Different types of stainless steel are being used increasingly widely and successfully in a variety of different marine environments. It is important to note that the long-term protection of stainless steels from corrosion in seawater requires complex corrosion protection engineering techniques and significant investment. Austenitic and martensitic stainless steels have long been used in superheater piping and turbine blades on marine power units. It is not easy to maintain a low chloride content in these installations, as the application technology for marine power units does not differ from the basic principles of general power generation. Stainless steel is also being used in large capacity chemical vessels on ocean-going merchant ships, where the conditions of use are in some respects very different to those of land-based chemical plants.
Most stainless steel grades can be applied under marine conditions with satisfactory results, but different grades are sensitive to stress corrosion cracking performance. Martensitic steels, represented by Type 410, and ferritic steels, represented by Type 430, rust within a few months under marine conditions. This uniform rusting can be removed by mechanical polishing. The more popular stainless steel is austenitic stainless steel because of its high resistance to rusting (with the exception of cracking due to stress corrosion). Austenitic stainless steel will also blacken over time. This blackening can also be removed by polishing if for aesthetic or other reasons. Stainless steel rarely rusts uniformly in seawater, so it is not a concern in practice.
The thrusters on tugboats and other vessels at sea can be made from cast stainless steel CF-8 (equivalent to dry type 304 stainless steel). When the ship is not sailing, from the thruster spindle through the bearing to the hull, forming a conductive metal circuit. The equivalent composition is 410 type stainless steel casting thrusters are often chosen and are widely used in other areas such as for icebreakers.
Recent developments in stainless steel technology have begun to use complex austenitic-ferritic stainless steel 20Cr-8Ni-3.5Mo to cast large propellers (weighing up to 3000kg) on ocean-going vessels.
Ships often working in port are particularly prone to encounter logs or other floating bodies on the sea and accelerate the damage to the propellers. Therefore, the use of austenitic stainless steel to produce propellers can be repaired by straightening or welding, which is a matter of importance in the selection of materials.
Has long been observed in the centrifugal pump in seawater working conditions, such as the use of stainless steel components, can show a certain degree of reliability. In keeping with the flow of seawater, the use of CF-8M casting stainless steel impeller (its composition is equivalent to 316 type stainless steel) as well as with 316 type stainless steel for the main shaft, can not be any problem. Crevice corrosion and pitting corrosion are likely to become serious problems when the pump stops working. But if you come to use the more active and rust-prone cast iron manufacturing a wall thickness of a fairly large pump box, cast iron in downtime can play a cathodic protection. When the pump work, cast iron box of cathodic protection under the role must be able to make under the stainless steel polarization, but the flow of water to maintain the cathodic protection role. In addition, long-term work of the pump may be due to alternate use, both when seawater replaced with fresh water and play a protective role.
Stainless steel has been used as a bulk bag container in freight transport for liquefied natural gas (LNG), chemicals, beverages etc. Containers for LNG in freight are customarily made of type 304L stainless steel, the purpose of which is not to resist corrosion but to take into account the mechanical properties in the low temperature state.
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 chemicals on land. If the vessel is part of a general unscheduled cargo ship, the container carrying the chemicals can also transport anything from acetic acid and bad slurry to xylene. Type 316L stainless steel is generally used for the valves, cargo pumps, pipe chases, heating coils and the vessels themselves. Containers can be made of stainless steel in its entirety or carbon steel clad with a layer of 0.06 to 0.08in (1.5-2.0mm) stainless steel plate. Before use, the plates must be carefully inspected for defects and thoroughly cleaned and passivated.
Tests have shown that vessels containing chemicals can be rinsed with sea water, but must be rinsed with fresh water soon afterwards. For any stainless steel heating devices inside the vessel, do not start them until the chlorine has been thoroughly flushed out to prevent stress corrosion cracking. Chemical containers should not be designed to hold seawater because of the risk of crevice corrosion. If seawater is required in the design, then a cathodic protection system must be considered to control the development of crevice corrosion. In this case, the stainless steel vessel may be difficult to remove limestone deposits, which is a problem worthy of attention.
Forced water circulation system coolers and power station steam condensers have been widely used austenitic stainless steel piping, the latter inlet is not suitable for copper alloy materials due to the high level of contamination. The more popular and good material is type 316 stainless steel. In coastal and harbour areas, large amounts of foreign clumps and silt enter the condenser pipework and are particularly likely to cause serious obstacles, a situation which must be removed by measures. One sensible measure is the use of rubber balls circulated through the pipes, which clean the pipe walls due to the squeezing effect they can create. When the flow rate of seawater is approximately 1m/S, it prevents organic marine debris from being drawn in, thus protecting the condenser pipework from pitting corrosion. Unlike other non-ferrous alloys, the use of stainless steel for the condenser pipeline is not limited by the maximum flow rate, but is related to the economic effect of the entire pumping unit.
Tandem electrical connectors and O-ring positioning devices are widely used 304 and 316 type stainless steel manufacturing, especially marine and military engineering. O-ring sealers can be cathodically protected by the hull, aluminium frame or other factors. Without cathodic protection, the O-ring can quickly (in some cases within a few weeks) develop gap rot and lead to serious failure.