Part A | 6.5
118 Part A Fundamentals
Table 6.4 Some factors influencing seawater corrosion of steel
Factor E i
Importance
Water velocity
Tends to increase corrosion in the first few weeks as rusts build up, pushing standard function upward and
thereafter little additional effect
Composition
(shows effect on
model parameters)
Cu
Al
V
Ni
Si
V
Cr +Mo
Mo
Cr +Mo
Mo <0.5%
Al
Cr
Ni
Cr
Cu
Mo
Al
Mn
P
V
Ni
Cu
Corrosion loss
Exposure period
cs
ca
ro
ra
rs
Salinity
Important but effect also influenced by water hardness. Some soft fresh waters are highly corrosive
Timing
Steel exposed, say in autumn or winter has initially a slower rate of corrosion. This pushes model (to the right)
in time by about 6 months
Pressure
No evidence that increased water pressure has any noticeable effect on corrosion. Any depth effects are due
primarily to temperature, DO and nutrient levels
Depth
Any depth effects are due primarily to temperature, DO and nutrient levels
Size/area
Precise shape or area involved in corrosion not discernible as important variable, for moderate-sized objects,
i. e., with distances insufficient for the water properties to change
Table 6.5 Values for model parameters based on calibration to field data (after [6.30, 31])
Model parameter
General corrosion
r 0 [mm/year]
r 0 D 0:076 exp.0:054T/
c a [mm]
c a D 0:32 exp.0:038T/
t a [year]
t a D 6:61 exp.0:088T/
r a [mm/year]
r a D 0:066 exp.0:061T/
c s [mm]
c s D 0:141 0:00133 T
r s [mm/year]
r s D 0:039 exp.0:0254 T/
only r 0 , and this clearly is not indicative of the longer
term behavior. From a practical perspective, the most
important parts of the model of Fig. 6.5 are the shortterm corrosion rate r 0 and the long-term rate r s together
with its intercept c s on the vertical (corrosion loss) axis.
Table 6.5 gives the relevant values as a function of
average seawater temperature. Similar curves and parameters are available for pitting corrosion of steel in
seawaters [6.32].
6.5 Other Influences on Steel Corrosion
The size and orientation of the steel being corroded can
be important. Most information about the corrosion of
steels has been obtained from coupons, that is, relatively small pieces of flat steel exposed in the marine
environment. It is known that in immersion and tidal
conditions, the size of the sample is not critical to measuring mass loss (general corrosion), but this is not necessarily the case for atmospheric corrosion. Also, long
steel strips (or sheet piling or similar) extending upward from the immersion zone, through the tidal zone
and into the splash zone or even the atmosphere zone,
will show corrosion characteristics quite different to individual, electrically isolated, coupons extending over
an identical vertical range. Figure 6.7 shows a typical
plot for mass loss for strips and coupons, standardized
to an equivalent coupon size. It is clear that while corrosion in the tidal zone is high for coupons, it is quite
low for strips. This demonstrates the importance of the
orientation of steel equipment relative to the tidal zone.
It also shows that electrical connectivity can be very important.
Figure 6.7 also shows the effect of higher nutrient levels in the seawater where steel strips or piling
is located. Typically, under higher levels of nutrients in natural seawater, microbiological activity is
enhanced such that in the zone immediately below
mean low tide there is increased corrosion. This is
known as accelerated low water corrosion. It is a major problem in seawater harbors located in regions
where high levels of seawater pollution occur [6.33],
has been associated with microbially induced corrosion (MIC) [6.34, 35] and has been shown to be closely
correlated with the level of dissolved inorganic nitrogen in the seawater. Typically, this is caused by
fertilizers that reach the seawater through agricultural
runoff or from sewage treatment plants [6.36]. Sim-
118 Part A Fundamentals
Table 6.4 Some factors influencing seawater corrosion of steel
Factor E i
Importance
Water velocity
Tends to increase corrosion in the first few weeks as rusts build up, pushing standard function upward and
thereafter little additional effect
Composition
(shows effect on
model parameters)
Cu
Al
V
Ni
Si
V
Cr +Mo
Mo
Cr +Mo
Mo <0.5%
Al
Cr
Ni
Cr
Cu
Mo
Al
Mn
P
V
Ni
Cu
Corrosion loss
Exposure period
cs
ca
ro
ra
rs
Salinity
Important but effect also influenced by water hardness. Some soft fresh waters are highly corrosive
Timing
Steel exposed, say in autumn or winter has initially a slower rate of corrosion. This pushes model (to the right)
in time by about 6 months
Pressure
No evidence that increased water pressure has any noticeable effect on corrosion. Any depth effects are due
primarily to temperature, DO and nutrient levels
Depth
Any depth effects are due primarily to temperature, DO and nutrient levels
Size/area
Precise shape or area involved in corrosion not discernible as important variable, for moderate-sized objects,
i. e., with distances insufficient for the water properties to change
Table 6.5 Values for model parameters based on calibration to field data (after [6.30, 31])
Model parameter
General corrosion
r 0 [mm/year]
r 0 D 0:076 exp.0:054T/
c a [mm]
c a D 0:32 exp.0:038T/
t a [year]
t a D 6:61 exp.0:088T/
r a [mm/year]
r a D 0:066 exp.0:061T/
c s [mm]
c s D 0:141 0:00133 T
r s [mm/year]
r s D 0:039 exp.0:0254 T/
only r 0 , and this clearly is not indicative of the longer
term behavior. From a practical perspective, the most
important parts of the model of Fig. 6.5 are the shortterm corrosion rate r 0 and the long-term rate r s together
with its intercept c s on the vertical (corrosion loss) axis.
Table 6.5 gives the relevant values as a function of
average seawater temperature. Similar curves and parameters are available for pitting corrosion of steel in
seawaters [6.32].
6.5 Other Influences on Steel Corrosion
The size and orientation of the steel being corroded can
be important. Most information about the corrosion of
steels has been obtained from coupons, that is, relatively small pieces of flat steel exposed in the marine
environment. It is known that in immersion and tidal
conditions, the size of the sample is not critical to measuring mass loss (general corrosion), but this is not necessarily the case for atmospheric corrosion. Also, long
steel strips (or sheet piling or similar) extending upward from the immersion zone, through the tidal zone
and into the splash zone or even the atmosphere zone,
will show corrosion characteristics quite different to individual, electrically isolated, coupons extending over
an identical vertical range. Figure 6.7 shows a typical
plot for mass loss for strips and coupons, standardized
to an equivalent coupon size. It is clear that while corrosion in the tidal zone is high for coupons, it is quite
low for strips. This demonstrates the importance of the
orientation of steel equipment relative to the tidal zone.
It also shows that electrical connectivity can be very important.
Figure 6.7 also shows the effect of higher nutrient levels in the seawater where steel strips or piling
is located. Typically, under higher levels of nutrients in natural seawater, microbiological activity is
enhanced such that in the zone immediately below
mean low tide there is increased corrosion. This is
known as accelerated low water corrosion. It is a major problem in seawater harbors located in regions
where high levels of seawater pollution occur [6.33],
has been associated with microbially induced corrosion (MIC) [6.34, 35] and has been shown to be closely
correlated with the level of dissolved inorganic nitrogen in the seawater. Typically, this is caused by
fertilizers that reach the seawater through agricultural
runoff or from sewage treatment plants [6.36]. Sim-