, exhibit almot the ame behaviour. After 24 h the TQ ample are already covered by corroion product, while ome pit are preent on NC and NC+O urface; on the impregnated ample there are any trace of corroion pit or product. Therefore, a little increment in the corroion reitance i given by nitrocarburizing and the pot-oxidation lightly improve the corroion reitance with repect to NC. Thee treatment improve the corroion reitance becaue they promote the uperficial formation of nitrocarbide and oxide that, in general, are le reactive that the material. A deciive corroion reitance improvement i given by lubricant oil impregnation and comparing 377 and TB oil, alo in thi cae, the bet behaviour i oberved in TB oil, in fact after 72 h only the 377 ample preent ome corroion pit and after 120 h the 377 ample how macrocopic area with corroion product, while the TB ample preent only ome corroion pit. Thi 14 ettembre 2008 << la metallurgia italiana

5 Memorie >> Trattamenti termici pore [7,8]. The deciive improvement meaured in cae of oil impregnation i acribed in cae of oil coating acting a an electrical barrier, blocking or lowing electronic tranfer at the treated material/olution interface [9]. CONCLUSIONS Fig. 6 Corroded ample after alt pray tet (fog). Campioni di dopo il tet di corroione in nebbia alina. fact probably occur becaue the 377 oil diffue ooner into olution, o it give a minor protection to the ample. In the Fig. 6 are howed the image acquired on ample at the beginning of the tet, after 48 h and after 120 h. The alt-pray tet reult are partially confirmed by the value of open circuit potential meaured in an electrochemical cell (FLAT CELL K0235, PAR), uing a working electrode the ample teted, the counterelectrode i a platinum grid, and a ilver/ ilver chloride or Ag/AgCl (atured) a the reference electrode. The electrolytic olution i HCl 0.1M. The potential (E) reported in the Tab. 5 are the potential value meaured at the beginning of the tet, a oon a the ample i in contact with the acid olution, and the potential value after 50 minute of degaing with a nitrogen flux at high partial preure. Uing the intrument convention, the nitrocarburizing, in both material, doen t improve the corroion reitance with repect to ample, unlike what emerged from the alt pray tet. Thi i probably due to the different media ued to carry out the two experiment. Intead the pot-oxidation and the lubricant oil impregnation determine a deciive corroion reitance improvement. During the 50 minute of degaing, the corroion protection given by lubricant oil remain, on the contrary the protection given by pot-oxidation treatment vanihe during the degaing tage. In general, the preent a better corroion reitance. In fact, nitrocarburizing, uually, doen t improve much the corroion reitance becaue the inner part of white compound layer i porou, and localized corroion phenomena are favourite [6]. Pot-oxidation treatment, in fact, i performed to improve corroion reitance, becaue thi treatment promote the formation of a very thin magnetite (Fe 3 ) layer (1-2 μm) above the nitrocarburized layer, and caue a partial contipation of pore, preent in the nitrocarburized layer, becaue thi oxide grow inide the New pot nitrocarburizing treatment, in particular pot-oxidation and impregnation in two different lubricant oil were performed on and, in order to improve wear and corroion reitance. It can be concluded: 1. The ga nitrocarburizing form a uperficially compound zone (15-20 μm thickne) laying on a diffuion zone that i about μm thick in the cat iron and μm thick in the teel. In the teel the nitrocarburized layer are homogenou and have contant thickne in whole the ample, but thi doen t occur in the cat iron. The pot-oxidation increae the diffuion layer thickne and the Fe 3 oxide formed fill partially the poroity of the nitrocarburized urface. 2. In the teel, all the treatment improve deciively the wear reitance, pecially the lubricant oil impregnation; intead between the nitrocarburized and the nitrocarburized and pot-oxidied ample there aren t relevant difference in term of ample volume removed. Between lubricant oil, the bet wear reitance i achieved uing TB oil. 3. The application of nitrocarburizing alone doe not ignificantly improve corroion reitance and it may even promote localized corroion. The ubequent pot-oxidation tep lead to a light decreae of the corroion rate, becaue it partially eal the porou nitrocarburized layer. The final oil impregnation tep reulted much more effective in further decreaing the corroion rate by and thi final treatment in ome cae improve alo the wear reitance. 4. In general the and treated preent a better wear reitance and a wore corroion reitance than. Regarding wear reitance all the treatment woren the ample volume removed, intead, the treatment greatly improve the corroion reitance, pecially the final lubricant oil impregnation. REFERENCES [1] A. Celik, S. Karadeniz (1996), Surface and Coating Technology, Vol. 80, p [2] S. Hoppe (1998), Surface and Coating Technology, Vol. 98, pp [3] M. Zlatanovic, N. Popovic, Z. Bogdanov (2004), Surface and Coating Technology, Vol , pp material treatment E (t = 0) TQ NC NC+O oil TB oil E (t = 50 min) material treatment E (t = 0) TQ NC NC+O oil TB oil E (t = 50 min) Tab. 5 Open circuit potential. With the notation intrument, the more the potential i poitive the more the material in noble. Potenziali a circuito aperto. Con la convenzione dello trumento, più il potenziale è poitivo e più il materiale è nobile. la metallurgia italiana >> ettembre

6 Trattamenti termici << Memorie [4] Doche M.L., Meynie V., Mazille H., Deramaix C., Jacquot P., Surface and Coating Technology, Vol. 154 (2002), pp [5] Zlatanovic M., Popovic N., Bogdanov Z., Surface and Coating Technology, (2004), Vol , pp [6] S.Hoppe, Surface and Coating Tenchnology, (1998) Vol.98, pp [7] Borgioli F., Galvanetto E., Foati A., Surface and Coating Tenchnology, Vol. 162 (2002) pp [8] M. Pellizzari, A. Molianari, and G. Straffelini: Wear, 2005,. vol. 259, pp [9] Doche M.L., Meynie V., Mazille H., Deramaix C., Jacquot P., Surface and Coating Technology, Vol. 154 (2002), pp [10] I. Lee, Surface and Coating Tecnology, (2004) [11] B. Jeong, M. Kim, Surface and Coating Tecnology, 141 (2001) [12] Zlatanovic M., Popovic N., Bogdanov Z., Surface and Coating Technology, (2003) [13] C. X. Li, T. Bell, Corroion Science, 48 (2006) [14] L. C. Gontijo, R. Machado, S. E. Kuri, L. C. Cataletti, P.A.P. Nacente, Thin Solid Film, 515 (2006) [15] C. Muratore, D. Leonhard, S.G. Walton, D.D. Blackwell, R.F. Fernler, R.A. Meger, Surface and Coating Tecnology, 191 (2005) [16]A. Alaran, H. Altun, M. Karakan, A. Celic, Surface and Coating Tecnology, 176 (2004) [17] E. Jeon, I. Park, I. Lee, Material Science and Engineering A, (2007) [18]S. Barella, M. Boniardi, F. D Errico, A. Sironi, Nitrurazione e pot-oidazione u acciai peciali da cotruzione: corroione, uura e fatica, La Metallurgia Italiana, aprile 2007 [19] G. Straffelini, Attrito e Uura, (2005) Tecniche nuove [20] A. Molinari, G. Straffelini, M. Pellizzari, M. Pirovano: Wear behaviour of the diffuion layer and of the compound layer of nitrided teel. Surface Engineering, 1998 Vol.14 No.6., 489 [21] A. Molinari, Bacci, P. Campetrini, M. Pellizzari, B. Tei: Plama nitriding of Fe-Cr-Mo teel, Powder Metallurgy, 1999, Vol.42 N.2, 119 [22] A. Molinari, M. Pellizzari, G. Straffelini, M. Pirovano, Corroion behaviour of a urface treated AISI H11 hot work tool teel in molten aluminium alloy. Surf. Coat. Technol., 126 (1) (2000) [23] G. Straffelini, G. Avi, M. Pellizzari: Effect of three nitriding treatment on tribological performance of 42CrAlMo7 teel in boundary lubrication, Wear, 252 (2002), [24] M.Zadra, M.Pellizzari, D.Anguillei, A.Molinari: Ottimizzazione del trattamento termico di una ghia feroidale autemperata per la realizzazione di egmenti per motori endotermici. La Metallurgia Italiana,Vol.1, 2004, [25] G. Straffelini, M. Pellizzari, A. Molinari: Influence of load and temperature on the dry liding behaviour of metal matrix compoite againt friction material. Wear, Vol. 256, N. 7-8, 2004, SVILUPPO DI TRATTAMENTI POST-OSSIDATION PER MIGLIORARE LA RESISTENZA A USURA E CORROSIONE Parole chiave: nitrocarburazione, pot-oidazione, impregnazione in olio, uura, corroione La nitrocarburazione ferritica è un noto trattamento termochimico di indurimento uperficiale che i eegue u leghe ferroe per incrementarne la reitenza a fatica e la reitenza ad uura. I materiali coì trattati motrano generalmente una reitenza a corroione non ottimale a caua della poroità dello trato dei compoti che i forma durante tale trattamento termochimico, promuovendo quindi fenomeni di corroione localizzata [1]. Per migliorare la reitenza a corroione, è oramai pratica comune potoidare i componenti nitrocarburati. Il miglioramento della reitenza a corroione è ottenuto grazie alla formazione di un ottile film di oido (Fe 3 ), che riempie parzialmente la poroità dello trato dei compoti [2]. Anche a eguito della pot-oidazione permane della poroità uperficiale reidua che può eere oggetto di un trattamento finale di impregnazione in oli lubrificanti, per apportare un ulteriore contributo alla reitenza a corroione e uura dei materiali trattati. Scopo del preente lavoro è lo viluppo, l ottimizzazione e la caratterizazione di tre trattamenti pot-nitrocarburazione, nella fattipecie, pot-oidazione, pot-oidazione eguita da impregnazione in olio lubrificante RUST VETO TB e pot-oidazione eguita da impregnazione in olio RUST VET77, al fine di incrementare imultaneamente la reitenza a uura e a corroione dell acciaio 20MnCr5 e della ghia feroidale. Accanto alle tradizionali indagini metallografiche (microcopia ottica ed elettronica, microdurezza Vicker, diffrazione a RX), la caratterizzazione dei campioni trattati e non trattati ha incluo tet di uura con un tribometro in configurazione ball-on-dik, prove di corroione in nebbia alina (ASTM B117) e la miurazione dei potenziali a circuito aperto in una oluzione acida di HCl 0.1M. ABSTRACT Le prove perimentali hanno motrato che: - La nitrocarburazione forma uno trato di diffuione, dello peore di μm, e uperficialmente uno trato dei compoti, peo μm. Nell acciaio lo trato dei compoti è omogeneo e preenta una peore cotante in tutto il materiale; al contrario la eibice uno trato dei compoti non omogeneo e con peore variabile, probabilmente a caua della preenza della grafite che inibice la diffuione del carbonio e dell azoto nel materiale (Fig. 1, Fig. 2, Fig. 3). La pot-oidazione in entrambi i materiali incrementa lo peore dello trato di diffuione o forma in uperficie un film di Fe 3, che riempie parzialmente la poroità dei materiali nitrocarburati (Fig. 4). - Tutti i trattamenti, nelle condizioni di prova coniderate, riultano eere più efficaci nell acciaio che nella ghia, ia per quanto riguarda la reitenza a uura che la reitenza a corroione. - Nell acciaio tutti i trattamenti apportano un decio incremento alla reitenza a uura, pecialmente l impregnazione finale negli oli lubrificanti, che determinano una diminuzione del tao di uura uperiore ad un ordine di grandezza; tra il tao di uura dei campioni nitrocarburati e pot-oidati non ci ono differenze rilevanti, mentre tra gli impregnati il materiale che manifeta la migliore reitenza ad uura è l impregnato nel RUST VETO TB. - In generale la non trattata eibice una migliore reitenza a uura ed un peggior comportamento a corroione ripetto al. Nelle condizioni di prova coniderate, infatti, tutti i trattamenti incrementano il volume aportato dopo il tet di uura e migliorano la reitenza a corroione ripetto alla non trattata. - La nitrocarburazione non migliora in modo ignificativo la reitenza a corroione ia del che della perché tende a promuovere fenomeni di corroione localizzata. La pot-oidazione diminuice enibilmente la velocità di corroione dei materiali, poiché riempie parzialmente la poroità della uperficie nitrocarburata. Ai fini della reitenza a corroione deciiva è l impregnazione finale nell olio RUST VETO TB, che inoltre garantice anche un bao tao di uura. 16 ettembre 2008 << la metallurgia italiana