Alloys that qualify Seawater systems material selection factors
Seawater is used in large quantities for quenching process in desalination industry. The marine conditions are considered to be the severely corrosive that they offer a great challenge to designers in choosing the right materials that can offer the suitable performance at the economical price.
Nickel based alloys, copper-nickel alloys and stainless steel grades have offered appropriate performance in several cases. The performance of materials in marine conditions has been traditionally related with ships. Over the time, essential new industries with latest materials issues have been seen. The common industries are desalination and offshore oil and gas production. Additionally, the need of large magnitude of cooling water by advanced plants have resulted in establishing plants near sea specifically in dry areas for example in Middle East.
This has driven the interest in the application of materials for dealing with salt water, specifically as the marine condition is the terribly corrosive natural media, it is tough for designers to consider the several factors including marine fouling, water velocity and aeration that have a major role in technical and economic choice.
Nickel based alloys for example Monel 400, austenitic stainless steels and other super alloys have offered supreme performance in the marine industry over the years.
Materials of choice in Seawater
Seawater contains complex inorganic salts, dissolved gases, suspended solids, organic matter and living organisms. It is found that living species have also a great impact on corrosion, so a layer of organism growth can decrease corrosion of carbon steel or may result into crevice corrosion on steel grades. Oxygen magnitude has a significant effect on the corrosive action of seawater however in desalination and oil supply systems, the brackish water or brine is mildly corrosive to materials although crevice corrosion on stainless steels may occur.
There are three significant commercial alloys of nickel and copper that offer good corrosion resistance specifically to running seawater. Resistance to deaerated seawater is a major factor considered in the desalination industry that serves under low oxygen brines and where cupronickel alloys are used for heat exchangers.
Nickel-Chromium alloys and stainless steels
The performance of austenitic stainless steels that basically contain iron and also include chromium and nickel. They offer better performance to crevice and pitting media. Stainless steels offer great resistance to pitting and crevice corrosion is enhanced by inclusion of molybdenum.
In the latest plants the whole distillate system is usually constructed in grade 316 stainless steel as the high purity water is severely corrosive to carbon steel. Use of stainless steel 316 also prevents stress corrosion cracking in the hot conditions in seawater systems. However this grade experienced crevice corrosion at the joints related with numerous modules used to develop these systems.
The nickel based alloy such as Monel bar resists pitting and crevice corrosion and offers wide reduction in corrosion in deaerated condition. Cupronickel alloys have been in service for heat exchanger tubes for several years such as in water boxes and large size pipes.
Other widely used nickel based alloys for example Hastelloy C, Inconel 625 and Incoloy 825 are common for essential components including bellows expansion joints, fasteners, exhaust systems and shaft seals