Immersion zone

Carbon and low-alloy steels are the predominant alloys used for construction of waterfront structures. In most applications the steels are protected by protective coatings in the atmospheric, splash, and intertidal zones (including metallic and elastomeric sheathing) and are protected by either cathodic protection alone or by a combination of coatings and cathodic protection in the immersion zone. Use of stainless steels is limited predominantly to mechanical applications rather than structural applications. The use of stainless steel fasteners is common. 

V 1 Marine atmosphere, splash, tidal and immersion zones... 

In the immersion zone (IZ) as well, the water is practically saturated with oxygen. In addition to fouling, solute contaminants, suspended solids and water currents are additional factors to be reckoned with here. 

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]. 

Difference between biggest and smallest sheet thickness in immersion zone, mm... 

Figure 3 Connection between corrosion mass ioss in the spiash zone and iocaiiy varying corrosion depth in the immersion zone after 3.5 years, materiais see Tabie 18 [15,16]...

Material Immersion zone Comparison IZ Tidal zone North Sea... 

The comparison of measurement results in the immersion zone of the North and Baltic Seas shows different values in the North Sea compared to the Baltic Sea. The comparison of tidal and immersion zones in the North Sea shows higher corrosion rates in the immersion zone with the exception of shipbuilding steel. In the contact corrosion tests it was seen that in the North Sea the potentials of the materials in contact with one other material are changed more than in the Baltic Sea. The mass losses per unit area of anode materials were greater in the pairings exposed in the North Sea than with the same pairings in the Baltic Sea (Table 13) [37]. Table 13 lists the factors by which the rate of corrosion is greater in contact corrosion than in free corrosion. 

The corrosion mass loss of the steels in Table 18 after exposure in the North Sea water off Helgoland in the splash, tidal, and immersion zones is shown in Figure 2. The scatter bands of the seven steels are shown. As was expected, the weather-resistant steel (O) with raised levels of copper, nickel, chromium and phosphoms, which favours the formation of denser mst layer, shows the lowest mst levels in the splash zone. A steel (A) with raised silicon and manganese contents shows nearly equivalent behaviour. The corrosion rates of both steels are at the lower limit of the scatter band. In the tidal and immersion zone, on the other hand, all steels fill out the scatter band uniformly. The resulting corrosion rates are hsted in Table 19. 

No dependence of the corrosion rates on the steel composition was found. The mean corrosion rates are higher in the tidal zone than in the immersion zone by a factor of 1.5. 

In principle, an improvement of rust corrosion rates in low-alloyed steels is possible as well in the tidal and immersion zone, but expectations should not be too high. The significant improvements frequently cited in the literature and patent registrations are usually based on tests in the laboratory and cannot be confirmed in practical applications or only in reduced form. It must also be noted that in the low-alloyed steels shallow pit corrosion or pitting corrosion is frequently observed in addition to uniform surface corrosion. Examples can be cited in which certain alloying elements reduce the uniform surface corrosion, only to cause increased pitting at the same time. 

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]. 

Whereas for unalloyed steels mean corrosion rates of 0.12 mm/a (4.72 mpy) in the immersion zone and 0.17 mm/a (6.69 mpy) in the tidal zone were recorded, the values for these low-alloyed steels were approx. 0.05 mm/a (2 mpy) for the immersion zone and 0.19 mm/a (7.5 mpy) for the tidal zone. The improvement in corrosion behaviour observed in the deep sea tests with 1% Cr -t 0.5% A1 by a factor of 2 can thus only be confirmed for the immersion zone. As was expected, the corrosion is most pronounced, and least dependent on the chemical composition of the steel, in the tidal zone. 

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 tidal zone, the effect of the higher sulphur content is correspondingly reduced, since here the rich oxygen supply required to oxidise the sulphur is only available at low tide and the influence of folding and cover layer formation increases with exposure duration. No influence of sulphur on corrosion rates is observed in the immersion zone. 

The different levels of oxygen access in the different water zones may, in extensive structures, for example sheet pilings or mooring posts, lead to the formation of aeration elements. Measurements have shown that the cathodic areas are usually on or just above the waterline [53]. The result is then increased corrosion in the tidal zone, which may be much larger than would be expected based on the different rusting rates in samples exposed separately in the tidal and immersion zones. 

Much more complex and difficult than the usually relatively small samples with approx. 0.02 nf surface are long samples with exposure extending from the splash zone (SZ) through the tidal zone (TZ) and into the immersion zone (IZ) or even into the silt zone/sea floor. Since the large-area aeration elements may form on such samples as described above, it can be expected that the results of corrosion tests in such long samples will differ clearly from those obtained with small individual samples. 

The results of comparative tests on both sample types for 4 years in the brackish water of Wilhelmshaven listed in Table 21 show that particularly pronounced differences occur in the tidal zone just above MTHW (mean tidal high water) and just below MTLW (mean tidal low water), whereas in the immersion zone, and above MTHW in the splash zone, the corrosion rates are the same, as was expected [54]. 

Splash zone (SZ) and tidal zone (TZ) Immersion zone ... 

Table 25 Results of stress corrosion cracking tests in bending samples after 12 months exposure time in the immersion zone (number of cracked samples in 3 parallel samples) [66]...

SZ splash zone TZ tidal zone IZ immersion zone FK free corrosion CCP cathodic corrosion protection... 

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]. 

Whereas in unalloyed steels the corrosion rates in the tidal zone are higher than in the immersion zone, the cast iron types show the highest corrosion rates in the immersion zone. This is also confirmed by exposure tests over a period of several years off Helgoland in which cast with lamellar graphite (GGL) and cast iron with spherical graphite (GCG) samples were exposed, in each case with and without addition of copper (Table 30) [86]. 

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]...

In the tidal zone and the immersion zone, no influence of graphite formation or addition of copper on the corrosion behaviour is recognisable. In the splash zone, on the other hand, the corrosion rates in the GCL samples are all at the lower limit of the scatter band. After the test period of 4.5 years, scatter ranges as follows were determined for surface corrosion ... 

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