Stainless Steel Applications in the Marine Field

Austenitic and duplex stainless steels are widely used in the marine field, including marine aquaculture, ship equipment, seawater desalination systems, fishing gear, bridges, and offshore platform structures.

Will stainless steel rust in the ocean, and how can this be addressed?
Various types of stainless steel are increasingly being used successfully in different marine environments. It must be noted that long-term corrosion resistance of stainless steel in seawater requires complex anti-corrosion engineering techniques and significant investment. Austenitic and martensitic stainless steels have long been used in superheater pipes and turbine blades in marine power plants. Maintaining low chloride levels in these systems is challenging, as the application technology for marine power plants does not fundamentally differ from that of conventional power generation systems. Stainless steel is also used in large chemical containers on ocean-going merchant ships, where the conditions of use differ significantly from those in land-based chemical plants in certain aspects.

Most stainless steel grades perform satisfactorily in marine conditions, but different grades exhibit varying sensitivities to stress corrosion cracking. Martensitic steels, represented by Type 410, and ferritic steels, represented by Type 430, can rust within months under marine conditions. This uniform rusting can be removed by mechanical grinding. Austenitic stainless steels are more popular due to their stronger rust resistance (except in cases of stress corrosion cracking). Over time, austenitic stainless steels may also darken. If necessary for aesthetic or other reasons, this darkening can be removed by grinding. Uniform rusting of stainless steel in seawater is rare, so it is generally not a practical concern.

Below are a few examples of stainless steel applications in marine equipment:

Propellers: Propellers for tugboats and other vessels can be made from cast stainless steel CF-8 (equivalent to Type 304 stainless steel). When a vessel is not in motion, a conductive metal circuit is formed from the propeller shaft through the bearings to the hull. Cast propellers with a composition equivalent to Type 410 stainless steel are also commonly used and are widely applied in other areas, such as icebreakers. Advances in stainless steel technology have led to the use of duplex austenitic-ferritic stainless steel 20Cr-8Ni-3.5Mo for casting large propellers (weighing up to 3,000 kg) for ocean-going vessels. Ships frequently operating in ports are particularly susceptible to damage from logs or other floating objects, which can accelerate propeller wear. Therefore, the use of austenitic stainless steel for propellers allows for repair through straightening or welding, which is an important consideration in material selection.

Pumps: Centrifugal pumps with stainless steel components have demonstrated considerable reliability in seawater applications. In flowing seawater, cast CF-8M stainless steel impellers (equivalent to Type 316 stainless steel) and Type 316 stainless steel shafts perform without issues. However, when the pump is idle, crevice corrosion and pitting corrosion can become significant problems. If a relatively active and rust-prone cast iron with substantial wall thickness is used for the pump casing, the cast iron can provide cathodic protection during idle periods. While the cathodic protection effect of the cast iron casing may not polarize the stainless steel during operation, the flowing water maintains the cathodic protection. Additionally, pumps used long-term may benefit from alternating between seawater and freshwater, which offers some protection.

Bulk Containers: Stainless steel has been used as bulk containers for transporting liquefied natural gas (LNG), chemicals, beverages, and other goods. Containers for LNG transportation typically use Type 304L stainless steel, primarily for its mechanical properties at low temperatures rather than its corrosion resistance. For chemical transport containers, stainless steel is chosen mainly for its corrosion resistance, which differs from the storage and transport conditions for land-based chemicals. If the vessel is a general-purpose cargo ship, chemical containers may also carry a variety of goods, from acetic acid and pulp to xylene. Type 316L stainless steel is commonly used for valves, cargo pumps, pipelines, heating coils, and the containers themselves. Containers can be made entirely of stainless steel or constructed from carbon steel clad with a 0.06–0.08 inch (1.5–2.0 mm) stainless steel plate. Before use, the plates must be thoroughly inspected for defects and subjected to meticulous cleaning and passivation.

Experiments have shown that chemical containers can be rinsed with seawater but must be promptly followed by a freshwater rinse. For any stainless steel heating devices inside the containers, they must not be activated until chlorides are completely flushed out to prevent stress corrosion cracking. Chemical containers should not be designed to hold seawater, as this could lead to crevice corrosion. If the design must accommodate seawater, a cathodic protection system should be implemented to control crevice corrosion. In such cases, stainless steel containers may develop difficult-to-remove limestone deposits, which is a significant concern.

Heat Exchangers: Austenitic stainless steel pipes are widely used in forced water circulation system coolers and power station steam condensers, particularly where copper alloys are unsuitable for inlet pipes due to high contamination levels. Type 316 stainless steel is a preferred material. In coastal and harbor areas, the ingress of foreign matter and silt into condenser pipes can cause serious blockages, necessitating measures to address this. One effective method is to circulate rubber balls through the pipes, as their squeezing action cleans the pipe walls. A seawater flow velocity of approximately 1 m/s can prevent the adhesion of marine organic matter, thereby protecting condenser pipes from pitting corrosion. Unlike other non-ferrous alloys, stainless steel condenser pipes are not subject to maximum velocity limitations but are influenced by the economic efficiency of the pump system.

O-Ring Seals: Series electrical connectors and related O-ring positioning devices widely use Type 304 and Type 316 stainless steel, particularly in marine and military engineering. These materials have delivered satisfactory performance. O-ring seals can receive cathodic protection from the ship’s hull, aluminum frame, or other factors. Without cathodic protection, the fine grooves of O-ring seals can quickly (some within weeks) develop crevice corrosion, leading to significant failure.