Application of stainless steel in marine installations

DataTime:[2019-7-26]

Various types of stainless steel are increasingly being used extensively and successfully in a variety of different marine environments. It must be pointed out that the long-term corrosion protection of stainless steel in seawater requires complex anti-corrosion engineering techniques and substantial investment. Austenitic and martensitic stainless steels have long been used for 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 the marine power plant does not differ from the basic principle of a general power generation device. Stainless steel is also being used in large-capacity chemical containers on ocean-going merchant ships, and its use is in some respects different from that used in land-based chemical plants.

Most stainless steel grades have satisfactory results in marine applications, but different grades are sensitive to stress corrosion cracking. Martensite steel, represented by Type 410, and ferritic steel, represented by Type 430, will rust within a few months under ocean conditions. This uniform rust can be removed by mechanical grinding. The more popular stainless steel is austenitic stainless steel because of its high resistance to rust (except for stress corrosion cracking). Austenitic stainless steels also black out over time. This blackening can also be removed by sanding for aesthetic or other reasons. Stainless steel rarely produces a uniform rust candle in seawater, so you don't have to worry about it in actual use.

Propellers Tugboats on the ocean and propellers on other vessels can be made of cast stainless steel CF-8 (equivalent dry type 304 stainless steel). When the vessel is not sailing, a conductive metal loop is formed from the propeller shaft through the bearing to the hull. Casting propellers, which are equivalent to type 410 stainless steel, are also often used, and are widely used in other applications such as icebreakers.

In the development of modern stainless steel technology, multiphase austenitic-ferritic stainless steel 20Cr-8Ni-3.5Mo has been used to cast large propellers (up to 3000kg) on ocean-going giant wheels.

Ships that often work in ports are particularly susceptible to encountering logs or other floating bodies on the surface of the sea to accelerate the damage of the propeller. 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 the selection of materials.

Pumps It has long been observed that centrifugal pumps operate under seawater conditions, such as stainless steel components, which show a certain degree of reliability. In the seawater that keeps flowing, the CF-8M cast stainless steel impeller (which is equivalent to 316 stainless steel) and the 316 stainless steel as the main shaft can be used without any problem. Crevice corrosion and pitting corrosion are likely to become serious problems when the pump stops working. However, if a pump box with a relatively large wall thickness is made from a relatively lively and rust-resistant cast iron, the cast iron can provide cathodic protection during downtime. When the pump is working, the cathodic protection of the cast iron box must be able to polarize the lower rust steel, but the flowing water maintains the cathodic protection. In addition, long-term working pumps may be used interchangeably, sometimes with the replacement of seawater into fresh water for protection.

Bulk containers Stainless steel has been used as a bulk container in freight to carry LNG, chemicals, beverages, etc. The container for LNG in freight is customary to use 304L stainless steel, the purpose of which is not to resist corrosion but to consider the mechanical properties at low temperatures.

For containers of marine chemicals, the purpose of using stainless steel is mainly to take into account its corrosion resistance, which is different from the storage and transportation of terrestrial chemicals. If the vessel is a general irregular ship, the container carrying the chemical can also transport any item, from acetic acid, slurry to xylene. 316L stainless steel is generally used as the valve, freight pump, tube, heating coil and the container itself. The container may be composed of integral stainless steel or a layer of 0.06-0.08 in (1.5-2.0 mm) stainless steel plate coated with carbon steel. Before use, it is necessary to carry out a detailed inspection of the board for defects and thorough cleaning and passivation treatment.

Experiments have shown that the chemical-filled passenger device allows flushing with seawater, but then it is annoying to rinse quickly with fresh water. For any stainless steel heating device in the container, do not start to prevent stress corrosion cracking before the chloride is completely flushed out. When designing a chemical container, it should not be considered for the purpose of holding seawater because it would lead to crevice corrosion. If the design stipulates that non-water supply is not acceptable, then a cathodic protection system must be considered to control the development of crevice corrosion. In this case, stainless steel containers may produce limestone deposits that are difficult to remove, which is a problem worthy of attention.

Heat exchangers Austenitic stainless steel pipes have been widely used in the forced water circulation system coolers and power station steam condensers. The latter inlets are not suitable for copper alloy materials due to high pollution. A more popular material is 316 stainless steel. In the coastal and harbor areas, a large amount of foreign agglomerates and silt enter the condenser pipeline, which is particularly prone to serious obstacles, and measures must be taken to eliminate them. A reasonable measure is to use a rubber ball to circulate through the pipe, and the pipe wall is cleaned because the rubber ball can be squeezed. When the flow rate of seawater is about 1 m/s, it is possible to prevent marine organic impurities from being sucked in, thereby protecting the condenser pipe from pitting corrosion. Unlike other non-ferrous alloys, pipes made of stainless steel as a condenser are not limited by large flow rates, but are related to the economics of the entire pump unit.

O-rings Series electrical connectors and O-ring locating devices are widely manufactured in Type 304 and Type 316 stainless steel, particularly marine and military engineering. With the above materials, satisfactory results can be obtained. The O-seal can be cathodically protected by hull, aluminum frame or other factors. Without cathodic protection, the O-ring seals quickly (some within a few weeks) produce crevices that cause severe failure.

Source: China Stainless Steel Pipes Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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