Influence of Copper on Iron Corrosion in Weakly Alkaline Environment Containing Chloride Ions

Transcription

1 Materials Transactions, Vol., No. () pp. 96 to # The Japan Institute of Metals Influence of Copper on Iron Corrosion in Weakly Alkaline Environment Containing Chloride Ions Je-Kyoung Kim, Atsushi Nishikata and Tooru Tsuru Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo -8, Japan The influence of copper content on the corrosion of iron has been investigated to make possible the use of scrap as a source material for reinforcing steel in concrete structures. An AC impedance technique was used to monitor the corrosion of iron, Fe. mass%cu and Fe mass%cu undergoing cyclic wet-dry conditions with a hour immersion in a ph solution of Ca(OH) containing. M NaCl followed by a hour drying at 98 K and % RH. The corrosion rate of iron is greatly accelerated by wet-dry cycles. This is because active FeOOH species produced by the oxidation of Fe(II,III)oxide in air during drying act as very strong oxidants that promote corrosion in the wet condition. In each cycle, shortly before the surface dried out, a large increase in the corrosion rate was observed. This can be explained by the acceleration of oxygen transport through the thin electrolyte layer. Fe. mass%cu showed a similar trend to iron. On the other hand, Fe mass%cu showed much lower corrosion rates and the corrosion was not accelerated by wet-dry cycling. Monitoring showed that the addition of. mass%cu does not diminish the corrosion resistance of iron and the addition of mass%cu improves it under these wet-dry conditions. Energy Dispersive Analysis of X-ray (EDX) results indicated enrichment of copper in an inner rust layer after exposure to 6 wet-dry cycles. (Received October 7, ; Accepted January, ) Keywords: scrap, iron, carbon steel, copper, concrete, corrosion, electrochemical impedance. Introduction There are several residual elements such as Cu, Sn and Sb in scrap steel. It is known that these elements are difficult to remove from scrap. Copper is usually considered a noxious contaminant for metalworking operations such as rolling. The amount of copper allowed in iron for rolling is limited to. mass%. However, rebar is not manufactured by rolling. This allows the use of scrap for rebar material. An advantage is that steels containing copper show high strength, which is desirable for rebar material. If it is not necessary to remove copper from steel scrap, the economical use of a large amount of scrap with high copper concentrations becomes possible. ) When concrete structures are used in atmospheric environments, they are exposed to cyclic wet-dry conditions. Corrosion tests of steel bars in concrete require long periods of time because of the very slow diffusion rate of oxygen through concrete. Therefore in this study tests were performed under cyclic wet-dry conditions employing simulated concrete solutions, and not in concrete. Application of electrochemical techniques to the study of atmospheric corrosion has been regarded as quite difficult because atmospheric corrosion proceeds under extremely thin electrolyte layers. The largest problem is that solution resistance becomes extremely high when the layer of electrolyte is very thin. The resulting extremely high ohmic drop leads to serious errors in the electrochemical measurements. However, published reports indicate ) that the AC impedance technique is very useful as an electrochemical tool for electrode systems with a high solution resistance, such as that encountered in atmospheric corrosion. When measuring AC impedance, the solution resistance is estimated from the high frequency impedance, while the sum of the polarization resistance and solution resistance is estimated from the low frequency impedance. The difference between them provides accurate polarization resistance information without incorporating an error due to ohmic drop. In this study, the corrosion of iron samples having different levels of copper content (,., mass%) was monitored by the AC impedance technique under a cyclic wet-dry condition in a concentrated simulated concrete solution in order to clarify the influence of copper content on the corrosion of carbon steel in concrete environments.. Experimental Method. Sample preparation and wet-dry condition The test electrodes were pure iron (99.99%), Fe. mass%cu and Fe mass%cu. The Fe Cu alloys were prepared from iron (99.99%) and copper (99.99%) by arcmelt. The specimens were hot-rolled to a thickness of mm, heat-treated at 8 K for h and then waterquenched. The final copper content as determined by chemical analysis was.6 mass% for Fe. mass%cu and.9 mass% for Fe mass%cu. Each sample was confirmed to be a single phase with optical microscopy and XRD. The wet-dry cycles comprised exposure to alternating periods of h immersion in a simulated concrete solution and h drying at 98 K and % RH. The concrete solution was a. M NaCl solution adjusted to ph with Ca(OH).. Experimental cell and procedure A schematic diagram of the electrochemical cell used in impedance measurement is shown in Fig.. A two-electrode cell arrangement was used in this study. A parallel pair of specimens ( mm mm) was embedded about. mm apart in epoxy resin, polished with emery paper up to #8, and ultrasonically rinsed in distilled water. A barrier wall. mm in height was then set around the electrode to ensure constancy of the water layer thickness at each starting point of the dry period. Three electrochemical cells, one each of iron, Fe

2 Influence of Copper on Iron Corrosion in Weakly Alkaline Environment Containing Chloride Ions 97 Barrier h=.mm mm Specimen mm.mm Epoxy resin Fig. Experimental cell used for the AC impedance measurement: top view, side view.. mass%cu and Fe mass%cu were simultaneously exposed to the cyclic wet-dry condition for h (6 cycles). During the exposure, the corrosion behaviors of the three specimens were monitored by the AC impedance technique. The corrosion potential E corr was also monitored only when the specimen was submerged for h in each cycle using an Ag/AgCl/saturated KCl reference electrode (SSE) with a conventional Luggin capillary. After the monitoring, the interface at the boundary between metal and corrosion products was analyzed using a Field Emission Scanning Electron Microscope (FESEM) equipped with Energy Dispersive Analysis of X-ray (EDX).. Electrochemical corrosion monitoring In the polarization resistance method, the corrosion current i corr is determined from the polarization resistance which is obtained from the slope (E=i) of the steady state polarization curve in the vicinity of the corrosion potential, according to Stern-Geary equation 8) given by i corr ¼ k=r P ðþ where k can be calculated from the Tafel slopes, b a and b c, for the anodic and cathodic portions of the polarization curve respectively, as follows; k ¼ b a b c =:ðb a b c Þ: ðþ For calculating i corr from R P using eq. (), the k value must be determined in advance. For example, for the corrosion of iron in bulk solutions of sulfuric acid, the k value is calculated to be. V, using b a ¼þ: V/decade 9) for the anodic reaction, Fe! Fe þ þ e ; ðþ and b c ¼ : V/decade 9) for the cathodic reaction, H þ þ e! H : In neutral bulk solutions of NaCl, the value of k is calculated to be.7 V from b a ¼þ: V/decade for the anodic reaction eq. () and b c ¼V/decade for the cathodic reaction, ðþ O þ H O þ e! OH : However, under thin electrolyte layers like those involved in the corrosion environment, it is much more difficult to measure accurate polarization curves over wide potential ranges. Accordingly, in this study, the obtained R p was used as an index of the corrosion rate. In this study, the AC impedance technique was used to monitor the. Only two frequency impedances, Z L at low frequency and Z H at high frequency, were measured. The polarization resistance R P was then calculated by subtracting Z L from Z H. A detailed description of the theory of corrosion monitoring using the AC impedance technique is described elsewhere. ) The impedance at khz (Z H ) and MHz (Z L ) was measured continuously using an electrochemical impedance measurement system consisting of a frequency response analyzer (FRA) and a potentiostat with a multiplexer, which was controlled by a computer through the GPIB interface. The amplitude of the applied ac voltage was mv in all measurements.. Results and Discussion. Corrosion behavior under cyclic wet-dry conditions Figures, and (c) show the results of corrosion monitoring for pure iron, Fe. mass%cu, and Fe mass%cu, respectively, subjected to alternating conditions of h immersion in ph Ca(OH) solution containing. M NaCl, and h drying at % RH and 98 K. From the R p vs. time curves of Fig., integration of which provides the index of total corrosion mass loss, it was found that pure iron and Fe Cu alloy showed drastic differences in corrosion mass loss. Red rust (FeOOH) was observed with the naked eye in the first wet-dry cycle for pure iron. [While it was found in the region II (Fig. ) for Fe mass%cu]. After that the corrosion rate decreased, leading to passivation.. Influence of wet-dry cycles.. and Fe. mass%cu The corrosion rates of pure iron and Fe. mass%cu, as shown in Fig., were greatly accelerated by the wet-dry cycles. This indicates that they are not passivated in an alkaline solution of ph in the presence of chlorides because the chloride concentration increases as drying progresses. However, the total amount of corrosion of Fe. mass%cu was similar to that of pure iron, indicating that the addition of. mass%cu does not impair the corrosion resistance of iron. This acceleration by wet-dry cycles is attributed to the formation of active FeOOH species. According to a model proposed by Evans, ) the active FeOOH species, which are produced by the oxidation of Fe(II, III)oxides during drying, act as very strong oxidants contributing to the corrosion of pure iron in wet conditions... Fe mass%cu The corrosion of Fe mass%cu during wet-dry cycles is divided into three regions, as shown in Fig. (c). In the initial stage (region I), corrosion was inhibited due probably to airformed oxide, which was produced on the alloy surface before the experiment. In region II, the corrosion was initiated by the breakdown of the air-formed oxide after ðþ

3 98 J.-K. Kim, A. Nishikata and T. Tsuru - / x Ω - / x Ω - / x Ω mass%Cu Region I (c) mass%cu Region III Region II Potential, E/V vs. SSE mass%cu Cycle Number, N c Fig. Difference of corrosion potential of pure iron and Fe mass%cu during the cyclic wet-dry process. cycle are shown. It can be seen in Fig. that the corrosion potential of Fe mass%cu was much higher than that of pure iron. This indicates that the reduction of the corrosion rate by added copper is caused by the inhibition of the anodic process rather than the cathodic. It has been proposed in the literature, ) that Cu enrichment at the metal/oxide interface during corrosion allowed the alloy to be passivated easily. To confirm this hypothesis, the cross section of Fe mass%cu was analyzed with EDX. Figure shows an FESEM photograph of the cross section of Fe mass%cu after the wet-dry cyclic test. The EDX analyses were performed at the squares numbered in Fig.. The results are shown in Table. The corrosion products consisted of two layers, a porous outer layer and a dense inner layer. From the EDX results, a copper oxide enriched layer was formed at the metal/oxide interface. The amount of copper oxide in Fe mass%cu was increased in the inner rust layer (# in Fig. ) compared with the outer (# in Fig. ). However, enrichment of metallic copper on the alloy side of the interface resulting from preferential dissolution of iron was not confirmed by the EDX analysis. Rusts Metal Fig. Changes in reciprocal of polarization resistance R p of pure iron, Fe. mass%cu and (c) Fe mass%cu during 6 wet-dry cycles. several wet-dry cycles. The corrosion was accelerated by the wet-dry cycles in this region and red rust was observed on the surface of the alloy. It seems that iron dissolves preferentially into the solution, leading to an increase of copper activity at the alloy surface. ) In region III, the corrosion of Fe mass%cu was strongly retarded, while that for pure iron was not inhibited until the final cycle. The reason may be either that anodic dissolution is hindered by the formation of protective oxide scales or the oxygen reduction reaction is kinetically slow on the oxide scales formed on the Fe Cu alloys. In both cases, the corrosion rate of the Fe Cu alloy will be slower than pure iron, but the corrosion potentials of the Fe Cu alloy should be higher for the former case, and lower for the latter. In Fig., changes in the corrosion potentials for Fe mass%cu and pure iron up to the th Fig. FESEM photograph of a cross section of corrosion products for Fe mass%cu Numbers indicate areas where EDX analysis of Fe, Cu and O was performed.

4 Influence of Copper on Iron Corrosion in Weakly Alkaline Environment Containing Chloride Ions 99 Table EDX analysis of cross section of Fe mass%cu after cyclic corrosion test. (Site numbers indicate numbers in Fig..) Site # Fe (at%) Cu (at%) O (at%) / Ω Corrosion behavior during one cycle Figure shows the enlarged plot of corrosion rate vs. time in the 9th and th cycles from Fig.. Generally, the rate of corrosion is accelerated by an increase in the rate of oxygen diffusion. Stratmann ) obtained similar monitoring results for carbon steel through corrosion potential monitoring by the Kelvin method and corrosion rate evaluation on the basis of oxygen consumption monitoring. Water film thickness decreases very rapidly immediately before drying up. Thus the presently observed features of the behavior pattern of corrosion seem to be rationally interpreted in terms of the relation between the atmospheric corrosion rate and the water film thickness proposed by Tomashov. ) Judging from observed patterns of variation of corrosion rate and corrosion potential, the influence on the corrosion process of NaCl concentration occurring during drying appeared to be rather insignificant, probably because of the simultaneous occurrence of a decrease in oxygen solubility and an increase in the anodic dissolution rate due to concentration of NaCl. With decreasing water film thickness, the rate of oxygen reduction tends to rise, and correspondingly, the partial cathodic current increases. Conversely, the partial anodic current of Fe dissolution tends to be suppressed with decreasing water film thickness due to the diminishing amount of electrolyte solution. Consequently, with decreasing water film thickness, the maximum corrosion rate emerges, and corrosion potential would shift in noble direction. As can be seen in Fig., the corrosion rate (R p ) of pure iron during drying displayed an increase as shown by an acceleration of cathodic oxygen reduction, leading to the maximum corrosion rate. In contrast, the corrosion rate of Fe mass%cu in Fig. is much smaller than that for pure iron and did not show a maximum just before the surface dried out, indicating that the anodic dissolution of Fe mass%cu is inhibited by the formation of dense corrosion products containing copper.. Potentiodynamic polarization measurement Figure 6 shows anodic and cathodic polarization curves with a mv/sec scan rate for pure iron, Fe Cu alloys and pure copper in a ph Ca(OH) solution with. M NaCl, which is the same solution used for corrosion monitoring in this study. The cathodic currents of pure iron and Fe mass%cu were similar. Meanwhile, the anodic current of Fe mass%cu was greatly decreased compared with pure iron. The rest potential of Fe mass%cu was : V (vs. SSE), more positive by. V than pure iron. From the rest potential to : V, the anodic polarization curve of Fe mass%cu was similar to that of pure copper. From the experimental results of Fe mass%cu shown in Fig. 6, the different anodic behaviors of pure iron and Fe mass%cu can be explained as activation for pure iron and passivation for the alloys. Possible equilibrium reactions involving Fe/Fe(II)/Fe(III) species and Cu/Cu(I) species, and their equilibrium potentials ) at ph and 98 K are as follows; - / Ω Fe-mass%Cu Current Density, i / Am.mass%Cu mass%cu Pure copper Potential, E / V vs SSE Fig. Reciprocal of polarization resistance R p vs. time for pure iron and Fe mass%cu during cycles 9 and. Fig. 6 Polarization curves of pure iron, Fe. masscu, Fe mass%cu and pure copper in Ca(OH) solution (ph) containing. M NaCl.

5 J.-K. Kim, A. Nishikata and T. Tsuru Fe(OH) þ H þ þ e Fe þ H O E ð6þ ¼ :67 V (vs. SHE) ¼ :86 V (vs. SSE) Fe(OH) þ H þ þ e Fe(OH) þ H O E ð8þ ¼ : V (vs. SHE) ¼ :9V (vs. SSE) Cu O þ H þ þ e Cu þ H O E ðþ ¼ :9V (vs. SHE) ¼ :8 V (vs. SSE) ð6þ ð7þ ð8þ ð9þ ðþ ðþ From the equilibrium potentials at ph, the reaction taking place in the potential range of : : V (vs. SSE) for Fe Cu alloys seems to be oxidation of Cu to Cu O [eq. ()], implying that for Fe Cu alloys, iron may dissolve preferentially to leave a Cu enriched layer on the surface, though the presence of a Cu enriched layer was not identified by EDX analysis (probably because the layer is extremely thin, as mentioned before). This is confirmed by the fact that the anodic polarization curves of Fe mass%cu alloy showed a similar anodic polarization to pure copper, as shown in Fig. 6.. Conclusions The atmospheric corrosion properties of pure iron, Fe. mass%cu and Fe mass%cu have been investigated under cyclic wet-dry conditions using simulated concrete solutions. The experimental results point to the following conclusions. () The corrosion rate of pure iron and Fe. mass%cu are greatly accelerated by wet-dry cycles, while Fe mass%cu shows low corrosion rates and high corrosion potentials. () The anodic dissolution of Fe mass%cu alloy is hindered by copper-enriched Fe rust layers. () From the point of view of steel corrosion, the presence of copper as the tramp element in scrap steel is desirable when the scrap is used as reinforcing bar in a concrete environment. REFERENCES ) M. Iwase and K. Tokinori: Steel Research 6 (99) 9. ) A. Nishikata, Y. Ichihara and T. Tsuru: Corros. Sci. 7 (99) ) A. Nishikata, Y. Ichihara and T. Tsuru: Electrochim. Acta (996) 7 6. ) A. Nishikata, Y. Ichihara, Y. Hayashi and T. Tsuru: J. Electrochem. Soc. (997). ) H. Katayama, A. Nishikata, Y. Tay, A. S. Viloria and T. Tsuru: Mater. Trans., JIM 8 (997) ) F. Mansfeld: J. Electrochem. Soc. (988) 8. 7) A. Nishikata, S. Kumagai and T. Tsuru: Zairy-to-Kankyo (99) ) M. Stern and A. L. Geary: J. Electrochem. Soc. (97) ) J. O M. Bockris and A. K. N. Reddy: Modern Electrochemistry A, pp. 9, B, pp. 667 (Kluwer Academic/Plenum Publishers, New York, ). ) U. R. Evans: Corros. Sci. 9 (969) 8 8. ) H. W. Pickering and C. Wagner: J. Electrochem. Soc. (967) ) M. Stratmann, K. Bohnenkamp and T. Ramchandran: Corros. Sci. 7 (987) ) N. D. Tomashov: Theory of Corrosion and Protection of Metals, (MacMillan, New York 966) pp ) M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solutions, (NACE, Houston, 996).