Hydraulic cylinder corrosion 1

[elevate]

Problem:
A seamless steel pressurized hydraulic cylinder approximately 12 inches outside diameter and 30 feet long with a .25 wall thickness and a prime coating of paint is inserted into a 16 inch inside diameter pvc casing which is sunk into a well hole in the ground. The PVC casing is capped at the bottom to prevent electrolysis/ corrosion from contact with the surrounding earth and ground water. Temperature of the surrounding earth and in turn the cylinder can be expected to vary between 55-80 degrees F. (dependent upon oil temperature inside the cylinder). The void between the steel cylinder has been filled with water which remains from the installation process. The top of this pressurized cylinder is connected to above ground pressure piping.

Question:
What is the liklihood that the walls of this pressurized cylinder (having minimal corrosion protection) will corrode to the point of compromising structural integrity through pitting, surface rust etc. Can this cylinder be corroded due to galvanic or electrolytic action even though it is protected from underground conditions? Will the contact with water described above have little effect?
Note: We will be pressure testing this cylinder every three years.

 

[EdStainless]

Can you give the outside of the cylinder a better paint job?
Can you add inhibitors to the water in the annulus?

I don't think that corrosion is likely to be severe, but there is the chance for MIC as well as some pitting. If you want this to last for a long time you should take some additional measures.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenvlach]

Agree with EdStainless's remarks. Need more info for more definite answers. Some comments and details:

Steel doesn't need other solids to galvanically corrode: It consists of 2 phases, ferrite + carbide, with carbide being cathodic, plus any mill & welding scale and rust are also cathodic vs. the ferrite. To illustrate the principle, look up the 'graphitization' type corrosion of cast iron.

Possibly, problems with water being acidic, high in salinity (Cl^-) or high in sulfate (feeds microbes). Have it analyzed (or use water from the nearest shallow well), or look up US Geological Survey hydrologic & subsoil pH data. Also, if wellwater is used thereabouts, plumbers can tell you whether the water is hard & alkaline (good w.r.t. steel) or not.

Steel alloy? Anything useful w.r.t. corrosion resistance (Cr, Ni, Mo)?

Primer paint: The cheap stuff typically used by steel fabricators is maybe worse than nothing when submerged. Maybe the best option w/o getting into coating systems: Zinc-rich primer (with 90 wt% metallic zinc dust). Offers galvanic protection, even if damaged, and can be field touched up.

Galvanic inhibitors: hang a magnesium or zinc rod or ribbon as a sacrificial anode alongside the pipe. Corrosion would ~stop once all oxygen was depleted.

Chemical inhibitors: W/o knowing more about the water and possible water quality regulations re potential aquifer contamination, I'd be hesitant about using.

Simple, cheap & effective: Slowly pour a 50-lb bag of lime into the water to buffer the pH at 12-12.5. Stops steel corrosion & kills most microbes. Seal off the top to keep air out. Use a rubber inner tube cut open circumferentially, with band clamps around outsides of the steel & PVC pipes. If you want to be thorough, use nitrogen gas to purge oxygen from the water (maybe through a 1/2" Sch. 80 PVC pipe going to the bottom) before sealing. You can also purge the air above the water via the air valve in the tube.