Page 291 - Marine Structural Design

Chapter 13 Collapse Analysis of Ship Hulls 267

13.4.4 Corrosion Rate Model
Corrosion rates depend on many factors including coating properties, cargo composition, inert
gas properties, temperature of cargo, and maintenance systems and practices. For this reason,
the corrosion rate model should be appropriately based on the statistics of measurement data.
Practically, the time-variant corrosion rate model may be divided into three phases. In the first
one, there is no corrosion because of the protection of coatings, and corrosion rate is zero. The
second phase is initiated when the corrosion protection is damaged and corrosion occurs,
which reduces the plate thickness. The third phase corresponds to a constant corrosion rate.
The present authors suggested a model as
I-Ti
-__
r(t)=rs(l-e ) (13.48)
where zi is the coating lifetime, z, is the transition time and rs is the steady corrosion rate.
Figure 13.8 shows the corrosion rate model.

T, t

Figure 13.8 Model of Corrosion Rate

Tf
Figure 13.9 Loss of Plating Thickness from Corrosion as Time Function.
By integrating Eq.(13.36), the corrosion depth can be obtained by:

t-ri
-_
d(t)=rs[t-(zi +z,)+z,e J (13.49)