3. 3
Corrosivity
• Oxygenated desalinated waters are
very aggressive to carbon steel,
particularly if deionization is
complete or if anions, specially
chlorides, are not eliminated
• The high aggressiveness is due to:
• Low content or absence of alkalinity
• Low content or absence of calcium
hardness
• Possible presence of chlorides (lime
softening and zeolite treatment)
• Insufficient calcium bycarbonate to
buffer the low pH environment formed
with corrosion reactions
4. 4
Corrosivity
• Lack of buffering capacity and calcium
carbonate in the water accelerate
exponentially the corrosion reactions
• Soft nature of formed iron oxides allows
continue water penetration and metal
dissolution
• Presence of chlorides in unbuffered waters
create the conditions for very high
corrosion rate
• The flow of anions, particularly Chloride ions,
to the anodes disrupts passivating oxides film
• Chlorides ions cause pitting by penetrating
through pores
• Stability of coordination complexes such as ZnCl4
-
5. 5
Treatment
In such unbuffered waters it is
essential to:
• Control very precisely the pH in a range
of 8 to 9. In low pH ranges corrosivity
will increase dramatically
• Addition of bicarbonate / carbonate will
reduce drastically the corrosivity of the
deionized waters
• If calcium is absent, addition of some
calcium hardness may help to easily treat
such type of waters
• To face the lack of buffer and calcium
hardness, strong cathodic and anodic
6. 6
Treatment
Basic treatment may depends from the
quality of the desalinated water
produced:
Water type Case Treatment
Bicarbonate/
carbonate
Ca
Hardness
pH
control
Corrosion
inhibitors
Cycles
Complete
deionization
1 100 – 150 ppm
as Malk
80 – 100 ppm 8.5 – 9.2 Low
Conc.
High
2 NO NO 8.5 – 9.5 High
Conc.
High
Lime
softned
min. 150 ppm
as Malk
min. 80 ppm 8.5 – 9.0 High
Conc.
Low f(Cl)
Ion exch.
zeolite
May be not
needed
50 - 100 8.5 – 9.0 High
Conc.
Low f(Cl)
7. 7
Treatment
Buffering capacity may be added by:
• Sodium bi-carbonate
• Sodium carbonate
• Sodium hydroxide (will form carbonate with carbon
dioxide adsorbed in the circulating water)
Calcium hardness may be added by:
• Calcium nitrate (nitrates may be limited at
discharge)
• Dolomite resin (some magnesium and carbonate are
also released)
• Calcium oxide/hydroxide (may create local
precipitation due to the high pH of the solution)
8. 8
Treatment
Corrosion inhibitors must face such high
water aggressiveness
• Base treatment
• Stabilized cathodic inhibitor
• Stabilized anodic inhibitor
• New generation line of organic cathodic
inhibitor Specific inhibitors stabilizers
• pH control (8 – 9)
• Alkalinity (100 – 200 ppm as CaCO3)
• Calcium hardness ( > 30 ppm as CaCO3)
• Non-phosphrous treatment using Silicates
( 30-40mg/l SiO2 in CW) can be
considered. pH control is vital: pH 8.5
