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-81
other types of degradation mechanisms. It is virtually impossible to model the complexity of the various
materials and interrelation with various chemical and contaminants in the water. Thus, it has been assumed
that, in general, the selection of appropriate alloy material combined with “correct” chemical treatment and
process control will render a negligible corrosion rate in the cooling water system.
In general, copper and its alloys are the most reliable and cost-effective alloys for many water services,
although dezincification needs to be specifically inhibited for brasses containing more than 15 % zinc.
Copper, red brass, inhibited admiralty brasses, aluminum brass, aluminum bronze, and cupronickels, in that
order, are used for water of increasing salinity and/or velocity. In the presence of dissolved oxygen, soft
waters can be highly corrosive to copper alloys. Also, copper can suffer pitting under some conditions, which
for fresh waters can be described as three types of pitting:
a) Type 1 pitting is apparently caused by residual carbonaceous films from the manufacturing process;
b) Type 2 pitting is associated with hot soft waters [>60 °C (140 °F)], and
c) Type 3 pitting may occur in cold water or high pH and low salt concentrations, for unknown reasons.
Another issue related to copper alloys is cracking. Admiralty brass is very susceptible to ammonia SCC and
might experience SCC with only a trace amount of ammonia present.
An important factor for copper-based alloys is maintaining operation within design velocity limits. Velocities
under the lower limit can lead to increased deposition and under-deposit corrosion, and velocities exceeding
the upper limit can cause damage to the protective surface film resulting in impingement attack.
2.B.11.2
2.B.11.2.1
Basic Data
Recirculating Cooling Water Systems
The data listed in Table 2.B.11.1 are required for determining the estimated corrosion rate for recirculating
cooling water service. If precise data have not been measured, a knowledgeable process specialist should
be consulted.
2.B.11.2.2
Once Through Cooling Water Systems
The data listed in Table 2.B.11.2 are required for determining the estimated corrosion rate for once through
cooling water service. If precise data have not been measured, a knowledgeable process specialist should
be consulted.
2.B.11.3
2.B.11.3.1
Determination of Corrosion Rate
Recirculating Cooling Water Systems
2.B.11.3.1.1
Corrosion Rate Equation
The steps required to determine the corrosion rate are shown in Figure 2.B.11.1. The corrosion rate is
computed using Equation (2.B.8). In this equation, the base corrosion rate, CR B , is adjusted for temperature
and flow velocity for each component in the system to calculate a final representative corrosion rate.
CR = CRB⋅FT ⋅ FV
(2.B.8)
The estimated corrosion rates need further adjustments in case construction material is other than carbon
steel. This has not been addressed within this paragraph.