the lower surfaceand are minimum at the upper surfaceofthe pipe buried
af ew feet below the soil surface.
The potential for steel is equal to K 0.85 Vv s. the copper-saturated
copper sulfate half-cell, or 0.53 Vo nt he standard hydrogen scale. The
theoretical open-circuit anodicpotential for other metals may be calculated
usingt he Nernst equation.S everal typicalc alculated values are shown in
Table 2.1.
2.7O verprotection
Overprotection of steel structures, to am oderate degree, usuallyd oes not
cause any problems. The primary disadvantages are waste of electric power
and increased consumption of auxiliary anodes.When the overprotection is
excessive, hydrogen can be generated at the protected structure in sufficient
quantities to cause blisteringoforganic coatings, hydrogen embrittlement of
the steel, or hydrogen cracking. Damage to steel by hydrogen absorption is
more prevalent in environments where sulfides are present.
Overprotection of systemswith amphoteric metals (e.g., aluminum, zinc,
lead, and tin) willd amage the metal by causing increased attacki nstead of
reduction of corrosion. This emphasizes the needf or making potential
measurements of protected structures.
2.8E conomics
Thec osto fc athodicp rotectioni sm oret hanr ecovered by reduced
maintenance costs or by reducedi nstallation costs, or both. For buried
pipelines, the guarantee that there will be no corrosiononthe soil side of the
pipe has made it economically feasible to transport oil and high-pressure
natural gas across the American continent. It has also permittedt he use of
thinner-walled pipe. Wall thicknesses need only be sufficient to withstand
TABLE 2.1
Calculated Minimum Potential, f ,f or Cathodic Protection
Metal
E 8 (V)
Solubility
Product, M(OH) 2
f H 2
Scale (V)
f vs. Cu–CuSO 4
Reference Electrode (V)
Iron
0.440
1.8! 10
K 15
K 0.59
K 0.91
Copper
K 0.337
1.6! 10
K 19
0.16
K 0.16
Zinc
0.763
4.5! 10
K 17
K 0.93
K 1.25
Lead
0.126
4.2! 10
K 15
K 0.27
K 0.59
Source:F rom H.H. Uhlig. 1971. Corrosion and Corrosion Control,2nd ed., New York: Wiley.
Cathodic Protection
53
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54
af ew feet below the soil surface.
The potential for steel is equal to K 0.85 Vv s. the copper-saturated
copper sulfate half-cell, or 0.53 Vo nt he standard hydrogen scale. The
theoretical open-circuit anodicpotential for other metals may be calculated
usingt he Nernst equation.S everal typicalc alculated values are shown in
Table 2.1.
2.7O verprotection
Overprotection of steel structures, to am oderate degree, usuallyd oes not
cause any problems. The primary disadvantages are waste of electric power
and increased consumption of auxiliary anodes.When the overprotection is
excessive, hydrogen can be generated at the protected structure in sufficient
quantities to cause blisteringoforganic coatings, hydrogen embrittlement of
the steel, or hydrogen cracking. Damage to steel by hydrogen absorption is
more prevalent in environments where sulfides are present.
Overprotection of systemswith amphoteric metals (e.g., aluminum, zinc,
lead, and tin) willd amage the metal by causing increased attacki nstead of
reduction of corrosion. This emphasizes the needf or making potential
measurements of protected structures.
2.8E conomics
Thec osto fc athodicp rotectioni sm oret hanr ecovered by reduced
maintenance costs or by reducedi nstallation costs, or both. For buried
pipelines, the guarantee that there will be no corrosiononthe soil side of the
pipe has made it economically feasible to transport oil and high-pressure
natural gas across the American continent. It has also permittedt he use of
thinner-walled pipe. Wall thicknesses need only be sufficient to withstand
TABLE 2.1
Calculated Minimum Potential, f ,f or Cathodic Protection
Metal
E 8 (V)
Solubility
Product, M(OH) 2
f H 2
Scale (V)
f vs. Cu–CuSO 4
Reference Electrode (V)
Iron
0.440
1.8! 10
K 15
K 0.59
K 0.91
Copper
K 0.337
1.6! 10
K 19
0.16
K 0.16
Zinc
0.763
4.5! 10
K 17
K 0.93
K 1.25
Lead
0.126
4.2! 10
K 15
K 0.27
K 0.59
Source:F rom H.H. Uhlig. 1971. Corrosion and Corrosion Control,2nd ed., New York: Wiley.
Cathodic Protection
53
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54