2019 Volume 60 Issue 5 Pages 732-736
Acids provide corrosive environments that have their own behavior and characteristics when reacted with metal. Much research has been conducted to determine corrosion behavior of metals in acidic environments; however, little has been published concerning the corrosion behavior of ultralow carbon steel in mixed acid solutions, although such systems are widely used in the textile, chemical, and fertilizer industries. Corrosion behavior of ASTM A1008 ultralow carbon steel in mixtures of HNO3, H2SO4, and HCl was investigated using immersion and polarization methods. The HCl concentration was held constant at 1 M and the HNO3 and H2SO4 concentrations varied to 0.1 M, 0.5 M, and 1.0 M. Under these conditions, a synergistic effect between HNO3 and H2SO4 was observed, which thinned the metal, while HCl caused pitting of the metal surface. The results showed that the corrosion rate of the metal was enhanced as the acid concentrations increased, especially that of HNO3. The corrosion products were identified as iron(III) oxide [Fe2O3] and rhomboclase [H5Fe3+O2(SO4)2·2(H2O)].
Fig. 2 Variation of pH with immersion time for different test solution compositions.
Interaction between a metal and its environment cannot be avoided. There are many applications of metals exposed to extremely corrosive environments, such as those of the textile, pickling, and fertilizer industries.1) Strong acids, such as HNO3, H2SO4, and HCl, can be dissociated in a galvanic cell and become ions that promote corrosion. Much research has been conducted to observe corrosion behavior of metals in acid solutions, so it would be interesting to consider the effects of acid mixtures and determine the nature of metal deterioration that occurs.
Achmad et al.2) stated that no chemical reaction occurred when two strong acid solutions were mixed, because the oxidation numbers of the components of their constituent ions did not change. Therefore, carbon steel that is exposed to mixed acid solutions, such as those of HNO3, H2SO4, and HCl, would react with the individual ions contained in the solution.
Nitric acid is an oxidizing agent, in which the anion is more involved in oxidation than the cation.3) It has high oxidation power, characterized by low pH values and high potential values.4) Wattanaphan5) showed that exposure of carbon steel to nitric acid solution gave iron(II) nitrate as the reaction product. Iron(II) nitrate is highly soluble,6) so it would redissolve, leaving a bare steel surface that would again react with the oxidizing agent to continue the same reaction. This was the reason why there was no nitrate content observed in the corrosion product.
Dilute sulfuric and hydrochloric acids are non-oxidizing3) and have a tendency to reduce metals in corrosion reactions.4) The anion of these acid solutions has weakly oxidizing power. If carbon steel is exposed to a sulfuric acid environment, iron(II) sulfate is the reaction product.5) Exposure of carbon steel to hydrochloric acid results in an autocatalytic reaction. An electrochemical cell exposed to an aerated environment was employed to ensure interaction between a metal and oxygen. The reduction of oxygen caused oxidation of the metal and produced a corrosion product.
The experiments used plate-shaped specimens of ASTM A1008 DS grade A ultralow carbon steel that contained 0.08% C. This was cut into 40 mm × 10 mm × 1 mm blocks and drilled with 3-mm-diameter holes for the immersion procedure. For the polarization method, the specimens were cut into 10 mm × 10 mm × 1 mm blocks. The properties of the specimen were confirmed by metallography, hardness testing, and optical emission spectroscopy. Figure 1 shows the metallographic microstructure of the specimen. Table 1 presents the hardness data and Table 2 provides the chemical composition of the steel specimen.
Microstructure of ASTM A1008 ultralow carbon steel.
The experiments were carried out using mixtures of strong acid solutions. The hydrochloric acid concentration was held constant at 1 M, while the nitric and sulfuric acid concentrations were varied (0.1 M; 0.5 M; 1.0 M). The matrix of solution compositions evaluated is presented in Table 3.
The immersion procedure followed the ASTM A 262-02,7) ASTM G 1-03,8) and ASTM G 31-729) standard methods. All specimens were prepared by grinding sequentially with sandpaper grades of 120, 600, and 1000, then rinsing in soap water, followed by demineralized water and then acetone.
A predetermined volume of the experimental solution, comprising 115 mL of the appropriate concentration of each acid (see Table 3), was measured and placed in the immersion container. The specimen was suspended in the solution and the container stoppered to avoid evaporation. Each immersion test was carried out for 2 days. The pH was checked three times per day. After the second day of immersion, the specimen was removed and pickled with concentrated hydrochloric acid. Its final mass was determined. It was cut, mounted, and polished to observe the cross-section. The solution was filtered to obtain any solid corrosion product. This was dried, powdered, and analyzed by X-ray diffraction (XRD) to determine the compounds of the corrosion product. The compound identification was carried out using Match! 3 software (Crystal Impact, Germany).
2.2.2 Polarization methodThe polarization procedure followed the specifications of ASTM G 5-9410) and ASTM G 59-97.11) The specimens were spot-welded with stainless steel wire, sequentially ground with sandpapers of grade 600, 1000, and 2000, and then polished and rinsed. The specimen was placed in a chamber with experimental solutions made up to the compositions shown in Table 3 using 100 mL of each acid. The initial pH value was recorded and then the reference (Ag/AgCl) and counter (graphite) electrodes were installed. All three electrodes were connected to a potentiostat (VersaSTAT 3, Princeton Applied Research, USA). The system was allowed to stabilize for 15 min and a polarization curve then obtained at a scan rate of 10 mV/min. The curve was analyzed using Origin 201 software (OriginLab Corp., USA) to determine the Tafel extrapolation and hence the corrosion potential and corrosion current density. Once the experiment was completed, the reference and auxiliary electrodes were switched off and the pH value checked. The surface appearance of the specimen was documented.
2.2.3 Determination of corrosion rateFor the immersion data, the corrosion rate was determined using the mass loss method. The mass of the specimen was measured before and after the corrosion test and used to calculate the corrosion rate according to eq. (1):
| \begin{equation} \mathit{CR} = kW/\rho At, \end{equation} | (1) |
For the polarization data, the corrosion rate was calculated using eq. (2):
| \begin{equation} \mathit{CR} = ki_{\text{corr}}W/\rho, \end{equation} | (2) |
Figure 2 presents the variation in pH value of the solutions during the 2-day tests.
Variation of pH with immersion time for different test solution compositions.
The pH of all solutions steadily increased due to the reduction of H+ ions. Table 4 presents the reaction products that were obtained from these experiments.
Figure 2 shows that solutions with higher concentrations of nitric acid always resulted in more significant increase of pH values than those with higher concentrations of sulfuric acid. This was interesting, because it proved that nitric acid dominated the reactions that occurred in the system. Reaction products were generated and contributed to the increase in pH. As stated earlier, nitric acid is an oxidizing agent that has a tendency to oxidize metal, so iron ions tended to react with nitrate in preference to sulfate ions.
3.1.2 Determination of corrosion rate using mass loss methodThe corrosion rates of the steel in various acid mixtures are shown in Table 5. The corrosion rate increased with all increases in acid concentration. Figure 3 graphically depicts the increase in corrosion rate with sulfuric and nitric acid concentrations. The corrosion rate of a specimen was always higher for mixtures that had a high concentration of nitric acid and low concentration of sulfuric acid when compared with that of solutions with a high concentration of sulfuric acid and low concentration of nitric acid.
Dependence of corrosion rate on (a) sulfuric and (b) nitric acid concentration of solution at constant HCl concentration.
Figure 4 shows polarization curves for corrosion in various solution compositions. A summary of the data is shown in Table 6. The corrosion potential increased with increases in the concentrations of HNO3 and H2SO4, as indicated by movement of the curve toward the more positive direction on the x-axis. A higher corrosion potential value means that the potential difference between the cathode and anode was larger. The current density simultaneously increased. A larger current indicated a higher corrosion rate, which enabled the oxide layer to be formed more easily.
Polarization curves for corrosion of steel in different acid mixtures, showing effect of sulfuric acid concentration at nitric acid concentrations of (a) 0.1 M, (b) 0.5 M, and (c) 1.0 M and a constant hydrochloric acid concentration of 1.0 M.
In the anodic region, the curve was not linear as the concentration of nitric acid decreased because the passive layer was destroyed. The passive layer decayed more easily on exposure to solutions with higher acid concentration, so the solution would be in direct contact with bare metal. This is why the curve was more linear for solutions that had higher acid concentrations. In the cathodic region, the curve was not linear because many reactions occurred at the cathode.
3.3 Microscopy observation of corroded cross-sectionFigure 5 presents a photograph of a specimen cross-section that was observed using a metallurgical microscope (Nikon Eclipse MA200, Japan). The specimen became thinner and formed pits on exposure to the acid mixture. Thinning appeared to be caused by the nitric and sulfuric acids, whereas the pits were attributed to hydrochloric acid activity.
Microscope cross-section of specimen exposed to a mixture of 0.1 M HNO3, 0.1 M H2SO4, and 1.0 M HCl.
XRD examination of the corrosion product obtained from an immersion test using a solution containing 1 M of each of the three acids showed the formation of iron(III) oxide [Fe2O3] and rhomboclase [H5Fe3+O2(SO4)2·2(H2O)]. The reactions believed to be responsible for these products are shown in eqs. (3) to (6) and (7), respectively:
| \begin{equation} \text{Fe} + \text{H$_{2}$O} + \text{1/2O$_{2}$} \rightarrow \text{Fe(OH)$_{\text{2(aq)}}$}; \end{equation} | (3) |
| \begin{equation} \text{Fe(OH)$_{\text{2(aq)}}$} + \text{H$_{2}$O} + \text{O$_{2}$} \rightarrow \text{Fe(OH)$_{\text{3(aq)}}$}; \end{equation} | (4) |
| \begin{equation} \text{Fe(OH)$_{\text{3(aq)}}$} \rightarrow \text{FeO(OH)} + \text{H$_{2}$O}; \end{equation} | (5) |
| \begin{equation} \text{2FeO(OH)} \rightarrow \text{Fe$_{2}$O$_{\text{3(s)}}$} + \text{H$_{2}$O}; \end{equation} | (6) |
| \begin{align} &\text{Fe$^{3+}$} + \text{2SO$_{4}{}^{2-}$} + \text{9H$^{+}$} + \text{2O$_{2}$}\\ &\quad \rightarrow \text{H$_{5}$Fe$^{3+}$O$_{2}$(SO$_{4}$)$_{2}{\cdot}$2(H$_{2}$O)}. \end{align} | (7) |
This study examined the corrosion of ASTM A1008 ultralow carbon steel in mixtures of nitric, sulfuric, and hydrochloric acids using immersion and polarization test methods. The main conclusions are as follows:
We thank Kathryn Sole, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.
