Introduction
Marine applications have been in great demand nowadays. The perfect material for marine applications is stainless due to the fact it resists rust much better than many other materials for example bronze, brass or galvanized steel. By it is now very known that stainless steels varying from AISI 316 as much as 6Mo and superduplex don’t always resist seawater.
Crevice corrosion and pitting may develop eventually. A good example of this may be that the 25Cr07Ni super duplex tubular heat exchanger inside a marine vessel demonstrated crevice corrosion within half-a-year and services information. A biofilm will build up around the metal surface in natural seawater and it’ll always promote the corrosivity from the water. Microbiological Caused Corrosion (MIC) frequently happens in seawater. Also galvanic corrosion is an issue available at ocean.
This short article describes the performance of stainless in marine atmosphere. Materials selection in marine atmosphere is quickly gaining recognition due to the worldwide trend to target major production facilities around ocean ports to be able to increase cooling capacity and save transport cost.
Localised Corrosion
Stainless won’t ever corrode uniformly in marine atmosphere. Corrosion is localized, I.e.: crevice corrosion and pitting corrosion. Localized corrosion frequently has been promoted with a biofilm.
Crevice corrosion is an issue in marine atmosphere due to the low resistivity from the water (seawater resistivity is all about .35 Ohm•m). Even 6% Mo SS at 30°C can suffer crevice corrosion in seawater. If chlorinated, seawater below 25°C won’t cause pitting corrosion to duplex stainless 2205 and alloys with greater PREN-value. PREN value is understood to be ‘Pitting Resistance Equivalent Number’:
PREN = %Cr 3.3%Mo • X%N where X = 16 for duplex and X = 30 for austenitic steels. The greater the PREN value the greater the pitting resistance.
Stress corrosion cracking
Stainless 304 and 316 are responsive to chloride cracking at temperatures above 60°C. Existence of oxygen is essential, meaning created water from gas or oil production doesn’t cause stress corrosion cracking, even at hot temperature. 6% Mo and duplex stainless tend to be less responsive to this phenomena, however under two opposites, I.e. high stresses and temperature and cold deformation it might occur. Sometimes stress corrosion cracking is carried out in the outdoors especially longitudinal welded pipes at greater temperature are responsive to this kind of ‘corrosion under insulation’.
Galvanic Corrosion
Like crevice corrosion, the reduced resistivity of seawater also promotes strongly galvanic corrosion. Galvanic corrosion is viewed as a significant concern for materials performance in marine atmosphere. A common example is bronze bearings in ships, where sacrificial zinc anodes have to safeguard the steel shell for galvanic corrosion. Also stainless can suffer galvanic corrosion, or it causes galvanic corrosion with other, less noble, alloys.
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