Civil Engineering Reference
In-Depth Information
TABLE 6.1
Estimated Maximum Corrosion Rates of Clean Steel in North
Sea Water at 7
°
C
O
2
Concentration
Linear Flow
Rate (m/s)
6 ppm
7 ppm
8 ppm
9 ppm
10 ppm
0
0.080
0.094
0.107
0.120
0.134
0.3
0.091
0.107
0.123
0.138
0.154
0.4
0.096
0.111
0.128
0.144
0.160
0.6
0.104
0.121
0.138
0.156
0.174
1
0.120
0.140
0.160
0.179
0.199
2
0.160
0.187
0.213
0.240
0.266
4
0.240
0.280
0.320
0.360
0.400
A value of 0.13 mm/year may be taken as reasonably representative in any part
of the world. However, pits may grow at 3 to 10 times that rate.
Many marine structures, particularly those in shallow waters, are simulta-
neously exposed to a number of discrete corrosive environments: the marine atmo-
sphere, the splash zone, the tidal zone, the fully submerged zone and the mud zone.
Commonly, the corrosion rate data are represented schematically, a practice that
derives largely from the work of
Humble (1949)
as shown in
Figure 6.6
. Peak cor-
rosion rates are found immediately below the mean low tide zone and in the splash
zone. The typical mean corrosion rate in the splash zone, in case of quiet sea con-
ditions, is estimated as 0.25 to 0.75 mm/year.
As shown in
Figure 6.3
, the corrosion rate is controlled by sluggish cathodic
kinetics: in this case, the rate of arrival of oxygen at the surface and the effect of
increasing oxygen availability.
The peak corrosion rate is often attributed to galvanic action between steel
in contact with oxygen-rich surface waters, which is the cathodic area, and steel
at somewhat greater depth exposed to waters of lesser oxygen content, which is
the anodic area. It is difficult to conceive that the change in oxygen concentra-
tion with depth is sufficiently great to cause such an effect, and it may be that
other factors come into play. It is essential to gather the following information
before starting the design of the CP system. In fact, this list of required data can
be considered a checklist for collecting information from the owner.
I. Structure design data
●
Design life for the cathodic protection system and the structure
Construction drawings with full details and dimensions
●
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