Alloy UNS(1) N07718, commercially known as Alloy 718, is the most applied nickel alloy in the oil and gas industry. It is mainly composed by nickel, chromium and iron with additions of molybdenum, niobium, titanium and aluminum, the last three elements being responsible for the precipitation of the hardening phases gamma prime and gamma double prime, which make this alloy a good option when high strengths are required. Its high corrosion resistance is primarily due to its elevated nickel and chromium contents. Nitrides, carbides and carbo-nitrides might form during the manufacturing process, and oxides, borides and sulfides can be seen in a less extension. It is known that each precipitate interacts with the matrix and the oxide protective film, but there is a lack of studies related to the mechanisms occurring during the initiation of localized corrosion. In this paper, we present first results of a study related to pitting formation in UNS N07718 in chloride-containing solution and the evolution of the first corrosion steps.
Being one of the most applied nickel alloy in the oil and gas industry, the UNS N07718 (Alloy 718) is mainly composed by nickel, chromium, iron, molybdenum, niobium, titanium and aluminum. UNS N07718 is used in the solution annealed and precipitation hardened condition, where a complex microstructure is formed, which is responsible for its very good mechanical properties, as well as its cracking and corrosion resistance.
In the Oil and Gas industry, UNS N07718 is used in many components such as completion tools, packers, hangers, valves, flanges, down hole measurement tools among others, where high tensile as well as corrosion properties are required for the materials.1,2
The nickel, together with the chromium and iron, form the austenitic matrix of alloy 718. The high share of nickel and chromium in the matrix allows for good general corrosion and cracking resistance, while the iron is used to control the metal cost increase. Molybdenum is alloyed to about 3 wt.-% in order to grant to the alloy acceptable resistance against localized corrosion in chloride-containing media. Niobium, titanium and aluminum are the three elements, together with a share of the nickel from the matrix, that are responsible for the precipitation of the intermetallic phases gamma prime, gamma double prime and delta.1 While gamma prime and gamma double prime are important for the improvement of the mechanical properties by forming a barrier to the movement of dislocations, delta phase is not desired in the oil and gas applications, due to its fame of negatively affecting the corrosion and cracking properties of this alloy.3,4,5
