Seawater corrosion-Serious Challenge to marine industries

Different materials have long been used in marine environments such as in ships. Recently, new industries with new materials problems have developed.

Major industries are desalination and offshore oil and gas production. The needs for large volumes of cooling water by advanced industry usually resulted in settling of plants near ocean specifically in dry areas like Middle East.

It has increased the use of materials for dealing with seawater, specifically as marine condition is proven as the most corrosive natural environment, the designers usually face problems due to seawater corrosivity however many factors like marine fouling, flow velocity and aeration that have be considered in making the suitable technical and economical selection.

Corrosion types in Seawater

General

Seawater contains complex inorganic salts, dissolved gases, suspended solids, organic matter and organisms. Living organisms have an effect on corrosion behaviour. So, a layer of marine growth can decrease corrosion on carbon steel or cause crevice corrosion on stainless steels.

Oxygen concentration has a significant effect on corrosivity of seawater and this is decreases, as in desalination and oil-well injection units, the seawater or brine is nominally corrosive to most materials. It allows designers to use materials like stainless steels that in aerated seawater could experience serious pitting.

Alloys containing copper in high concentration have high corrosion and pitting resistance in low and medium velocity seawater and are not sensitive to pitting in stagnant conditions. At high velocities, they are attacked at high rates. The nickel-copper alloy is sensitive to pitting in stagnant conditions however becomes passive in running seawater and has high resistance even at 40m/sec.

Resistance to deaerated seawater by Monel bar is significant. This type of resistance is now of major interest with the thriving desalination industry that serves mainly in low oxygen brines and here copper base alloys are the definite choice for heat exchanger components.

Lab tests and service experience show that these deaerated environments are less corrosive than natural seawater to copper-base alloys, infact also with increase in temperature.

Copper base alloys are slightly affected by changes in oxygen concentration. So, it can said that copper-nickel alloys also offer good resistance to seawater with a low general corrosion rate and excellent resistance to pitting in static conditions, good resistance at moderate flow rates and quick corrosion at high flow rates. The nickel-copper alloys have a potential to pit in static conditions, however have outstanding resistance at moderate and high flow rates.

The austenitic stainless steels which are basically iron base alloys contain chromium and nickel are similar to Ni-Cu alloys, however they can receive pitting attack in static conditions , specifically crevice corrosion is higher. The corrosion resistance of stainless steels can be enhanced by the addition of molybdenum. Addition of nitrogen has also been found advantageous in enhancing resistance to crevice corrosion. Alloys that resist crevice corrosion also resist pitting. Hence the performance of alloys can be significantly improved.