An idiots guide to cathodic protection

What the heck IS cathodic protection in the first place???

Cathodic protection is an electrical way of stopping rust.

Rust is chemical and electrical. Metal dissolves in some solutions and gives off electricity. Metal can be 'plated' onto other metal electrically.
All 'batteries' work on this principle and everyone knows that batteries drive loads of the things we use daily.
Not many people know that our gas and oil comes to us through pipes that are inclined to rust, but are protected by 'cathodic protection'.
Some people know that metal boats are protected by cathodic protection, and have seen lumps of metal attached to hulls for this purpose. These lumps of metal dissolve in the water and give off electricity which prevents the hull from rusting.
When you put two different metals in contact and submerge them in liquid (or wetness) one of the metals dissolves and discharges an electrical current into the liquid. The liquid (or damp material) is the 'electrolyte' and gets 'charged up' with electricity. It's 'electrical potential' is increased.
Electricity works by 'pressure' and anything with a higher 'pressure' gives off electricity to anything with a lower 'pressure'.
The electrolyte is then at a higher electrical 'pressure' than the metal that is not dissolving and so the electricity passes into it.
The metal that is dissolving is the 'anode' from which the electrical current passes into the electrolyte and the other metal is the cathode into which the current passes because the electrical pressure must be balanced out. (everything tries to equalise).
The dissolving metal is sacrificed to prevent the subject metal from corrosion, and this method is known as 'sacrificial cathodic protection'.
There are limits to which sacrificial cathodic protection can be used but the same principle can be used by causing a manufactured electrical pressure which is 'impressed' into the electrolyte. The electricity is then 'drained' out of the subject metal....... boat hull or pipeline.... and this interferes with the natural tendency of the metal to dissolve....or rust!

Impressed current cathodic protection

Electricity is generated by a sort of pumping action which causes it to flow backwards and forwards in 'waves', but this is no use for our purposes so we have to get it going in one direction through a circuit known as a 'rectifier'. At the same time we can control the amount of current by transforming it, so the apparatus is know as a transformer-rectifier.
A transformer-rectifier can be regarded as an electrical pump which is sucking the electricity out of the pipeline (etc) and pumping it into the ground (or sea ... or swamp... or wherever else you want to pump it).
The effect of this is amazing. It stops rust! And it's cheap!
But there are some snags.
Because it's so good, it gets installed .... then ignored...... well most people don't even know it exists... and because it's cheap some people don't think it's important.

But it 's life and death to some.

The villagers in the picture are gathering water from outside a flowstation in Nigeria. A pipeline in Nigeria leaked petrol and local people collected the petrol in cans and washing up bowls and the site drew hundreds of women and children until the petrol was accidentally ignited.... cooking up to 1000 people.

Cathodic protection IS important. A couple of years before this incident a pipeline in the USSR exploded and blew a train off it's tracks, killing many and causing ecological devastation. This was thought to be caused by corrosion.

by : Roger Alexander (my great Teacher)

Newsflash 5th December 2000

*** Natural gas spewing in Texas MONT BELVIEU, Texas (AP) - A pipeline ruptured and released a
potentially explosive cloud of natural gas, forcing evacuations of
about 40 homes and the rerouting of airplane flights around the
area. Several minor injuries were reported Monday night when the
pipeline, owned by Channel Industries Gas Co., blew open near
Houston Raceway Park. The blowout was felt and heard as far away as
Baytown, more than 10 miles to the south. There was no fire, said
Baytown police Sgt. Keith Dougherty. However, residents of the
immediate area were told to evacuate and flights east of Houston
were kept at least miles from the site as a precaution, said Texas
Department of Public Safety spokesman Richard Vasser.

Full article at: http://www.infobeat.com/fullArticle?article=405225991

Cathodic Protection

Pages

Dangerous Coating Breakdown Factor

Cathodic protection is important

An idiots guide to cathodic protection

What the heck IS cathodic protection in the first place???

Impressed current cathodic protection