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ISIJ International, Vol. 31 (1991), No. 2, pp. 229-232

High Resistance of Amorphous Fe-Cr-P Alloys

S. VIRTANENand H. BOEHNl

Institute of Materials Chemistry and Corrosion, Swiss Federal Institute of Technology, ETH-Hoenggerberg,CH-8093Zurich, Switzerland.

(Received on July 6, 1990; accepted in the final form on October 12. 1990)

Amorphousiron-base alloys typically contain large amounts of (B,P,C). The role of the metalioids is to facilitate the -formation but they very strongly influence the corrosion behaviour, too. Addition of phosphorus to - alloys modifies the passivation meohanismand to self-passivation even in strongly acidic deaerated solutions. Ahighly stabl6 chromiumoxyhydroxide passive filmwith significant amountsof oxidized phosphorus incorpo- rated is formed on the surface of the P-containing alloy. Theamorphousalloy Fe-10Cr-13P-7Cshowsan extremely high resistance against localized attack in Cl--containing solutions. This is due to a low pit initiation rate and a high repassivation ability of the P-containing alloy.

KEYWORDS:amorphousalloys; passivity; phosphorus; Iocalized corrosion.

diffraction its 1. Introduction was used to confirm amorphousstruc- ture. The samples were mechanically polished with Metallic are often expected to show a su- abrasive paper (1 200 grit), rinsed with distilled water, perior corrosion resistance comparedto conventional ethyl alcohol and acetone, and dried. crystalline alloys, since the glassy structure is free of The polarization curves and the potential decay crystalline defects such as grain boundaries and dis- measurementswere performed with a conventional locations, and since the glassy alloys are chemically electrochemical set-up as described earlier in Refs. very homogeneous. Onthe other hand the specific 9) and 10). The scanning rate for the polarization composition of the alloys required to form an amor- curves was 0.1 mV/s. For the potential decay mea- phous structure also very strongly affects the corro- surements the samples were passivated at E=+600 sion behaviour of these alloys compared to conven- mVSCEfor I h. After the passivation the regulat- tional ones. In iron-base alloys an addition of a ing circuit wasopenedand the potential wasrecorded large quantity (15-25 ato/o) of semi-metallic compo- as a function of time. The impedancemeasurements nents is necessary for the formation of the amorphous were performed with a SchlumbergerSolartron poten- structure by rapid quenching from the liquid. In tiostat ECI 1286 and a transfer frequency analyzer such metal- alloys alloying with film-forming FRA1250 in the frequency range from 60 kHZ to elements (especially Cr) is usually necessary for a good 6mHZwith a perturbation amplitude of 10mV. corrosion resistance.1-3) The different glass-forming For the transient measurementsthe samples were elements (P, B, Si, C) show drastically different ef- polarized at a constant potential below the pitting fects on the corrosion behaviour of the alloys. Al- potential. The current was monitored by a digital ready in the first works on the corrosion behaviour of oscilloscope and evaluated using a computer-aided amorphousFe-Cr-metalloid alloys it was shownthat transient technique (CAT). Further experimental the corrosion resistance wasthe highest for alloys con- details on the transient measurementsare described carried taining phosphorus and carbon as metalloids.2) The in Refs. 10) to 12). A11 the experiments were positive effect of the phosphorus on the passivation out at roomtemperature. and on the localized corrosion resistance of the amor- phous Fe-Cr alloys has been verified by manyau- 3. Results and Discussion thors.4~10) It can thus be concluded that the com- Potential in Acidic Solu- position of the amorphous alloys has got a very 3.1. Behaviour at the Corrosion significant effect on the corrosion properties. Sub- tions sequently the electrochemical behaviour of an Fe- The electrochemical behaviour of the alloy Fe- Cr-P-C alloy will be considered in moredetail. lOCr-13P-7Cis strongly time-dependent. In deaerat- ed I NH2S04the corrosion potential of the amorphous is shifted positive direction with time 2. Experimental alloy into the whereas a crystalline 17Cr-containing shows The amorphousalloy had the following nominal a very constant corrosion potential with practically composition (ato/o): Fe-10Cr-13P-7C. The alloy no change with time,13) The shift of the corrosion was prepared by melt-spinning as ribbons with a potential of the amorphousalloy is connected with a width of l2 cmand a thickness of 30-50 Ilm. X-ray coverage of the surface by a black which was

C 1991 ISIJ 229 ISIJ International, Vol. 31 (1991), No. 2 found be rich of oxidized iron first to and phosphorus mVSCE). In the measurementswhenthe sur- thus suggesting it to be a kind of an iron-phosphate. face is covered by the black layer the capacitance In Fig. I the changes of the open-circuit potentials of values are high suggesting the black layer to be highly the amorphousalloy Fe10Cr-13P-7Cas a function porous. The gradual dissolution of the black layer of time in aerated and deaerated I NH2S04Solution can be seen in the decrease of the capacitance values. are shown. In the aerated solution the potential is The time-dependence of the open-circuit potential rapidly shifted positive potentials. very to high In and the polarization resistance shows that the amor- both solutions the surface is originally covered by the phous alloy Fe-lOCr-13P-7C is self-passivating in black layer, but the surface becomesgradually shiny both aerated and deaerated solution containing only again little residual oxidizing species. . very as The shift The of the potential into the positive direction self-passivation is connected with the formation of an is connected with an increase of the polarization re- insoluble, porous pre-cursor layer of iron-phosphates. sistance (Fig. 2). The polarization resistance in- The pre-cursor layer covers the surface decreasing the creases from about 200 O¥cm2 after 0.5h at the anodically active area and leading to a low critical open-circuit potential (E=-350 mVSCE) up to current strength needed for passivation. Cathodic >100~.cm2 after 72 h in the solution (E=+200 polarization measurementson an uncovered and cov- ered alloy surface showedthat cathodic reactions (H2- 400 evolution and 02-reduction) can take place on the UJ pre-cursor layer.13) Since the layer is (/)O pre-cursor o2 highly porous, the cathodically active area becomes >E 200 very large leading to high cathodic current strengths. .~! N2 Dueto the small anodically active area and the large a) o cathodically active area altready small amounts of '~o oxidizing species as present in deaerated solutions ~ -200 to passivation. '~5

a) c~ 3. 2. Stability of the Passive o Film -400 O 2000 4000 6000 Open-circuit potential decay measurementswere carried in order to study the stability of the Time (min) out pas- sive fllms. The potential decay for the crys- Fig. l. Changeof the open-circuit potential of the curves amor- talline steel and for the amorphousalloys Fe-lOCr- phous alloy Fe-lOCr-13P-7C in aerated and de- 13P-7C in H2S04and in Cl--containing solu- aerated I NH2S04' I N tions are shownin Fig. 3. The reduction of the pas- o/o 5 sive fllm formed on the 17 Cr steel takes place in three stages, and the film becomestotally reduced \ \,72 \ without external cathodic polarization. The poten- 4 \~24 \ tial of the alloy the \ decay curve Fe-10Cr-13P-7C, on ~ J:: \ contrary, shows clearly defined potential arrest; 3 \ no o \ instead the potential gradually falls \ to a minimum 0.5 \ value of 100 rises afterwards again. ~e02 h t~ \ E=- mVSCEand In about 20 h the potential has reached the value H ~~ \ E=+200 SCE. The value of the flnal potential l \ *\ mV dependson the 02-content and on the stirring of the O -2 -1 O 1 2 3 4 5 6 800 --'1- Fe-17Cr in I N H2S04 10g ,,, --dl- Fe-17Cr in I NH2S04+ 0.1 NNaCl 90 400 in *~,, 72 Fe-10Cr*13P*7C i N H2S04 / /' \ '-Q- \\24 Lu Fe-10Cr-13P-7Cin I NHCl ~ o(,) c~o l \ >E O _1' 1:, 1 \ CQ \ 17 45 L \ h q) cQ \ \ o-o -400 p*~ 0.5 h \ \ \ \ \ \ \ -800 o o 30 60 90 120 150 -2 -1 O l 2 3 4 5 6 Time (min ) log os Fig. 3. Open-circuit potential decay curves for the crys- Fig. 2. Impedancespectra of the alloy Fe-10Cr-13P-7C talline 17 o/o Cr steel and the amorphousalloy Fe- after various times at the open-circuit potential in lOCr-13P-7C after passivation in Cl--free and Cl-- deaerated I NH2S04' containing solutions.

230 ISIJ International, Vol. 31 (1991), No. 2 electrolyte changing into the positive direction by tionally an analysis of the shape ol' the single transients rising 02-content and by stirring. that lifetimes stronger The po- shows the of the transients of the amor- tential decay thus that after the measurements show phous alloy are typically about a factor ten shorter amorphous alloy has been passivated in a solution than those of the crystalline stainless (Fig. 5). it where originally wasactive, the passive state will be The lower transient rate of the amorphousalloy stable indicating that the active state of the amor- comparedwith the AISI 316 indicates phous alloy is stable. is Fe-lOCrl3P-7C not This that less pits initiate on the amorphousalloy. This in good agreementwith the self-passivating ability of can be partly due to the lack of surface heterogeneities the alloy. in the amorphousalloy, since pits often initiate at in- Potential decay in C1--containing measurements clusions for instance. The results of transient mea- solutions showthat the stability of the passive film of surements on other amorphousalloys showclearly that the crystalline steel is negatively affected by the chlo- the localized corrosion resistance of the amorphous rides. Even if the chlorides were added to the solu- alloys is dependent on the composition of the alloys after tion the passivation in a Cl--free solution, ac- and the highly homogeneousamorphous structure tivation the crystalline steel is accelerated.11,12) of cannot guarantee a high localized corrosion resis- potential decay the alloy tance.13) The curve on amorphous Amorphousalloys containing no phosphorus the other hand, is practically Fe-10Cr-13P-7C, on are very susceptible to crevice corrosion and the re- identical in I NH2S04and in I NHCIsolutions. sistance against Cl--attack is the best whenthe alloy contains phosphorus and carbon as metalloids.11-13) 3. 3. Localized Corrosion Resistance An XPSstudy has shown that the passive fllm localized The corrosion resistance in C1--contain- formed on the amorphousalloy Fe-10Cr-13P-7Cin ing solutions wasstudied in detail with the computer- 1NH2S04and I NHCIcontains remarkable amount aided transient technique. results of the The tran- of oxidized phosphorus enriched as phosphates in the sient measurementsshowthat the transient rate of the outer layer of the film (film/electrolyte-interface).14) alloy amorphous Fe-1OCr-13P7Cdecays muchfaster Sakashita and Sato have shown on hydrated iron- than on a crystalline steel AISI 316 indicating a much and chromiumoxides that adsorption and incorpora- lower numberof pit nucleation sites (Fig. 4). Addi- tion of oxyanions such S0~ PO~-, as , Cr0~ , MoO~- and W0~-are capable of converting anion-selective 20 hydrated metal to cation-selective phases.15-17) Later the model of the so-called bipolar passive fllm has discussed E~ been by Clayton and Lu to explain the effect of chromiumand molybdenum the localized ~ on corrosion resistance of stainless steels.18,19) Thus the Q' 10 incorporation of phosphates into the +, outer layer of the passive film can hinder the penetration of the aggressive anions into the film by rendering the ca- tion-selectivity of this layer or the phosphates can at H least repel the chlorides from the surface. This was verified by which that o XPSmeasurements showed O Iao 200 anions from the electrolyte (SO~- and Cl-) are pres- Time (min) ent in a muchlesser amount on the surface of the Fig. 4. Transient rate in ilJ:nction of time for the crystalline P-containing amorphousalloy than on the surface of AISI 316 steel and the amorphousalloy Fe-10Cr- the crystalline 17Cr-steel.* A model of the passive 13P7Cin 0.1 MNaCl at E=+220mVSCE. fllm on the amorphousP-containing alloys is shown

0.7

0,6 AISI 302 :i 0,5 H 0.4 UJZ O: 0.3 H LUZ O: O( D O( O 0.2 Fig. 5. ~)~ Single transients of the crystalline 0.1 l*- AISI 302 steel the 2s and amorphous o H alloy FelOCr-13P-7C in 0,lM 3Q.o 30; ,o., 3Q,e 3Q8 3lx 3i2 31/ sl.e 8]¥ :a TIME NaCl at E=+220mVSCE. TIME (sec)

* " The samples for the XPSmeasurementswere passrvated In I NHSO wrth OI MNaC1at E +500mVSCEfor I h Onthe surface steel of the clystallme the concentratron ato//o the Cl concentratron at o/ anion concentrations SO was =5 , was = I Io . The (S0~- and Cl-) on the surface of the amorphousFe-10Crl3P7Calloy were hardly detectable."

231 ISIJ International, Vol. 31 (1991), No. 2

Electro[yte Acknowledgements Cl - The authors are pleased to acknowledge the cation- Schweizerischer Nationalfonds zur Fdrderun~)or der wis- selective P043~ o senschaftlichen Forschung for supporting this research layer O Passive o within its national research Nr. 19 (Werk- anion. program Fe/Cr( OH)3 film also selective o stoffe fur die Bedurfnisse von morgen). We ac- layer O knowledge the Institute of Physics at the University of Basel (Prof. Gtintherodt) for the production of the Meta] / alloy amorphousalloys. ~~;~4////////~4 Me"' ~///// ' Fig. 6. Model of the bipolar passive film of the amorphous REFERENCES alloy Fe-1 OCr-13P7C. l) M. Naka, K. Hashimoto and T. Masumoto: Corrosion- JV:ACE, 32 (1976), 146. in Fig. 6. 2) M. Naka, K. Hashimotoand T. Masumoto: ./. JVlon-Cryst. Solids, (1978), 403. The short lifetimes of the transients of the amor- 28 3) and T. J. JV;on-Cryst. phous P-containing alloys indicate tha.t the repassiva- M. Naka. K. Hashimoto Masumoto: Solids, 31 (1979), 355. tion of small pits takes place rapidly. At local very 4) V. Yu. Vasil'ev, N. I. Isaev. V. N. Shumilov, A. N. Klo- defects of the passive fllm the dissolution rate is high chko. A. I. Zakharov and A. V. Revyakin: Prot. Met., 19 the dissolved and consequently the concentration of (1983), 203. solubility iron and phosphorus rapidly exceeds the 5) V. Yu. Vasil'ev. A. N. Klochko and Yu. A. Pustov: Prot. product ofthe sparingly soluble iron-phosphates. The Met., 21 (1985), 163. formation of the iron-phosphate pre-cursor layer leads 6) R. B. Diegle and D. M. Lineman: J. Electrochen:. Soc., 131 to repassivation in the sameway as it facilitates the (1984), 106. self-passivation. The rapid repassivation due to the 7) S. Virtanen. B. Elsener and H. Boehni: Proc. Eurocorr formation of the iron-phosphate layer impedes the 87. DECHF*MA,Frankfrut an Main, (1987), 763. 8) S. Virtanen, B. Elsener and H. Boehni: J. I.ess-Colnmon stabilization of the pits by the precipit.ation of an iron- Met., 145 (1988), 581 chloride salt fllm as has been suggested to occur by . 9) S. Virtanen, B. Elsener and H. Boehni: Proc. of the 3rd pitting oi' conventional Fe r alloys .20-22) - C Int. Symp. on Electrochemical Methods in Corrosion Re- search, Materials Science Forum44!45, Trans Tech Pub- 4. Conclusions lications, Zurich, (1989), l. 10) S. Virtanen, B. Elsener and H. Boehnl: Proc. Symp. The effect of phosphorus in amorphousFeCr-P-C Transient Techniques in Corrosion Scienc,e and Engineer- alloys is to facilitate the passivation and increase the ing, ed. by W.H. Smyrl, D. D. Macdonald and W.J. localized corrosion resistance in Cl--containing solu- Lorenz, Electrochem. Soc., Pennington, NJ, (1q. 89), 306. Corrosion tions. This positive effect of phosphorus can be sum- ll) L. Stockert and H. Boehni: Proc. 8th European Congress, Vol. 2, Centre Fran~ais de la Corrosion, Paris, marized as follows : (1985), 22. (1 Alloying with phosphorus leads to self-passiva- ) 12) R. Holliger and H. Boehni: Proc. Symp.ComputerAided tion in deaerated acidic solutions containing only even Acquisition and Analysis of Corrosion Data, Vol. 85-3, little oxidizing species. This is due to of a coverage Electrochem. Soc., Pennington, NJ, (1984), 200. surface insoluble, iron-phosphate the by an porous 13) S. Virtanen: Ph.D. thesis Nr. 8924 to Swiss Federal Inst. layer in the active state of dissolution which decreases Tech., (1989). the anodically active area and strongly increases the 14) A. Rossi, D. DeFilippo, S. Virtanen and B. Elsener: Proc. cathodically active area. The passivation leads to a 1Ith Int. Corrosion Congress. Vol. 3, Assoc. Itali. di Metall., formation of a highly stable passive fllm and thus to Milano, (1990), 3.539. Passivity oi' Metals, ed. by very low dissolution rates of the alloy. 15) M. Sakashita and N. Sato: J. Soc., Pen- (2) The high localized corrosion resistance of the R. P. Frankenthal and Kruger. Electrochem. nington, NJ, (1978), 479. alloy Fe-10Cr-13P-7Cis due to a low pit initiation Sci., (1977), 473. in- 16) M. Sakashita and N. Sat0: C,rros. 17 rate and to a rapid repassivation of the pits. The 17) N. Sato: Corrosion-JV:ACF., 45 (1989), 354. corporation of oxidized phosphorus (P0~-) into the 18) A. R. Brooks, C. R. Clayton, K. Doss and Y. C. Lu: .J. layer of the passive fllm renders it to be cation- outer Elecf.,'ochen?.. Soc., 133 (19 87), 2459. selective least repellent towards electrolyte anions or at 19) C.R. Clayton ancl Y. U. Lu: J. Eleclroclte,n. Soc., 133 thus hindering the adsorption and penetration of the (1987), 246J'. is chlorides into the film. The rapid repassivation 20) H.-H. Strehblow and J. Wcnners: I+'1ec!roch'i,1't. Actct, 22 due to a destabilization oi' the active state by phos- (1977), 421. phorus. 21) G. S. Frankel, L. Stockert, F. Hunkcler and H. Boehni: Corrosion, 43 (1987), 429. 22) H. Boehni and L. Stockert: Werkstof. Ii:'orros., 40 (1989), 63.

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