Cathodic Protection

[Knobhead]

Following a talk in the background with Chr!s I’m opening this as a discussion not a question.
The aim is to share peoples experience and knowledge of cathodic protection and earthing systems.
For myself, my experience is slightly off kilter to where I’m hoping the thread will go.

Basically cathodic protection is the use of sacrificial anodes that by giving up molecules to provide a coating to the metal requiring protection from normal corrosion. It’s used a lot in underground pipelines, structural steel work, etc.

As I said my involvement is off topic right from the start. For 12 years I was involved in the manufacture of cast iron pipes which relied on zinc as the protection. But the way we did it made the iron self healing. It’s galvanic protection with a diference.

After being prodded in to making this post, I hope Chr!s will soon respond.

 

[Andy 7671]

Dirty work...

Me, in my ignorance thought the reference cells, via telemetry to the monitoring station then back to the anodes provided the right amount of m/voltage to be applied to keep the electrolysis (ie rust) in suspension by not allowing a PD.

There is a train of thought whereby a laid mesh attracts salts away from rebars and the like. I have thought this information to be floored but have heard it more than once.

Still confused as to whether hovercraft need sacrificial anodes...(joke)

 

[MarkieSparkie]

Corrosion and Cathodic Protection Principles:
Corrosion always develops at the anode, where current leaves the metal and enters the electrolyte, whilst a protective effect occurs at the cathode. Thus if the whole metal surface is made sufficiently cathodic, corrosion will not occur. This is the basic principle of Cathodic Protection. In marine structures, such corrosion cells may result from the use of dissimilar metals. Usually, however, localised anodic and cathodic areas arise on the surface of the same metal through differences in the metal itself, variations in protective films or changes in the electrolyte. ie: aeration, temperature and salinity. Corrosion may be prevented by removing one or more of these corrosive elements and for marine structures, the most practicable method is to apply a protective coating, thus introducing an electrical resistance between the metal and the electrolyte. Paint in various forms normally provides the first level of protection. However, even the most efficient coatings are subject to defects during application or service, with inevitable corrosion of the exposed metal. It is therefore generally accepted that cathodic protection, in conjunction with a high performance paint system provides the most effective and economic safeguard against corrosion on larger structures, vessels or platforms.

 

[Chr!s]

Well corrosion takes place in what they call cells, Galvanic and Electrolytic.

The Galvanic Cell has a Anode, cathode and electrolyte and a return current path where the corrosion current returns via the cathode. Its not just dissimilar metals but also different metal conditions, stresses, surfaces etc.

Electrolytic Cell also comprises of a Anode,Cathode and Electrolyte, the corrosion current is caused by an external potential. So current flows into the electrolyte and and creates a positive anode and returns via the cathode to the source.

The Two systems that i am aware of are Sacrificial Protection to which Tony refers and Impressed current protection to which Andy above refers.

Sacrificial protection requires a Anode which is higher in the Galvanic series of metals, Zinc being towards the top of the table.

Impressed current basically current is impressed on the structure, this opposes current from leaving the structure preventing corrosion, a potential of 850 mV deems the structure to be protected unless Anaerobic.

 

[Knobhead]

The system I’ve been involved with doesn’t use a sacrificial anode but forms a self-healing coating to the surface of ductile iron pipes.

The pipes were metal sprayed with a layer of zinc at the rate of 200gm/M². the spray is created by bringing two pure zinc wires in contact with each other with a high current passing between them (2000A) at the point of contact an air jet directs the metal to the surface of the iron. Over the zinc a layer of bitumen would be applied.
Normally the bitumen provides the protection but if it gets damaged galvanic action will occur bonding the zinc to the iron. If the damage is severe enough to expose the iron then zinc from the surrounding area will migrate to the area and form a protective blister.
In essence the same system as outlined by Mark with the exception that no first coat is applied but relies on iron oxide from the initial casting.

 

[Engineer54]

Sticking to ''passive'' cathodic protection, it's generally accepted that the best cost effective method of protecting metalwork and general sized pipework that is surfaced mounted is the ''Hot dip'' zinc coating. Thickness of the zinc coating is proportional to the time left in the molten zinc bath but there is a limit to the thickness you can apply. Obviously fabrication sections that need to be immersed in these Zinc baths need to be carefully designed for the free flow of escaping air, along with the flow of the zinc to all hidden sections without causing air pockets. Any air pockets can be very dangerous to the hot dip bath operatives, with blobs of molten zinc shooting up out of the bath.

Anyway, a suitably hot dip zinc coated structure, will be expected to have around a 30 year life before any remedial work should needed. If subjected to corrosive atmospheres the same structure would also require a suitable over-coating with a protective paint system, but could still reduce expected lifespan to 20 to 25 years before remedial works would be needed.

In extremely high risk areas, a similar method is used, but the protective medium will be Magnesium in place of zinc. It's a far more expensive coating and reserved for the very top end situations. Magnesium is also not as robust as Zinc and in some situations not suitable due to environmental conditions...

Lot more to this type of protection, and not even scratched the surface here.... lol!!