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charged hydroxyl ions, resulting in iron FIGURE 2 PRACTICAL GALVANIC SERIES
hydroxide (rust). A surplus of electrons
at the cathode will combine with
hydrogen ions to form a polarisation METAL VOLTS*
film, which protects against corrosion.
Commercially pure magnesium -1.75
Potential Magnesium alloy (6% Al, 3% Zn, 0.15% Mn) -1.60
When a metal is placed in an electrolyte Zinc -1.10
it develops a ‘potential’. The potential Aluminium alloy (5% Zn) -1.05
depends on the individual metal and Commercially pure aluminium -0.80
the electrolyte properties. A voltage Mild steel (clean and shiny) -0.5 to -0.8
series of all common metal/alloys has Mild steel (rusted) -0.4 to -0.55
been developed to show their potential Cast iron (not grashitized) -0.50
in volts against a reference electrode Lead -0.50
(see Figure 2).
A material that possesses a more Mild steel in concrete -0.20
negative natural potential is said to be Copper, brass, bronze -0.20
more active and will therefore corrode Mill scale on steel -0.20
more readily than a material with a less
negative potential.
Commercially, pure magnesium FIGURE 3 METAL POTENTIAL CHANGE
sacrificial anodes are often buried
alongside and connected to steel pipelines.
The difference in potential between the metal 1 v metal 2
two materials creates a driving voltage, (natural potential -500 mV) (natural potential -1500 mV)
which enhances the current flow
between the anode and cathode.
When anodes are connected to the
pipeline, a shift in potential occurs
and the steel pipeline develops a more
negative potential. These potential
readings are measured at intermittently
spaced test posts using a multi-meter
and a copper/copper sulphate reference
electrode placed into the soil.

Compliance and protection criteria when the two are connected:
There are two aspects of protection
criteria for a cathodic protection scheme potential of metal 1 becomes more negative
to be deemed compliant. A reading potential of metal 2 becomes less negative
can be taken with the current flowing,
deemed the ‘on potential’. However, a shift in potential occurs, positively charged ions leave metal 2 (the anode)
this reading will have an inaccuracy and flow through the electrolyte to metal 1 (the cathode)
due to an ‘IR error’. The current passing
through the soil encounters resistance,
which creates a potential drop. networks now owned by Cadent. start of the second financial year.
A more accurate reading can be A principle contractor/designer was As a result, they were relieved from
obtained by switching all current engaged, briefed and mobilised to their duties and all ongoing design work
sources off. A true ‘polarised potential’ assess the status of the medium and was to be conducted by the Cadent
can be noted at the instant when the low pressure pipeline cathodic pipelines team. Prior to this adjustment
current has been stopped. protection schemes and then in contractor usage, the Cadent
Cathodic protection systems should commence remediation. pipelines team in the North West had
be maintained at a more negative on been successfully managing several
potential than -1250mV measured Contractors other civils contractors to ensure
without interruption of the current The sheer scale of the project and delivery of the high and intermediate
Pipe to soil potentials must be volume of required delivery across pressure pipelines cathodic protection
maintained at a more negative polarised five distribution networks soon proved remediation activities. Therefore, the
(instant off) potential than -850mV unmanageable for the appointed same methodology for delivery is now
principal contractor/designer. Shortly also used for the medium and low
Challenges after the end of the first financial year pressure pipeline cathodic protection
Following the improvement notice following mobilisation, the North remediation activities.
served by the HSE, significant investment West network concluded that the
totalling approximately £10.5 million arrangements had to change. The Data
has been sanctioned by the business. principal contractor/designer had Data errors have been identified
This has enabled the recruitment of continued to under-deliver and failed as a significant cause of non-
more staff in each of the distribution to implement recommendations at the compliant schemes. An in-depth data

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