Caustic Cracking | Materials And Corrosion Control
| Damage Mechanism | Caustic Cracking |
| Damage Description | · Caustic cracking is a form of stress corrosion cracking characterized by surface cracks t initiating in piping and equipment exposed to caustic, primarily adjacent to non-PWHT’d welds.
· Cracking is sometimes related to the use of caustic injection in crude units as well as MEROX treatments for MERCAPTAN removal. · Cracking of stainless steels has been related to caustic carryover from MEROX systems & possibly within deadlegs, following soda ash neutralization · Cracking is always intergranular in carbon steels but can be either intergranular or transgranular in stainless steels and is therefore very difficult to separate from Cl-SCC of SSs. · Concentration and Temperature Effects on Cracking Probability: (1) Caustic concentration exceeding 5 wt% in the aqueous phase can produce SCC in CS. Caustic SCC can occur at lower concentrations, where local concentration effects occur; · Caustic is used in boiler feed water treatment to control the pH. The amount added is usually harmless to carbon steel but under steam blanketing conditions is known to cause caustic gouging or even SCC. (See NALCO book) |
| Affected Materials | Carbon steel, low alloy steels and 300 Series SS are susceptible. Nickel base alloys are more resistant but still susceptible for given conditions of high caustic concentration, high temperature, and high residual stresses. Commercially pure Nickel is immune. |
| Control Methodology | · Cracking can be effectively prevented in carbon steels by means of a stress-relieving heat treatment (e.g., PWHT, up to a given service temperature that depends on caustic concentration). A heat treatment at 1150°F (621°C) is considered an effective stress relieving heat treatment for carbon steel. The same requirement applies to repair welds and to internal and external attachment welds. · Above certain caustic concentrations and relatively high service temperatures, 300 Series SS offer little advantage in resistance to caustic cracking over CS. · Nickel base alloys are more resistant to cracking and may be required at higher temperatures and/or caustic concentrations. · Steam out of non-PWHT’d carbon steel piping and equipment must be avoided. Equipment should be water washed before steam out to remove all traces of caustic. Where steam out is required, only low-pressure steam should be used for short periods of time to minimize exposure. · In crude units, caustic is sometime injected downstream the desalters as a primary neutralizationprocedure. Proper design and operation of the caustic injection systems is required to ensure that caustic is properly dispersed before entering the high-temperature crude preheat system. |
| Monitoring Techniques | · Inspect for cracking at weld HAZs at deadlegs/drains & other locations where caustic concentrations could occur
· Crack detection is best performed with WFMT, EC, RT or ACF techniques. Surface preparation by grit blasting, high pressure water blasting or other methods is usually required. · Crack depths can be measured with a suitable UT technique including external SWUT. · AET can be used for monitoring crack growth and locating growing cracks. |
| Inspection Frequency | Internally inspect vulnerable locations every T&I |
| KPIs | · Ensure PWHT’d CS piping & equipment is specified within caustic injection piping circuits or where caustic carryover can occur · Document process upsets that could lead to caustic carryover |
| Reference Resources (Standards/GIs/BPs) | · API RP 571 (DM #18)
· API RP 581-2008 · NACE SP0403-2008 |
Ammonium Bisulfide Corrosion | Materials And Corrosion Control(Opens in a new browser tab)
Wet H2S Cracking | Materials and Corrosion Control(Opens in a new browser tab)