Seawater tidal zone

In the tidal zone and the spray zone (known as the splash zone), cathodic protection is generally not very effective. Here thick coatings or sheathing with corrosion-resistance materials (e.g., based on NiCu) are necessary to prevent corrosion attack [4]. The coatings are severely mechanically stressed and must be so formed that repair is possible even under spray conditions. Their stability against cathodic polarization (see Section 17.2), marine growths, UV rays and seawater must be ensured [4,5]. 

In seawater, calcium aluminate cement is more durable than Portland cement Conversion also takes place here, but it is usually veiy slow, except in the tidal zone or in warm waters (Baker and Banfill, 1992). If seawater is employed as mixing water the initial hydration may be retarded, but the final microstmcture of the hardened material is very similar to that made with fresh water. Chloroaluminates are formed in the hydration reaction (Raise and Pratt, 1986). 

Tidal—The tidal zone is an environment where metals are alternately submerged in seawater and exposed to the splash/spray zone as the tide fluctuates. In the submerged condition, metals are exposed to well-aerated seawater and biofouling does occur [11,121. A continuous cover of biofouling organisms protects some metal surfaces such as steel, while the presence of biofouling on stainless steel surfaces can accelerate localized corrosion. Steel is influenced by tidal flow, where increased movement due to tidal action causes an increased steel corrosion rate [121. Curve (b) in Fig. 1 shows that steel corrosion at exposed coating defect sites is as severe in the tidal zone as it is in the splash/spray zone. 

Evaluation of this seawater must consider its movement as well as its oxygen content, since both factors are of equal importance when it comes to transporting oxygen to the steel surface. This is why the corrosion rates in the tidal zone (TZ) are greater by a factor of 1.5 than in the immersion zone (IZ) [15]. 

Extensive exposure tests in the North Sea on the influence of the elements copper, chromium, aluminium, nickel and silicon on corrosion in seawater showed that the corrosion in the immersion zone in seawater is significantly reduced by suitable combinations of the alloying elements Cr -t Al, Cr -t Al -t Cu and Cr -t Si. At longer exposure times, the corrosion rates can be reduced to as little as 20% of the rates for unalloyed steel. In the tidal zone (TZ), however, only the combination Cr -t- Si results in an improvement, albeit of only 20% after four years. In the splash zone (SZ) improvements by a factor of 2 can be achieved [50]. 

For the seawater corrosion resistant steels sold commercially under various different designations, the reported improvements in corrosion behaviour by a factor of 2 or 3 also apply only to the immersion or non-immersion zone. In the tidal zone, the corrosion rates practically fall into the general scatter band of the low-alloyed steels [47, 51]. Therefore, these steels also require corrosion protection in the tidal zone. 

In the immersion zone in seawater, iron-carbon cast alloys show somewhat less corrosion than steels. Since cast parts usually have thicker walls, such structural elements often show longer useful lives than rolled materials. In the splash and tidal zones, the corrosion rates are, however, as much as one-third lower than is observed in unalloyed steel types [23, 86]. 

The pitting depths listed in Table 39 show that pitting corrosion with pronounced fouling in the tidal zone can be worse than in the immersion zone and that stagnant conditions can constitute a much more critical situation than flowing seawater... 

In tropical waters, the corrosion values for nickel are higher than in the temperate climatic zones. The pitting depths reach approx. 3 mm after only 1 year, after which the penetration rate drops. Figure 43 and Figure 44 show the results of exposure tests of nickel and nickel alloys in the seawater of the Panama Canal Zone. Whereas in the immersion zone pitting depths of over 3 mm were reached, the values in the tidal zone were about 1.6 mm [192]. 

Depending on composition brackish waters can be more aggressive than seawater. In tidal estuaries, the highest corrosion rate of carbon steel is just below the tidal zone, while in open seawater the highest corrosion rates are in the splash zone [8]. 

The corrosion resistance of nickel alloys has been extensively explored in seawater and saltwater (brackish water). Although stainless steel 316 is known to resist pitting in seawater, stainless steels are, in general, susceptible to pitting in the tidal zones of seawater. The nickel alloys, more expensive than steels, have been extensively used in seawater service. Inconel alloy 625 offers an excellent resistance to corrosion in seawater. It also offers an excellent resistance to SCC. Nickel alloys are best used for pump shafts, bodies and impellers while other materials, like 90-10 Cu-Ni and austenitic steels are used for other parts, such as heat exchangers and valves. Table 9.47 shows the classification of selected nickel alloys in seawater service. 

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. 

This chapter covers information applicable to zinc corrosion behavior in general. Chapter 2 covers corrosion in the atmosphere—which is the most important group of environments in which zinc is used. Attack is usually approximately linear with time, but often with some reduction of rate as protective films form. Many results are available, and tables have been prepared for the guidance of designers. Water corrosion follows in Chapter 3, with distinctions between hard and soft tap water (hot and cold), temperate and tropical seawater, and tidal and splash zones. Buried structures—together with a section on earth reinforcement—follow in Chapter 4, and conditions appropriate for zinc sacrificial anodes are included in both Chapters 3 and 4. 

Environmental zones involving seawater include marine atmosphere, splash/spray, tidal, submeiged/shallow ocean, deep ocean, and mud. Figure 1 illustrates the relative... 

Figure 34 Mass losses in cast iron samples from Table 30 after exposure in the splash, tidal and immersion zones in seawater off Helgoland [86]...

From this viewpoint, the natural environments should be better identified, in terms of ecological habitats, and test approaches should in turn be based on the typical conditions experienced by the plastic products when entering each habitat, since the microbial population (and thus biodegradation activity) could be quite different. Tosin and co-workers [13] have identified 6 habitats where plastic waste can reside when littered in the marine environment 1) pelagic domain (the plastic products float freely in estuaries and the open ocean water), 2) eulittoral zone (tides and storm waves bring great quantities of plastic waste to the shoreline, where plastic products get partly buried and kept wet by tidal inundation and waves), 3) supralittoral zone (the plastic products are washed onto the beach, exposed to a sandy soil with a low moisture level), 4) sublittoral zone (plastic products settle on marine sandy sediment where they are exposed to the seawater/sediment interface), 5) plastic products can otherwise sink to the bottom of the deep sea and 6) plastic products can be slowly buried within sediments on the sea-floor. 

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