Nickel flowing seawater

Nickel is resistant to chloride-induced SCC, but subject to caustic cracking in aerated solutions under high stress. Nickel is highly resistant to corrosion in natural fresh water and flowing seawater. Pitting occurs under stagnant or crevice conditions. 

Figure 4. 1. Complex plane impedance diagrams for 90 10 Cu Ni alloy in flowing seawater as a function of exposure time. Flow velocity = 1.62m/sec, [O2] = 0.045 mg/1, specimen area = ll.OScm T = 26°C exposure time = 50h. (From B. C. Syrett and D. D. Macdonald, The Validity of Electrochmical Methods for Measuring Corrosion Rates of Copper-Nickel Alloys in Seawater. Reprinted with permission from Corrosion, 35, 11, [1979], NACE, Houston, TX.) Numbers next to each point to frequency in hertz.

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... 

Seawater. Titanium-nickel is not affected when immersed in flowing seawater however, in stagnant seawater, such as foimd in crevices, the protective film can break down, which results in pitting corrosion. 

Alves et al. [744] determined vanadium, nickel, and arsenic in seawater in the 10-20 000 ppt range using flow injection cryogenic desolvation ICP-MS. 

Komatiite lava flows are very rich in MgO. They contain significant iron oxide, and are typically associated with nickel sulfides and chromite. Hydrothermal systems in highly magnesian rocks can be very alkaline, with very high pH. Thus, it would be expected that rain falling onto komatiite flows, or flows into shallow (low-pressure) seawater, would generate very alkaline outflows of hot or warm water. 

Table 7.5 Corrosion rates in seawater at different flow velocities (data from International Nickel Co). (Adapted from Fontana and Greene [7.1].)...

For industrial use, nickel coatings are applied without chromium on top, and the coatings are most often thicker, usually 25-250 pm, and up to 500 pm for some applications. Such coatings are most suitable for environments that tend to cause general corrosion with reaction products of low solubility (environments without chlorides). However, it is also stated that nickel performs well in seawater provided that the flow rate is high. Furthermore, the experience with nickel in various alkalis, neutral and alkaline salts and organic chemicals is good. 

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 corrosion behaviour of nickel and 14 nickel alloys was tested in comparison to titanium and stainless austenitic steel (Table 66) by means of exposure in stagnant and flowing natural seawater (flow rate 0.23 m/s) over a period of 1.6 years [205]. 

Copper-based alloys. The copper-based alloys are velocity-limited, as impingement attack occurs when the hydrodynamic effect caused by seawater flow across the surface of such alloys exceeds the value at which protective films are removed and erosion-corrosion occurs. Thus, if these alloys are to exhibit high corrosion resistance, they must be used at design velocities below this limiting value. A more detailed coverage of the marine usage of two important copper-nickel alloys is presented in the section on copper alloys. 

The protective film continues to become more protective with time, as indicated by corrosion rate measures made over several years. Studies in quiet seawater show that the time span approaches 4 years before the decrease in corrosion rate becomes negligible. In flowing water, the corrosion rate was found to decrease continually over at least a 14-year period, the effect being similar for both 90-10 and 70-30 alloys. The normal corrosion product film is thin, adherent, and durable. Once fully formed and reasonably mature, the film on copper-nickel alloys will withstand considerable excursions in water velocity, pollution, and other conditions normally adverse to the good performance of copper alloy tubing. Copper-nickel alloys remain resistant to corrosion in deaerated seawater at low pH, as has been experienced in numerous distillation-type desalination plants. ... 

Effect of velocity. The combination of low general corrosion rates and high resistance to pitting and crevice corrosion ensures that the copper-nickel alloys will perform well in quiet, clean, and aerated seawater. As the flow rate of seawater increases, the corrosion rate remains... 

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