Electrochemical Behaviors of Various Alloys in Ferric Sulfate Solution
Authors: Xiao Qian , Chen Junjie , Chen Zhen , Lu Zhanpeng 
Published: February 2020

A508III low alloy steels (LAS) are often used as reactor pressure vessel materials in nuclear power plants. LAS could be exposed to primary water of nuclear power plants when stainless steel or Ni-based alloy corrosion resistant layers failed or as a result of small leaks in flanged joints valves, fittings, etc. Primary water contains boric acid and lithium hydroxide. Under leaking conditions, the primary water could be concentrated due to evaporation and become aerated as the results of contacting with the environment, producing varying degrees of external corrosion. There could be corrosion product such as rust or ferric ions at the leaking sites. Previous work has conformed that the corrosion rates of A508III steel increased dramatically when ferric sulfate was added in boric acid solution. While in the ferric sulfate solution, adding boric acid had no significant effect on the electrochemical behavior and corrosion rates, suggesting that ferric ions promoted the subsequent corrosion process.
A508III LAS contains about ~97 wt.% Fe, with Cr, Ni, Mn and other elements added to improve the corrosion resistance, mechanical properties and processing performance, etc. The present work investigated the electrochemical behavior of various alloys and their main alloying elements in 0.05 mol/L Fe2(SO4)3 solution at 25 oC. Pure iron, Cr, Ni, A508III LAS, 430 steel, 316L steel, and Alloy 690 were used as working electrodes. Open circuit potential (OCP), electrochemical impedance spectroscopy and anodic polarization curve were measured.
OCP values of A508III steel and iron in 0.05 mol/L Fe2(SO4)3 solution were lower than -0.52 V(SCE). The OCP of A508III steel is a little higher than that of iron. OCP values of the other materials were stable at relatively higher values that range from 0.37 V(SCE) to 0.56 V(SCE), and the ascending order was that 316L steel, 430 steel, Cr, Alloy 690, and Ni. The anodic polarization curves for various materials in 0.05 mol/L Fe2(SO4)3 solution are shown in Figure 1. Anodic polarization curves for A508III and iron were nearly the same, and neither current peak nor dissolution/passivation transition could be observed. Active-passivation transition appeared and single current peak were observed on the curves for 316L and 430 steel. The anodic polarization curves for Cr, 690 alloy and Ni showed different trends at different potential ranges.
The EIS Nyquist plots of various materials in 0.05 mol/L Fe2(SO4)3 solution are shown in Figure 2. The Nyquist plots of A508III steel and iron exhibited two capacitive loops at higher and intermediate frequencies, and one inductive loop at low frequencies. The second capacitive loop was smaller than the first which suggest that the protectiveness of surface film is not good. The second capacitive loop of iron was a little larger than that of A508III steel and this suggest that protectiveness of iron was slightly better. The Nyquist plots of Cr, 316L steel, 430 steel and Alloy 690 exhibited two capacitive loops. The second capacitive loop was far greater than the first one, suggesting that the high protectiveness of the surface films. The size of second capacitive loop decreased in the order as 316L steel, Alloy 690, 430 steel, and Cr. This suggests that when immersed in 0.05 mol/L Fe2(SO4)3 solution, the surface film protectiveness of Cr was good. The Nyquist plots of Ni exhibited two capacitive loops at high and intermediate frequencies, and Warburg characteristics at low frequencies. The first capacitive loops were nearly the same for all materials in this context. The second capacitive loop of Ni was significantly greater than those of A508III steel and iron, but far smaller than those of 316L steel, Alloy 690, 430 steel and Cr.
The amounts of Cr and Ni in A508III LAS were not enough to significantly affect the electrochemical behavior in the Fe2(SO4)3 solution. The electrochemical behaviors of A508III steel and iron were nearly the same. Protective films could form on the electrode surfaces at OCPs in 0.05 mol/L Fe2(SO4)3 solution if there was a certain Cr content in alloys. The reactivity of materials with certain Cr element was higher than that with certain nickel element under anodic polarization.

Figure 1