Effect of Elevated Temperature on the Corrosion Polarization of NO7718 and NO7208 Nickel Alloys in Hot Acid Chloride Solution

Abstract

The effects of ambient and elevated temperatures (35 °C, 45 °C, 65 °C, 85 °C and 100 °C) on the corrosion resistances of NO7718 and NO7208 nickel alloys were evaluated in 2 M H2SO4/6% aqueous media using potentiodynamic polarization method and micro-analytical studies. Data obtained showed temperature increase restricted the passive film formation on NO7718. Beyond 45 °C, passivation disappeared leading to significant localized corrosion and comparatively higher corrosion rate values. NO7208 retained its passivation characteristics up to 100 °C with minimal change in corrosion rate. Metastable pitting activity was visible on NO7208 with respect to temperature. Negative entropy values for NO7718 and NO7208 show the activation complex is an association between the alloy cations resulting in slower corrosion from reactants to the activation complex at specific localized corrosion reaction sites. The exothermic nature of the corrosion reactions was shown from the negative enthalpy values. Lower enthalpy of NO7208 compared to NO7718 shows that heat of corrosion reaction is lower due to slower rate of corrosion. The activation energy for NO7208 is lower than the value for NO7718 due to its more resistant passive property. The exothermic nature of the corrosion process is responsible for the positive Gibbs free energy of both steels. Superficial oxide with numerous corrosion pits appeared on the morphology of NO7718 at 35 °C compared to NO7208 morphology of which showed the presence of fewer pits. At 100 °C, NO7718 morphology was severely degraded with respect to its intermetallic phases coupled with visible corrosion pits, and intergranular cracks, while on NO7208, the corrosion pits and intergranular cracks where relatively superficial.