Application of corrosion resistant alloys in deep sea downhole conditions

High strength corrosion resistant alloys for deep sea downhole applications have been in use since 1980. Although cold processed nickel chrome alloys were used in tubing forms, their machining into complex components was not possible. Further low alloy and stainless steels were used, Monel k500  was also used for high strength and corrosion resistance, however it was corroded in hydrogen sulfide conditions at high temperatures. In fact nickel-chrome alloys  received corrosion in this field. Oil applications needed subsurface protective vales that could deal with temperatures up to 150oC and pressures up to 140MPa. For this precipitaton hardened Inconel 718 was used for its excellent properties.

Nickel alloys are austenitic in nature and are widely engineered to enhance their microstructures. However the composition of these alloys varies extensively, there are major compositional elements such as iron, cobalt, tungsten, molybdenum, chromium, and rhenium.

Inconel alloy 718 is popular for its outstanding performance at high temperatures and resistance to mechanical and chemical corrosion. It has large grains and a clean microstructure. The microstructure and hence the function of an alloy are determined by its development and processing background. Aging cycles fine-tune the microstructure for specific applications.

The melting and casting processes provide an initial microstructure with the precipitation of secondary phases. These processes need careful control because it is not easy to dissolve few precipitates with the after heat treatments. Inconel 718 is often vacuum induction melted and remelted through vacuum arc remelting and electro slag remelting. The remelting process manages improves the uniformity and purity of the microstructure. High temperature and forging treatments are used to organize the microstructure by dissolving second phases to undo segregation that occurs during solidification.

Heat processing methods are applied to achieve a specific property of an alloy. Inconel 718 is processed via two types of methods: Direct Ageing and solution annealing with double ageing processing. Direct ageing enhances the strength but it cannot dissolve the harmful precipitates.

Inconel 718 is heat treated by: annealing up to 1040oC for approx 1 hour, water cooled and precipitation hardened up to 800oC for ten hours and furnace cooled to bring temperature down to 650oC for 8 hours. The annealing temperature of this material is slightly higher to increase the magnitude of dissolved precipitates. After, aging processing is performed to enhance the gamma double prime precipitation.

Nickelbased superalloy Inconel 718 offers outstanding resistance to localized corrosion, intergranular attack and SCC. The extent of resistant is based on the alloy’s microstructure and composition. It is hence essential to consider the compositional elements of nickel alloys and their effect on the corrosion resistance.

Nickel prevents chloride based SCC and molybdenum and copper prevent corrosion in reducing conditions. Chromium prevents localized attack by developing chromium oxide layer.

Materials used in oil and gas plants are exposed to sulfur based conditions for example hydrogen sulfide in downhole pipes. These conditions cause sulfide corrosion and attack the alloys at the higher rate. To prevent this corrosion, nickel alloys with higher chromium content should be used.