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-83
Corrosion of steel increases with chloride content of the water and reaches a maximum at approximately
6000 ppm. Above that level the chloride effect is offset by diminishing solubility of dissolved oxygen.
2.B.11.3.1.3
Temperature Factor
The corrosion rate of carbon steel has shown to increase almost linearly with temperature from 27 °C to
79 °C (80 °F to 175 °F). This classical correlation has been used to adjust the calculated corrosion rates.
Therefore, to calculate the temperature adjustment, the ∆T is calculated by subtracting 24 °C (75 °F) from the
actual metal temperature, T OP , or:
∆T = T OP − T adjust
(2.B.12)
This ∆T is used to determine the temperature correction factor, F T , using Table 2.B.11.5. Note that the F T
values are different between open and closed systems at high temperatures. In an open system, heating
above room temperature initially increases corrosion rate for steel but also reduces solubility of dissolved
oxygen, which allows oxygen to escape. Therefore, at temperatures of 79 °C (175 °F) and greater, the
corrosion rate decreases. However, in a closed system, the corrosion rate increases with temperature
because of retention of small amounts of dissolved oxygen under pressure.
2.B.11.3.1.4
Flow Velocity Factor
Velocity is one of the prime variables influencing waterside corrosion. At very low velocity, biofouling or
deposit buildup can occur promoting under-deposit type of attack or MIC. Even if fouling deposits do not
occur, low velocity encourages higher metal temperatures that results in an increase in the corrosion rate.
For carbon steel there is a range of flow velocities [see Equation (2.B.14)] where temperature does not have
an effect on the corrosion rate. If flow velocities are outside these limits the velocity factor may be
determined from Table 2.B.11.6 or calculated using the following equations where V a is the actual velocity.
For SI units, use Equations (2.B.13) through (2.B.15):
F V = 1+ 1. 64 ⋅(0. 914 − Va)
for V a < 0.914 m/s (2.B.13)
F V = 1
for 0.914 m/s ≤ V a ≤ 2.44 m/s (2.B.14)
F V = 1+ 0. 82 ⋅( V a − 2.
44)
for V a > 2.44 m/s (2.B.15)
For U.S. customary units, use Equations (2.B.16) through (2.B.18):
F V = 1+ 0. 50 ⋅(3 − Va)
for V a < 3 ft/s (2.B.16)
F V = 1
for 3 ft/s ≤ V a ≤ 8 ft/s (2.B.17)
F V = 1+ 0. 25 ⋅( Va
− 8)
for V a > 8 ft/s (2.B.18)
This represents a fairly coarse and conservative way of factoring in the velocity effect in the corrosion rate
prediction model. In reality this effect is a product of a much more sophisticated interrelation between
temperature, dissolved oxygen, pH, and velocity. However, the trend shown in Table 2.B.11.6 does comply
with actual testing described in Reference [52] for velocities up to 2.13 m/s (7 ft/s). Note that for carbon steel
in seawater, the velocity is even more a governing factor for the corrosion rate.