Page 313 - Corrosion Engineering Principles and Practice

284 C h a p t e r 8 C o r r o s i o n b y W a t e r 285

hundredfold. Knowing that the level of Ca ions in seawater is 0.01 molal and
2+
that the diffuse layer is approximately 5 mm thick, calculate:
a) The current density that would be required to force the precipitation of
insoluble aragonite onto the steel close to the water line.
Answer
One has to use Eq. in which n = 2, F = 96485, D = 5 × 10 cm s , and δ = 5 ×
−5
2
−1
−4
10 cm.
2−
The concentration of CO ions C can be obtained with the solubility product of
3
aragonite K described in Eq. (8.12).
sp
K 6.7 × 10 -7
5
-8
C 2− = sp = = 6.7 × 10 mol kg − 1 = 6.7 × 10 mol cm − 3
−
l
CO 3 C Ca 2+ 0.01
Hence the limiting current density when the concentration of carbonate ions
−2
is high enough to precipitate with the calcium ions is 1.29 10 A cm or
−3
1.29 mA cm −2
b) The number of sacrificial anodes/m required to provide this initial
2
protective current.
Answer
For a 1 m surface the required current would be 10 000 × 1.29 × 10 A = 12.93 A
2
−3
Each anode provides 1.817 A
The number of anodes required is therefore = 12.93/1.817 = 7.1 anodes
c) What would be the impact on the current density requirements of attempting
to deposit aragonite in an agitated sea?
Answer
Agitation would force the limiting current to increase and the number of anodes
to do the same.
Question 4: Calculate the current density required to precipitate aragonite at the
bottom of the pillar. Assume that the K of aragonite has not changed.
sp
Answer
Only the diffusion coefficient for the carbonate ions would change.
Since D /D 25°C = 0.355
t
The new limiting current density would therefore be 1.29 × 0.0355 = 0.459 mA cm .
−2
8.3.2 Treated Waters
Potable Water
Potable water is fresh water that is sanitized with oxidizing biocides
such as chlorine or ozone to kill bacteria and make it safe for
drinking purposes. In general, this is done by public water utilities
that are responsible for the treatment and distribution of water to
communities. Although in developed countries less than 1 percent
of potable water is consumed, all water delivered to homes for food
preparation, bathing, washing, watering gardens, heating, and
cooling is treated to potable standards.