Stainless steel applications in Marine environments

A variety of different types of stainless steel are being used increasingly commonly and triumphantly in a variety of different Marine environments.It must be pointed out that stainless steel in seawater to avoid corrosion for a long time requires complex anticorrosive engineering technology and a lot of investment.

Austenitic and martensitic stainless steel has long been used in superheater pipes and turbine blades on Marine power installations. It is not easy to maintain a low chloride content in these installations because the application technology of Marine power installations is not different from the underlying principles of ordinary generation installations.Stainless steel is also being used in large capacity chemical containers on ocean-going merchant ships, whose application conditions are in some ways completely different from those of land-based chemical plants. Most stainless steel grades can achieve satisfactory results in Marine applications, but different grades are sensitive to stress corrosion cracking.

Martensitic steel, as represented by type 410, and ferritic steel, as represented by type 430, rust within months under Marine conditions.This average corrosion can be removed by mechanical grinding.

Compared to the welcome stainless steel is austenitic stainless steel, because of its corrosion resistance to strong (but due to stress corrosion cracking is an exception). Austenitic stainless steel also darkens over time.If for aesthetic or other reasons, this blackening can also be polished to remove.Stainless steel rarely produces average rust candle in seawater, so there is no need to worry about it in practice.Propeller Thrusters on tugs and other vessels on the ocean can be made of cast stainless steel CF-8 (quite dry type 304 stainless steel).When the boat is not flying, a conductive metal loop is formed from the propeller spindle through the bearing to the hull.     The cast propeller equivalent to the composition of 410 stainless steel is also often used, and is widely used in other aspects, such as for icebreakers, etc.With the development of modern stainless steel technology, 20Cr-8Ni-3.5Mo of multiphase austenitic ferritic stainless steel has been used to cast large thrusters on ocean ships (weighing up to 3000kg).

Ships that often work in ports are particularly vulnerable to damage to their thrusters from logs or other floating objects on the surface. Therefore, the propeller made of austenitic stainless steel can be repaired by straightening or welding, which is a problem worth paying attention to in material selection. The pump has already seen that the centrifugal pump in seawater working conditions, such as the use of stainless steel components, can show a certain degree of reliability. In the sea water adhere to the activity, the CF - 8M casting stainless steel impeller (its composition is equivalent to 316 stainless steel) and 316 stainless steel as the spindle, can not cause any problems.     When the pump stops working, slot corrosion and spot corrosion are likely to become serious problems. But if a pump box with a fairly thick wall is made of a more vivid and rust-prone cast iron, the cast iron can play the role of cathode maintenance during downtime.When the pump is working, the iron box under the action of cathode maintenance can polarize the rust steel, but the active water maintenance is waiting for the cathode maintenance. In addition, pumps that have been operating for a long time may be protected by alternate use, sometimes switching from seawater to fresh water.    

Stainless steel in bulk containers is increasingly used as a bulk container in cargo for liquefied natural gas (LNG), chemicals, beverages, etc. It is customary to use 304L stainless steel as container for shipping Zhongsheng LNG, which is not for corrosion resistance but for mechanical properties at low temperature. As for shipping chemical containers, the purpose of using stainless steel is mainly to consider its corrosion resistance, which is different from the storage and transportation of chemical products on land. If the ship is an ordinary tramp, the container carrying chemicals can carry anything from acetic acid to pulp to xylene. Generally, 316L stainless steel is used for valves, freight pumps, tubes, heating coils and containers themselves. The container can be made of monolithic stainless steel or carbon steel coated with 0.06 to 0.08in. (1.5 -- 2.0mm) stainless steel plates. Before use, it is necessary to stop carefully checking whether the plate can have defects and stop thorough cleaning and passivation disposal. Tests indicate that containers containing chemicals are allowed to be stopped by sea water, but must soon be flushed with fresh water. With respect to any stainless steel heating installation in the container, do not start until the chloride has been thoroughly washed away to prevent stress corrosion cracking. Chemical containers should not be designed to hold seawater due to the risk of crevice corrosion. If the rules of the design plan must be used to hold seawater, then it is necessary to consider cathodic maintenance systems to control the development of crevice corrosion. Under these conditions, stainless steel vessels may produce calcareous deposits that are difficult to remove, which is a concern. Austenitic stainless steel pipe has been widely used in coolers of forced water circulation system of thermal AC and steam condensers of power stations. The intake of the latter is not suitable to use copper alloy material because of high pollution. The good data is 316 stainless steel. In coastal and harbor areas, a large number of external clumps and silt enter the condenser pipeline, which is especially easy to form serious obstacles. This situation must be removed by measures. A reasonable measure is to use rubber ball circulation through the pipe, because the rubber ball can produce extrusion and thus clear the pipe wall. When the moving speed of sea water is about 1m/S, it can avoid the inhalation of Marine organic debris, so as to keep the condenser pipe from point corrosion. Unlike other non-ferrous alloys, stainless steel condenser pipe is not limited by the maximum flow rate, but it is related to the economic effect of the whole pump installation. O-ring series electrical connector and the positioning and installation of the O-ring are commonly made of 304 and 316 stainless steel, especially in Marine and military engineering. Satisfactory results can be obtained by using the above information. The O - shaped seal can be used for cathodic maintenance through the hull, aluminum frame or other elements. If there is no cathode maintenance, the O-ring will soon (some in a few weeks) crack decay and serious failure.