Cathodic Corrosion Protection

[mc5w]

Does anybody know anything about cathodic ( eletrolytic ) corrosion protection of natural gas and oil pipelines? I know the basic principles but have very little knowledge of how much voltage, how much current, and where to buy the carbon anodes.

One of my applications would be to protect the electrical grounding system in a foundry or other corrosive environment. I would probably need about twice as much current and voltage as a gas pipeline needs because gas and oil pipelines are electrically insulated on the outside to conserve on current and the concentrate the voltage gradient at the pipe surface. In the case of grounding electrodes, the metal is bare which would require more current all other things being equal.

A also know that the employee showers would need to be rebuilt with a grounding mat in the floor and a piece of grounded metal pipe in the drain all bonded to the water supply pipes and the electrical system ground.

In a foundry environment there is ammonia gas and aqueous solution which is murder on copper. This is because of one of the ingredients in sandless molding material that is less hard than sand is in machine tools when a machine tool encounters a sand pocket in the surface of a casting. Fluxing chlorides in combination with ammonia, oxygen, and water makes for a very nasty corrosive.

Mike Cole

 

[ScottyUK]

If you have an effective grounding grid then applying CP to it is essentially impossible unless you are looking to inject a very large current into the ground. We are undergoing trials at present to sort out our CP system and one of the problems we have is the presence of the huge copper earthing grid for the power station. Current preferentially flows to copper rather than buried steel because of the relative electrochemical potentials of the two metals. In our case we wanted the current to flow onto the steel, but had to overcome the drain from the copper. Initial design work by our contractor suggested that the brute force approach to overwhelm the copper required in the order of 5000A injected into the ground. While technically possible (we were talking to a company who supplies rectifiers for aluminium smelter) the problems which would arise included stray current flowing in hydrocarbon lines, spark potentials occuring in hazardous areas, possible effects on instrumentation and telecoms, and so on.

Can you move the grounding grid to a location where the chemical problem is less acute? Either deep drilled or at a location away from the process?

I strongly suspect CP won't prevent chemical attack from compounds like ammonia, but you might want to pose the question in the chemical engineering forum. Is using something more chemically compatible with NH₃ like a bronze an option?

Cumberland Engineering and Corrpro both have some informative stuff on their websites, but neither reference use of CP to reduce chemical attack rather than electrochemical action. NACE publish a book on CP which is a good reference even if it isn't exactly bedtime reading.

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Sometimes I only open my mouth to swap feet...

 

[mc5w]

A place where I worked processes and recycles foundry wastes from aluminum foundries. Ammonia dissolves the copper in bronze and brass and this plant has a "no bronze" and "no brass" specification for the plumbing in the wastewater treatment room.

The combination of ammonia, water, oxygen, and chlorides dissolves all metals except indium, possibly also bismuth, and remotely copper free aluminum that is coated with urethane varnish. The wastewater treatment room at this facility has the ability to dissolve stainless steel.

Scotty, one strategy would be to put carbon anodes nearby the foundations that contain steel so that soil resistance sends 1/4 or whatever of the current to the foundations. Each foundation would need to be equipped with its own DC power supply. If lets say your copper wire and copper rods grid is 10 feet by 10 feet ( about 3 meters by 3 meters ) you could put each carbon anode around the foundation on the diagonal of the copper wire square and about 1.5 meters from the foundation and somewhat deeper than the copper wire mat.

You might need to set up a testing area to get carbon anode locations that would send about 1/4 of the current into the steel. The rest of the current would then help stop the sulfuric acid from catalytic converters in motor vehicles from corroding the copper wire.