Buried structures, cathodic protection

The cathodically protected primary structures may be the hulls of ships, jetties, pipes, etc. immersed in water, or pipes, cables, tanks, etc. buried in the soil. The nearby unprotected secondary structures subjected to interaction may be the hulls of adjacent ships, unbonded parts of a ship s hull such as the propeller blades, or pipes and cables laid close to the primary structure or to the cathodic-protection anode system or groundbed. 

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

The switching-off method for 7/ -free potential measurement is, according to the data in Fig. 3-5, subject to error with lead-sheathed cables. For a rough survey, measurements of potential can be used to set up and control the cathodic protection. This means that no information can be gathered on the complete corrosion protection, but only on the protection current entry and the elimination of cell activity from contacts with foreign cathodic structures. The reverse switching method in Section 3.3.1 can be used to obtain an accurate potential measurement. Rest and protection potentials for buried cables are listed in Table 13-1 as an appendix to Section 2.4. The protection potential region lies within U[[Pg.326]

Cathodic protection cannot work with prestressed concrete structures that have electrically insulated, coated pipes. There is positive experience in the case of a direct connection without coated pipes this is protection of buried prestressed concrete pipelines by zinc anodes [38], Stability against H-induced stress corrosion in high-strength steels with impressed current has to be tested (see Section 2.3.4). 

Saponification Paints are most commonly used to protect steel from corrosion by seawater in marine applications and soil in the case of buried structures. Additional protection is often supplied by the application of cathodic protection to the steel. Any paint coating used in conjunction with cathodic protection must be resistant to the alkali which is produced on the steel at defect sites in the coating. The amount of alkali generated depends on the potential to which the steel is polarized. Some paint binders such as alkyds and vinyl ester are very susceptible to saponification, and should not be used on cathodically protected structures. Cathodic disbondment testing should be undertaken if the relevant information is not available. 

Cathodic Protection of Buried and Submerged Structures, CP 1021 1973, British Standards Institution, London... 

If a continuous metallic structure is immersed in an electrolyte, e.g. placed in the sea or sea-bed or buried in the soil, stray direct currents from nearby electric installations of which parts are not insulated from the soil may flow to and from the structure. At points where the stray current enters the immersed structure the potential will be lowered and electrical protection (cathodic protection) or partial electrical protection will occur. At points where the stray current leaves the immersed structure the potential will become more positive and corrosion may occur with serious consequences. 

Stray-current electrolysis occurring as a result of the application of cathodic protection to a nearby immersed or buried structure is known as cathodic-protection interaction and is described in Section 10.6. 

Current due to the anode effect The potential of the earth near the ground-bed of a cathodic-protection system becomes more positive as the groundbed is approached (see Fig. 10.7, p. 10 10). A structure buried near the ground-bed will pick up current due to this variation in the soil potential and current will flow in the structure in each direction away from a point close to the groundbed (Fig. 10.39). The upper curve AGA in Fig. 10.39 shows how the current in the unprotected structure changes owing to the anode effect. 

The results of these experiments have been considered by the Joint Committee for the Co-ordination of the Cathodic Protection of Buried Structures and, in view of the various types of buried structures concerned and the circumstances in which field tests are conducted, the Committee decided not to amend its provisional recommendation that when cathodic protection is applied to a buried structure the maximum permissible potential change in the positive direction on a nearby pipe or cable should be 20 mV. If there is a history of corrosion on the unprotected installation no detectable positive change in structure/soil potential should be permitted. These criteria of interaction have been adopted in the British Standard Code of Practice for Cathodic Protection . 

If both buried structures have been subject to corrosion damage the best solution may be to install a joint cathodic-protection scheme with sufficient current output to provide adequate protection for both installations. The application of separate cathodic-protection schemes to structures buried... 

Recent experience has confirmed that, by adopting the recommendations of the British Standards Institution or similar codes of practice operating in other countries, the likelihood of corrosion damage to buried structures adjacent to cathodically protected installations is negligible. This is because recently installed cathodically protected structures are usually coated with eflicient and durable insulating coverings such as epoxy resins and the protective current applied is consequently small. In many cases the small protective currents that can be applied by means of galvanic anodes is adequate. 

Cathodic Protection Rectifier transformer-rectifier arrangement for supplying the direct current which flows between a groundbed and a buried structure which is receiving cathodic protection. 

Corrosion Interaction (or interaction) increase (or decrease) in the rate of corrosion of a buried or immersed structure caused by interception of part of the cathodic protection current applied to another buried or immersed structure. 

Groundbed in cathodic protection of underground structures, a buried mass of inert material (e.g. carbon), or scrap metal connected to the positive terminal of a source of e.m.f. to a structure. 

Primary Structure a buried or immersed structure cathodically protected by a system that rpay constitute a source of corrosion interaction with another (secondary) structure. 

Buried Structures There has been no dramatic improvement in the protection of buried structures against MIC over the last several decades. Experience has been that coating systems, by themselves, do not provide adequate protection for a buried structure over the years for best results, a properly designed and maintained cathodic protection (CP) systemmust be used in conjunction with a protective coating (regardless of the quality of the coating, as applied) to control... 

Cathodic protection. Another way of keeping metal in a cathodic state is to connect it to an external direct current source. For smaller structures such as boats and domestic water water heaters the current is supplied by a sacrificial anode made of aluminum or zinc. For larger extended structures such as piers and buried pipelines, an external line-operated or photovoltaic power supply is commonly used. (All interstate oil piplines in the U.S. are required by law to employ cathodic protections.)... 

Cathodic protection has many applications, e.g. in refineries, power stations, gas, water, and oil utilities on marine structures, e.g. jetties, piers, locks, offshore platforms, pipelines, ships hulls, etc. and on land structures, e.g. buried pipeline, storage tanks, cables, etc. For each use, the cathodic protection system requires careful design, either impressed current, sacrificial anodes, or a combination of both may be chosen. There may also be other protection systems, e.g. paint, the nature of which will affect the design parameters and must be taken into consideration. 

D. P. Riemer and M. E. Orazem, Application of Boundary Element Models to Predict the Effectiveness of Coupons for Accessing Cathodic Protection of Buried Structures, Corrosion, 56 (2000) 794-800. 

Galvoline [Dow], TM for a cored magnesium ribbon used as a continuous anode for the cathode protection of buried pipelines and other metal structures. Combustible. 

Paint/coating and cathodic protection for submerged or buried structures or piping. 

Cathodic protection is an anticorrosion technique widely used in ships and in buried or submerged pipe work. This method seeks to reduce the rate of corrosion of the structure to be protected by joining it to sacrificial anodes. In other words, the structure is joined to another metal (an anode) that corrodes more readily, effectively diverting the tendency to corrode away from the structure. 

For practical applicability, several aspects have to be considered such as the anode material (sacrificial (e.g. zinc) or inert (e.g. Pt/Ti or graphite)), the conductivity of the medium and the current distribution. Cathodic protection is typically used for buried constructions (e.g. pipelines), off-shore structures or ship hulls. 

Corrosion due to stray current— the metal is attacked at the point where the eurrent leaves. Typically, this kind of damage can be observed in buried structures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

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