Pitting potential of Inconel 718 in chloride base aqueous solutions
Inconel 718 is one of the most essential super alloys and is widely used in aerospace and aviation for its high mechanical strength, good resistance to fatigue and creep, supreme corrosion resistance and potential to serve regularly at the high temperatures. In this post, we evaluate the pitting corrosion resistance potential of Inconel 718 before and after double aging heat processing.
From beginning, nickel base super alloys are widely used in aerospace industry for their excellent characteristics. Inconel alloy 718 has been used in nuclear, cryogenic, oil and various aerospace industries such as in jet turbines for civil and military use.
Chromium and aluminium develop their adherent oxides, Inconel 718 surface passivation is received that features its corrosion resistance. Localized corrosion such as pitting occurs when the passivation layer is penetrated commonly in defects like additions, dislocations, grain boundaries and others.
First process is solid solution that promotes homogenization and redistribution of alloy elements and increases the grain size. Aging promotes precipitation of secondary phases primarily gamma prime and gamma double prime.
Traditional metallographic procedures of grinding and polishing were used prior every corrosion testing. For the corrosion test and electrolytic cell with three electrodes was used, a working electrode of Inconel 718 wire. The corrosion medium was 3.5% wt NaCl aqueous solution without stirring at ambient temperature and pH 6.05. In order to feature the pitting development, the surfaces were tested. Conventional metallographic procedures of grinding and polishing were used prior every corrosion testing. For the corrosion test an electrolytic cell with three electrodes was used, a working electrode, a reference electrode of Ag/AgCl and a counter electrode of platinum. To feature the pitting development, the surfaces were tested by using a Tescan electron microscope equipped with EDS system of microanalysis. Cyclic voltammetry and chronoamperometric tests were also conducted after one hour of every surface to the electrolyte solution.
The process ASTMF746-04 was used to find the pitting corrosion potential. After this procedure, initially he OCP is determined after which the potential is transferred to higher value. To find accurate critical pitting potential, a chronoamperometric procedure was performed. However before that the breakdown potential was received through cyclic voltammetry. It was utilized as starting potential in the chronoamperometric tests. Inconel 718 surfaces were subjected to the chloride based aqueous solutions. There is no considerable change was found in the corrosion trend of alloy 718 before and after the heat processing.
Chronoamperometric results show that for the as received Inconel 718 pitting corrosion is higher than double aged sample. Pitting potential of as received Inconel 718 is nobler than that of double aged material. So thermodynamically the double aged surfaces are more prone to pitting developments as compare to the as received materials. Additionally, at the pitting corrosion potential, the induction time for pitting. The procedures suggested in standard ASTM F746-04 separately are not sufficient to receive information about pitting resistance of Inconel 718 wire in chloride based aqueous solution, possibly due to high corrosion resistance of this alloy.