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-61
mole/mole. In MDEA units, and particularly those used for selective H 2 S removal in sulfur plant tail gas
cleanup, rich loadings are often below these levels. As with most corrosion mechanisms, higher temperature
increases the corrosion rate.
Another important factor in amine corrosion is the presence of amine degradation products, usually referred
to as “heat stable amine salts” or HSAS. These amine degradation products act in two ways. On the one
hand, they reduce the amount of active amine available to absorb acid gas, resulting in higher acid gas
loadings. In addition, some amine degradation products themselves are corrosive. In MEA and DEA
systems, HSAS above 0.5 wt% can begin to increase corrosion although a common operating limit is 2 wt%.
Corrosion can be particularly significant, even at low acid gas loadings, at >2.0 wt% HSAS. MDEA will also
form HSAS, but the primary influence on corrosion in these units is organic acid contaminants (formate,
oxalate, and acetate). Thermal reclaimers are often provided in MEA units to reduce HSAS, but DEA and
MDEA salts are more stable and cannot be thermally reclaimed. DEA degrades less readily than MEA and
MDEA. Velocity or turbulence also influences amine corrosion. In the absence of high velocities and
turbulence, amine corrosion can be fairly uniform. Higher velocities and turbulence can cause acid gas to
evolve from solution, particularly at elbows and where pressure drops occur such as valves, resulting in more
localized corrosion. Higher velocity and turbulence may also disrupt protective iron sulfide films that may
form. Where velocity is a factor, corrosion may appear either as pitting or grooving. For carbon steel,
common velocity limits are about 1.52 m/s (5 ft/s) for rich amine and about 6.01 m/s (20 ft/s) for lean amine.
Austenitic stainless steels are commonly used in areas that are corrosive to carbon steel with good success
unless temperatures, amine concentration, and degradation product levels are particularly high. Common
applications for stainless steels are reboiler, reclaimer, and hot rich-lean exchanger tubes as well as
pressure let-down valves and downstream piping/equipment. 12 % Cr steels have been used for scrubber
(absorber) tower internals successfully. Copper alloys are subject to accelerated corrosion and SCC and are
normally avoided.
2.B.8.2 Basic Data
The data listed in Table 2.B.8.1 are required to determine the estimated corrosion rate for amine service. If
precise data have not been measured, a knowledgeable process specialist should be consulted.
2.B.8.3 Determination of Corrosion Rate
The steps required to determine the corrosion rate are shown in Figure 2.B.8.1. The corrosion rate may be
determined using the basic data in Table 2.B.8.1 in conjunction with Tables 2.B.8.2 through 2.B.8.5.
The estimated corrosion rate for carbon steel should be obtained from Table 2.B.8.2 for 20 wt% MEA and
30 wt% DEA and from Table 2.B.8.3 for 50 wt% MDEA. If higher amine concentrations are used, the
corrosion rate obtained should be multiplied by the appropriate factor from Table 2.B.8.4.
The estimated corrosion rate for stainless steel may be obtained from Table 2.B.8.5. Note that at extreme
conditions of amine concentrations, temperatures, and levels of degradation products, the corrosion rate of
stainless steel can be as much as 200 times the value in the Table 2.B.8.5.
2.B.8.4 References
See References [23] (Appendix B—Considerations for Corrosion Control), [115], [116], [117], [118], [119],
[120], [121], [122], [123], and [124] in Section 2.2.