ENVIRONMENTAL EFFECTS ON THE SUSCEPTIBILITY AND CRACK GROWTH OF NEAR NEUTRAL PH STRESS CORROSION CRACKING

Environmental Effect on the Transformation of Shallow to Deep Stress Corrosion Cracks
Jeffrey Xie*, Chris Foy
NOVA Chemicals Research & Technology Center (NRTC)
2928 16 Street NE, Calgary AB CANADA T2E 7K7
* Email: xiejeff@novachem.com
Tel: 1-403-250-4511
Bob Worthingham
TranCanada Pipelines Limited
450 1^st Street SW, Calgary AB CANADA T2P 5H1
Mark Piazza
Pipeline Research Council International
1401 Wilson Blvd, Suite 1101, Arlington, VA USA 22209
The vast majority of near-neutral pH SCC cracks on pipelines are shallow, and many appear to be dormant. The concept of dormancy is consistent with the field observation: very few service failures compared to the very large number of shallow cracks that have been found.  Although several mechanisms for re-activating a dormant crack have been demonstrated, this phenomenon is not completely understood.
A combination of mechanical testing, hydrogen permeation and potential drop measurements were performed to characterize the susceptibility and crack growth rates of near-neutral pH SCC. The investigated environmental parameters were mill scales, dissolved CO₂, HCO₃^-, pH, sulfur containing species (sulfide and sulfur oxyanions) and organics (AQDS, laurentian humic and fulvic acids).

Hydrogen concentration was directly proportional to the concentration of sulfide (HS^-), and the %RA-ratio decreased linearly with the logarithm of sulfide concentration. Organic compounds, like AQDS, LHA and LFA, had an inhibitive effect on the crack growth rates, and also reduced the hydrogen concentration. Higher CO₂ concentrations led to a decrease in solution pH. However the addition of HCO₃^- may result in the formation of a ferrous carbonate film that could protect the underlying steel and slow the corrosion process.