Material Selection for Seawater Applications

Hello again M&C colleagues, hope everyone is always in fantastic condition.

Today I would like to summary my understanding about material selection for seawater applications. Seawater is considered complex and unpredictable, it has variations in temperature, oxygen content, flow, salinity, biological activities, pollution, etc, that can cause significant difference in corrosion properties.

There is no silver bullet for selecting materials for seawater, what I tried to do is to list several commonly used materials I know and highlight their main issues. Contents of the table are just rules of thumb and likely stringent for cautionary purposes. They may be relaxed in some cases, further expert recommendations and studies are highly advised.

The table is readily to be printed in A3 paper for quick reference. Materials listed inside may not available in all product forms, it is responsibility of engineers to design fit for purpose solution. Also, there are many improved corrosion resistant materials for seawater such as hyper duplex SS, alloy 686, alloy 654, etc, but I don’t put in there as not enough data available.

Please right click on the image and select open in new tab for original resolution.

Some additional design notes:

• Some ways to reduce seawater corrosivity to metallic material is through deaearation until several ppb dissolved oxygen and chemical treatment addition such as inhibitor, but in practice is rarely economical for open design.
• Seawater is an excellent electrolyte, minor potential difference may easily cause galvanic corrosion, dissimilar metal contact shall be avoided, some copper alloys even galvanically incompatible to each other such as brass to CuNi.
• Graphite containing gasket and washer are not recommended since it is an excellent cathode because very electropositive in aerated near-neutral solutions and doesn’t form oxide film.
• Special attention for small details such as elastomers, pump parts or valve components, sometimes they are not made from seawater & chlorine resistant materials.
• The use of threaded connection for metallic material in seawater shall be minimized because of crevice corrosion risk
• Biological growth takes time, likely several days at minimum, intermittent chlorination is recommended rather than continuous so that there will be enough time to stabilize protective layer in metallic materials.
• Biofouling also another major issue in seawater, for example can cause macrofouling that can block pipe easily, microfouling also cause MIC and reduce heat transfer in exchangers, hence chlorination is often required, but since it is highly oxidizing, almost all material require injection limit to avoid more corrosive condition.
• CP in combination with coating is one of the most cost efficient way to mitigate seawater corrosion, it generally applies limited to external surface of material, although internal is possible but only for non-complex design such as storage tanks.
• Maximum velocity limit in material is rarely achieved because most of the time, engineering design economics such as pump sizing and friction loss resulted in lower velocity requirement, also there is risk for water hammer. In general, bigger pipe can tolerate higher velocity and best practice is to keep velocity 1 m/s minimum to avoid stagnation, fouling, and deposit.

I hope they are useful and enjoy the read!

Seawater corrosion is very huge topic and I definitely welcome any improvement suggestions and also corrections.

Thank you for reading!

Further reading:

• EFC 63 - The Corrosion Performance of Metals for the Marine Environment: A Basic Guide
• Seawater Corrosion Handbook
• ISO 21457 - Materials Selection and Corrosion Control for Oil and Gas Production Systems

Thanks to Mushahid & Anupkumar for reminder about FRP UV degradation concern.