The corrosion nature of metals in marine water is influenced by natural factors related with specific marine conditions. The more crucial factors are temperature, dissolved oxygen, marine organisms and velocity. It is tough to isolate the effects of these factors in the nature seawater tests due to effect of the uncontrollable conditions for example weather, seasons etc. Besides these limitations, the marine corrosion tests, using natural marine water is recommended to conduct studies for applied corrosion analysis.

Velocity is one of the more essential environmental factors of metals and alloys. In the marine applications, velocity can affect over a wide range. Considering, for instance, a hydrofoil craft with fixed struts and foils. The construction materials should prevent attack in stagnant marine water when the ship is at anchor, moderate velocities prior take off and large velocities while flight. Additionally to complicate the conditions, in any given velocity state there exists the feasibility of facing various velocities over the different regions of the component due to shape and fabrication such as development of crevice in which a nominal volume of stagnant water may be stored with the remainder of the surface subjected to a specific velocity.

The chosen data received over a length of time have been gathered and integrated to find the response of metallic materials to marine velocity.      

The performance of austenitic stainless steels that are basically iron-base alloys comprising of chromium and nickel is similar to nickel-copper alloys however their capacity to pit in the static conditions specifically in crevices is larger.

The pitting and crevice resistance offered by stainless steels is enhanced by the inclusion of molybdenum. In the latest times, the inclusion of nitrogen has also been discovered advantageous in enhancing the crevice corrosion resistance.

The crevice resistant alloys will also prevent pitting so only crevice corrosion resistance is observed. Better understanding of the crevice corrosion mechanism has contributed in manufacturing the high alloy stainless steels with crevice corrosion resistance approaching that of high molybdenum nickel base alloys however at low price.

In the running sea water stainless steels and nickel base alloys produce an adherent security layer and for velocity higher than 1 m/sec, pitting doesn’t occur on the surfaces subjected to seawater and at 40 m/sec, corrosion rate is nominal.

In deaerated marine water, lab tests and service experience describe that resistance to pitting and crevice corrosion improves. It is because these mechanisms based on the differential aeration effects that are avoided when the oxygen content is small. Following table shows behavior or stainless steel and nickel base alloy in aerated and deaerated marine water. The significant reduction in corrosion in deaerated conditions is generally of that observed in this kind of material.

In the recent times, major interest has been seen in austenitic-ferritic-duplex stainless steels. These materials have a greater strength than austenitic stainless steels and their chemistry can be modified to offer high resistance to marine attack. A commonly used alloy is Duplex stainless steel 2205. Following table shows the mechanical characteristics of 2205 duplex steel and SS 316.

 Following table shows the crevice corrosion resistance data in tests on Swedish North Sea coast

 Further interest of these alloys is their significant resistance to stress corrosion cracking. It is usually not an issue in the common marine operations as the service temperatures are lower than the widely accepted limit for cracking about 60oC. Although some applications in oil and gas units can cause exposure above the least value and therefore usually selected.

Uses of alloys in marine water

Cupronickels

Cupronickel alloys 90/10 and 70/30 are chosen for heat exchanger tubing for several years. The requirement for high consistency in major plants has preferred the use of cupronickels instead the traditional brass. The chief significance has been the growth in the desalination market. Around 75% of this capacity includes distillation processes- multistage flash distillation is the common process.

These units are basically big heat exchangers and they offer a significant market for tubing. As every cubic meter/day of output needs about 25 kg of tubing, the installed capacity shows about 200,000 tonnes of tubing, about 75% of which is cupronickel primarily 90/10 however with considerable magnitudes of 2% iron and 2% manganese, cupronickel 70/30.

The reasons for the selection of cupronickel are clear from the following table:

 Corrosion rate is in mm/yr

The data describes that the cupronickels are mildly affected by changes in oxygen magnitudes than aluminum brass – a material that is rarely used in the less severe parts of the desalination units.

From real desalination units confirm the mild corrosivity of warm deaerated marine water:

 Failure rates and tube replacements

 Besides of tubing applications, there has been a major growth in other applications where weldability and fabricability are needed besides of corrosion resistance. Cupronickel 90/10 offers significant weldability and can be produced into complicated shapes like water boxes and ship hulls. Major development has been observed in the recent times in the applications where production is included, significantly in seawater pipes. Large magnitudes of cupronickel 90/10 pipes have been delivered to offshore oil plants.

Traditional platforms in North Sea followed US Gulf of Mexico use galvanized steel marine systems which proven to be unreliable and in the aggressive natural conditions in the North Sea maintenance costs were very large. Later on the platforms 90/10 cupronickel marine units utilized which offered reliable and cost-effective service.  The popularity of cupronickel has now spread beyond the North Sea to the regions of Arabian Gulf and Brazil.

Cupronickel 90/10 is used as clad plate such as carbon steel cladding with a cupronickel layer. This product has been used in large magnitudes in the desalination industry for water boxes and large diameter pipes and has met the corrosion resistance requirements in the early coated steel water boxes and pipes.

In Italy, an array of fireboats with hulls is constructed from 90/10 cupronickel clad steel which offered resistance to fouling and corrosion and gave service for above 10 years without fouling or corrosion issues. The popular characteristics of cupronickel 90/10 with its fabrication and welding ensure that this flexible material is widely used in the marine applications.

Nickel-Copper Alloys

The wrought alloys such as Copper-Nickel 70/30 have been broadly used in the critical marine operations. They are featured by high strength and marine attack resistance, specifically in conditions of high water velocity.

It is utilized for fasteners, propeller and pump shafts and valve trims. It is also used in cast form and in pump impellers where significant resistance to running marine water is utilized for benefit. In the desalination units, it has been employed for demisters where high corrosion resistance to brine and incondensable gases is required.

Of main interest, Monel 400 is used for enclosing offshore steel structures for corrosion resistance in the splash region. Installations with this alloy have been in operation in US and Gulf area for above 20 years and are widely used in the Arabian Gulf where they prove to be an economical method for security in the splash region.

A common offshore oil production issue involving splash region corrosion occurred about ten years ago when damage of cement covering on pipe carrying hot oil production issue on the large attack rates that could take place in the splash zone when cold oxygenated seawater splashed onto the hot surface. Perforation of the pipe wall of thickness many mm occurred in a few months. Enclosing the splash region of these hot oil pipes with Monel 400 pipes proven to become a suitable choice of security and it is not widely used in the North Sea oil unit and elsewhere.

Nickel-Copper alloy 400 is offered age hardening by allowing with aluminum and titanium. With suitable heat processing, Monel K-500 can be reinforced and room temperature tensile strength can be readily doubled of traditional alloy. Tensile strength of 1100 N/mm3 can be obtained in hot rolled and heat processed bar and are beneficial in applications like shafts and fasteners specifically for marine operations.

Stainless steels

Issues related with pitting and crevice attack have made designers unwilling to use stainless steels in marine water. Benefits can be taken of its other characteristics, although, resistance to quickly flowing marine water for outstanding service in some applications is nominal. For example pump impellers and shafts. The pumps use stainless steel 316 for parts and offered the pump does not withstand for prolong durations, issues with crevice attack are nominal.

For static conditions, benefit is taken of the cathodic security offered to austenitic stainless steels by Ni resist cast irons. The high nickel cast irons have suitable corrosion resistance in marine conditions and are usually used for pump casing components for example inlet bell mouths, column pipes and discharge heads in big vertical pumps for example as utilized for circulating marine water in MSF and electricity units.

Ideal factors apply to valve steels and valve trim that need large resistance to turbulent marine water makes stainless steels popular. In the desalination industry, large magnitudes of stainless steels are utilized in applications for example vent pipes, distillate trays and transfer troughs, evaporator vessel linings and demisters.

In the latest units, the complete distillate system involves the use of Stainless steel 316 because high purity water can be of attacking nature to carbon steel. The minor oxygen levels in these units allow stainless steels to be used to deal with hot seawater and brine water. It also avoids stress corrosion cracking in the hot conditions.

In the recent applications, stainless steels have been used in the reverse osmosis, a membrane process that allows freshwater to be made from marine water by pressure. In the RO procedure, natural aerated marine water is utilized that states that SS 316 suffers crevice attack at the different joints related with multiple modules utilized to develop these units.

The pipe material in these units is upgraded to high alloy stainless steels. Other common marine application markets are in the ship containers, fittings for pleasure craft and fasteners.

Nickel base alloys

Nickel base alloys for example Hastelloy C, Inconel 625 and Incoloy 825 are widely used for crucial parts for example bellows expansion joints, fasteners, exhaust systems and shaft seals. Hastelloy C and Inconel 625 offer large resistance to marine conditions however they are very costly for some applications whilst the resistance can be beneficial.

A technique for preventing this issue is to use the alloys as weld overlay on less costly materials to offer crucial regions subjected to attack with a crevice corrosion resistance. These areas are shaft surfaces in a way of seals and crevice regions on pipes and pump casings.

For example Inconel 625 overlay on a warship fin shaft boss. This boss had been intensely attacked while in use and was mended with Inconel alloy 625. After use for many years, the welded surface was in ideal condition. This method of weld overlaying of critical regions with an extremely resistant material like Inconel 625 is commonly used for seawater and offshore applications.

Outline

Superalloys for example cupronickel, stainless steels and nickel alloys offer suitable properties for marine applications. The latest demands for high consistency and small life cycle costs are likely to result in increased use of these alloys in the marine area.