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-79
There are several strategies for controlling corrosion. These include:
a) keep metal surfaces free from deposits,
b) create and maintain an environment that is not conducive to corrosion,
c) incorporate corrosion inhibitors into the treatment program,
d) pre-passivate new and recently cleaned equipment.
In practice it is common to simultaneously employ several of the above strategies. For example, an effective
treatment program frequently incorporates corrosion inhibitors, maintenance of a benign environment, and
steps to keep metal surfaces clean by using dispersants and side-stream filtration.
Many variations of basic cooling water treatment programs are being practiced. In general, they all include
fouling control, corrosion control and microbiological control. The most common method to control the
microbiological population in a cooling system is to treat the system with one or more biocides. Biocides can
be classified into oxidizing and non-oxidizing.
2.B.11.1.9
Indicators for Corrosion Issues and Their Deterrence
The following parameters might be used as indicators for potential issues in the cooling system, depending
on the water treatment program in place.
a) If process-side temperature is greater than 60 °C (140 °F), then a scaling potential exists. However,
note that at the measured pH, one can calculate the temperature at which the water begins to scale, by
solving for the value of C 2 in Equation (2.B.11) using the coefficients provided in Table 2.B.11.3.
b) Dissolved Solids—The higher the level of dissolved solids, typically an indicator of chlorides, the higher
the corrosion rate.
c) Velocity should be maintained at minimum 1 m/s (3 ft/s) through all parts of the system. For example, if
cooling water is on the shell-side of a shell-and-tube exchanger, some regions within the shell will have
a low velocity.
d) Iron levels greater than 5 ppm in the recirculating water could be an indication that the applied
dispersants and flocculating agents are insufficient to keep the formation of deposits at an acceptable
level. Other means of deposit control may be required.
e) In open recirculated systems, suspended solids above 100 ppm will cause settlement in heat exchanger
equipment and become a site for under-deposit corrosion.
f) Chlorine content may be dissolved intentionally in water as a biocide. It has little effect on carbon steel if
pH is maintained above 7 to suppress formation of acid hydrolysis products by Cl 2 +H 2 O → HCLO+HCL.
However, chlorine will attack copper alloys, even at higher pH, presumably by reaction with the Cu 2 O
surface film.
g) Ammonia Content—Results primarily from contamination by process leaks. Could also be present if
NH 4 OH is used as an acid neutralizer; however, this is not recommended due to the volatility of
ammonia and its use as a food source by microorganisms. It has little effect on iron and steel but has a
strong effect on copper alloys. Ammonia forms complexes with copper that can cause rapid general
corrosion and/or SCC of copper alloys. For example, admiralty brass is very susceptible to ammonia
SCC and might experience SCC with only trace amount of ammonia present.