Cathodic protection potential

The IR drop is caused by current flow through the soil, pipe coating, and metallic path [5]. AH cathodic protection potential measurements contain an IR drop component when CP current or interference current is present. The measured potential ( J is the sum of polarized potential ( p) measured near the cathode electrolyte interface and IR drop along the... 

Cathodic protection is an electrochemical technique in which a cathodic (protective) potential is applied to an engineering structure in order to prevent corrosion from taking place. This implies that Ohm s law, E = IR, can be used to control the potential, as well as the current. Hence, metal oxidation is prevented since the potential must be below the corrosion potential (E < Ecorr)- This is the main reason for this potential-control technique. In principle, aU stmctures can be protected cathodicaUy, but stmctural steels being the most common ferrous materials used to build large stmctures are cathodicaUy protected by an external potential (impressed potential). 

Titanium has potential use in desalination plants for converting sea water into fresh water. The metal has excellent resistance to sea water and is used for propeller shafts, rigging, and other parts of ships exposed to salt water. A titanium anode coated with platinum has been used to provide cathodic protection from corrosion by salt water. 

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

The basic standard for cathodic protection was laid down for the first time in DIN 30676 to which all the application areas of the different branches of protection can be referred. In this the most important point is the technique for accurately measuring the object/soil potential [58]. The usual off-potential measurement method for underground installations has been slowly implemented and enforced in Europe since the 1960s [59]. 

When cathodic polarization is a result of negative total current densities 7., the potential becomes more negative and the corrosion rate lower. Finally, at the equilibrium potential it becomes zero. In neutral water equilibrium potentials are undefined or not attainable. Instead, protective potentials are quoted at which the corrosion rate is negligibly low. This is the case when = 1 flA cm (w = lOjUm a ) which is described by the following criteria for cathodic protection ... 

The terms protection current and protection current densities refer to any values of total cathodic currents that meet the criterion in Eq. (2-40). However, in the field, and for designing cathodic protection stations, another term is of interest, the protection current requirement. This term is concerned with the lowest value of the protection current that fulfills the criteria in Eqs. (2-39) or (2-40). Since with an extended object having a surface S the polarization varies locally, only the current density for the region with the most positive potential has the value J. In other regions 17. 1 > 7. . For this reason, the protection current requirement 4 is given by ... 

For Fe it follows from Eq. (2-530 that = -0.64 V. This value is 0.21 V more negative than the protection potential i/cu-cuso4 = -0.85 V which is adopted in practice (see Fig. 1-12) [22,23]. This more positive potential results because 5is actually greater and the cathodically generated surface films give additional protection [24-26]. 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

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