Erosion corrosion seawater

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

Since the formation nature and breakdown of protective surface films depends on both material and environmental parameters such influences on erosion corrosion will be discussed together. Particular attention will be paid to the copper/seawater and carbon steel/water (steam) systems. 

Briefly the important developments in copper alloys with respect to their erosion corrosion behaviour in seawater have been ... 

The corrosion rate of carbon steel increases with increase in velocity until a critical velocity is reached. This behavior is different from that of the carbon steel in fresh water where the corrosion rate decreases beyond a critical velocity due to the formation of a passive him. In seawater passive films are not formed because of the presence of high concentrations of chloride. The erosion corrosion occurs after critical velocity 20 m/s is reached. The maximum corrosion rate of 1,0/mm/yr is reached at velocities up to 4 m/s. 

Figure 7.46 Critical velocities for erosion corrosion of different materials in seawater. (From Bemhardson et al. [7.41].)...

Syrett BC. Erosion corrosion of copper-nickel alloys in seawater and other aqueous environments - A literature review. Corrosion, 32(6), June 1976. 

With higher flow rates, the corrosion rate increases up to around 40 ft/s (12 mm/s) where the attack changes to erosion-corrosion, which means that any protective oxide or adsorbed layer is stripped away and bare steel is open to accelerated attack. Turbulence has a similar effect. Figure 3.3 and Figure 3.4 show the effects of increasing flow velocity for distilled water and seawater. At 39ft/s the corrosion rate in distilled water at 50°C (122°F) exceeds 200 mpy (5 mm/year). 

Flow velocities play an important part in seawater corrosion. Velocity-dependent erosion corrosion is often associated with turbulance and entrained particles. Recommended velocities for continuous service are as follows ... 

The protective oxide film of most metals is subject to being swept away above a critical water velocity. After this takes place, accelerated corrosion attack occurs. This is known as erosion-corrosion. For some metals, this can occur at velocities as low as 2-3 ft/s. The critical velocity for titanium in seawater is in excess of 90 ft/s. Numerous corrosion-erosion tests have been conducted and all have shown that titanium has outstanding resistance to this form of corrosion. 

Figure 6.40 Erosion-corrosion of a brass tube carrying out seawater. (Courtesy of Defence R D Canada-Atiantic)...

Aluminum bronzes containing 7% Al, 2% Ni, show an outstanding resistance to de-alloying and cavitation corrosion in most fluids and seawater, because of nickel addition which is highly resistant to corrosion. Aluminum bronze, such as 76 Cu-22 Zn-2 Al, are used for marine heat exchangers and condenser because of its excellent corrosion resistance. Aluminum is responsible for increased corrosion resistance. But the velocity must not exceed a safe threshold to avoid erosion-corrosion. 

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. 

Titanium is fully resistant to natural seawater regardless of chemistry variations and pollution effects (i.e., sulfides). Twenty-year corrosion rates well below 0.0003 mm y have been measured on titanium exposed beneath the sea and in splash or tidal zones. In the sea, titanium alloys are immime to all forms of locahzed corrosion and withstand seawater impingement and flow velocities in excess of 30 m s Table 8.43 compares the erosion-corrosion resistance of unalloyed titanium with two commonly used seawater materials. In addition, the fatigue strength and toughness of most titanium alloys are unaffected in seawater, and many titaniinn alloys are immime to seawater stress corrosion. 

Experience with tubular heat exchangers in aluminium alloys of the series 3000, 5000 and 6000 used for desalination of seawater has shown that aluminium can withstand a flow speed in the order of 2.5-3 m s at temperatures up to 130 °C with no erosion corrosion. This range corresponds to the usual flow speed in industrial installations. Tests with distilled water at 100 °C have shown that the erosion of aluminium starts at a flow speed in the order of 12-15 m-s [42]. 

The velocity of the environment plays an important role in erosion-corrosion Table 2.2 shows the effect of velocity on a variety of materials and alloys exposed to seawater. These data show that the effect of velocity can range from nil to extremely great. 

This type of damage is dealt with comprehensively in Section 8.8. It can be particularly severe in seawater giving rise to cavitation corrosion or cavitation erosion mechanisms, and hence can be a considerable problem in marine and offshore engineering. Components that may suffer in this way include the suction faces of propellers, the suction areas of pump impellers and casings, diffusers, shaft brackets, rudders and diesel-engine cylinder liners. There is also evidence that cavitation conditions can develop in seawater, drilling mud and produced oil/gas waterlines with turbulent high rates of flow. 

Seawater-based utility systems for condenser and process cooling systems in power plants exhibit serious corrosion, erosion and fouling problems. Equipment made from carbon steel and even stainless steel shows sign of degradation from galvanic effect, corrosion, erosion and microbiological induced corrosion (MIC). Corrosion... 

Ship hulls Painting cannot always protect hostile marine conditions, in ships and, areas above keel blocks. Stem and mdder areas suffer erosion and corrosion due to the high turbulence caused by the propeller coupled with the galvanic effects of the noble bronze propeller. Effective cathodic protection of ship hulls and similar marine structures in seawater against corrosion can be apphed using either aluminum or zinc alloy sacrificial anodes. Twenty percent of the anodes required for full hull protection are required for stern protection only. 

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