Nickel-chromium alloys flowing seawater

Removal of the corrosion product or oxide layer by excessive flow velocities leads to increased corrosion rates of the metallic material. Corrosion rates 2ire often dependent on fluid flow and the availability of appropriate species required to drive electrochemical reactions. Surface shear stress is a measure of the force applied by fluid flow to the corrosion product film. For seawater, this takes into account changes in seawater density and kinematic viscosity with temperature and salinity [33]. Accelerated corrosion of copper-based alloys under velocity conditions occurs when the shear surface stress exceeds the binding force of the corrosion product film. Alloying elements such as chromium improve the adherence of the corrosion product film on copper alloys in seawater based on measurements of the surface shear stress. The critical shear stress for C72200 (297 N/m, 6.2 Ibf/ft ) far exceeds the critical shear stresses of both C70600 (43 N/m, 0.9 Ibf/ft ) and C71500 (48 N/m, 1.0 Ibf/ft ) copper-nickel alloys [33]. 

Alloys such as 304 and 316 stainless steel or nickel-chromium cdloys exhibit deep pitting in low flow conditions, yet at high seawater velocities their corrosion rate decreases to less than 25 pm per year. Contrary to this, iron and copper show significantly lower corrosion rates at low flow velocities than rmder high seawater flow conditions [37]. 

Nickel-copper and nickel-chromium-molybdenum alloys are the nickel-base alloys that are t5fpically used in seawater. The nickel-copper alloys have good corrosion resistance in high velocity seawater, but do exhibit localized corrosion in quiescent seawater [79]. Alloy 625, a nickel-chromium-molybdenum alloy, is susceptible to crevice corrosion in both quiescent and flow conditions [97-700]. Other nickel-chromium-molybdenum alloys, such as Alloys C-276, C-22, 59 and 686 have increased seawater crevice corrosion resistance as compared to Alloy 625 [97,98],... 

The austenitic cast iron alloys with 13-35% nickel and 1-5% chromium, also known under the commercial name Ni-Resist [93], show, due to their austenitic structure, much better corrosion behaviour in stagnant and flowing seawater than unalloyed castings. The corrosion resistance increases markedly with increasing chromium content. The carbon may be present either as graphite in lamellar form... 

Chromium has proven to be most beneficial toward improving the properties of the passive film of ferrous and nickel-based alloys while molybdenum, when added to these alloys, improves their pitting resistance. Oxide passive films that contain insufficient molybdenum, such as in many nickel-based alloys and stainless steels, are susceptible to pitting in stagnant and low-flowing seawater, but perform well on boldly exposed surfaces at intermediate and high flow velocities. In oilfield conditions, fluid velocity acts in... 

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