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-5
2.B.2.5 Tables
Table 2.B.2.1—HCl—Basic Data Required for Analysis
Material of Construction
pH or Cl − concentration
Maximum temperature, °C (°F)
Presence of air or oxidants
(Yes or No)
Determine the Material of Construction of the Equipment Item
pH is preferred for estimating the corrosion rate at dilute concentrations for carbon
steel and 300 series stainless steels. Table 2.B.2.2 may be used to estimate pH from
the Cl − concentration if it is known. Note that the presence of neutralizing agents may
elevate the pH, however.
For high alloy materials, Cl − concentration is used to estimate the corrosion rate.
NOTE The pH used should be of the separated acid phase within this equipment or nearest
equipment downstream, e.g. the overhead accumulator boot water downstream of the overhead
condenser.
Determine the maximum temperature present in this equipment/piping. This may be
the maximum process temperature, but local heating condition such as effect of the
sun or heat tracing should be considered.
Presence of air (oxygen) may increase corrosion rates, particularly for Alloy 400
and Alloy B-2. Other oxidants such as ferric and cupric ions will have a similar effect
on these alloys.
Table 2.B.2.2—HCl Corrosion—Determination of pH from Cl − Concentration
Cl − Concentration (wppm)
pH
3,601 to 12,000 0.5
1,201 to 3,600 1.0
361 to 1,200 1.5
121 to 360 2.0
36 to 120 2.5
16 to 35 3.0
6 to 15 3.5
3 to 5 4.0
1 to 2 4.5
<1 5.0
NOTE Assumes no alkaline agent present (NH 3 ,
neutralizing amines or caustic).