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-43
2.B.4.6 Figures
Start
• Material
• Temperature
• H 2 S Concentration
Determine estimated
corrosion rate from Tables
2.B.4.2 through 2.B.4.7 as
applicable.
Type of Hydrocarbon
(except 12% Cr steel
and 300 Series S.S. )
Estimated
Corrosion Rate
Figure 2.B.4.1—High Temperature H 2 S/H 2 Corrosion—Determination of Corrosion Rate
2.B.5
Sulfuric Acid Corrosion
2.B.5.1 Description of Damage
Sulfuric acid (H 2 SO 4 ) is one of the most widely used industrial chemicals. One common use of concentrated
H 2 SO 4 is as a catalyst for the alkylation process. H 2 SO 4 is a very strong acid that can be extremely corrosive
under certain conditions. The corrosiveness of H 2 SO 4 varies widely, and depends on many factors. Acid
concentration and temperature are the foremost factors that influence corrosion. In addition, velocity effects
and presence of impurities in the acid, especially oxygen or oxidants, can have a significant impact on
corrosion.
Although H 2 SO 4 corrodes carbon steel, it is the material typically chosen for equipment and piping handling
concentrated H 2 SO 4 at near ambient temperatures. The corrosion rate of steel by H 2 SO 4 as a function of
acid concentration and temperature under stagnant conditions is provided in NACE Publication 5A151 [101] .
Stagnant or low flow (<0.91 m/s or 3 ft/s) conditions typically cause general thinning of carbon steel. The
ferrous sulfate corrosion product film is somewhat protective, and as it builds on the metal surface the
corrosion rate decreases. The mass transfer of ferrous sulfate away from the corroding steel surface is the
rate-limiting step for the corrosion. Acid solution velocity above approximately 0.91 m/s (3 ft/s) (turbulent
flow) has a significant impact on this mass transfer rate and thus the corrosion rate. Corrosion rates for steel
pipelines carrying H 2 SO 4 at various conditions and velocities have been calculated from a well-established
mathematical model [13] . The calculated rates were based on pure H 2 SO 4 solutions with no ferrous sulfate
present in the acid solution. These rates for turbulent flow in straight pipes were then multiplied by a factor of
3 (based on experience cited in Reference [13] to account for the enhanced localized corrosion that occurs
at elbows, tees, valves, and areas of internal surface roughness such as protuberances at welded joints.
This provides maximum estimated corrosion rates. Actual corrosion rates could be 20 % to 50 % of these
estimated maximum corrosion rates.