Effect of Inclusions on the Corrosion Properties of the Nickel-Based Alloys 718 and EP718

Abstract

Inclusions in steels and alloys are known to lower the resistance to deformation, as well as to lower the mechanical, corrosion and other properties. Studies of inclusions in nickel-based alloys are important since these materials could suffer from corrosion degradation in harsh operational conditions. This, in fact, could lead to a pitting initiation around the inclusions. Two industrial Ni-based alloys (alloy 718 and EP718) were investigated to determine the harmful effects of different inclusions on the corrosion resistance of Ni-based alloys. Specifically, the inclusion characteristics (such as composition, morphology, size, number and location) were determined for inclusions collected on film filters after electrolytic extraction and dissolution of a metal matrix around different inclusions on surfaces of metal samples after electrolytic extraction (EE). It was found that both Ni-based alloys contain various inclusion types: carbides (large size NbTi-C and small multicomponent carbides), nitrides TiNb-N and sulphides (TiNb-S in EP718 alloy). The most harmful effects on the corrosion resistance of metal were detected around sulphides and small carbides containing Mo, W, Cr. Dissolution effects were also observed around large carbides and nitrides, especially around inclusions larger than 10 µm. Moreover, the dissolution of a matrix around inclusions and clusters located on the grain boundaries were found to be 2.1–2.7 times larger compared to inclusions found inside of grains of the given alloy samples.

Pitting corrosion resistance of additive manufactured alloy 718 in alkaline brines at elevated temperatures

“…[ 26 ] As shown in recent studies, Nb‐rich particles are considerably larger (0.5–30 µm) on wrought 718. [ 26,27 ] Furthermore, Nb‐rich carbides have been recognized as potential initiation sites for pitting corrosion in wrought 718. [ 27–29 ] However, the microstructural examinations conducted inside the pits formed during the polarization scans have shown that these particles remain intact during the anodic dissolution (Figures 12 and 14), which is in line with the observations made by Alekseeva et al [ 27 ] Therefore, not the particle but their interface to the austenitic matrix constitutes most likely the initiation site for localized corrosion and the reason for the observed preferential dissolution of the matrix (Figures 10–12).…”

On the Influence of the Microstructure upon the Fatigue and Corrosion Fatigue Behavior of UNS N07718

“…However, in the present study electrochemical results alone could not be utilized to infer the CF behavior of alloy 718. Since the corrosion behavior of alloy 718 is mainly influenced by inclusions [26,27], and conventional electrochemical techniques do not have enough resolution to assess the influence of these microstructural particularities, the obtained electrochemical results should be taken carefully. Nevertheless, the excellent pitting corrosion resistance of 718 observed during the cyclic potentiodynamic polarization tests correlates well with the absence of pitting determined on the specimens after CF testing.…”

Corrosion Resistance of 13Cr Steels

“…Metal samples were used for three-dimensional (3-D) investigations of non-metallic inclusions extracted using the electrolytic extraction (EE) technique. The electrolytic extractions were carried out at the KTH Royal Institute of Technology (Stockholm, Sweden) by using the following extraction parameters: electrolyte, 10%AA (10% acetylacetone-1% tetramethyl-ammonium chloride-methanol); electric current, 40~60 mA; voltage, 2.9~3.8 V. After an electrolytic dissolution of metal matrix, the non-metallic inclusions, which did not dissolve in the given electrolyte, were collected on a surface of a membrane polycarbonate film filter (with a 0.4 µm open-pore diameter) during filtration of the electrolyte after the completed EE [20,31]. The characteristics of inclusions (such as size, morphology, and chemical composition) were analyzed by using an SEM combined with EDS.…”

“…With a decrease in the carbon content, the ductility of the steel increases, the impact resistance increases, and the machinability improves, while the hardness and strength decrease. Additional alloying of steels of the supermartensitic 13Cr type with strong carbide-forming elements in an amount of up to 0.05% contributes to a significant increase in strength properties due to secondary hardening without a negative effect on the ductile and corrosion properties [29][30][31].…”