Is stainless steel suitable for seawater?

Application of Stainless Steel in Marine Installations

Various types of stainless steel are increasingly widely and successfully used in various marine environments. It must be pointed out that the long-term protection of stainless steel from corrosion in seawater requires complex anti-corrosion engineering technology and a large amount of investment. Austenitic and martensitic ss have long been used for superheater pipes and turbine blades in marine power plants. It is not easy to maintain a low chloride content in these devices, because the application technology of marine power devices is not different from the basic principles of general power generation devices. Stainless steel is also being used for large-capacity chemical containers on ocean-going merchant ships, and its use is in some respects completely different from the use conditions of land-based chemical plants.

Most stainless steel grades can get satisfactory results when used in marine conditions, but different grades are sensitive to stress corrosion cracking. Martensitic steel represented by type 410 and ferritic steel represented by type 430 will rust in a few months under ocean conditions. This uniform rust can be removed by mechanical polishing. The more popular stainless steel is austenitic stainless steel because of its strong corrosion resistance (except due to stress corrosion cracking). Over time, austenitic stainless steel will also blacken. If due to aesthetics or other reasons, this blackening can also be removed by sanding. Stainless steel rarely produces uniform rust candles in seawater, so there is no need to worry about it in actual use.

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AISI 304; AISI  301; AISI 316;AISI 316L…

 

Propellers The tugs on the ocean and the propellers on other ships can be made of cast stainless steel CF-8 (very dry type 304 ss). When the ship is not sailing, it forms a conductive metal loop from the main shaft of the propeller to the hull through the bearings. Cast propellers equivalent to 410 stainless steel are also often used and are widely used in other areas such as icebreakers.

 

The development of modern stainless steel technology has begun to use complex austenitic-ferritic stainless steel 20Cr-8Ni-3.5Mo to cast large propellers (up to 3000kg) on ocean-going ships.

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Vessels that often work in ports are particularly susceptible to encountering logs or other floating bodies on the sea surface to accelerate the damage of their propellers. Therefore, the use of austenitic stainless steel to make the propeller can be repaired by straightening or welding, which is a problem worthy of attention in material selection.

 

Pumps have long been observed that centrifugal pumps work under seawater conditions, such as using stainless steel components, they can show a certain degree of reliability. In the seawater that keeps flowing, using CF-8M cast stainless steel impeller (its composition is equivalent to 316 type stainless steel) and 316 type sss as the main shaft, there is no problem. When the water pump stops working, crevice corrosion and pitting corrosion are likely to become serious problems. However, if a more active and rusty cast iron is used to make a pump box with a relatively large wall thickness, the cast iron can play a cathodic protection effect during the downtime. When the pump is working, the cathodic protection of the cast iron box must be able to polarize the lower rusty steel, but the flowing water remains the cathodic protection. In addition, long-term pumps may be used alternately, and sometimes sea water is replaced with fresh water to play a protective role.

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Bulk Containers Stainless steel has always been used as bulk bag containers in freight transportation to loading liquefied natural gas (LNG), chemicals, beverages, etc. 304L stainless steel is used to hold LNG containers in freight transportation. The purpose is not to resist corrosion but to take into account the mechanical properties at low temperatures.

 

For the containers of marine chemicals, the purpose of using stainless steel is mainly to consider its corrosion resistance, which is different from the storage and transportation of terrestrial chemicals. If the vessel is a general non-scheduled freighter, the container carrying chemicals can also transport anything, from acetic acid, lees to xylene. 316L stainless steel is generally used for valves, freight pumps, pipes, heating coils, and the container itself. The container can be made of integral ss or made of carbon steel covered with a layer of 0.06 to 0.08in (1.5 to 2.0mm) stainless steel plate. Before use, the board must be carefully inspected for defects and thoroughly cleaned and passivated.

 

Experiments have shown that the container with chemicals is allowed to be washed with seawater, but then it must be washed with fresh water soon. For any stainless steel heating device in the container, do not start it to prevent stress corrosion cracking before the chloride is completely washed away. When designing chemical containers, they should not be considered for holding seawater because doing so will lead to the risk of crevice corrosion. If the design plan stipulates that it must be used to hold seawater, then it must be considered to adopt a cathodic protection system to control the development of crevice corrosion. In this case, stainless steel containers may produce lime deposits that are difficult to remove, which is a problem worthy of attention.

 

Austenitic stainless steel pipes have been widely used in the cooler of the forced water circulation system of the heat exchanger and the steam condenser of the power station. The water inlet of the latter is not suitable for copper alloy materials due to high pollution. The more popular and good material is type 316 stainless steel. In coastal and harbor areas, a large number of foreign clumps and sludge entering the condenser pipe are particularly likely to cause serious obstacles. This situation must be eliminated by measures. A reasonable measure is to use rubber balls to circulate through the pipeline because the rubber balls can produce a squeezing effect to clean up the pipe wall. When the flow speed of seawater is about 1m/S, it can prevent marine organic debris from being sucked in, thereby protecting the condenser pipe from pitting corrosion. Unlike other non-ferrous alloys, the use of stainless steel as the condenser pipe is not limited by the maximum flow rate, but it is related to the economic effect of the entire pump device.

 

O-ring series electrical connectors and related O-ring positioning devices are widely made of 304 types and 316 types stainless steel, especially for marine and military engineering. Satisfactory use effects can be obtained by using the above materials. The O-shaped seal can be cathodic protection through the hull, aluminum frame, or other factors. If there is no cathodic protection effect, the fineness of the O-ring seal will soon (some within a few weeks) produce cracks and cause a serious failure.

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