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

2.B-78 API RECOMMENDED PRACTICE 581
common minerals used as corrosion inhibitors in water treatment programs and can form mineral scale if
not properly controlled.
c) Deposits
1) Mud and Silt—Enters the cooling tower in makeup water or scrubbed from the air.
2) Corrosion Products (Iron Oxide)—Formed as a result of corrosion. Forms a porous deposit that is
frequently the site for pitting corrosion.
3) Bio-mass—A mixture of bacterial slime and other material. Typically mud and iron oxide accumulate
in low flow regions, causing a favorable environment for microorganisms to form colonies on the
surface that potentially can promote accelerated pitting corrosion. Process leaks of hydrocarbon can
often accelerate bacterial growth and lead to rapid formation of bio-mass.
2.B.11.1.7
Types of Corrosion
Corrosion of carbon steel in cooling water systems is dominated by pitting. The following are other more
common types of corrosion that can be seen.
a) Under-deposit Corrosion—A form of pitting corrosion that occurs beneath deposits.
b) Crevice Corrosion—Pitting and preferential attack at a crevice such as at the tube to tubesheet crevice,
etc.
c) Galvanic Corrosion—The enhanced corrosion of one metal in electrical contact with another kind of
metal in an electrolyte. This form of corrosion depends on the metals having a dissimilar corrosion
potential that causes one metal to be polarized into a potential region where it corrodes more quickly.
As an example, copper alloy tubes in a carbon steel tubesheet may cause more rapid corrosion of the
tubesheet.
d) Dealloying—Corrosion process that appears to selectively dissolve one of the constituents of an alloy.
When admiralty brass experiences dealloying, zinc is removed leaving copper (referred to as
dezincification).
e) MIC—MIC is a complex issue. Strictly speaking, MIC is not a specific type of corrosion. Rather, it
consists of both direct and indirect effects on materials by bacteria, fungi, algae, and yeasts. Many types
of bacteria can be implicated involved in MIC processes. Perhaps the best known type of MIC attack is
the corrosion process involving SRB. These organisms are anaerobic (cannot tolerate oxygen) and
thrive beneath deposits. They metabolize sulfate creating a highly acidic local environment that leads to
metal corrosion. Other types of bacteria that contribute to MIC include slime forming bacteria, nitrifying
bacteria, manganese reducing bacteria, and iron reducing bacteria.
f) SCC—SCC mechanisms require the coming together of three things—a tensile stress, a susceptible
alloy, and an environment that promotes SCC in that alloy. The tensile stress may be either applied or
residual, and residual stresses from welding commonly contribute to SCC. Many alloys can be
susceptible to SCC in at least one environment with 300-series stainless steels and copper alloys being
the most common susceptible materials used in cooling water systems. The environments most
associated with SCC of these alloys are aqueous chloride environments for 300-series stainless steels
and ammonia for specific copper alloys.
2.B.11.1.8
Corrosion Monitoring and Control in Cooling Water Systems
The largest potential for problems in the cooling system is associated with scaling, deposition of suspended
solids, and MIC. Usually, under conditions with controlled water treatment and operating parameters, only
negligible corrosion is expected. However, even with these parameters in control, MIC or buildup of solids
may cause unsuspected corrosion. This fact emphasizes the need for an effective corrosion monitoring
program in addition to the water treatment and process control programs.