Corrosion Protection

Question 1

What are the methods of preventing corrosion

Answer 1

There are various methods for preventing corrosion:
 Coatings.
 Inhibitors.
 Selective Design.
 Anodic Protection.
 Cathodic Protection.
Before considering these, a look at the way in which the
corrosion process is influenced by the two main variables
will assist in understanding the various protection methods.
Electrode potential - pH of the Electrolyte.

Question 2

Show the Pourbaix Diagram

Answer 2

Corrosion
At intermediate electrode potentials and over a very wide range of pH values
corrosion takes place and metal is removed.

Passivity
At higher electrode potentials and over a wide range of pH values, there is a
passivity region. This is the region in which a corrosion product film is formed,
that in most cases is an oxide film. It is worth noting that the diagram only
indicates that an oxide film is formed; it does not mean that the oxide film
gives protection. The properties of the film must be known in order to
determine this.

Immunity
At low electrode potentials and over almost the whole of the pH range, the rate
of corrosion is so low that the metal is said to be immune.

Question 3

What are the first things that a CP systems designer looks at

Answer 3

The CP System Designer starts by determining an
acceptable corrosion rate.
 This data is used to fix a value for current density.
 The potential to achieve this current is approx. -800mV.
 It may seem that potentials more negative than -800mV would produce even less metal loss.
 There are two reasons why this is not prudent.

Question 4

What are the hazards with high cp values

Answer 4

1. At potentials more negative than -1100mV,
   the possibility of Hydrogen evolution exists &
   this can cause Hydrogen Embrittlement.
2. Large currents are associated with more
   negative potentials that produce Hydroxyl
   ions, these damage paint coatings
   (Disbondment).

Question 5

Explain Sacrificial Anode System.

Answer 5

Sacrificial Anode System.
This method of CP is straightforward.
We create a galvanic corrosion cell, in our
favour, by attaching less noble materials to the
structure, making it the cathode.
The materials most commonly used:
 Aluminium (Uncoated structures).
 Zinc (Coated Structures).
 Magnesium (Rarer, may not last for the design
life).

A natural phenomenon does occur, which assists
in the protection of structures.
 Calcium, Magnesium & other metal ions are present in sufficient quantities in seawater to
react with hydroxyl ions.
 This produces insoluble Calcium & Magnesium salts, known as Calcareous deposits.
 These form a strongly adherent film, that reduces the current requirement.

Question 6

What are the advantages of a sacrificial anode system

Answer 6

1) No external power required
2) Active from day of immersion
3) No running costs
4) No danger of over protection
5) Well proven, reliable method

Question 7

What are the disadvantages of a sacrificial anode system

Answer 7

1) Current output decreases with time
2) Comparatively high initial costs
3) Adds considerable weight & drag
4) Comparatively difficult to increase protection by retro- fitting anodes

Question 8

What type of anodes are used in an ICCP

Answer 8

Anode Material.
 Sacrificial anodes corrode.
 In the ICCP system, the surface area is very
important, so the anode must not corrode.
 Anode material is selected from the top of the
galvanic series not the bottom.
Platinum Sheathed Titanium.
Lead/Silver Alloys.

These anodes are capable of supplying high
current densities.
 A structure could be protected with a few large
anodes supplied with a high current.
 The high current density present in the vicinity
of a single large anode, could cause
disbondment & possibly embrittlement.
 So, many smaller anodes are distributed over
the whole structure, which reduces the current
density for each.

Question 9

What role does geometry play in regards to an ICCP

Answer 9

Offshore structures have complicated geometries.
 Making it difficult for corrosion engineers to
predict the total distribution potentials.
 More anodes are used, each protecting a smaller
area, to minimise the areas at risk of inadequate
protection.
 If engineers have any doubts about protecting
any particular area of the structure, sacrificial
anodes may be installed to work with the ICCP
system.

Question 10

What is the correct way to hook up an ICCP

Answer 10

Polarity.
It’s vitally important the power supply is connected
correctly.
Negative terminal must be connected to the
structure.
Positive terminal must be connected to the anode.
Get it wrong & the structure will corrode
catastrophically.

Question 11

Platform-Based Mode ICCP

Answer 11

Platform-Based.
 Numerous anodes are attached to the
structure at intervals around it.
 They are insulated from the structure. By a di-electric shield
Two problems are associated with this method.
 Areas of inadequate protection - Shadow
Areas.
 May be solved by adding sacrificial anodes.

Second is that of current flowing directly from
the anode to the adjacent structure.
 This could cause embrittlement & coating
damage.
 Dielectric Shields are deployed to insulate the
structure electrically.
 Current is limited by design, each anode being
positioned to protect the local area only.

There is a diver safety consideration
These anodes have an output of between:
30 – 80 Volts DC
300 – 1000 Amps
If divers are deployed adjacent to any of these anodes they should be isolated from the system
No need to switch off the whole system

Question 12

Remote System. ICCP

Answer 12

 A number of anodes are placed on the seabed
at a designated distance from the structure.
 Avoids the possibility of current flowing
directly from anode to the structure.
 Being fewer of them, the current density is
higher so, still a possibility of disbondment &
embrittlement.
 Designs generally favour more anodes
distributed around the structure.

Question 13

Advantages of an ICCP

Answer 13

1) Fewer required
2) Reduced weight - loading
3) Output can be adjusted
4) Can be remotely monitored

Question 14

Disadvantages of an ICCP

Answer 14

1) No power – no protection
2) Continuous monitoring required
3) Danger of over protection
4) Cables are vulnerable
5) Hazardous to divers
6) Possibility of shadow areas
7) May increase marine growth

Question 15

Reference Electrodes ?

Answer 15

are commonly:
 Zinc
 Silver/silver-chloride (Ag/AgCl).
 Copper/copper-sulphate (CSE).
 They are a vital component of any ICCP system.
 They determine the current required from the power source.

Question 16

Protective Coatings?

Answer 16

Coatings.
Form a barrier between the electrolyte & the 
metal.
They may be:
 Paints:
 Organic films.
 Varnishes.
 Metal coatings.
 Enamels.
 Sheathing.

Question 17

Paints ?

Answer 17

Paint presents a barrier to air, moisture & ions
aggressive to the metal.
 Paint cannot provide a complete barrier to
oxygen or water.
 In time, these will penetrate through to the
surface of the metal.
 Paint used underwater must have a strong

Question 18

What types of metallic coatings

Answer 18

Metallic Coatings
Impose a continuous barrier between the metal surface & surrounding environment.
These may be applied by:
 Electroplating.
 Hot Dipping.
 Spray Coats.
 Diffusion.

Question 19

What types of cladding are there

Answer 19

Cladding metal skins laminated onto the metal.
The skin can be applied by:
 Rolling.
 Explosive welding.
 Buttering (building up a welded coat).
 Sheathing.

Monel Sheathing.
Is a copper/nickel alloy,
about 2mm thick, welded
to the riser.
Inert gas is injected
between it & the riser.

Question 20

What are 3 methods for preventing corrosion

Answer 20

The Pourbaix diagram indicates 3 methods for 
preventing corrosion:
 Making the electrode more positive.
 Making the electrode more negative.
 Changing the electrolyte pH.

Question 21

How can you affect the electrolyte

Answer 21

Properties of the electrolyte that can be affected
by using inhibitors are:
 The conductivity.
 The pH.
 The interaction with the metal surface, either
attacking or strengthening passive films.

Question 22

In Summary

Answer 22

 Avoid all unnecessary bimetallic corrosion cells.
 Avoid differential-aeration cells (crevices,
debris traps, poor drainage).
 Avoid stray currents from electrical machinery.
 Choose the material with the best properties
for the environment.