Page 317 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION CONTROL IN THE CHEMICAL, PETROCHEMICAL 295

The original saltwater condenser tube made of admiralty brass was found to be
susceptible to erosion–corrosion at tube ends. Aluminum brass containing 2% alu-
minum was more resistant to erosion in saltwater. Inhibition with arsenic is necessary
to prevent dezincification as in the case of admiralty brass. The stronger naval brass
is selected as the tube material when admiralty brass tubes are used in condensers.
Cast brass or bronze alloys for valves and fittings are usually Cu–Sn–Zn composi-
tions, plus lead for machinability. Aluminum bronzes are often used as tube sheet
and channel material for exchangers with admiralty brass or titanium tubes exposed
to cooling water.
The 70/30 cooper–nickel alloy is used for exchanger tubes when better salt-
water corrosion resistance than in aluminum brass is needed, or when high metal
temperatures in water-cooled exchangers may cause dezincification in brass.
Monel is 67:30 of nickel: copper has very good resistance to saltwater and, under
nonoxidizing conditions, to HCl and HF acids. Monel has a better high-temperature
resistance to cooling water than does 70/30 Cu–Ni alloy. Monel cladding and Monel
trays are commonly specified at the top of crude towers to resist HCl vapor at a
∘ ∘
temperature below 205 C. Above 205 nickel-based alloys are attacked by H S. For
2
high-temperature strength and/or corrosion resistance, several nickel-based alloys
are used for expansion bellows in FCC process units (Alloy 625), stems in flue gas
butterfly valves (Alloy x750), and in springs exposed to high-temperature corrosives
(Alloy X).
Titanium has excellent resistance to seawater and it is also used for tubing in crude
tower overhead condensers. Use of titanium is limited because of the high cost.

4.28 CORROSION CONTROL IN THE CHEMICAL, PETROCHEMICAL,
AND PHARMACEUTICAL INDUSTRIES

The types of corrosion experienced in the chemical manufacturing industry, the
petrochemical manufacturing industry, and the pharmaceutical manufacturing indus-
try are similar in many respects. The most common types of corrosion encountered
are caustic and chloride cracking, oxidation, sulfidation, corrosion under thermal
insulation, ammonia cracking, and hydrogen-induced cracking.
The corrosion failure mode with an average frequency of occurrence is given in
Table 4.46.
The data in Table 4.47 show the SCC failures of different alloys.
Plant and process design involves the materials of construction, equipment design,
process conditions, and recommended operating practices to minimize the risk of
corrosion.
In large companies, an internal project team may design the plant, otherwise con-
tractors provide the design. In any case, the corrosion engineer must be involved from
the beginning of the project. The materials of construction must be chosen to satisfy
the process condition with respect to corrosion and its control. Figure 4.11 shows the
phases of corrosion control.