Kudo, Takeo (Sumitomo Metal Industries) | Tanaka, Teruyuki (Sumitomo Metal Industries) | Sakaguchi, I. (Sumitomo Metal Industries) | Hirano, Masahiro (Sumitomo Metal Industries) | Takahiro Kushida, Kushida (Sumitomo Metal Industries) | Inaba, Yoji (Sumitomo Metal Industries)
electrochemical impedance spectroscopy, corrosion rate, CO2-contaminated hydrazine, decomposition
Corrosion behavior of iron (Fe), nickel (Ni), cobalt (Co) and titanium (Ti) alloys in hydrazine (N2H4) and CO2-contaminated N2H4 were studied using Electrochemical Impedance Spectroscopy (EIS). The corrosion rate of Fe and Ti alloys increased in hydrazine and CO2-contaminated hydrazine as a function of time and CO2 concentration; however, the corrosion rate of Ni and Co alloy decreased as a function of time in contaminated hydrazine. The corrosion rate of Ni and Co alloys were significantly higher when compared with corrosion rates of Fe and Ti alloys in CO2-contaminated hydrazine. T1-leeffects of CO2 concentration on corrosion rate has been explained in terms of alloy composition and the role of CO2 in forming carbazic acid and its metal complexes. The polarization values obtained from EIS studies were used to calculate the exchange current density and decomposition rate of hydrazine.
Previous investigations have indicated that presence of relatively small amounts of carbon dioxide impurity in hydrazine in contact with metal alloys can lead to significant metal corrosion and to an increase in the concentration of dissolved metal ions. The involvement of carbon dioxide in promoting corrosion of alloys by hydrazine and the consequent increase in its decomposition rate maybe due to the formation of carbazic acid (N2H5, N2H3CO2-)which has been claimed to be an effective catalyst3 for decomposition and metal dissolution.
A long-term real-time study of the compatibility of liquid propellants with spacecraft materials has been reported for hydrazine, hydrazinium nitrate, and nitrogen tetroxide. In general, only two methods for compatibility testing have been previously used: Real-time tests over periods extending several years and accelerated testing using temperature as an independent variable to increase the reaction rate between the material and the propellant. The former method is inconvenient, because the time required to evaluate material performance is too long. The latter method is deficient, because the mechanism for the corrosion and propellant decomposition may be changed.
For the current study an electrochemical technique, serves the dual purpose of measuring the corrosion rate of alloys and decomposition rates of electrolytes. Corrosion of alloys and decomposition of hydrazine can be described in terms of an oxidation-reduction mechanism, which implies that an electron transfer is taking place at the metal-hydrazine interface. Hydrazine decomposition and metal dissolution rates are proportional to the rate of electron transfer, and therefore, are directly proportional to an equilibrium current flow. This natural equilibrium exchange current flow can be upset by altering the potential energy barrier at the metal-hydrazine interface with an extreme]y small amount of externally applied potential to obtain a current-voltage curve at different frequencies. The mechanism of the reaction is not changed, since the applied potential in the polarization resistance measurement is very low (~ 50 millivolts (mV)). The EIS technique, in particular, ha:; an advantage over other techniques, as it uses 10 mV amplitude.
The objective of this investigation was to apply EIS to determine the compatibility of N2H, and C02- contaminated N2H4 with various alloys. The corrosion rates of 304L and 17-7PH Stainless Steel (SS) (iron alIoys), MP35N (cobalt alloy), Udimet 720 (nickel alloy) and TiGr5 (titanium alloy) are of interest.