Application of stainless steel in marine installations

Knowledge / 2020-07-09 09:42:01

Various types of stainless steels are increasingly being 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 substantial investment. Austenitic and martensitic stainless steels have long been used in superheater pipes and turbine blades on marine power plants. It is not easy to maintain a low chloride content in these devices, because the application technology of marine power plants is no different from the basic principles of general power plants. Stainless steel is also being used in large-capacity chemical vessels on ocean-going commercial vessels, and its usage is quite different from that of land-based chemical plants in some respects.
 
Most stainless steel grades can obtain satisfactory results under marine conditions, but different grades are sensitive to stress corrosion cracking performance. Martensitic steel represented by type 410 and ferritic steel represented by type 430, in the ocean Under conditions, it will rust within a few months. This uniform rust can be removed by mechanical grinding. The most popular stainless steel is austenitic stainless steel because of its strong resistance to rust (but the exception is due to stress corrosion cracking). Over time, austenitic stainless steel also It will go black. 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 you do not need to worry in actual use.
 
Propellers Tugboats on the ocean and propellers on other ships can be made of cast stainless steel CF-8 (a fairly dry type 304 stainless steel). When the ship is not sailing, it forms a conductive metal circuit from the main shaft of the propeller to the hull through the bearing. Casting propellers equivalent to type 410 stainless steel are also often selected, 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 giant ships. Vessels that often work in ports are particularly easy to encounter Logs or other floating bodies on the sea accelerate damage to the propeller. Therefore, the thrusters made of austenitic stainless steel can be repaired by straightening or welding, which is a matter of importance in the selection of materials.
 
It is observed that the centrifugal pump can show a certain reliability if it uses stainless steel components under seawater working conditions. In the seawater that keeps flowing, the CF-8M cast stainless steel impeller (its composition is equivalent to 316 stainless steel) and 316 stainless steel as the main shaft can be used without any problems. When the pump stops working, crevice corrosion and pitting corrosion are likely to become serious problems. However, if you use a more lively and rusty cast iron to make a pump box with a relatively large wall thickness, the cast iron can play a role in cathodic protection during downtime. When the pump is working, the stainless steel box must be polarized under cathodic protection, but the flowing water maintains the cathodic protection. In addition, the pumps that work for a long time may be used alternately, and sometimes the seawater is replaced by freshwater to play a protective role.
 
Bulk container stainless steel has been used as a bulk bag container in freight transportation for loading liquefied natural gas (LNG), chemicals, beverages, etc. LNG containers used in freight transportation are used to use 304L stainless steel. The purpose is not to resist corrosion but to consider the mechanical properties at low temperatures.
 
For the container 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 land chemicals. If the vessel is a general non-scheduled cargo ship, the container carrying chemicals can also transport any item, from acid, slurries to xylene. Generally, 316L stainless steel is used for valves, freight pumps, pipe grooves, heating coils and the containers themselves. The container can be made of integral stainless steel or made of carbon steel covered with a layer of 0.06~0.08in (1.5-2.0mm) stainless steel plate. Prior to use, the board must be carefully checked for defects and thoroughly cleaned and passivated.
 
Experiments have shown that containers containing chemicals are allowed to be rinsed with sea water, but then must be rinsed with fresh water as soon as possible. For any stainless steel heating device in the container, before the chloride is completely washed away, do not start it to prevent stress corrosion cracking. The design of chemical containers should not be considered for holding seawater because doing so will cause the risk of crevice corrosion. If the design plan stipulates that it is not necessary to hold seawater, then a cathodic protection system must be considered to control the development of crevice corrosion. In this case, the stainless steel container may produce calcareous deposits that are difficult to remove, which is a problem worthy of attention.
 
The cooler of the forced water circulation system of the heat exchanger and the steam condenser of the power station have widely used austenitic stainless steel pipes. The inlet of the latter is not suitable for copper alloy materials due to high pollution. The more popular good material is 316 stainless steel. In the coastal and harbour areas, a large number of foreign lumps and sludge enter the condenser pipeline, which is particularly susceptible to serious obstacles. This situation must be eliminated by taking measures. A reasonable measure is to use a rubber ball to circulate through the pipe, because the rubber ball can produce a squeezing effect to clean the pipe wall. When the flow rate of seawater is about Im/S, it can prevent marine organic debris from being sucked in, thereby ensuring that the condenser pipes are protected from pitting corrosion. Unlike other non-ferrous alloys, pipes using stainless steel as the condenser are not subject to the maximum flow rate, but it is related to the economic effect of the entire pump device.
 
The 0-type sealing ring series electrical connector and the relevant 0-type sealing ring positioning device are widely made of 304 and 316 stainless steel, especially in marine and military engineering. All the above materials can be used satisfactorily. O-shaped seals can be cathodic protected by hull, aluminum frame or other factors. If there is no cathodic protection, the fineness of the O-ring seal will soon (some within a few weeks) will produce cracks and cause serious failure.

Application of stainless steel in marine installations

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