Nickel based materials in marine and related conditions
The demand for large volumes of water for quenching of process treatments has resulted in the diverse chief industries to be located near the coastlines. The marine condition is found as the most corrosive natural media that causes problems in choosing materials that offer good functionality at the economical price.
Nickel based materials for example copper-nickel alloys, stainless steels and nickel alloys have been noticed to offer suitable techno-economic solutions in several cases.
The application of materials in marine conditions has been related with ships. In the mean time, although, essential new industries with latest materials issues have been introduced. Major industries are desalination and offshore oil and gas production. Needs for large volumes of quenching water by latest industry have resulted in establishment of plants near oceans specifically in dry areas for example Middle East.
It has given rise in the choice of materials for dealing with seawater specifically in marine environments as the corrosive natural conditions, designers usually discover problems not only from attacking nature of seawater even also of several factors including marine fouling, speed of flow and aeration that is taken into account in making the suitable and economic choice.
Nickel based materials like cupronickels, austenitic stainless steels and nickel base alloys have been used in the marine industry for several years. In the recent time, quick increase in application of cupronickel 90/10 that has proven to be a flexible material in several applications including heat exchanger, tubing, marine water and steam pipes, evaporator bodies, water boxes, boat hulls and hydraulic pipes.
The recent development of high alloy stainless steels, resistance to pitting and crevice attack in marine water.
The selection of nickel and nickel alloys as marine materials and their choice in major applications has been discussed.
Seawater is an aggressive mixture of inorganic salts, dissolved gases, solids, organic matter and organisms.
The tests in salt solutions simulating seawater can deliver abnormal results and it is now understood that the living organisms also create additional corrosive effect. So a layer of marine growth can decrease the corrosion of carbon steel or crevice attack on stainless steels.
Magnitude of oxygen has a remarkable effect on attacking nature of marine water. In desalination and oil-well injection units, marine water or brine is nominally attacking to the common materials. It allows designers to use materials including stainless steels that in aerated marine water could be affected by intense pitting.
Nickel and Copper Alloys
Commonly used commercial copper-nickel alloys are Cupronickel 90/10, Cupronickel 70/30 and Monel 400. Cupronickel alloys contain deliberate amount of iron to enhance their corrosion resistance specifically in running seawater. The nickel based alloys also comprise of nominal magnitudes of iron and manganese.
The copper bas alloys offer high resistance to corrosion and pitting in mild and medium velocity marine water and are not susceptible to pitting in the stagnant conditions. At high velocities they are attacked at the large rates. Monel is prone to pitting attack in sluggish conditions however it becomes resistant in running seawater and offers excellent resistance even at the speed of 40 m per sec.
70/30 cupronickel is widely used in North America. It was originally produced in United Kingdom for applications need to prevent of attack in cooling waters containing high sand amount. Considerable iron content although offers good resistance to impingement attack in clean marine water and is usually used due to this feature.
Deaerated seawater corrosion resistance is a chief interest with the development of seawater industry that operates primarily under small oxygen brine waters and where copper base alloys are a common selection for heat exchanger tubing. Lab tests and operation performance state that these deaerated conditions are mildly attacking than natural marine water to copper-base alloys however temperature may be increased. Following table shows copper base alloys in warm deaerated seawater.
|Alloy||Corrosion rate mm/yr|
|20 ppb oxygen||100 ppb oxygen||200 ppb oxygen|
From the above table it is evident that cupronickels are mildly affected by the changes in oxygen concentration than aluminum –brass that is occasionally used in mildly aggressive components of desalination units.
It is proven that copper-nickel alloys offer adequate resistance to marine water with small general corrosion rate and large resistance to pitting under static conditions, suitable resistance at moderate flow rates and quick attack at high flow speeds. Nickel-copper alloys describe a tendency to pit in the static media however offers outstanding resistance at moderate and large flow rates.