Cathodic protection

Introduction

A metal consists of uncharged "building blocks" known as atoms. When a metal corrodes in a liquid, positively charged atoms flow from the metal and dissolve in the liquid.

Negatively charged electrons remain in the metal. If free oxygen (O2) or positively charged hydrogen atoms (H+) are contained in the liquid, the electrons are consumed in the metal and corrosion proceeds. Reactions with oxygen for any metal (Me) as shown below:

If the electrons are not consumed, the negative charges become so high that the atoms cannot leave the surface of the metal because of the electrostatic forces of attraction.

Corrosion can be prevented by eliminating the oxygen or the positively charged hydrogen ions. But this cannot be carried out in practice. A better method is to create an excess of electrons in the metal, so that positively charged metal atoms cannot leave the surface of the metal because of the electrostatic forces of attraction.

This method of preventing corrosion is known as cathodic protection. Two methods are available for applying cathodic protection: - impressed current and sacrificial galvanic anodes.

Impressed current

When impressed current is applied, the component which is to be protected is connected to an electrical power supply source. The impressed current flows from an electrode (normally inert), through the liquid and to the component to be protected (see below).

This system must be carefully controlled to give the correct current density and to allow for the potential drop across the electrolyte. An electrode which senses the current density is usually connected to this system and its function is to adjust the current to the correct value. Incorrect adjustments can cause damages on paints and roller bearings. This type of systems is owing to that more suitable for large installations but can also be used in fish farms where the use of sacrificial anodes is restricted. The electrodes generally employed are made of graphite, titanium alloys, silicon iron, etc.

Sacrificial anodes

Sacrificial galvanic anodes can be used to produce the necessary current to protect the component. They are self regulating and give the right current during variable situations. In this case, the necessary current is produced by connecting the metal which is to be protected to a less noble metal (see galvanic series table in the chapter Corrosion theory). An example how this can be done is shown below.

Types of water

Cathodic protection can be applied in various types of water, i.e. sea water, acidic- and alkaline waters. In the succeeding, the description of the use of cathodic protection will be restricted to sacrificial galvanic anodes and their use in sea water and other chloride containing waters.

Resistivity of the water

An important parameter in the provision of cathodic protection is the resistivity of the water. The resistivity is specified in ohm × cm. The table below shows the resistivity etc. in various seas at 20 oC.

Water from Resistivity
Ohm x cm
Conductivity
uS/cm
Salinity
%
Chlorides
mg/l or ppm
River and lake waters

25 000 - 5000

50 - 220

0.0

10 - 50

The Baltic sea

200 - 80

4500 - 12 500

0.3 - 0.8

1 500 - 4 500

The Caspian sea

53

19 500

1.3

7 200

The Black sea

32

31 500

2.2

12 200

The Pacific

20

47 000

3.4

19 000

The Atlantic

20

48 000

3.5

19 500

The Mediterranean

19.5

51 500

3.8

21 000

The Red sea

19

53 500

4.1

22 500

The Dead sea

3.1

122 000

34

120 000

The majority constituent of sea water is sodium chloride (table salt), and the salinity of the sea water can be estimated by using the following relationship: Salinity (%) = 0.18 × chlorides (g/kg)

Restrictions

When sacrificial anodes are used, the resistivity of the water should not exceed 200 ohm×cm, because of the otherwise poor current distribution. In case of higher resistivity more anodes per area can be used. Increased corrosion can also sometimes be accepted because low salt content give low corrosion rate.

Other restrictions for the use of anodes are that the pH must be between 5.5 and 11 and the temperature not exceeds 60 oC. If paint is used together with anodes the temperature limit is 50 oC.

Protected metals

In principle, cathodic protection can be applied to all metals, with the aim of reducing the corrosion rate. The following metals will be discussed in the succeeding:

  • Plain carbon steels and unalloyed cast iron
  • Stainless steels and copper alloys
  • Aluminium

Depending on the resistivity of sea water and its velocity, satisfactory protection of these metals may require different current densities (mA/m2).

A painted metal surface needs a lower current density, since only pinholes and damage to the paintwork need be protected. The table below shows the current densities necessary to suppress the corrosion of different metals in diluted sea water with a resistivity up to 200 ohm × cm. For flowing water the current density must be multiplied by (V+1), where V correspond to the water velocity expressed in m/s.

Necessary current density (mA/m2)

Uncoated

Coated

Carbon steel and cast iron

70

15

Stainless steel

100

15

Copper alloys

200

30

Aluminium alloys

25

5

Anode performance

Various metals are used as sacrificial anodes for protecting components against corrosion in sea water. The basic condition is that the sacrificial anode must be less noble than the metal which it is designed to protect.

The most common metals used as sacrificial anodes are zinc, magnesium and aluminium. The chemical composition of the sacrificial anode is very important if the correct effect is to be achieved. For example, the iron content of zinc anodes must be very low, since the zinc anode will otherwise not produce the correct current density.

The geometry of the anodes is also important. Long and narrow anodes are best, to produce good current distribution. The capacity of an anode material is expressed in ampere hours per kilo (Ah/kg). This describes the number of ampere hours available per kg of anode material. The capacity is used to calculate the amount of anode metal required in the design of cathodic protection. The capacities of different anode materials are tabulated below.

Anode material

Capacity in Ah / kg

Magnesium

1 100

Aluminium

1 500

Zinc

750

Magnesium is used in high-resistivity water i.e. drinking water but not in sea water, since it would be consumed too rapidly and may give rise to explosive gas mixtures.

Zinc anodes should not be used at temperatures higher than 60°C. Above this temperature, the polarity of zinc in relation to iron can change, and the zinc will become "more noble" than steel and the corrosion rate on the iron will increase dramatically.

Calculation for cathodic protection with sacrificial anodes

The following method of calculation can be used to estimate the amount of anodes necessary to protect a submersible pump by sacrificial anodes.

The water should have a resistivity below 200 ohm × cm. In water with higher resistivity the same anode weight can be divided up in smaller units to maintain protection of the complete area.

Calculate the surface area to be protected. Coated and uncoated areas separately. Take into consideration that the inside of the pump house has high water velocity and that pipes and discharge connections may also consume anodes. Calculate separately the protective current necessary for the inside and outside surfaces.

The weight (M) of anodes necessary to provide protection is calculated with following formula:

The anode geometrical factor is 0.9 for long thin anodes and 0.85 for others.

An example of a calculation below, that could be a small pump, shows the total amount of zinc required to protect uncoated cast iron of 1 m2 in stagnant water and 0.5 m2 with a water velocity of 5 m/s for one year:

The right amount of anodes must be mounted in their respectively area.

The recommendation for change of the anodes is when little more than half of their volume are consumed. The zinc weight in the example should therefore be somewhere between 6 and 7 kg depending of suitable sizes of the anodes.

Location and mounting of the anodes

On submersible pumps the anodes should be fitted to the outside as well as to the inside. If the anodes cannot be located inside the pump casing, they should be fitted as close as possible to the inlet. Important to note is that the anode must be located in areas where the water velocity is low, otherwise they can be attacked from erosion-corrosion.


Sacrificial anodes mounted at the inlet of a pump.

The anodes must be in electrical contact with the metal to be protected. The current should flow from the anode, into the water, to the surface to be protected and through the metal back to the anode.

The anodes need not be secured directly to the metal to be protected, although this is preferable. A piece of wire or a steel strip can be fitted to conduct the current.

To ensure good current distribution to all parts of the pump, more than one anode must be used. Corners, baffles, etc. counteract good current distribution.

Painting

Various organic coatings can be used together with cathodic protection, but epoxy coating is preferred. Cast iron and steel are easy to paint on sand blasted surfaces. Stainless steel and aluminium are more difficult and require special primer.

The sacrificial anodes will protect the metal from corrosion in areas where the paint has pinholes or damages. The zinc anodes must remain bare, since a coat of paint on them would destroy their protective effect. They must also be electrical connected to through the paint onto the metal.

Anode consumption

The anodes must be inspected regularly and more often for a new installation to get an idea of the consumption rate. Change of anodes should be done when about 70% of the anode mass is consumed.

The anode consumption is affected by parameters such as salinity, oxygen content, pH, temperature, water velocity etc, but most of all from the size of the protected area. External unpainted constructions can cause heavy consumption of the anodes. The useful life of the anodes in sea water may therefore be difficult to estimate. On new installations, the anodes should be inspected after, for example, one month, two months and six months, to estimate the consumption rate.


Strong galvanic action from an armoured concrete wall and a ship on aluminiumpumps.

If exceptional consumption of the anodes is present, it may be caused by one of the following causes:

  • An electrical contact with other large metal parts in the neighbour-hood like steel piling etc.
  • A current on the earth connection of the pump from an imperfect earth.
  • A stray current from another current source.
  • To low or to high pH.