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BS Cathodic protection.
BS Code of BS Part bw Code of BS Code Steel Constru Application to buried structures 4. Reinforcing steel in concrete 5. Application to 3761-1 6. Immersed structures excluding off-shore applications 7. The internal protection of plant 8. Measures to safeguard neighbouring structures 9. Commissioning, operation and maintenance It represents a revision of CP Cathodic protection is a means bbs preventing the corrosion of most metals wherever they are in contact with a mass of water or moist materials.
It should not, however, be inferred that cathodic protection, wherever applicable, is necessarily advantageous. Unless the need for cathodic 7361-11 is already established, every case should be carefully examined and an evaluation made of the economic advantage 736-11 cathodic protection, compared with other methods of avoiding or 73661-1 corrosion, such as sheathing or coating, the use of other materials of construction etc.
Included in the assessment should be the cost and consequences of allowing the expected corrosion to proceed. This may be possible, for example, in the case of steel pilings, while entirely ruled out in the case of high pressure pipelines. This Part of BS 7361-11 good up-to-date practice and contains both guidance on general principles and detailed information on the cathodic protection of particular hs of structure or plant, excluding those involved in off-shore applications.
Even where sufficient evidence based on actual installations has not been obtained to enable detailed information to be given, it should not be assumed that cathodic protection is unsuitable if there is sufficient economic incentive. Lack of evidence has also prevented firm guidance being given at several points in this Part of BSnotably on the test methods needed to avoid excessive error due to potential drop through the electrolyte when the effectiveness of cathodic protection is being assessed by measuring the potential of the metal relative to a reference electrode.
Attention is drawn to the importance of considering any measures necessary to prevent corrosion during the early stages of 3761-1 design of structures and possible future extensions.
Design to prevent corrosion, for example by selection of materials, avoidance of unsuitable shapes or combinations of metals, or by using metal spraying or protective coatings is important, but is outside the scope of this standard. The corrosion protection of iron and steel in this context is covered by BS This revision has been occasioned by developments in a variety of industries sinceas well as increasing sophistication in the associated measuring techniques.
Whilst the main body of the first edition has been found to be sound, detailed changes have been required throughout the document, and additional experience in areas such as pipe coatings, anode materials and the cathodic protection 736-1 reinforcing steel in concrete, and methods of assessing cathodic protection, has necessitated the introduction of new text.
Despite further consideration by the committee no change is proposed to the 20 mV criterion for limiting corrosion interaction.
A major area for cathodic protection is that concerned with off-shore applications. The scale of this work and the size of the corresponding text to cover it requires the preparation of Part 2 of this standard and this is intended to be accomplished by participation in corresponding work that has been initiated in Europe under the aegis of CEN.
It has been assumed in the drafting of this British Standard that the execution of its recommendations is entrusted to appropriately qualified and experienced people, for whose guidance it has been prepared. Users of British Standards are responsible for the correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations.
In particular attention is drawn to the following regulations: Regulations for electrical installations, published by the Institution of Electrical Engineers subsequently referred to as the IEE Wiring Regulations are not statutory, but compliance may satisfy the requirements of certain statutory regulations. It is advisable therefore to comply with these regulations where appropriate.
This standard has been updated see copyright date and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. It does not deal with the cathodic protection of off-shore structures. The code indicates general circumstances in which the application of cathodic protection is likely to be economical as a method of corrosion prevention. It covers general principles, their application, the special problems arising in the protection of particular types of structure, the safeguarding of neighbouring structures, electrical measurements, the commissioning, operation and maintenance of cathodic protection systems, and safety aspects.
Off-shore applications are to be covered in Part 2 see foreword. NOTE 1 Where detail of 7361–1 is shown in the figures, the purpose is to assist in the reading of the text by indicating typical arrangements.
The figures are not intended to establish preferred types of equipment. Similarly, any dimensions shown are to provide an indication of size, not to establish preferred dimensions. NOTE 2 The titles bz the publications referred to in this code are listed on the inside back cover. The titles of related British Standards are listed in Appendix B for information. In the latter case it is used to connect two sections of a structure which would otherwise be disconnected during a modification or repair.
In addition to the standards referred to in the text of this code, relevant British Standard specifications and codes of practice are listed in Appendix B. This does not apply to plant protected internally by cathodic protection. There are also possibilities b electrical interference with telecommunication circuits and of causing false operation of railway signalling circuits see 9.
Cathodic protection equipment may give rise to certain hazards, particularly in areas where there are flammable liquids or gases. In such cases, consultation between the installer and owner sb necessary.
BS Cathodic protection. Code of practice for land and marine applications_图文_百度文库
Where the equipment is required to be flameproof, the relevant provisions of BS apply. There is, inevitably, some over-simplification. A number of reference books dealing with the subject are included in the bibliography see references  to .
This variation is referred to as polarization. The potential difference is also dependent on the types of chemical 7361-11 occuring at the metal surface.
Since the measured potential difference will also depend on the type of reference electrode which is used see The polarization of a metal surface may be conveniently sb by plotting potential 7316-1 the logarithm of current flowing to or from the metal surface, i.
A schematic representation is shown in Figure 2. Although the current of the cathodic reaction is in the negative direction, it is shown in the figure in sb positive direction for convenience.
It will be easily seen that at the corrosion potential Ecorr the anodic and cathodic currents are equal in magnitude and opposite in sign. Hence all the electrons produced by the anodic reaction are consumed by the cathodic reaction, which is the condition existing on vs freely corroding metal surface. Icorr is the corrosion current at the corrosion potential Ecorr. From the figure it can be seen that if the potential of the metal can be reduced by external means, the corrosion anodic reaction rate will decrease and the cathodic reaction rate will increase.
Conversely, the corrosion rate is 731-1 by making the metal more positive. Bw electrons are left in the metal. The process is shown for iron in equation 1. Any area at which current flows in this direction is referred to as an anodic area, and the reaction is called an anodic reaction.
The metallic ions may react with negative ions in the electrolyte to give insoluble corrosion products for example, rust in the case of steel. Such reactions do not materially affect the corrosion process except where insoluble corrosion products are able to stifle further corrosion attack see 2. Overall electric neutrality has to be maintained.
For the corrosion reaction to proceed, therefore, the movement of electrons into the metal and positive ions into the electrolyte at the anodic areas has to be counterbalanced by the passage of current from the solution to the metal and the consumption of electrons at other areas, known as cathodic areas. Various reactions can occur at cathodic areas see 2.
Thus during the corrosion process, electrons are transferred through the metal from one site on the metal surface to another electronic conduction and positive ions are transported through the electrolyte electrolytic conduction. The current flowing round the circuit is proportional to the corrosion rate. For example, in the case of iron or steel, two electrons are released for each atom going into solution, as shown in equation 1, and a corrosion current of 1 A corresponds to a 3761-1 of about 9 kg of metal a year.
An increased concentration of oxygen tends to make the potential of a metal more positive; thus variation of soil density and porosity is a common cause of corrosion sb. Arrangements giving rise to such cells are shown in Figure 4. The size of the cells may vary greatly.
In Figure 4 cfor example, the anodic area may be small. The resultant pitting can, however, lead to rapid penetration of the metal. Figure 2 — Schematic polarization curves 2. Metal A will therefore be the anode and will be corroded, while metal C acts as the cathode. Commonly used metals and conducting materials gs listed below in such order that each normally acts as an anode with respect to all the materials which follow it: Thus, for example, the connection of magnesium to iron 7361–1 in a cell in which the magnesium acts as an anode and the iron as a cathode.
The corrosion resistance of stainless steels, for example, is due to protection by oxide films. The metals aluminium, titanium and tantalum also form highly resistant and adherent oxide films which restrict the rate at which metal ions can pass into bbs.
Both tend to make the 731-1 alkaline near the cathode excess of hydroxyl ions over hydrogen ions: In 73611 to the anodic reaction for example equation 1cathodic reactions do not involve the passage of metal into solution.
bw Hence, in general, corrosion does not occur at cathodic areas. In aerated, near-neutral conditions, the iron ions produced at the anode react with the hydroxyl ions formed at a nearby cathodic site to produce ferrous hydroxide. However, ferrous hydroxide is readily converted be rust in the presence of oxygen, thus the overall reaction which proceeds through a series of intermediate steps is that given in equation 4.
For example, if the relevant reaction is that given by equation 3, replenishment of oxygen may be the controlling factor.
BS – Cathodic protection. Code of practice for land and marine applications
In near-neutral anaerobic soils, sulphate-reducing bacteria give rise to a further type of cathodic reaction and soils of this kind are often particularly aggressive to iron and steel. It is possible by determining pH and redox potential, to assess whether conditions are such that sulphate-reducing bacteria are likely to be active see Appendix A. The reactions occurring at cathodes do not directly result in corrosion. It should be noted, however, that the environment of the metal is altered, for example it becomes more alkaline see above.
In the case of aluminium and occasionally lead, corrosion may result. Alkalinity may also cause deterioration of paints and other coatings by saponification.