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-33
2.B.4
2.B.4.1
High Temperature H 2 S/H 2 Corrosion
Description of Damage
High temperature H 2 S/H 2 corrosion is a form of normally uniform corrosion that can occur at temperatures
typically above about 204 °C (400 °F). This form of sulfidation corrosion differs from high temperature sulfidic
and naphthenic corrosion described in Section 2.B.3. H 2 S/H 2 corrosion occurs in hydroprocessing units, e.g.
hydrodesulfurizers and hydrocrackers, once sulfur compounds are converted to hydrogen sulfide via catalytic
reaction with hydrogen. Conversion of sulfur compounds to H 2 S/H 2 typically does not occur to a significant
extent in the presence of hydrogen, even at elevated temperatures, unless a catalyst is present. The
corrosion rate is a function of the material of construction, temperature, nature of the process stream, and
the concentration of H 2 S.
In H 2 S/H 2 environments, low levels of chromium (e.g. 5 to 9 % Cr) provide only a modest increase the
corrosion resistance of steel. A minimum of 12 % Cr is needed to provide a significant decrease in corrosion
rate. Further addition of chromium and nickel provides a substantial increase in corrosion resistance. The
nature of the process stream is a factor in determining the corrosion rate. In H 2 S/H 2 environments alone (all
vapor), corrosion rates may be as much as 50 % greater than in the presence of hydrocarbons as suggested
by the referenced NACE committee report. Nevertheless, the correlations developed by Couper and Gorman
are used for estimating corrosion rates in both hydrocarbon-free and hydrocarbon-containing services. The
predicted rates in both services are very high at high H 2 S levels and temperatures, and the one set of data is
satisfactory for risk-based inspection assessment purposes of either situation.
2.B.4.2
Basic Data
The data listed in Table 2.B.4.1 are required to determine the rate of corrosion in high temperature H 2 S/H 2
service. If precise data have not been measured, a knowledgeable process specialist should be consulted.
2.B.4.3 Determination of Corrosion Rate
The steps required to determine the corrosion rate are shown in Figure 2.B.4.1. The corrosion rate may be
determined using the basic data in Table 2.B.4.1 in conjunction with Tables 2.B.4.2 through 2.B.4.7.
The estimated corrosion rates in H 2 S/H 2 environments in these tables were determined using data from the
correlations developed by Couper and Gorman.
2.B.4.4 References
See References [95], [98], [99], and [100] in Section 2.2.