API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

2.B-44 API RECOMMENDED PRACTICE 581
Although the performance of many alloys in H 2 SO 4 service is primarily related to the acid concentration and
temperature, velocity and the presence of an oxidant can play a significant role as well. This is because these
alloys often depend upon formation of a protective oxide film to provide passivity and, therefore, corrosion
resistance. The presence of an oxidant usually improves the corrosion performance in H 2 SO 4 service of alloys
such as stainless steel and many nickel alloys. This is not the case with Alloy B-2, which can suffer drastically
high corrosion rates if an oxidant is present in the acid. The corrosion rates provided in these tables are from
published literature, and the corrosion rates for non-aerated acid services are used to provide conservatism,
except for Alloy B-2. This conservatism is appropriate because other acid contaminants and velocity can affect
the material’s passivity. The effect of velocity on corrosion rates is assumed to hold over a wide range of
conditions for very little information on the effect of velocity is published.
2.B.5.2 Basic Data
The data listed in Table 2.B.5.1 are required to determine the estimated corrosion rate for H 2 SO 4 service. If
precise data have not been measured, a knowledgeable process specialist should be consulted.
2.B.5.3 Determination of Corrosion Rate
The steps required to determine the corrosion rate are shown in Figure 2.B.5.1. The corrosion rate may be
determined using the basic data in Table 2.B.5.1 in conjunction with Tables 2.B.5.2 through 2.B.5.7.
Note that the corrosion rates of Alloy B-2 can increase drastically in the presence of an oxidant (e.g. oxygen
or ferric ions), which is not reflected in Table 2.B.5.7. For this environment, a corrosion engineer should be
consulted to establish an estimated corrosion rate.
2.B.5.4 References
See References [106], [101], [102], [103], and [104] in Section 2.2.
2.B.5.5 Tables
Table 2.B.5.1—H 2 SO 4 Corrosion—Basic Data Required for Analysis
Basic Data
Material of construction
Acid concentration (wt %)
Maximum temperature (°C:°F)
Velocity of acid (m/s: ft/s)
Oxygen/oxidant present?
(Yes or No)
Comments
Determine the material of construction of the equipment/piping.
Determine the concentration of the H 2 SO 4 present in this equipment/piping. If
analytical results are not readily available, it should be estimated by a
knowledgeable process engineer.
Determine the maximum temperature present in this equipment/piping. This may
be the maximum process temperature, but local heating conditions such as effect
of the sun or heat tracing should be considered.
Determine the maximum velocity of the acid in this equipment/piping. Although
conditions in a vessel may be essentially stagnant, the acid velocity in flowing
nozzles (inlet, outlet, etc.) should be considered.
Determine whether the acid contains oxygen or some other oxidant. If in doubt,
consult a knowledgeable process engineer. These data are only necessary if the
material of construction is Alloy B-2. For carbon steel and other alloys, the
corrosion rates in the tables assume the acid does not contain oxygen/oxidants.