Marine Stainless Steel Applications
Marine applications are in great demand these days. The ideal material for marine applications is stainless steel because of the fact that it resists rust better than other materials such as bronze, brass or galvanized steel. By now it is very known that stainless steels ranging from AISI 316 up to 6Mo and superduplex do not always resist seawater.
Crevice corrosion and pitting may develop sooner or later. An example of this could be that a 25Cr07Ni super duplex tubular heat exchanger in a marine vessel showed crevice corrosion within half-a-year of service. A biofilm will develop on the metal surface in natural seawater and it will always promote the corrosivity of the water. Microbiological Induced Corrosion (MIC) often occurs in seawater. Also galvanic corrosion is a major problem found at sea.
This article describes the performance of stainless steel in marine environment. Materials selection in marine environment is rapidly gaining popularity because of the worldwide trend to concentrate major industrial facilities around sea ports in order to increase cooling capacity and save transport cost.
Stainless steel will never corrode uniformly in marine environment. Corrosion is localized, I.e.: crevice corrosion and pitting corrosion. Localized corrosion often is being promoted by a biofilm.
Crevice corrosion is a major problem in marine environment because of the low resistivity of the water (seawater resistivity is about 0.35 Ohm•m). Even 6% Mo SS at 30°C can suffer crevice corrosion in seawater. If chlorinated, seawater below 25°C will not cause pitting corrosion to duplex stainless steel 2205 and alloys with higher PREN-value. PREN value is defined as 'Pitting Resistance Equivalent Number':
PREN = %Cr + 3.3%Mo • X%N where X = 16 for duplex and X = 30 for austenitic steels. The higher the PREN value the better the pitting resistance.
Stress corrosion cracking
Stainless steel 304 and 316 are sensitive to chloride cracking at temperatures above 60°C. Presence of oxygen is a must, which means that produced water from gas or oil production does not cause stress corrosion cracking, even at high temperature. 6% Mo and duplex stainless steel are much less sensitive to this phenomena, however under extreme conditions, I.e. high stresses and high temperature and cold deformation it may occur. Sometimes stress corrosion cracking occurs from the outside; especially longitudinal welded pipes at higher temperature are sensitive to this type of 'corrosion under insulation'.
Like crevice corrosion, the low resistivity of seawater also promotes strongly galvanic corrosion. Galvanic corrosion is seen as a major concern for materials performance in marine environment. A well known example is bronze bearings in ships, where sacrificial zinc anodes need to protect the steel hull for galvanic corrosion. Also stainless steel can suffer galvanic corrosion, or it causes galvanic corrosion to other, less noble, alloys.
For more details duplex 2205 pipe, check the links https://www.steeltubemanufacturing.com/duplex-stainless-steel-pipe.html
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