Page 45 - Handbook of Materials Failure Analysis

38 CHAPTER 2 Failure analysis in chemical industries

600.0 m

400.0 m
Re-passivation
200.0 m
I(A/cm 2 )
0.00
Electrochemical scratch
−200.0 m Passivation

−400.0 m
0.000 s 50.00 s 100.0 s 150.0 s 200.0 s
(a) T (s)
600.0 µ

400.0 µ

Re-passivation
200.0 µ
I(A/cm 2 ) 0.00

Electrochemical scratch
−200.0 µ Passivation

−400.0 µ
0.000 s 50.00 s 100.0 s 150.0 s 200.0 s
(b) T (s)
FIGURE 2.6
Potentiostatic polarization scans of line pipe material in (a) spent caustic solution (b) tap
water.
to the presence of high chloride and alkalinity in spent caustic solution. The electro-
chemical testing results also validated the possibility of spent caustic attack to the
surface and were in support to predict corrosion phenomenon. The passivation
was prevented by high concentration of chloride and high alkalinity.

3.1.7 Conclusions
1. The near-stagnant conditions in a horizontal 4-in. line pipe at the junction of
CFL400 were formed due to reduced velocity 0.0345-0.0690 m/s, hence settling
of suspended solids and corrosion product at 6 O’clock position resulted in the
formation of aeration cells.
2. The differential aeration cells underneath the deposit resulted in the breakdown
of passivity and presence of high chloride content may produce acidic
environment locally at the pit sites.