Experimental Demonstration of Corrosion Phenomena Part II. Corrosion Phenomena of Steel in Aqueous Media Elsa M. Arce, Roman Ramirez, and Felipe C o r t k lnstituto Politecnico National, ESIQIE, Div. ingenieria Metalurgica, Ap. Postal 75-874, Mexico, D.F., Mexico Jorge G. Ibaiiezl Universidad Iberoamericana, Depto. de lng. y C. Quimicas, Prol. Reforma 880, 01210, Mexico, D.F., Mexico Modern applications of steel are innumerabk, from basic construction materials to the fabrication of so~histicated reaction vessels used in highly aggressive environkents. Unfortunately, corrosion affects steel to the ~ o i n that t it has been estimated that approximately 25% of the world production of steel is lost due to corrosion ( I ) . In Part I (2), we showed the phenomena of corrosion, passivation and pitting of an iron electrode in aqueous media. In the experiments described below, we use a sample of carbon steel SAE 1065 (or UNS-G-10650 using the Unified Numbering System (3)) with an approximate weight percent composition as follows (4): C 0.60-0.70, M n 0.30-0.60, P (max) 0.040, and S (max) 0.050 (the rest is Fe) to show the effect of aeeressive ions .... (e.g., CI-) and inhihiting ions (e.g., NO?-) upon its corrosion behavior. bv w i n e the ootentiodvnamic anodic ~olarization technique (2,5,6j. I n addition, the effect that ihe C1- concentration has uDon the time reauired for the initiation of the breakdown the passive filmon the surface of the steel electrode (induction time, T ) is put in evidence. Experlrnental A 0.5 M HzSOa stock solution was prepared with deionized water, and three aliauots where taken: (a) with no additives. h) enoueh NaCl was ad& to make a 1.0 M KaCl solution, and (r; I\'~cIu& addednsin h,nndao wnsNaNOlnstomakra0.5M UaNO~solutiun. All reagents were anal>~ieal grade. The experiments were performed in a conventional three-eleetrode (working, WE; auxiliary, AE, and reference, RE) Pyrex cell, with an approximate capacity of 100 mL. The working electrode was a steel 1065rod ($ = 11.1mm), encapsulated into an eooxv . . resin matrix: the exnosed electrode surface was mirror-polishedbeforr earh run with sandpaper s6OO and wirh sureersively liner alumina suspensions tdom t o 0.0:) rm) and washrd wirh deionized water. The auxiliary electrode wa- a aplral madeof platinum wire, prevrously cleaned h y immersion in hot aqua rrgia tnirrohvdrorhl~rricacid). The reference electrode was a raturated calomel electrode W E ) ,and all the wtentials measured were given with reference to the SCE. Each soluiion was de-aerated with N. before each run. and a Ng atmos~herewas maintained inside the eeil durine the wh& exoeriknt. ?he tem~eraturewas maintained at 15 T. For earh sysrrm thun generated, the corrosion plmntial (E,,,,) was measured at open circuit hetween the W E and the R E from this potential, the potentiodynamie anodic polarization experiments were performed for earh system, at a scan rate of 5U m\' s :.The applied potential was controlled with a BAS potentiostat model CV.2". and the obtained rirnal (, vs. E ) was plottrd with a BAS X-Y recorder; however, pra>tically any simpier equipment could be used as well. As for the induction time experiments, the 0.5 M Hi301 stoek solution and the cell described above were also used following the same procedure. A potential was then applied to the WE so as passivate it at E = 1.25 V during 10 min. Enough NaCl was then added to different aliquots of the H2SO4stock solution to prepare M, (b) 1 Clsolutions with the following concentrations: (a) 5 X ~~~~~~~~~~
-~~~
~~~
.
Presented at the 199th Meeting of the American Chemical Society. Boston. MA. April 22-27. 1990. Author to whom correspondence should be addressed.
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Figure 1. Anodic potentlodynamlc polarization curves of 1065carbon steel in: (a) 0.5 M H.S04. (b)0.5 M H.SOdl.0 M NaCI, and (c)0.5 M H2S0,11.0M NsCII 0.5 M NaN03.
X M, and (c) 5 X M; the electrodes were then immersed in each solution, and the current produced at E = 1.25 V was recorded. The time elapsed from here to the onset of the current production is taken as the pitting induction time, 7 (7).
Results and Dlscuslon T h e anodic polarization c w e obtained for the 1065 steel electrode in 0.5 M H2SOais shown in Figure la. This c w e shows an active dissolution zone up to a critical point where the current starts decaying drastically due to the formation of a passivating layer that reduces corrosion in a small passive zone (2, 7,8). As the scan is continued, a zone is reached where the current increases due to either the solvent decomposition and/or to the breakdown of the passivating layer and the dissolution of the metallic substrate (transpassive zone) (5). When the C1- ions are added to the system, the passive zone disappears (see Fig. Ib) due to the localized breakdown of the passivating layer (pitting) (2, 7,8). When the Nos- ions are added to the steell0.5 M HzS04/1.0 M NaCl system, the active dissolution zone is followed by a drastic current decrease, by a small peak and by a wider passive zone (Fig. le). The first two features were explained above, whereas the small peak observed is due to the initiation of pitting by the C1- ions, which is then inhibited by the NO3- ions; the presence of the Nos- induces the repassivation of the steel Volume 68
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M
/.XI@
I
,,/-ixl0-/-"
ly Several shown hy corrosion-related using the anodic phenomena potentiodynamic of steel may polarization he easitechnique. These experiments can he performed in a 3-h lab session. Acknowledgment
E= 1.25V
"
5
10
15
20
25
30
TIME ( MIN.) Figure 2. Potentiastatic ivs. f cuwes (at E = 1.25 V) for 1065 carbon steel in 0.5 M HzS04with different CI- concentrations:(a) 5 X M. (b) 1 X M, and (c)5 X M.
(7). As for the induction-time experiments, the time required for the onset of the pitting of the passivating layer ( r ) decreases as the concentration of the aggressive ion increases (Fig. 2).
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Journal of Chemical Education
One of us (JI)acknowledges partial support from PEMEX and from Project INQ-050 of the Universidad Iheroamericana. Literature Clted 1. J. chem.Educ. SteftJ.Cham.Edue. 1979.56.673-674. 2. Solona. 0:Ibhdoe. J. G.; Olivares, L. J. Cham.Educ. 1991,68,175-177. 3. ~ i l l ~ ~P.. CCorrosion . Control in the ChamicdPracers Industrie8;McGrav-Hill: New York, 1986: p 97. 4. Norden. R. B. In ChemicolEnginear's Handbook, 5th ad.; Perry, R. H.; Chilton, C. H., Eds.:MeGraw-Hill: New York. 1975;Senion 23, p51. 5. Uhlia, H. H.: Revie, R. W. Corrnrion and Corrosion Control; Wiley: Neu York, 1985: chapters 5 and 6. 6. E. G, and G. princeton AppliedResearch. Appliestio" Notecon. 1,Bosics of corroaian ~ ~ ~ ~ ~~ r ~i n ~~ ~NJ, ~t o1982: e ~ ,n p 2.t s : 7. Gaivele. J. R. 1" Passivity ofMetais: FrankenUlal, R. P.: Kruger,J., Eda.: The Elrctrochemical Society: Princeton, NJ, 1977;p 285. 8, wrang~en. G. an mrwiuction to corrosion and ~ ~ o t r c t i ooinM ~ I ~ Ichapman Z: and Hall: London, 1985: Chapter 5.
