CORROSION MECHANISM UNDER ACCELERATED ATMOSPHERIC CONDITIONS

Transcription

1 Journl of Optoelectronics nd Advnced Mterils Vol., o., Mrch, p. - CORROSIO MECHAISM UDER ACCELERATED ATMOSPHERIC CODITIOS G. Vourlis,. Pistofidis, G. Stergioudis, E. K. Polychronidis *, D. Tsips Deprtment of Physics, Aristotle University of Thessloniki, Greece Mechnicl Engineering Deprtment, Aristotle University of Thessloniki, Greece Corrosion experiments, which were conducted under ccelerted tmospheric conditions, re presented nd discussed. A slt spry chmer (terntive imte Test Chmer type SC-) equipped with sltwter tnk nd wter or stem injector ws chosen for this purpose. Series of ferrous nd non-ferrous mterils were sujected to controlled corrosion lsted t lest nd t mximum dys. As prmeters the temperture of the test room nd the temperture of the humidified ir were chosen. The s corroded specimens fter rough oservtion with opticl microscope (Olympus ) were exmined y mens of X-Ry Diffrction (XRD) nd Scnning Electron Microscopy (SEM). Results re discussed on the sis of pssivity of the films of corrosion products formed on the surfce of the specimens nd their rekdown nd mechnism is proposed. According to this, in spite tht protective films re formed during oxidtion process, these films do not ensure sufficient protection to the mterils, s corrosion mechnism nd especilly pit corrosion propgtion destroy the continuity of the pssive films. (Received Septemer, ; ccepted Ferury 9, ) Keywords: Corrosion, Slt-spry chmer, Ferrous nd non-ferrous metls. Introduction Corrosion is n electrochemicl process in which metl rects with its environment to form n oxide or other contminnt compound. The cell, which cuses this process, hs three essentil constituents: n node, cthode nd n electrolyte (n electriclly conducting solution). The node is the site where corrosion tkes plce, the electrolyte is the corrosive medium nd the cthode is the second electrode of the cell, which is not corroded. Usully the pirs of nodes nd cthodes (corrosion cells) re very smll nd scttered everywhere t the surfce of metllic component. The different electrodes my rise from differencies in the constituent phses of the metl itself, from vritions in surfce deposits or cotings on the metl or from vritions in the electrolyte [-]. In the cse of given sustrte exposed in the tmosphere, the wter, which is condenced on it from the tmosphere, cts s n electrolyte. The corrosion rte under these circumstnces is minly determined y the temperture, the reltive humidity nd the composition of the wter, which is condenced, since corrosion is ccelerted y dissolution of solule ions in the electrolyte. Very usul contminnts, especilly in res close to the se, re the - ions, which even t reltively low concentrtion destilize the pssive films nd cuse pitting corrosion [-]. Despite the gret interest towrds understnding pitting corrosion, its mechnism hs not een totlly clrified yet; therefore predictions for the extent of corrosion of ny metl in prticulr sitution re rther limited. In the present work experiments were conducted in slt spry chmer in order to study the corrosion mechnism of luminum, copper, cronsteel St-7, stinless steel nd glvnized steel. Efforts were lso mde to elucidte the role of the concentrtion of prticles contined in the environment of the indoor corrosion. * Corresponding uthor: polychr@uth.gr

2 G. Vourlis,. Pistofidis, G. Stergioudis, E. K. Polychronidis, D. Tsips. Experimentl The mterils studied were pieces of luminum, copper, stinless steel, crons-teel St-7 nd glvnized steel. Every smple ws sndlsted nd ultrsoniclly clened efore the corrosion experiments, which were crried out in Slt Spry Chmer (terntive imte Test Chmer type SC-) with solution of % wt. in distilled wter s corrosive medium. The specimens were exmined fter nd dys in the chmer under the conditions of Tle. Tle. Conditions in the slt spry chmer. Time (dys) Chmer temperture ( o C) Humidifier temperture ( o C) The smples fter eing cross-sectionlly cut, were mounted in kelite, polished, etched nd exmined with n opticl microscope (Olympus ), scnning electron microscope ( kv JEOL A equipped with n OXFORD ISIS EDAX nlyzer) nd -cycle diffrctometer (SEIFERT ) with FeK rdition ( =.9 Å).. Results nd discussion.. uminum When luminum is exposed in the tmosphere under usul circumstnces, it is covered y film of O, which is formed in very short period of time nd protects the sustrte from further corrosion y isolting it from the compounds of the tmosphere [9]. However fter the exposure in the slt spry chmer, the O film is hevily influenced y the - ions (Fig. nd ). XRD ptterns (Fig. nd ) nd SEM micronlysis (Fig. ) reveled the penetrtion of - nd + ions to depth more thn m fter dys. This phenomenon ws more intense fter dys in the sme environment nd could cuse pitting corrosion. The mechnism proposed ws sed on the fct tht sufficient mount of chloride ions ws found t O / interfce. Therefore the internl oxidtion [-] of the sustrte ws proposed. ITESITY (ARBITRARY UITS) O O O O ITESITY (ARBITRARY UITS) O O O O 7 θ d e g re e s 7 θ d e g re e s Fig.. X-Ry Diffrction pttern of fter () nd dys ()... Copper Copper seems to e corroded uniformly (Fig. c nd d). During the initil stge of exposure in the slt spry chmer O (prite) nd chlorides were formed (Fig. ). After prolonged period of time in the chmer new corrosion products such s (OH) (Fig. ) were formed. SEM nlysis showed tht slt ions penetrted the copper mtrix to depth of out m. l the oservtions concerning corrosion led to the conclusion tht the min fctor which influences the

3 Corrosion mechnism under ccelerted tmospheric conditions 7 mechnism of this phenomenon is the high concentrtion nd the high moility of chloride ions (Fig. ). ITESITY (ARBITRARY UITS) XR D ITESITY (ARBITRARY UITS) 7 θ d e g re e s 7 θ d e g re e s Fig.. X-Ry Diffrction pttern of fter () nd dys (). The peks noted with () refer to (OH)... Ferrous metls In the cse of stinless steel (Fig. g nd h) it seems tht rrier minly consisting of Fe oxides, djcent to the sustrte ws first formed, followed y nother lyer comprised of precipitted phses, which incorporte metl ctions nd chloride nions (Fig. ). The diffusion of lloying elements into the upper lyer growing outwrds from the rrier proly delys the trnsmission of ions through the rrier. So fter n extended induced corrosion exposure, the structure of the lyers does not chnge essentilly (Fig. ). Due to the fct tht oth lyers re formed in ulk form the formtion nd description of the mechnism in detils is rther impossile so only rough nlysis ws ttempted. Regrding St-7 (Fig. e nd f) complex multilyer scle ws formed (Fig. ), even from the erly stge of corrosion, which undergoes further degrdtion (Fig. ). In oth cses wter sorption plys decisive role in the mechnism of oxidtion, fcilitting the diffusitivity of the ions (Fig. c nd d) through the corrosion products towrds the metl/corroded lyer interfce. In the cse of the glvnized steel (Fig. i nd j) the ferrous sustrte is protected y zinc coting, which cts s rrier nd scrificil node []. However this coting is hevily ffected y the ggressive environment s the XRD ptterns imply (Fig. nd ). The - ions penetrtion is very intense (Fig. 7e). This phenomenon could destilize the lyer of zinc compounds, which protects the glvnized coting. ITESITY (ARBITRARY UITS) XR D Tenite (Fe,i) Tenite (Fe,i) ITESITY (ARBITRARY UITS) Tenite (Fe,i) Tenite (Fe,i) 7 7 θ d e g re e s 7 7 θ d e g re e s Fig.. X-Ry Diffrction pttern of stinless steel fter () nd dys (). The peks noted with () refer to Fe chlorides.

4 G. Vourlis,. Pistofidis, G. Stergioudis, E. K. Polychronidis, D. Tsips ITESITY (ARBITRARY UITS) Fe ITESITY (ARBITRARY UITS) Fe 7 θ d e g re e s 7 θ d e g re e s Fig.. X-Ry Diffrction pttern of St-7 fter () nd dys (). The peks noted with () refer to Fe chlorides, with () to Fe hydroxy chlorides, with () to Fe oxides nd with () to Fe hydroxides. Fig.. X-Ry Diffrction pttern of glvnized steel fter () nd dys (). The peks noted refer to hydroxy Zn chlorides, to Zn oxides, Fe oxides nd Fe hydroxyl chlorides. m m m m c d m m m m e f g h i j Fig.. SEM microgrphs of the following metls: ) fter dys, ) fter dys, c) fter dys, d) fter dys, e) St-7 fter dys, f) St-7 fter dys, g) SS- fter dys, h) SS- fter dys, i) glvnized steel fter dys nd j) glvnized steel fter dys.

5 Corrosion mechnism under ccelerted tmospheric conditions 9 Concentrtion %wt. Surfce of pure Concentrtion %wt. Surfce of pure Distnce from the surfce of the corrosion products ( m) Distnce from the surfce of the corrosion products ( m) c Surfce of pure steel d Concentrtion %wt. Surfce of pure steel Concentrtion %wt. Distnce from the surfce of the corrosion products ( m) Distnce from the surfce of the corrosion products ( m) e Concentrtion %wt. Surfce of pure Zn Distnce from the surfce of the corrosion products ( m) Fig. 7. Concentrtion (wt%) of the - ions in the lyer of the corrosion products for ), ), c) St-7, d) SS- nd e) glvnized steel.. Conclusions In the cse of, the - ions penetrte in the O lyer tht protects the sustrte nd destilize it, leding to pitting corrosion.

6 G. Vourlis,. Pistofidis, G. Stergioudis, E. K. Polychronidis, D. Tsips corrodes uniformly. The corrosion products re minly composed y O nd + chlorides, which re lter oxidized to + compounds such s (OH). - ions re lso highly diffused in the scle lyer of copper. Stinless steel is protected from rrier consisting minly of Fe oxides. Corrosion is lso delyed y the diffusion of the lloying elements to the outer surfce of the metl. - ions hve the sme ehviour s in the cse of nd. A lyer composed minly y hydrted Fe chlorides nd oxides covers St-7. References [] M. G. Fontn, Corrosion Engineering, d Edition, McGrw-Hill, ew York (9). [] E. Mttson, Bsic Corrosion Technology for Scientists nd Engineers, d Edition, IOM Communictions, London (99). [] J. S. Rodriguez, F. J. S. Hernndez, J. E. G. Gonzlez, Corrosion Science, 799 (). [] Q. Qu, C. Yn, Y. Wn, C. Co, 79 (). [] A. R. Mendoz, F. Corvo, (). [] J. Oh. Sei, D. C. Cook, H. E. Townsend, Corrosion Science, 7 (999). [7] V. Ligier, et l, Corrosion Science, 9 (999). [] K. A. Vn Oetern, Glvnotechnik 7, (9). [9] G. Chryssoulkis, D. Pntelis, Science nd technology of metllic mterils (in Greek), Ppsotiriou Athens (99). [] P. Kofstd,, High Temperture Corrosion, Elsevier Applied Science Pulishing, London (9). [] F. Gesmundo et l., Oxid. Met., (99). [] F. Gesmundo, F. Vinni, Y. iu, Oxid. Met., 9 (999). [] F. Gesmundo, F. Vinni, Y. iu, Oxid. Met., (99). [] A. R. Mrder, Progress in Mterils Science, 9 ().