Metal finishing Q&As since 1989

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Aluminum vs. Steel for Salt Water Piping

Q. I was given a project to design the piping to carry seawater from the ocean to an RO unit then to a storage tank. We have a length of aluminum pipe and steel of two different diameters. I cannot find a clear answer on which would be more resistant to salt water corrosion. The specific alloys were not mentioned as this is not a materials course, so I would believe that is up to your imagination. I would imagine the steel being something on the order of 316 stainless. I often heard 5140 Aluminum (used in ship hulls) in conjunction with corrosion, but I am not sure if it is actually used in piping. So my question is this: In general, is aluminum piping more resistant to corrosion than steel piping?

William Kenneth Sumerford
Student - Amory, Mississippi
2005

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A. Hi William, Why metals, Will? Why not plastics? Yes, steel, aluminum and stainless are all more rigid than plastics... but the right plastics will last much longer... so consider PVC. You didn't mention size... but up to 4" diameter, you have a plethora of fittings, valves, caps, etc. But there are the salt resistant metals such as Monel... and, I'm told, they come with a range of fittings, too. Freeman Newton [deceased] (It is our sad duty to advise that Freeman passed away April 21, 2012. R.I.P. old friend). 2005 A. ASTM B241 "Standard Specification for Aluminum and Aluminum-Alloy Seamless Pipe and Seamless Extruded Tube" includes the 5000-series alloys 5052, 5083, 5086, 5454 and 5456. These are all fairly resistant to seawater, of good strength, and weldable. Suppliers may be found in the Thomas Register books or online at www.thomasnet.com. A quick check found Al 5086 pipe available in 20 foot lengths in diameters up to 12 inches. Welded construction, rather than flanged or threaded connections, is advisable for joining the lengths of pipe in order to avoid crevice corrosion. Flanges are welded on to enable pump and valve connections. The low-carbon version 316L is preferable to regular 316 stainless steel for corrosion resistance, especially when welded. 316L is more corrosion resistant than Al alloys in most applications. Mild steel pipe has a much lower cost than either Al or SS, but easily rusts. You would perhaps include an internal lining and outer protective coating. Or, simply use a thicker wall pipe, i.e., have a greater corrosion allowance, and plan on a shorter lifetime (interval before replacement). Steel can also be protected from corrosion by sacrificial anodes, or made cathodic by applying a DC electrical current, but the details are likely beyond the scope of your project. Finally, one would not necessarily use the same piping material for both seawater and RO water. Ken Vlach [deceased] - Goleta, California Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015. Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

Thank you so much for the feedback. The project was geared around using the available material which was 8" 'aluminum' and 6" and 4" 'steel' pipe. PVC would have been great but it was not an option. The project was set around the scenario of providing 500,000 lbs of water a day to support the troops after the D-day landing at Omaha Beach. RO wasn't around at the time, so that is a bit of an anachronism. It helps a lot knowing that there is a good aluminum alloy that is fairly resistant to corrosion. We were given about 2000 ft of that piping so it was plentiful and with it being the largest diameter available the fluid traveled at a low velocity which means that the pump doesn't have to work too hard. I came up with about a 5 1/2 hp to pump enough seawater. Thanks so much for the information. About how long would this pipe last in constant contact with salt water?

William Kenneth Sumerford [returning]
- Starkville Mississippi
2005

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Q. How to protect carbon steel pipelines from saltwater in a creek environment.The type and severity of corrosion problems that I should expect to arise in service, and alleviation measures.

Lekan Daniel
student - Warri, Delta, Nigeria
October 7, 2008

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Q. At oil refinery where I work we have a real problem with sea water fire fighting system specially to fire hydrants, fire monitors, underground carbon steel pipes, and check valves(NRV). Most of fire hydrant valves are butterfly valves and almost all sea water pipes are cement coated internally, but we still have pitting corrosion; pipes are of 4" and 6", rating 150 psi.
Any ideas to improve piping against sea water corrosion?

Thanks

Nedal Gamal
- Kuwait
May 19, 2009

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Q. Considering galvanized steel, aluminum, and stainless steel, which has the least amount of corrosion in a high sea salt atmosphere?

Richard MacVeigh
- Loveland, Ohio, U.S.A.
November 20, 2012