3.18 CO2 Corrosion Flashcards
CO2 corrosion results when CO2 dissolves in water to form_____________. The acid may lower the pH, and sufficient quantities may promote ______________________ of carbon steel.
carbonic acid (H2CO3) ; general corrosion and/or pitting corrosion
Affected Materials
Carbon steel and low-alloy steels
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.
12 % Cr ; Type 410 SS. 300 series austenitic SS
Critical Factors (7 factors)
- Liquid water must be present
- Partial pressures of CO2.
- pH
- locations where CO2 condenses from the vapor phase.
- Temperature.
- Oxygen contamination..
- Velocity and turbulence of fluids.
Increasing partial pressures of CO2 result in ________ pH and, therefore, _______________
lower , higher rates of corrosion
Corrosion occurs in the liquid water phase, often at locations where CO2 _____________________
condenses from the vapor phase
Increasing temperatures _____________corrosion rate.
increase ; ( up to the point where CO2 is driven off.)
Oxygen can _____________ corrosion rates. Oxygen should be limited to __________ to avoid accelerating corrosion
acelerate ; 10 ppb
High velocity and turbulence can cause _________________
accelerated, localized corrosion
Affected Units or Equipment
- BFW and condensate systems.
- shift converters in hydrogen plants
- Overhead systems of regenerators in CO2 removal plants
- crude towers
- areas downstream of control valves, and changes in piping direction or diameter.
- Locations where a cooling effect can cause condensation
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.
300 °F (150 °C) ; 1000 mpy
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 _____________ ).
control valves ; elbows and tees ; reducers
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 ______________
damaged; beyond the insulation ; attach to piping
Localized ________________ and/or _________________ normally occurs in carbon steel.
general thinning ; pitting corrosion
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.
root of piping welds
Corrosion may initiate where water first condenses and may be ___________severe at water/vapor interfaces
most
It may appear as a number of flat-bottomed pits, sometimes called “____________”-type pitting
mesa
For prevention/Mitigation
- Corrosión inhibitors.
- Increasing condensate pH above 6.
- upgrading to stainless steel (300, 400 series and duplex are hifhly resistant).
- correcting or improving the operating conditions and/or water treatment program.
- Ensure insulation and jacketing are in good condition.
- Internal coatings
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.
general and local loss
For inspections of preferential corrosion of welds may require
angle beam UT (SWUT or PAUT) or RT.
Monitor water analyses as ______________ to determine changes in operating conditions.
(pH, Fe, O2, etc.)
The use of______________ can be effective for locations with limited or no direct line-of-sight (e.g. in boiler tubes
remote video probes
