3.18 CO2 Corrosion

Question 1

CO2 corrosion results when CO2 dissolves in water to form_____________. The acid may lower the pH, and sufficient quantities may promote ______________________ of carbon steel.

Answer 1

carbonic acid (H2CO3) ; general corrosion and/or pitting corrosion

Question 2

Affected Materials

Answer 2

Carbon steel and low-alloy steels

Question 3

Increasing the level of chromium in steels offers no major improvement in resistance until a minimum of _________ is reached, i.e. _______________________is highly resistant to CO2 corrosion.

Answer 3

12 % Cr ; Type 410 SS. 300 series austenitic SS

Question 4

Critical Factors (7 factors)

Answer 4

1. Liquid water must be present
2. Partial pressures of CO2.
3. pH
4. locations where CO2 condenses from the vapor phase.
5. Temperature.
6. Oxygen contamination..
7. Velocity and turbulence of fluids.

Question 5

Increasing partial pressures of CO2 result in ________ pH and, therefore, _______________

Answer 5

lower , higher rates of corrosion

Question 6

Corrosion occurs in the liquid water phase, often at locations where CO2 _____________________

Answer 6

condenses from the vapor phase

Question 7

Increasing temperatures _____________corrosion rate.

Answer 7

increase ; ( up to the point where CO2 is driven off.)

Question 8

Oxygen can _____________ corrosion rates. Oxygen should be limited to __________ to avoid accelerating corrosion

Answer 8

acelerate ; 10 ppb

Question 9

High velocity and turbulence can cause _________________

Answer 9

accelerated, localized corrosion

Question 10

Affected Units or Equipment

Answer 10

1. BFW and condensate systems.
2. shift converters in hydrogen plants
3. Overhead systems of regenerators in CO2 removal plants
4. crude towers
5. areas downstream of control valves, and changes in piping direction or diameter.
6. Locations where a cooling effect can cause condensation

Question 11

In hydrogen plants, the corrosion usually occurs when the effluent stream drops below the dew point at approximately ___________. Corrosion rates as high as ______________ have been observed.

Answer 11

300 °F (150 °C) ; 1000 mpy

Question 12

Locations where high velocity, impingement, or turbulence can create increased susceptibility include areas downstream of ___________, and changes in piping direction (e.g. at __________) or piping diameter (i.e. at _____________ ).

Answer 12

control valves ; elbows and tees ; reducers

Question 13

Locations where a cooling effect can cause condensation and resultant CO2 corrosion include where insulation is ___________, where portions of blind flanged nozzles extend ________________ and thus cool below the dew point, and where pipe supports ______________

Answer 13

damaged; beyond the insulation ; attach to piping

Question 14

Localized ________________ and/or _________________ normally occurs in carbon steel.

Answer 14

general thinning ; pitting corrosion

Question 15

Corrosion generally occurs or is worse in areas of turbulence and impingement. It is sometimes seen at the _________________. Carbon steel may suffer deep pitting, grooving, or smooth “wash out” in areas of turbulence.

Answer 15

root of piping welds

Question 16

Corrosion may initiate where water first condenses and may be ___________severe at water/vapor interfaces

Answer 16

most

Question 17

It may appear as a number of flat-bottomed pits, sometimes called “____________”-type pitting

Answer 17

mesa

Question 18

For prevention/Mitigation

Answer 18

1. Corrosión inhibitors.
2. Increasing condensate pH above 6.
3. upgrading to stainless steel (300, 400 series and duplex are hifhly resistant).
4. correcting or improving the operating conditions and/or water treatment program.
5. Ensure insulation and jacketing are in good condition.
6. Internal coatings

Question 19

VT, UT, and RT (preferably profile RT) can be used for ________________ in thickness where water wetting is anticipated. Permanently mounted thickness monitoring sensors can be used.

Answer 19

general and local loss

Question 20

For inspections of preferential corrosion of welds may require

Answer 20

angle beam UT (SWUT or PAUT) or RT.

Question 21

Monitor water analyses as ______________ to determine changes in operating conditions.

Answer 21

(pH, Fe, O2, etc.)

Question 22

The use of______________ can be effective for locations with limited or no direct line-of-sight (e.g. in boiler tubes

Answer 22

remote video probes