Although corrosion is a potential hazard in a number of applications,
few environments are more aggressive than those in a desalination
plant. That’s why these stainless steels have been developed, refined
and selected with the utmost care and attention. Below you’ll find
information on the various types of corrosion, including common
pitfalls and how best to avoid them.
Localized corrosion
Chloride containing environments increase the risk of localized corrosion in the form of pitting, crevice corrosion and stress corrosion cracking (SCC). The most important alloying elements for improving resistance to this type of corrosion are chromium, molybdenum and nitrogen. The relative resistance of stainless steel to pitting and crevice corrosion can be calculated by means of a PRE-number, based on the chemical composition of the steel as shown in table below.
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Field test in a desalination plant
Corrosion resistance is shown by test results from a .eld exposure in an MSF once-through plant. For a period of 28 months, five stainless steel grades were put through their paces in a first-stage
flash chamber where the feed, air-saturated seawater, had a chloride content of 20,000 ppm, a residual chlorine content of 0.2 ppm and the temperature was 100° C.
The flash chamber shells at the first stages of this plant were made of 254 SMO with 654 SMO flanges. Results were satisfactory. The remaining stages, however, utilized 1.4439, when Duplex 2205 would’ve been a more corrosion-resistant and cost-efficient choice.
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* Out of total 120 sites
** P=pitting; CC= crevice corrosion |
Materials selection
The selection of an appropriate grade for a variety of components – except for those exposed to the highest temperatures in distillation plants – can be based on engineering diagrams. You can evaluate grades not included in the diagrams or the table, abowe, by calculating their PRE numbers and comparing them with the grades included.
The consequences of selecting an inappropriate grade are dramatically illustrated in the adjacent pictures, for example crevice corrosion on a high-pressure pipe made of 904L after just one year of service in a Spanish SWRO plant or SCC on 1.4404 in
ventilation systems in a destillation plant. Conventional austenitic grades can be susceptible to stress corrosion cracking at high temperatures. Duplex stainless steels and highly alloyed austenitic grades are not.
Uniform corrosion
Generally, stainless steel offers excellent resistance to uniform corrosion in seawater – as shown by the rates measured in the field test described previously, less than 0,00005 mm/year. Consequently, there’s no measurable release of toxic metal ions – copper, chromium or nickel – into the sea when stainless steels are used for evaporator vessel walls or condenser tubes.
Galvanic corrosion
There is no major difference in corrosion potential between highly alloyed stainless steels and titanium. However, stainless steel is generally ‘nobler’ than copper-base alloys and may cause galvanic corrosion in cupronickel, brass and bronze.
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