Guide to Metal Selection in Marine Applications

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Users and designers of marine equipment have available a large backlog of published data and experience for the selection of economical materials that will give reliable and durable service. Charts and summaries of these data are presented that permit the designer to screen his initial materials selection in terms of general wasting away, pitting, crevice effects, fouling, velocity effects and cavitation, galvanic effects, selective attack, stress corrosion cracking, deep immersion, and cost. These principles are applied in the selection of materials for sea water heat‐exchange systems, propellers, stern tube bearings, marine hardware, fasteners, marine wire rope, buoys, and floating platforms.

marine-grade-metals

MARINE GRADE METALS

Marine applications are very demanding. Stainless steel is the ideal materials for marine applications due to the fact that resists rust better than other materials such as brass, bronze, or galvanized steel. By now it is common knowledge that stainless steels ranging from AISI 316 up to 6Mo and superduplex do not always resist seawater.

When a metal is exposed to underwater or wet environments, marine grade metals may be necessary to prevent the material from failing under the conditions. To be considered a marine grade, the material must be able to resist corrosive effects that are common in a water environment. To achieve this, special alloying elements are added to these grades in order to defend against corrosion. Marine grade metals include:

  • Carbon Steel and Alloy Steel
  • Aluminum
  • Stainless Steel
  • Copper
  • Bronze
  • Brass
  • Galvanized Steel

Carbon Steel and Alloy Steel

carbon-and-alloy-steel

Most carbon steel is not well-suited for marine environments, however, there are several marine grade carbon steels available. AH36, DH36, and EH36 are all examples of commonly used marine grade carbon steels approved by the American Bureau of Shipping. These grades will have slightly more alloying elements such as manganese and chromium compared to their ASTM grade counterparts, which helps achieve higher strength and more corrosion resistance. There are also marine grades of alloy steel as well. Grades MD, ME, MF, MG, and others can provide the strength that normal alloy steel is known for, and have also been approved by the American Bureau of Shipping for use in shipbuilding applications.

Applications: structural shipbuilding and other offshore structural applications

Aluminum

aluminum

There are several different grades of aluminum that are suitable for marine conditions. Marine grade aluminum, for the most part, is limited to the 5XXX and 6XXX grades. Grade 5052 is an excellent marine grade for when formability is required. If strength is more of a concern, grade 5083 is a good alternative. 6061-T6 is an all-around popular grade of aluminum that is also commonly used in marine applications. It has very good corrosion resistance and is precipitation hardened. These grades, for the most part, have additional chromium and manganese to help protect them from corrosive marine conditions.

Applications: structural shipbuilding, boat hulls, boat lifts, docks, and other offshore structural applications

Stainless Steel

stainless-steel

All grades of stainless steel have some degree of corrosion resistance, however, there are several grades of stainless steel that are far more suitable for use in marine environments than others. Grade 316 is probably the most common grade of stainless steel used in marine applications. It has more molybdenum than other austenitic stainless steels which helps it to resist pitting and other corrosive effects of salt water. Grade 304 is another marine grade stainless steel, although it has less molybdenum than grade 316 making it a less desirable choice in chlorine-rich environments.

Applications: marine fittings, marine fasteners, and marine structures

Copper

copper

There are several copper-nickel alloys suitable for marine applications. Examples include C70600 which contains nickel and manganese to help resist corrosion. C71500 is also able to withstand marine conditions, which has a similar makeup to alloy C70600, except that it has even more nickel in its chemical composition.

Applications: water lines, desalination equipment, marine fittings, fasteners, valves, and pumps

Bronze

bronze

Silicon bronze alloys like C65500 and aluminum bronze alloys like C95400 are developed to better withstand marine environments than other bronze alloys. Alternatively, the phosphor bronze alloy C51000 is another common grade suitable for marine applications.

Applications: propellers, propeller shafts, pipes, and marine fasteners

Brass

brass

Marine brass, or naval brass as it is commonly referred to, comes in a variety of alloys. Alloys C46200 and C46400 have additions of zinc and tin which allow the brass alloy to resist corrosion in wet or even underwater applications. Alloys C48200 and C48500 are similar except that they have a larger amount of lead in their chemical composition.

Applications: pipes, marine fittings, marine fasteners, and pumps

Galvanized Steel

galvanized-steel

Galvanized steel can be suitable for some marine environments. Zinc coatings typically fare better under cool temperatures in fresh water than warm temperatures in salt water. Salt water contains chlorides which cause zinc to corrode rapidly. A high temperature will enable the corrosion to occur more rapidly.

Applications: marine structures

Corrosions

Crevice corrosion and pitting may develop sooner or later. For example, a 25Cr07Ni super duplex tubular heat exchanger in a marine vessel showed crevice corrosion within 6 months of service. In natural seawater a biofilm will develop on the metal surface and it will always promote the corrosivity of the water. Microbiological Induced Corrosion (MIC) often occurs in seawater. Also galvanic corrosion is a major problem at sea.

Materials selection in marine environment is quickly gaining interest because of the worldwide trend to concentrate major industrial facilities around sea ports in order to save transport cost and increase cooling capacity.

Pitting Corrosion

In seawater, 316 will perform well up to around 30çC while the more highly alloyed S32750 will not suffer corrosion at all in seawater up to boiling point.

Crevice Corrosion

In situations where crevices exist, such as at propeller shaft glands and bearings where bolts and chainplates pass through the hull or deck, or where barnacles can grow, severe crevice corrosion can occur. As a general rule, crevice corrosion will occur at around 15-20çC below the temperature at which pitting occurs.

In many cases, grade 316 proves satisfactory. The diagram above shows that crevice corrosion can be expected in grade 316 at temperatures above 10-15çC in seawater, thus making it unsuitable for immersed applications where crevices exist. For this reason, propeller shafts made from 316 are usually galvanically protected. This may be with a separate zinc anode, but a bronze propeller on the 316 shaft could provide the same effect – the bronze slowly corrodes, protecting the shaft and allowing it to perform satisfactorily.

Stress Corrosion Cracking

This form of corrosion is quite common in grades 304 and 316 in wet diesel exhausts above 60çC. There have also been cases reported in cold-worked 304 and 316 rigging, chain links, deck fittings and chainplates under operating conditions. This is most usually overcome by using duplex grades or, in some special applications, high nickel alloys.

Stagnant, aerated sea water is a very corrosive medium and equipment should be designed to self-drain when not in use. It may be necessary to flush and blow dry components such as cooling systems if the less resistant grades are used.

The following list of applications is a practical guide to the suitability of various stainless steel grades for specific purposes. The availability of suitable products and cost limitations have been taken into account.

The following general rules apply:

  • 304 may be used for fully exposed components, frequently washed with fresh water;
  • 316 may be used for all hull and deck fitting applications above the water line;
  • S31803 offers higher strength and, therefore, lighter weight components for the same applications as 316 and can additionally be used up to 60çC in wet exhaust systems and in fully submersed applications; and
  • S32750 offers even higher strength and weight savings, and can handle all marine applications with no risk of corrosion even in tropical waters and hot, wet exhaust applications.

While stainless steel grades 304, 316 and duplex steels can all be used in marine applications, they do provide varying degrees of corrosion resistance and durability. The key to their successful application is, therefore, being aware of the strengths and limitations of each grade and how they can be used to their optimum potential.

The corrosion resistance of all stainless steels also improves if they are kept clean. The buildup of salt encrustations, grease or dirt allows corrosion to occur in these regions. After use in marine situations, it is good practice to wash down with clean water and to remove any deposits on the surface.

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