3.12 Carbonate Stress Corrosion Cracking Flashcards
Carbonate SCC, or alkaline carbonate SCC (ACSCC), referred to as carbonate
cracking, occurs in process units containing:
free water phase with carbonate ions and some amount
of H2S is invariably also present.
Affected Materials are
Carbon steel and low-alloy steels with welds or cold-worked areas.
A critical factor refers to weld or cold -Work area must have ________ and must be in contact with __________.
tensile residual stress ; liquid water
The _________ is a critical factor affecting the likelihood of carbonate SCC
water chemistry
Carbonate cracking can occur at relatively low levels of residual stress, but it usually occurs at
welds or coldworked areas that have not been stress relieved.
Critical factor are:
pH, H2S, Ammonia (NH3) , Carbonate ion concentration, sulfide to carbonate ratio.
ACSCC does not occur below pH 7. Susceptibility exists in the pH _____ to _____ range; however, most failures have occurred in the pH range of _____ to ______.
7.5 to 11 ; 8 to 10
Streams with higher NH3 and lower H2S will have a higher pH, which increases the percentage of total carbonate present as carbonate ion and thereby
increases the likelihood of ACSCC.
Cracking occurs above a certain threshold concentration of carbonate ions. It has been suggested that levels above
100 ppmw
Low ratios of S2− to CO32− weaken the protective sulfide scale on steel and
increase the likelihood of cracking.
Cracking can occur with or without the presence of
cyanides
There is no evidence to suggest that ammonium polysulfide injection
increases or decreases the potential for ACSCC.
FCC unit feed quality and unit operation appear to have an effect on cracking susceptibility. In general, cracking is associated with low ________ and __________ ; on the other hand ______ is usually higher in cases where ACSCC
Sulfur and S-to-N Ratio (less than 5) ; Total nitrogen
In FCC unid Full Burn vs Partial Burn
Full burn increases the regenerator O2 level, which increases the amount of CO2 This can directionally increase the tendency for cracking
most prevalent Affected Units or Equipment are __________________________________. Adicionally ACSCC has been observed in ________________________ units
main fractionator in FCC units, however, piping, vessels, and floor of tanks are also affected. ; pumparound-type SWS
Carbonate cracks are surface breaking and typically propagate
parallel to the weld in the HAZ or adjacent base metal within 2 in. (50 mm) of the weld.
The pattern of cracking on steel surface is sometimes described as a________________, which often initiate at or interconnect with ___________________
spider web of small cracks ; weld-related flaws that serve as stress risers.
Carbonate cracks are usually found further away from the toe of the weld than cracks resulting from __________ and may show up as multiple cracks parallel to a weld.
SSC or SOHIC
Carbonate cracks are predominantly ____________and typically appear as a network of very fine, oxide-filled cracks similar in appearance to caustic SCC and amine SCC.
- Transgranular
- Intergranular
- Intergranular
Application of a post-fabrication stress-relieving heat treatment at ___________ is a method of preventing ACSCC
1200 °F to 1225 °F (650 °C to 665 °C)
Cracking can be eliminated through the use of effective barrier coatings as:
solid or clad 300 series SS, Alloy 400, or other corrosion-resistant alloys
Sampling and analysis fot monitoring parameters as _____________ is the most practical and cost-effective method to locate areas where there is a potential for carbonate cracking to occur
pH, CO32 levels
Although cracks may be seen visually, crack detection is best performed with _____________ on the other hand _______________ is not recommended
WFMT or ACFM, Angle beam UT (SWUT or PAUT) and ECT, and AET ; PT
Electrical resistance type instruments that measure crack depth are
not effective because the cracks typically contain magnetic iron oxide.
Field welds may have ____________ for cracking if high restraint is introduced when making the joint fit-up.
higher potential
