corrosion of stainless steel - Damstahl

Chapter 6 - Corrosion of Stainless Steel
Pressure testing
of pipe systems
neutral water” (with e.g. 250 mg/l chloride, the maximum in EU tap water)
is used, it is seen from Figure 6.10 that steel type 4301 only lasts to approx.
50°C without suffering from pitting corrosion. If the chloride content is
increased to 500 mg/l, the temperature is reduced to 38°C and at 2000 mg/l
(= 0.2%) only 25°C is possible.
The higher the chloride concentration, the poorer the corrosion resistance,
which needs to be taken into consideration for e.g. pressure testing of pipe
systems. It is possible that the steel will be fine in standard tap water with
max. 250 mg/l during the pressure testing itself, but remaining drops of
water may cause problems. The water evaporates, but the salts do not,
and the chloride content can easily become much higher than the original
250 mg/l in the drops of water. This is the same evaporation effect that over
thousands of years has created the Dead Sea, and several times corrosion
has occurred due to evaporation of the pressure testing water. Corrosion at
the bottom of pipes after pressure testing indicates such damage.
Figure 6.11:
Remaining drops of water from e.g. pressure testing of pipe systems may
cause unpleasant surprises. As the water evaporates, the salt content
increases, and the conditions gradually become more corrosive. The risk
of bacterial growth does not improve the situation.
Extreme salt
concentrations
give low oxygen
solubility
If you look at the CPT curves in Figure 6.10, it can be seen that very
high chloride concentrations almost make the curves run completely
horizontally. This is a sign that there is no great difference between the
corrosion conditions at 1% Cl – and 2% Cl – . At the lower end, a doubling of
the concentration has much greater effect which is not really as strange as
it seems. With more than 1% chloride, the corrosive agents are so dominant
that a bit more makes no difference.
In practice, one may even encounter a 'softening effect' at very high salt
concentrations as the solubility of oxygen is lower in strong solutions than
in weak ones. This often means that a high salt content is accompanied
by a lower corrosion potential which may even improve the resistance to
corrosion (see below in Section 6.2.3). Concentrated salt solutions are
thus not necessarily worse than diluted ones as the increase in chloride
is compensated for by a reduction in the corrosion potential - the driving
force of the corrosion.
If strong oxidants are present (e.g. peroxide or hypochlorite), the reduced
solubility of oxygen unfortunately does not play a role, and the corrosion
conditions uniformly become worse at higher concentrations of chloride.
Trying to improve the corrosion conditions by increasing the salt
concentration is therefore not recommended.
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RS for alle.indb 87
9/29/2011 12:44:36 PM