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

2.B-80 API RECOMMENDED PRACTICE 581
With fresh waters, corrosion of steel is governed by dissolved oxygen over a broad pH range (4.5 to 9.5) at
relatively low temperatures. Below pH 4.5, the corrosion is controlled by hydrogen evolution under acid
conditions. Above pH 9.5, an insoluble film of ferric hydroxide suppresses corrosion. In buffered solutions,
where ionic concentrations are high without a corresponding drop in pH (as with carbonic and other weakly
ionized acids), corrosion with hydrogen evolution may occur in the pH 5.0 to 5.5 range. In the intermediate pH
range of 4.5 through 9.5, a loose, porous, ferrous-oxide deposit shelters the surface and maintains the steel
surface pH at about 9.5. The corrosion rate remains nearly constant and is determined by uniform diffusion of
dissolved oxygen through the deposit. At the metal surface under the deposit, oxygen is reduced cathodically.
Cathodic reduction of dissolved oxygen produces a slightly alkaline surface condition, which precipitates
calcium carbonate (CaCO 3 ). These precipitates in turn inhibit further cathodic reduction and corrosion.
The corrosive effect from the water in closed recirculating cooling water systems is easily mitigated. These
systems may be treated either by rendering them sterile and anaerobic or by either oxidizing or non-oxidizing
inhibitors.
Corrosion control in open recirculating cooling water systems can be achieved through the continuous use of
treatment chemicals containing scale and corrosion inhibitors and polymeric dispersants. The regular use of
broad spectrum microbiocides is typically used for controlling microbiological populations.
Once through cooling water systems present special problems because chemical treatment of the water may
not be possible or practical. In addition, such systems often use seawater or other inherently corrosive water.
Corrosion control in once through systems is principally achieved by proper alloy selection, CP, and/or the
use of epoxy-phenolic coatings.
2.B.11.1.10
Assumptions
Cooling water systems can be very complex, and this risk-based inspection model does not attempt to
address every issue that must be considered. For the purposes of this model, the following assumptions
have been made.
a) Low alloy steels are rarely used in cooling water systems and will not be addressed in the remainder of
this paragraph. However, most of the content for carbon steel applies to low alloy steels.
b) This model does not consider degradation of alloys other than carbon steel. Beyond some general
comments, SCC and pitting of stainless steels and dealloying of copper alloys are not considered.
c) If coupon measurement results are available, these should be used instead of this model. As a rule of
thumb for carbon steel, the pitting rate is a factor of 5 to 10 times the coupon general corrosion rate,
(calculated by weight loss).
d) If corrosion inhibitors are being used, it is assumed that the program is designed and operated to
adequately control corrosion of carbon steel and alloy materials.
e) An effective microbiological control program is in place, and corrosion driven by MIC is negligible, i.e.
can be set to < 0.13 mm/y (5 mpy), pitting.
f) Water pH is kept within the range 6.5 to 9.5. Outside this pH range, the corrosion is assumed caused by
other means than what is covered in this paragraph.
g) In the event the RSI value is < 6, it is assumed that corrosion is retarded by scale formation, but can still
be estimated on the basis of the chloride content, temperature and flow velocity.
h) There is no deposition and no local low flow areas.
2.B.11.1.1
Corrosion Rates for Copper Alloys Components
This paragraph is not intended to cover material of construction other than carbon steel. The introduction of
other materials such as stainless steel, copper, nickel, zinc, or aluminum-based alloys, etc. might introduce