Cathodic Protection of Buried Structures

Buried metallic structures are continuously exposed to soil environments that can cause electrochemical corrosion. Soil moisture, electrical conductivity, differences in oxygen concentration, salts, microorganisms and stray currents can create corrosion cells on the surface of underground steel structures.

Over time, corrosion may lead to metal loss, pitting, perforation, leakage and structural failure.

Italcorr provides cathodic protection of buried structures, including engineering, installation, testing, monitoring and performance assessment of cathodic protection systems for underground metallic infrastructure.

What Is Cathodic Protection of Buried Structures?

Cathodic protection is an electrochemical technique used to control corrosion by making the protected metallic structure the cathode of an electrochemical cell.

For buried structures, cathodic protection is normally used together with a protective coating. The coating reduces the area of metal exposed to the soil, while the cathodic protection system provides protective current to coating defects and other exposed metallic areas.

Cathodic protection can be applied using two main methods:

Impressed Current Cathodic Protection (ICCP);
Sacrificial Anode Cathodic Protection (SACP).

The appropriate system is selected according to the characteristics of the structure, soil conditions, coating quality, required protection current and expected service life.

Structures That Can Be Protected

Cathodic protection can be applied to a wide range of buried metallic infrastructure, including:

underground pipelines;
buried storage tanks;
tank bottoms;
underground steel vessels;
firewater pipelines;
industrial piping systems;
water supply networks;
metallic foundations;
buried sections of industrial facilities;
underground utility systems;
other steel structures in contact with soil.

Each structure requires an individual assessment because electrical continuity, coating condition, soil resistivity and interaction with neighbouring structures can significantly influence cathodic protection performance.

Corrosion of Buried Metallic Structures

Soil is an electrolyte capable of supporting electrochemical corrosion.

Different areas of a buried metallic structure may develop different electrochemical potentials. Where anodic areas form, metal dissolution occurs and corrosion progresses.

The corrosion rate can be influenced by several factors, including:

soil resistivity;
moisture content;
pH;
chloride and sulphate concentration;
oxygen availability;
temperature;
microbiological activity;
coating condition;
DC stray currents;
AC interference;
proximity to other buried structures.

For this reason, the design of a cathodic protection system should be based on actual site conditions rather than only on general design assumptions.

Impressed Current Cathodic Protection

Impressed Current Cathodic Protection (ICCP) is commonly used for large structures or installations requiring relatively high protection current.

An ICCP system typically includes:

transformer-rectifier or other DC power source;
impressed current anodes;
anode groundbed;
cables and electrical connections;
test stations;
reference electrodes;
monitoring equipment.

The DC power source supplies current to the anodes. The current flows through the surrounding soil toward the protected structure and returns through the electrical connection to the rectifier.

The output can be adjusted according to the protection requirements of the structure.

Sacrificial Anode Cathodic Protection

Sacrificial Anode Cathodic Protection (SACP) uses galvanic anodes electrically connected to the protected structure.

Magnesium and zinc alloys are commonly used for buried applications, depending on soil conditions and system requirements.

The galvanic potential difference between the anode and the protected steel generates the protective current without an external power source.

Sacrificial anode systems can be particularly suitable for:

relatively small buried structures;
underground tanks;
short pipeline sections;
isolated structures;
locations without electrical power;
supplementary cathodic protection.

The number, type and location of anodes must be calculated according to the required current output and design life.

Cathodic Protection System Design

Effective cathodic protection system design for buried structures requires evaluation of both the structure and its surrounding environment.

The engineering process may include:

collection of structural and operational data;
determination of the surface area to be protected;
coating assessment;
soil resistivity measurements;
estimation of protection current requirements;
evaluation of electrical continuity;
identification of insulating joints;
assessment of neighbouring metallic structures;
stray current investigation;
AC interference assessment;
selection of cathodic protection method;
anode system design;
cable and test station design;
calculation of expected system life.

The objective is to provide adequate protection while avoiding excessive polarization and unnecessary current consumption.

Soil Resistivity Measurements

Soil resistivity is an important parameter in cathodic protection design.

Low-resistivity soil generally allows current to flow more easily, while high-resistivity soil may require different anode configurations or groundbed designs.

Measurements can be performed at different depths and locations to evaluate variations along the structure.

The results are used for anode selection, groundbed design and estimation of cathodic protection current distribution.

Potential Measurements

The effectiveness of cathodic protection is normally evaluated by measuring the structure-to-electrolyte potential using an appropriate reference electrode.

Measurements may include:

ON potentials;
Instant OFF potentials;
depolarization measurements;
potential monitoring at test stations;
potential measurements along buried pipelines.

Instant OFF measurements can help reduce the influence of voltage drops in the soil and provide a more representative assessment of the polarized structure potential.

Stray Current Interference

Buried structures may be affected by stray DC currents generated by external electrical systems.

Potential sources include:

DC railway and tram systems;
neighbouring cathodic protection systems;
industrial DC installations;
grounding systems;
other buried metallic infrastructure.

Where current leaves a metallic structure and enters the soil, accelerated corrosion may occur.

Cathodic protection surveys can therefore include stray current interference testing and evaluation of mitigation measures.

AC Interference

Buried pipelines and other long metallic structures located near high-voltage AC power lines may be exposed to induced AC voltage.

AC interference can create electrical safety risks and, under certain conditions, contribute to corrosion.

Assessment may include AC voltage measurements, current density evaluation and verification of mitigation systems.

Testing and Commissioning

After installation, the cathodic protection system should be tested and commissioned to verify correct operation.

Commissioning activities may include:

verification of electrical connections;
polarity checks;
rectifier output measurements;
anode current measurements;
structure-to-soil potential measurements;
Instant OFF measurements;
insulating joint testing;
verification of electrical continuity;
adjustment of system output;
identification of interference with neighbouring structures.

The system can then be adjusted to achieve the required protection criteria.

Monitoring and Maintenance

Cathodic protection systems require periodic monitoring throughout their operating life.

Regular inspections help identify changes caused by coating deterioration, anode consumption, electrical faults, environmental conditions or modifications to surrounding infrastructure.

Monitoring activities may include:

potential measurements;
rectifier inspection;
current output measurements;
reference electrode verification;
cable and connection testing;
insulating joint testing;
anode performance assessment;
remote monitoring;
evaluation of historical measurement trends.

Correct maintenance helps maintain effective corrosion control and extends the service life of buried assets.

Advanced Cathodic Protection Surveys

For buried pipelines and extensive underground systems, cathodic protection assessment can be integrated with specialized diagnostic surveys.

Italcorr can combine cathodic protection measurements with:

CIPS Survey (Close Interval Potential Survey) – evaluates the distribution of cathodic protection potentials along a pipeline.

DCVG Survey (Direct Current Voltage Gradient) – detects and evaluates defects in pipeline coatings.

PCM Survey (Pipeline Current Mapping) – evaluates current attenuation and can help identify areas of abnormal current loss.

Pearson Survey – assists in locating coating defects on buried pipelines.

Combining several techniques provides a more complete assessment of coating condition, cathodic protection effectiveness and corrosion risk.

Applicable Standards

Cathodic protection of buried metallic structures should be designed, operated and evaluated according to applicable European and international standards and project requirements.

Depending on the type of structure, relevant standards may include EN 12954, EN ISO 15589-1 and other standards applicable to cathodic protection and corrosion control.

Protection criteria and measurement procedures should always be selected according to the specific structure and operating conditions.

Cathodic Protection Services by Italcorr

Italcorr provides cathodic protection services for buried structures, from initial site investigation and engineering through installation, commissioning and long-term monitoring.

Our services include:

cathodic protection system design;
impressed current cathodic protection;
sacrificial anode systems;
soil resistivity measurements;
installation and commissioning;
potential measurements;
cathodic protection performance assessment;
insulating joint testing;
stray current investigation;
AC interference assessment;
CIPS, DCVG, PCM and Pearson surveys;
cathodic protection maintenance;
technical reporting and recommendations.

A properly designed and maintained cathodic protection system can significantly reduce external corrosion, extend the service life of underground metallic infrastructure and improve operational reliability and safety.