3.12 Carbonate Stress Corrosion Cracking

Question 1

Carbonate SCC, or alkaline carbonate SCC (ACSCC), referred to as carbonate
cracking, occurs in process units containing:

Answer 1

free water phase with carbonate ions and some amount
of H2S is invariably also present.

Question 2

Affected Materials are

Answer 2

Carbon steel and low-alloy steels with welds or cold-worked areas.

Question 3

A critical factor refers to weld or cold -Work area must have ________ and must be in contact with __________.

Answer 3

tensile residual stress ; liquid water

Question 4

The _________ is a critical factor affecting the likelihood of carbonate SCC

Answer 4

water chemistry

Question 5

Carbonate cracking can occur at relatively low levels of residual stress, but it usually occurs at

Answer 5

welds or coldworked areas that have not been stress relieved.

Question 6

Critical factor are:

Answer 6

pH, H2S, Ammonia (NH3) , Carbonate ion concentration, sulfide to carbonate ratio.

Question 7

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 ______.

Answer 7

7.5 to 11 ; 8 to 10

Question 8

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

Answer 8

increases the likelihood of ACSCC.

Question 9

Cracking occurs above a certain threshold concentration of carbonate ions. It has been suggested that levels above

Answer 9

100 ppmw

Question 10

Low ratios of S2− to CO32− weaken the protective sulfide scale on steel and

Answer 10

increase the likelihood of cracking.

Question 11

Cracking can occur with or without the presence of

Answer 11

cyanides

Question 12

There is no evidence to suggest that ammonium polysulfide injection

Answer 12

increases or decreases the potential for ACSCC.

Question 13

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

Answer 13

Sulfur and S-to-N Ratio (less than 5) ; Total nitrogen

Question 14

In FCC unid Full Burn vs Partial Burn

Answer 14

Full burn increases the regenerator O2 level, which increases the amount of CO2 This can directionally increase the tendency for cracking

Question 15

most prevalent Affected Units or Equipment are __________________________________. Adicionally ACSCC has been observed in ________________________ units

Answer 15

main fractionator in FCC units, however, piping, vessels, and floor of tanks are also affected. ; pumparound-type SWS

Question 16

Carbonate cracks are surface breaking and typically propagate

Answer 16

parallel to the weld in the HAZ or adjacent base metal within 2 in. (50 mm) of the weld.

Question 17

The pattern of cracking on steel surface is sometimes described as a________________, which often initiate at or interconnect with ___________________

Answer 17

spider web of small cracks ; weld-related flaws that serve as stress risers.

Question 18

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.

Answer 18

SSC or SOHIC

Question 19

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

Answer 19

• Intergranular

Question 20

Application of a post-fabrication stress-relieving heat treatment at ___________ is a method of preventing ACSCC

Answer 20

1200 °F to 1225 °F (650 °C to 665 °C)

Question 21

Cracking can be eliminated through the use of effective barrier coatings as:

Answer 21

solid or clad 300 series SS, Alloy 400, or other corrosion-resistant alloys

Question 22

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

Answer 22

pH, CO32 levels

Question 23

Although cracks may be seen visually, crack detection is best performed with _____________ on the other hand _______________ is not recommended

Answer 23

WFMT or ACFM, Angle beam UT (SWUT or PAUT) and ECT, and AET ; PT

Question 24

Electrical resistance type instruments that measure crack depth are

Answer 24

not effective because the cracks typically contain magnetic iron oxide.

Question 25

Field welds may have ____________ for cracking if high restraint is introduced when making the joint fit-up.

Answer 25

higher potential