Volume 4 Preprint 9
Keywords: cathodic protection, tanks, pipelines, ships, solar powered systems, on-potentials, off-potentials, CIPS, maintenance, monitoring
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CATHODIC PROTECTION CONFERENCE
UMIST, Manchester 10th – 11th February 2003
Anticorrosion Engineering Limited 4 Langley Drive Bayston Hill
Shrewsbury SY3 0PR firstname.lastname@example.org
This paper describes some cases where elementary errors in the
design, installation or operation of cathodic protection systems have
subsequently lead to underperformance, or even failure.
Keywords: cathodic protection, tanks, pipelines, ships, solar powered
systems, on-potentials, off-potentials, CIPS, maintenance, monitoring.
Cathodic protection (CP) involves a very simple concept. If the
potential difference between a structure and its environment is made
more negative then corrosion is controlled, or even stopped
altogether. As has been discussed at this conference, the sacrificial
anode version of the technique has been known since early in the 19th
century; and the century had not closed before the first forays into
impressed current CP were made.
Despite its sound theoretical basis, CP took some time to become
widely established. However, it is now applied to the majority of buried
or immersed steel structures; either routinely or because regulations
require it. Usually it works well; but there can be problems.
Is CP Needed?
In many cases, for example steel pipelines transporting hazardous
fluids, the decision to apply CP is imposed by the relevant national
regulations. However, in other instances the responsibility for that
decision rests with the owner or operator. Two contrasting examples
show how, on occasions, the thinking behind the decision can become
Case 1 – Water Storage Tank – Coastal Location
The author’s company carried out a formal corrosion risk
assessment for a fertilizer complex in the Middle East. The
assessment concluded that the majority of the piping and
equipment fell into low or medium risk categories. Among the
few exceptions were the water storage tanks which, because of
the prevailing soil conditions and absence of CP, were predicted
to suffer high corrosion rates. Since an adequate supply of water
was essential to maintain production, the assessment concluded
that the prospective failure of the tank placed it in the high risk
category. It was recommended that CP be installed.
In the event, the client was reluctant to accept and act upon the
outcome of the report. Just under a year later the raw water
storage tank perforated due to soil-side corrosion. The resulting
loss of water caused a very costly two-week unscheduled
interruption in production and a belated decision to install CP.
Case 2- Water Storage Tank – Desert Location
A raw water tank at an oil company desert station, described as
very old, collapsed as a result of corrosion thinning of the
internal shell. The company’s corrosion department advised
installing CP systems for the internal surface (magnesium
anodes) and the external base (impressed current) of the
The need for the internal anodes had been effectively
demonstrated by the collapse of the original tank. However, the
decision to apply CP externally was taken without having to
suffer the inconvenience of studying the soil conditions at the
At about that time, this author happened to visit and took the
opportunity to view the remains of the old tank in the scrap
yard. Due the way the debris had been dumped, about 50% of
the underside of the base could be inspected. It had originally
been galvanized. There was some white dusting of zinc
corrosion product and a few small rust spots. There was no
general thinning of the plate from the soil side, nor any evidence
of pitting attack. In other words, the original tank base had done
very well for a long time in the absence of CP. This was not
surprising in view of the benign soil conditions. As a result the
corrosion department were persuaded to delete the requirement
for external CP.
Who Should Design the CP System?
Having made a decision to apply CP, someone needs to be appointed
to design the system. In an ideal world this would be a CP specialist.
Unfortunately, such people are not always available, so an acceptable
alternative might be a competent engineer with access to the relevant
However, on one occasion the design was left up to the company
accountant. As the following example illustrates, this was not a good
Case 3 – Effluent Treatment Tank.
Figure 1 shows a painted steel effluent water treatment tank at a
meat processing factory. The tank contained an internal weir
structure fabricated in stainless steel (Figure 2). The water had a
high conductivity and was well aerated (as indicated by the
growth of algae). This design led to a classic case of galvanic
corrosion, with a large cathode (the stainless steel weir) coupled
to a small area of anode (the areas of damaged paint on the tank
wall). Localized penetration of the tank soon occurred.
The owner sought advice from the tank manufacturer, and was
informed (quite correctly) that the problem could be remedied
by the installation of sacrificial anodes. A request to procure
anodes was then passed over to the accountant who saved
money by dispensing with the cost of involving a CP design
engineer. Instead he ordered four “suitable” anodes from a yacht
chandler. These were installed by the company’s own
maintenance fitters. Sadly, however, there was no discernable
reduction in the frequency of leaks (Figure 3).
This author’s subsequent potential measurements confirmed
that the undersized anodes were failing to exert any useful
effect on the galvanic corrosion.
How Should We Use Design Guidelines?
Fortunately there are plenty of published guidelines to help engineers
produce CP designs for offshore[2,3,4,5,6], coastal or onshore[8,9]
structures, as well as for concrete reinforcement. Moreover, design
houses can easily implement the published guidelines in the form of
spreadsheet or Mathcad® design tools. This has the significant
advantage that, providing the tool has been validated under an inhouse quality system, its results are guaranteed to be arithmetically
The disadvantage is that the design exercise becomes mechanical and
is often carried out, checked and approved by engineers with no
practical experience in CP. Moreover, because the computerised
design calculations are presented with a high degree of precision, the
engineers responsible are apt to lose sight of the fact that the
underpinning guidelines embody a compilation of predictions about
how CP system components and, more particularly, protective coatings
will perform in future. It is sometimes worth remembering that these
predictions are often little more than guesses endorsed by
committees. The following two cases make the point that guidelines
can be misleading.
Case 4 Coatings that Under-Perform
Figure 4 shows a section of 6-inch EPDM rubber coated flowline
installed in the North Sea some fifteen years ago. The CP was
provided by sacrificial anode bracelets which had been designed
according to the accepted guidelines of the day. However, a
survey carried out two years after installation showed that,
despite the anodes yielding more than the design current, the
line had failed to polarize to its protection potential.
Subsequent investigations revealed that the cause of the
problem was the carbon black used as a filler in the rubber
coating. Laboratory measurements showed that, although the
polymer itself was expected to have a specific electrical
resistivity of 1014 Ωm, the value for the carbon-loaded coating
was 10 Ωm at atmospheric pressure, falling to 2 Ωm at 16 bar
(the water pressure at the installation depth). What is more, as
expected from the galvanic series, the carbon was acting as a
cathode: draining current from the anodes and potentially
putting the pipeline at risk.
Even before the cause of the problem could be fully elucidated it
was clear that there was a need to install additional CP. This
necessitated a costly retrofit using the (then) innovative
approach of magnesium anode arrays connected to the pipeline
via voltage limiting diodes.
Fortunately the above case was one of only a few such instances. The
lesson has been learned, and insulating coatings now have very low
It is more usual for adherence to the published coating breakdown
guidelines to result in excessively conservative designs. For an
offshore installation, some degree of over-design is usually justified. It
represents a small increment to the capital cost, and provides
insurance against the financial penalty of having to carry out remedial
work sub-sea. On the other hand, over-design is less easy to justify
for on-shore CP systems.
Although there are guidelines[2,6] for predicting coating breakdown and
cathodic current densities for offshore structures and pipelines, there
are no corresponding documents for onshore CP. In the latter cases
some designers use traditional, but nonetheless arbitrary, methods
such as assuming 5% coating breakdown and a bare steel current
demand of 10 mA/m2. Other designers refer to less conservative
historical data, for example those summarized statistically by
Hewes. However, it needs to be understood that historical data have
been gained predominantly on pipelines coated with earlier generation
materials such as asphalt, coal tar enamel or tape wraps. The modern
three-layer polyolefin coatings, applied under carefully controlled
factory conditions, will inevitably perform better.
Although the improved durability of modern coatings is to be
welcomed, it is not without its problems when it comes to designing
an onshore impressed current CP system.
Case 5 The Coating That Was Too Good.
In 1997 the author was involved in resolving a dispute between
an oil company and a CP contractor. The issue involved a new
30-inch oil trunk line passing through predominantly desert
terrain. The line was protected by a good quality factory-applied
polyethylene coating, plus carefully applied shrink-wrap at the
CP was provided by solar powered stations, the number and
output of which were calculated on the basis of coating
breakdown estimates relevant to previous generation coating
The CP contract required that, at commissioning:
all pipe potentials were to be within the range -0.95 V to 1.4 V (vs Cu|CuSO4), with
all CP stations operational.
The problem was that the output of each CP station was virtually
uncontrollable at levels below about 10% of the rating. However,
even this minimum level of output was so far above the need of
the very well coated line that the least negative “instant”
current-off potentials that could be achieved were in the region
of -2.5 V. Inevitably the unfortunate CP contractor found that
the twin requirements imposed by the contract were mutually
This case provided an illustration of the largely unquestioned practice
of sizing impressed current CP systems according to the maximum
(i.e. end-of-life) anticipated current demand. Whereas this is sensible
for offshore systems, where retrofitting is prohibitively expensive, it is
invariably uneconomic for onshore systems. It involves the excessive
up-front capital expenditure of installing CP capacity that will not be
needed for decades, if at all. Moreover, there is the additional cost due
to the inefficiency of operating large capacity CP units at near the
bottom of their rated outputs.
Although it might be contractually inconvenient, it would make more
sense to start with a small system appropriate to the early
requirements, and then to provide supplementary CP stations where
and when the CP monitoring indicated a need.
What Happens When The System is Handed Over?
As noted, it is by no means certain that designers of a CP system will
be familiar with the electrochemical principles underpinning their
design. It is even less certain that the future operator of the system
will have anything more than an imperfect understanding. As a matter
of personal experience, many of the problems found with CP systems
derive from the combination the owner’s lack of understanding and
the supplier’s lack of motivation to impart that understanding. The
following recent case is typical.
Case 6 CP Only Works When It is Switched On
Ships’ propellers are often fabricated from a bronze alloy with
good general corrosion resistance to seawater. On ships with
impressed current systems, the propeller can further be
protected by electrically bonding it to the cathodically protected
hull through a slip-ring connector on the shaft.
In the case of a fast ferry constructed in 2001, however, the
propellers were made in austenitic stainless steel (UNS S31600);
a material selected because of its superior mechanical
characteristics. Quite correctly, the vessel’s designers had
recognized that that particular grade of stainless steel does not
possess adequate inherent corrosion resistance for seawater
service. Accordingly, the design included a slip-ring device
bonding the propeller into the hull’s impressed current system.
However, what the designers either failed to appreciate, or else
failed to communicate to the builders, was that the stainless
steel was at risk during the period between launch and the
commissioning of the CP. In the event, the vessel lay afloat with
the CP system inactive during the four month fitting out period.
Unfortunately, although this period was relatively short in
shipbuilding terms, it was sufficient for pitting corrosion to
initiate on the propeller hubs (Figure 5). This caused a delay in
the delivery of the vessel, and an acrimonious dispute between
the shipyard and the buyer.
The lesson was learned in time for the launch of the sister ship,
for which temporary sacrificial anodes were provided to protect
the propeller during outfitting (Figure 6).
The Management of CP Systems.
As a broad generalization offshore CP systems are based on sacrificial
anodes. As such, the only system management involved is periodic
surveying and inspection to confirm that all is well. Conversely, if
problems are found, there are few remedial options other than an
expensive upgrade. Consequently, there is a tendency, sometimes
justified, on the part of system owners to regard sacrificial systems as
Land-based CP systems, on the other hand, are usually impressed
current. Such systems need to be managed and maintained by suitably
trained personnel. This fact is all too often overlooked by system
Perhaps the most common example of the lack of training among
operators is the persistent failure to appreciate the importance of
taking account of the IR-error when measuring potentials. This is a
subject considered in detail in another paper at this conference. The
following case is one of many examples of an operator believing that
all was well, when the converse was the case.
Case 7 The Importance of Off-Potential Measurements
Figure 7 shows the results of a close interval potential survey
(CIPS) along a high pressure gas pipeline. In the original survey
the data were collected at I m intervals and plotted on sheets
each showing 500 m of line. In Figure 7, however, the data for
each kilometre have been averaged so that the condition of the
entire 112 km of the pipeline can be seen in a single plot.
Prior to the CIPS, the operator had only ever measured onpotentials at test posts 5 km apart; and was of the opinion that
the line was well protected. It was only when the CIPS was
carried out, and the on-potentials (green line in Figure 7)
compared with the off-potentials (purple line) did it become
apparent that the line was suffering a dangerous stray current
interference problem. (This is revealed by the off-potentials in
the region of km 60).
CP field engineers can catalogue numerous instances of neglect or
vandalism relating to land-based CP systems. These can range from
the common fault of failing to carry out routine monitoring and
maintenance, or failing to turn CP systems back on after they have
been shut down for maintenance work, to more intractable problems
such as the theft of hardware. The following case relates to a
systematic lack of maintenance that went undetected for a
Case 8 Dead Batteries
Solar power units of the type illustrated in Figure 8 provide a
convenient source of electrical power for CP systems in remote
desert locations. However, it is not always appreciated that the
function of the solar panels is not only to provide the CP current
but, more important, to charge the batteries (Figure 9).
Unfortunately, the remote locations of these CP stations mean
that it is all too easy for routine maintenance, including the
topping up of battery electrolyte, to be overlooked.
In one case involving a solar powered CP system for an oil trunk
line, it appears that the batteries had never been topped up.
Inevitably, they eventually became completely dry. However, the
problem went unnoticed for a long time; possibly several years.
This was because the (infrequent) CP inspections were only
carried out in the day-time, when the solar units were delivering
current. Moreover, the inspectors only recorded on-potentials,
which gave an unrealistically optimistic indication of the level of
It was only when the author’s company audited the CP
monitoring and inspection records that the problem came to
light. Subsequent monitoring of on- and off-potentials using a
data logger over a 24-hour cycle revealed under-protection, and
confirmed that the sun does not shine at night!
Cathodic protection is a well established technique for controlling
corrosion of buried or immersed structures. It usually works well.
However, as the cases outlined above illustrate, there can be
problems. Moreover, these problems could easily have been avoided
with a very small investment in training the system designers in the
underlying principles of CP, and educating operators about the need
for effective monitoring, inspection and maintenance.
Figure 1 Effluent Water Treatment Tank
Figure 2 Stainless Steel Weir in Tank
Figure 3 Result of Inadequate Internal CP
Figure 4 Section of 6-inch nb flowline with 25 mm EPDM rubber coating
Figure 5 Pitting damage to a stainless steel propeller hub – fast ferry newbuilding
Figure 6 sacrificial anodes to protect stainless steel propeller during outfitting
Potential V (cse)
Figure 7 Potential distribution along a high pressure gas transmission line
Figure 8 Solar Panel Array
Figure 9 Storage batteries
1 J.D. Scantlebury Humphrey Davy and the origins of cathodic
protection. (This Conference).
2 DNV RP B401 Cathodic Protection Design 1993
3 European Standard EN 12495 Cathodic protection for fixed steel
offshore structures 2000
4 European Standard EN 12473 General principles of cathodic
protection in sea water 2000
5 European Standard EN 12474 Cathodic protection of submarine
6 NORSOK Standard M-503 Common Requirements for cathodic
protection Rev 2 Sep. 1997.
7 European Standard EN 13174 Cathodic protection for harbour
8 British Standard BS 7361 Cathodic protection Part 1 Code of practice
for land and marine applications 1991
9 European Standard EN 12954 Cathodic protection of buried or
immersed metallic structures – general principles and applications for
10 European Standard EN 12696 Cathodic protection of steel in
11 W.R. Jacob and C.G. Googan High current CP retrofits for a
submarine flowline Proc. Conf. Corrosion and the Environment, Bath
(1998) NACE International.
12 F.W. Hewes Cathodic protection of pipelines – underground in
Cathodic Protection – Theory and Practice ed. V. Ashworth and C.J.L.
Booker Ellis Horwood (1986)
13 R.A. Cottis The real meaning of off-potentials (This conference)