Cathodic protection criteria

The electrochemical foundation for cathodic protection is based on the criterion described by eq. (3.29), = -ic = icorr Ecorr or icarr = io + c = 0. 

This model states that the corrosion rate on a metal surface is zero when the forward and reverse current densities become exactly the same at equilibrium E = Ecorr)- This criterion was recognized by Mears and Brown [3 in 1938 and it has become a common practice for protecting steel sUuctures. 

Half-Cell Potential Criterion For steel stmctures, cathodic protection is achieved when polarized at the iron (Fe) equilibrium half-cell potential [3]. In neutral environments (soil and seawater), the half-cell potential is based on the following reactions and it is determined by the Nemst equation 

Criterion The above theoretical potential for polarizing a steel structure can be determined nsing a differmt approach. If pH is defined as given in Table 2.6, then 

Once more, the Nemst equation gives the theoretical potential for polarizing iron and steel 

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Technical Committee Reports of the National Association of Corrosion Engineers, USA, on pipeline corrosion control, including Statement on Minimum Requirements for Protection of Buried Pipelines , Some Observations on Cathodic Protection Criteria , Criteria for Adequate Cathodic Protection of Coated Buried Submerged Steel Pipelines and Similar Steel , Methods of Measuring Leakage Conductance of Coatings on Buried or Submerged Pipelines , Recommended Practice for Cathodic Protection of Aluminium Pipe Buried in Soil or Immersed in Water ... 

B. Linder, Cathodic protection criteria for passivated metals ,... 

Electrochemical Basis for Cathodic Protection Criteria The corrosion rate of a steel structure tends to zero when it is polarized to the equilibrium potential because the rate of forward and reverse reactions becomes equal at this potential. For a neutral electrolyte, the calculated potential for the reaction of Fe is —0.59 V (versus saturated hydrogen electrode, SHE), which corresponds to —0.90 V (versus Cu-saturated CUSO4 electrode), not much varied from —0.85 V... 

Cathodic Protection Criteria-A Literature Survey, NACE International, Houston, Tex., 1989. 

The basic design of sacrificial CP system includes calculation of cathodic protection circuit resistance, potential difference between the anode and structure, anode output, number of anodes, and the anode life expectancy. A schematic of the cathodic protection test is given in Fig. 15.11. To estimate current requirements, a test is needed to determine the current i ) necessary to provide adequate protection for the pipeline. This can be done by applying current using a temporary test setup and adjusting the current from the rectifier until the cathodic protection criteria is reached. 

The variation of potential with distance along with the cathodic protection criteria is given in Fig. 15.14. 

R. A. Gummow, Cathodic protection criteria—a critical review of NACE standard RP-01 -69, Mater. Performance 25 (1986) 9—16. 

L.P. Sudrabin, F.W. Ringer, Cathodic protection criteria. Corrosion 12 (1957) 825t-828t. 

Cathodic Protection Criteria—A Literature Survey, NACE, Houston, TX, 1989. 

K.P. Fischer. Cathodic protection criteria for saline mud containing SRB at ambient and higher temperatures. Paper No. 110, Corrosion 1981, NACE, Houston, TX, 1981. 

R. Bianchetti (ed.) (2000), Peabody s Control of Pipeline Corrosion, 2nd Edition, NACE, Houston, TX. Cathodic Protection Criteria—A Literature Survey (1989), National Association of Corrosion Engineers. Mars G. Fontana and Norbert D. Greene (1978), Corrosion Engineering, 2nd Edition, McGraw-HUl, London. 

Structure-to-electrolyte potential measurements are analyzed to determine whether a structure is cathodically protected these measurements are made by the use of cathodic protection criteria. Unfortunately, no one simple criterion has been accepted by all cathodic protection engineers that can be practicably measured in the field under all circumstances. Guidelines for selecting the proper criterion under various circumstances will be provided below. Guidance concerning the criteria of cathodic protection for external corrosion control on underground structures is found in two recommended practices (RPs) published by the National Association of Corrosion Engineers (NACE). These are RP-01-69 and RP-02-85. A summary of the criteria for steel and cast iron structures follows [8]. 

Bennett, J.E. and Broomfield, J.P. (1997). Analysis of Studies on Cathodic Protection Criteria for Steel in Concrete. Materials Performance, 36(12) 16-21. 

Among several cathodic protection criteria recommended by NACE, the most common criterion is the half-cell potential, which predicts the theoretical potential to be —0.59 Vshe = —0.90 Vb /c so. for iron in neutral environments. This theoretical value is close to NACE potential (—0.85Vfc /csO4) On the other hand, overprotection at < corr can be harmful because of hydrogen evolution and diffusion of atomic hydrogen into the stmcture causing hydrogen embrittlement. 

Due to the more accurate verification and criticism of cathodic protection criteria used up to now, the possibility of introducing new, more rational criteria is being considered. The so-called kinetic criteria will probably be adopted. Modern investigation techniques using the theory of kinetics of electrode processes will probably find application in direct measurements of the corrosion current on cathodically polarized structures. Instead of intermediate control of the effectiveness of anticorrosion protection on the basis of potential measurements, it seems significantly more purposeful to maintain a corrosion rate of the cathodically protected structure in an accepted range. 

Table 13.1 Cathodic Protection Criteria for Steel in Concrete.

Eischer KP (1981) Cathodic protection criteria for saline mud containing SRB at ambient and higher temperatures. Paper No. 110. CORROSION/81, NACE International, USA de Romero ME, Parra J, Ruiz R, Ocando L Bracho M de Ricon OT, Ramero G, QuinteiD A (2006) Cathodic polarisation effects on sessile SRB growth and iron protection. Paper No. 06526. CORROSION 2006, NACE International, USA... 

TABLE 2.32 Cathodic Protection Criteria for Steei in Concrete... 

Different protection criteria are required for different material-environment combinations. Other construction materials commonly used in buried applications, such as copper, aluminum, and lead, have different potential criteria than those given for steel above. Table 11.2 provides a comprehensive listing of cathodic protection criteria for different materials and environments. It should be noted that excessively negative potentials can be damaging to materials such as lead and aluminum and their alloys, due to the formation of alkaline species at the cathode. 

TABLE 11.2 Selected Cathodic Protection Criteria for Different Materials... 

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