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Aluminum vs. Steel for Salt Water Piping

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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


First of two simultaneous responses -- +++++

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 portrait Freeman Newton
- White Rock, British Columbia, Canada

freeman newton died

Second of two simultaneous responses -- +++++

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
- Goleta, California  

Ken received a special
"Contributor of the Year" award
from finishing.com for his numerous
helpful and well researched responses

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thumbsup2Thank 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


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A. Re pipe sizing: An 8" pipe diameter is excessive. Figuring 1.25 volumes seawater to produce 1 volume RO water, and allowing for some internal paint thickness, the linear flow rate is only 1/3 ft/second. Trade in the 8" Al pipe for 4" and send me 10% of the refund!

Re pump size: I suggest one 2.5 HP pump for intake and pre-RO treatment, and another 2.5 HP pump for the RO unit.

Anti-corrosion coating. In 1944, there's a good chance that Al & steel piping would have been coated inside and out with a bitumen paint for protection from seawater. Stainless steel piping may have been coated on the outside.

Corrosion rates. With suggested coatings and pipeline burial near shore to avoid sand abrasion, I suggest using these corrosion rates: SS 0.1, aluminum 0.25, and steel 0.6 mm/year/surface. Estimate piping lifetimes from Schedule 40 pipe thicknesses. Corrosion would be mostly of the pitting type, so leakage would be noticed well before any catastrophic failure.

Anyone with marine pipeline (or naval, etc.) knowledge is invited to contribute more accurate numbers.

History lesson: Look up Operation PLUTO (Pipe-Lines Under The Ocean), a World War II operation which constructed and laid undersea oil pipelines under the English Channel between England and France.

Ken Vlach
- Goleta, California


October 7, 2008

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


May 19, 2009

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


June 2, 2009

Dear Sirs:
RO of seawater will concentrate the salt content up to a factor 2, the regular recovery is 45% the use of 316L is not guaranteed, more exotic alloys like 904L or super duplex is currently used. Please remember that the membranes as well as the piping need regular cleaning with high pH cleaners as well as acid cleaning. In my opinion aluminum cannot be used.

Hans Thomassen
- Windermere, Florida, USA


November 20, 2012

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 24, 2012

A. Hi Richard. Surely the better grades of stainless steel are the most corrosion resistant. The 904L mentioned by Hans may be best.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey

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