Effective corrosion protection: Keep your pipes rust-free. 

The term “corrosion” describes an electrochemical process that results in a reaction of a metal with its environment and hence in a change of its properties. In everyday language one would say "the steel is rusting". The expert would say that the steel is thermodynamically unstable, certain oxidising agents like oxygen and acids, or a polarisation with anodic currents, cause the iron atoms contained in the metal matrix to migrate to the surrounding electrolyte solution (soil). This process continues for as long as there is a supply of oxidising agents.

Experience has shown that the steel surface of a buried structure without cathodic protection corrodes at a rate of 0.1 to 0.2 mm/a in those areas where the coating is damaged. Using cathodic protection, the corrosion rate can generally be reduced to less than 0.01 mm/a (= 1 mm in 100 years). If there is a risk of corrosion due to alternating currents, cathodic protection can reduce the corrosion rate to less than  0.03 mm/a.

The structure to be protected, e.g. a buried pipeline, is connected to the negative terminal of a DC source and thus becomes the cathode in a DC circuit. The positive terminal of the DC source is connected to an anode buried in the ground and via which a DC current (protective current) flows to the “holidays” (= coating holidays = defects) where the steel surface comes into contact with the soil. This triggers various electrochemical processes at the pipe-to-soil interface which interact depending on the specific environmental conditions to reduce the corrosion rate:

• The potential of the steel surface is made more negative
• The oxygen at the steel surface is reduced (=“consumed”)
• The pH of the steel surface increases

Steel structures (e.g. pipelines) can be equipped with cathodic protection (CP) systems, if their surface is in contact with an electrolyte solution (soil etc.) via which a cathodic current can be delivered to the structure. As well as buried or offshore pipelines, the interior surfaces of water tanks or the reinforcing elements of reinforced concrete structures can also be protected against corrosion using cathodic methods. For cost and electrical engineering reasons it is often practical to provide the surfaces to be protected with a firmly adhering coating made of insulating plastics. The effect of the cathodic corrosion protection can then be restricted to the smaller defects occurring randomly during pipeline operation.

This term is a rather imprecise way of describing the measured value for the voltage between a reference electrode and a structure (e.g. a buried pipeline). As the potential of the reference electrode remains constant under all circumstances, the above mentioned measured value is simply designated “potential”, whereby the type of reference electrode against which the measured value has been determined must be specified (generally a saturated Cu/CuSO4 electrode is used with a voltage of 320 mV against the standard hydrogen electrode). The potential of the structure that has been determined in this way is initially “only” a kind of mean value of the electrochemical conditions of all pipe coating defects. By using a suitable measuring technique (so-called "intensive measurements", similar to a “close interval survey”) the potential of a single coating defect can often be determined with a good degree of accuracy. As the potential is a parameter for the electrochemical conditions at the steel/soil phase boundary, the measured value provides information about the corrosion protection or a possible corrosion risk at this location.

This term covers all corrosion protection measures used to isolate steel surfaces to be protected from the surrounding (corrosive) medium. Typical passive corrosion measures include the coating of pipes with 3-ply polyethylene systems for below-ground applications or corrosion protection coatings for pipes and fittings above ground. Generally, a distinction is made between coatings applied by the pipa manufacturer (factory coatings) and those applied on site (field-applied coatings). Local damage to a factory or field-applied coating that exposes the steel surface underneath is often referred to as a “holiday”.

Stray current interference results from all structures that cause DC currents to flow in the ground. These include, for example, cathodically protected pipelines and their anodes, DC-powered trams, but also reinforced concrete foundations. The DC currents cause local changes to the potential of the soil, as a result of which the potential of a third-party structure/system in this area is influenced, i.e. changed. Generally, a positive change in potential leads to an increase in the corrosion rate (stray current corrosion), while a negative change of potential reduces the corrosion rate.

High-voltage interference occurs when an AC voltage is induced on an intersecting or parallel pipeline due to the operation of an AC system (three-phase AC high-voltage line, transformer substation, electrified rail line etc., frequency 50 Hz or 16 2/3 Hz). In this context, a distinction is made between interference mechanisms: ohmic (due to currents in the ground), inductive (due to the magnetic field of the AC currents) and capacitive (due to the electrical field of the high-voltage system). The magnitude of the AC voltage induced in a pipeline has to be limited to 60 V to protect against electric shock. Voltages of up to 1000 V are only permissible for short-term faults (< 0.2 s) on the high-voltage systems. Induced AC voltages are also the cause of AC corrosion.
High-voltage direct current transmission (HVDC) systems can also affect buried piping. Direct currents induced in the pipeline via the soil can also lead to stray current corrosion; switching processes in HVDC lines and systems cause a short-term inductive effect.

This term describes a corrosion mechanism whereby, in addition to a protective current (from the CP system), an alternating current (due to high-voltage interference) many times higher than the protective current also flows into a coating defect via the steel/soil phase boundary. The current densities, i.e. the protective and AC currents referred to the surface of the defect, are decisive for evaluating the risk of AC corrosion. Under unfavourable conditions, an increased corrosion rate will result due to the repeated build-up and subsequent destruction of a passive layer (oxide layer) at molecular level during every anodic cycle of alternating current. Sufficient protection against AC corrosion can be achieved through targeted control of protective and alternating current densities.

Provided that the necessary conditions (longitudinal conductivity, electrical isolation from earthed structures, good coating, contact with ground all round) are observed when building a structure like a buried pipeline, cathodic protection is a cost-effective way of preserving the value of your investment. The relevant regulations prescribe the use of CP for high-pressure gas pipelines and pipelines carrying media hazardous to water, making CP both an operational safety and an environmental protection measure.