HIC risk assessment using hydrogen flux

Corrosion of carbon steel in sour, cyanidic bisulfide and HF service can generate high hydrogen activity in affected steel. This activity determines the hydrogen flux generated through the steel. Also, expressed in bar, the activity is a direct measure of the hydrogen’s propensity to inititiate hydrogen induced cracking (HIC). To obtain a HIC risk, first determine the hydrogen activity with the charts above (top, oF, bottom, oC) by looking up the flux-thickness of the service steel on the horizontal axis, and temperature on the vertical axis. From this hydrogen activity and the table overleaf to determine the hydrogen damage steel.  No, flux-thickness is obtained by multiplying the flux in pL/cm2/s by the thickness in cm.  It has units of pL/cm/s.

activity a0

Carbon steel quality:

Is steel more than 18 mm thickness?

100 to 10,000 bar

10,000 to 1,000,000 bar

More than 1,000,000 bar

Poor steel, or steel poor weld HAZ

yes

no

2

1

3

2

3

3

Average steel, or good weld HAZ

yes

no

1

0

2

1

3

2

sour service

yes

no

1

0

1

0

2

1

Table hydrogen damage risk: 0 = zero, 3 = high

Key risk factors:

0: No risk of active damage at site of measurement.

1: Small risk of crack initiation, some risk of crack propagation, especially in steel known to be susceptible.

2: Moderate risk of crack initiation. Likely crack propagation.

3. High risk of crack initiation and certain crack propagation.

Standard NACE TM0284 generates an activity of about 1,300,000 bar. Generally, hydrogen cracks are initiated in poor quality, non-sour service steels, and welds, at activities as low as 10000 bar, whereas sour service steels can withstand at least 1,000,000 bar. After cracks have appeared, much lower activities are needed to propagate them, indeed, any flux may contribute to further crack growth. In order to ensure close correspondence between flux measurements and active corrosion, it is recommended that measurements are carried out on unblistered steel.