Seawater corrosion factors immersion

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. 

Metallurgical factors affect metal loss and tend to corrode at a lower rate with higher alloy content. Atmospheric corrosion resistance of steel was improved by alloying with Cu, P or Cr to form passive oxide layer [50]. Studies have shown that these steels show superior corrosion resistance in particular during atmospheric exposure but not so much for immersed exposure as in seawater and close to the coastline in the presence of high chloride concentrations. Alloying elements like... 

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]. 

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