API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-85
Corrosion of carbon steel in seawater is controlled by the availability of oxygen to the metal surface. Under
static conditions (zero velocity), carbon steel corrodes at rates between 0.10 and 0.20 mm/y (4 and 8 mpy),
depending on the local oxygen and temperature variations. As the velocity causes a mass flow of oxygen to
the metal surface, corrosion is very dependent on flow rate and can increase by a factor of 100 in moving
from static condition to a velocity of about 39.6 m/s (130 ft/s). Galvanizing confers only limited benefit under
flow conditions, as corrosion of zinc also increases with velocity. For the thickness normally used in seawater
piping, it will extend the life of the pipe for about 6 months only.
Thus, velocity is the most important single factor influencing design of carbon steel components in seawater
systems. The chosen design velocity controls the dimensions of many components, such as piping and
valves. When the corrosion rate is subject to mass transfer control, flow velocity at the metal surface
becomes the rate-determining factor.
Based on test results reported [30, 35, 36] , Equations (2.B.19) and (2.B.20) may be used to calculate the
corrosion rates on carbon steel in seawater systems as a function of the velocity, V a . Values for the corrosion
rate as a function of velocity using this equation are shown in Table 2.B.11.7.
For SI units, use Equation (2.B.19):
2 25 .
= 01318 + 0 3108⋅ a −0 0579⋅ a + 0 01208⋅ a
(2.B.19)
CR . . V . V . V
For U.S. customary units, use Equation (2.B.20):
2 25 .
= 51885 + 3 7293⋅ a −0 21181⋅ a + 0 02439⋅ a
(2.B.20)
CR . . V . V . V
Equations (2.B.19) and (2.B.20) were developed based on the data specified in Reference [35], assuming
seawater temperature of about 21 °C (70 °F) and an oxygen concentration of 6 to 8 ppm.
With high flow rates, the corrosion rate increases up to around 12.2 m/s (40 ft/s), where the attack changes
to erosion-corrosion. However, it is assumed that cooling water systems in the refining industry will not
experience water flow velocities in excess of 6.1 m/s (20 ft/s).
2.B.11.3.3
Groundwater
Groundwaters are not specifically addressed in this paragraph. However, the following can be a quick
guideline for determining the level of corrosivity for such waters. The assessment needs to be made by a
competent person for water corrosivity issues.
Groundwaters may contain well water, geothermal springs, or produced water (i.e. waters of brines coproduced
with oil or gas). Although groundwaters can be considered one category, they might vary largely in
chemistry. Groundwaters are often characterized in terms of total key species (TKS), which is a sum of
chloride, sulfate, dissolved CO 2 , bicarbonate, carbonate, sulfide, and ammonia concentrations. TKS is a
measure of corrosivity. Depending on the TKS value, the corrosivity towards steel can be rated as follows.
a) Low [<0.03 mm/y (1 mpy)] corrosion rate.
b) Medium [0.03 to 0.25 mm/y (1 to 10 mpy)].
c) High [0.25 to 1.27 mm/y (10 to 50 mpy)].
d) Very High [>1.27 mm/y (50 mpy)].
The subject of specific TKS values vs corrosivity has not been addressed in this paragraph.