Page 312 - Corrosion Engineering Principles and Practice

282 C h a p t e r 8 C o r r o s i o n b y W a t e r 283

Calculation Example. For this problem, imagine a situation where
you would have to design a sacrificial CP system for the protection of
the four main pillars of an oil-drilling platform. These pillars are
basically sealed steel cylinders, 1 m in diameter and 25 m long in their
immerged section. The sacrificial anodes are zinc bars 100 cm long, 12
cm wide and 12 cm thick, with their back tightly screwed onto the
steel surface. This problem is a particular application of the blocking
or passivation of an electrode surface by forcing the accumulation of
some ions that can eventually precipitate and block the surface.
Answer the following questions knowing that the temperature of
the seawater decreases exponentially as a function of depth, changing
from 25°C at the surface to 10°C at a depth of 25 m and that the diffusion
coefficient of CO can be described by the following expression:
2−
3
D = D (1 − 0.043 × (25 − t)) (8.16)
t 25 C
o
where D is the diffusion coefficient of CO at temperature t
2−
t
3
t is the temperature in °C
D is the diffusion coefficient of CO at 25°C (5 × 10 cm s )
2−
−1
−5
25°C 3 2
Question 1: Given that the corrosion rate of unprotected steel is 1.1 mm y ,
−1
estimate the total corrosion current for one pillar.
Answer
Surface area of one pillar = p × diameter × length = 3.1416 × 1 × 25 m 2
Surface area of one pillar = 78.54 m 2
Corrosion current = surface area × corrosion rate in current density units
−1
−2
Since 1 mm y = 0.0863 mA cm or 0.863 A m −2
Corrosion current = 1.1 × 0.863 × 78.54 = 74.56 A
Question 2: Given that the zinc anodes can provide a sacrificial current
corresponding to a corrosion rate of zinc of 7 mm y , evaluate the number of
−1
anodes that would be required to reduce the corrosion of steel by a factor of ten.
Assume that the total corrosion current calculated in Question 1 remains the
same to balance a constant cathodic process, the reduction of oxygen.
Answer
Surface of each anode = face + sides + ends
Surface of each anode = (12 × 100) + 2 × (12 × 100) + 2 × (12 × 12) = 3888 cm
2
Conversion of corrosion rate into current units: 1 mm y = n × density × 0.306/M
−1
For zinc, n = 2, density = 7.133 g cm , M = 65.38 g mol −1
−3
Corrosion current density of anode = 7 × 2 × 7.133 × 0.306/65.38 = 0.4674 mA cm −2
Corrosion current/anode = 0.4574 × 3888 = 1.817 A
The current required is 90 percent of the current calculated in Question 1
or 0.9 × 74.56 A, that is, 67.1 A
Number of anodes required = 67.1/1.817 = 36.9 anodes
Question 3: In order to reduce the consumption of anodes over time you would
like to force the precipitation of calcareous deposits onto your steel surface
because you know that by doing so you can cut down the corrosion of steel a