Top of the Line Corrosion (TLC) | Materials And Corrosion Control - PDFYAR

Top of the Line Corrosion (TLC) | Materials And Corrosion Control

Damage Mechanism	Top of the Line Corrosion – TLC
Damage Description	· Corrosion of steel in wet multiphase gas lines operated in stratified flow due to water condensation as a result of external cooling. · Other important parameters for top-of-line corrosion are operating temperature, condensation rate, CO₂ and H₂S ratio, and content of organic acids. · Normally general corrosion but can be localized.
Affected Materials	Primarily Carbon Steel
Control Methodology	· Restrict use of organic acids for well stimulation. · Use nickel based alloys (solid or cladding), internal coating with continuous and batch chemical inhibition, or continuous and batch chemical inhibition. · Use heat insulation in exposed areas of the pipeline. · Forced condensation to prevent TLC in mainlines
Monitoring Techniques	· Inline Inspection (ILI) · Monitor inhibitor residual · OSI · Corrosion probes and coupons
Inspection Frequency	· 3-5 years for ILI · UT, coupons and visual inspection at T&I
KPIs	· Corrosion rate of less than 5 mpy for inhibited system. · Inhibitor residual (100 ppm)
Reference Resources (Standards/GIs/BPs)	· OU TLC JIP

Additional Information

Major improvements have been made in understanding sweet TLC mechanisms over the past
10 years:

1. Crucial influence of the condensation process: No TLC can be expected if the water condensation rate is equal or less than 0.25 ml/m²s in pure CO₂ environment and
  025 ml/m²s if acetic acid is present. The presence of non-condensable gases greatly decreases the water condensation rate. The difference between the pipe wall temperature (at which corrosion reactions take place) and the gas temperature can be quite high. Increasing the gas velocity increases the water condensation rate. There is no critical water acetic acid level for TLC; the more acid there is, corrosion is expected to get worse.
2. Good understanding of chemical and electrochemical reactions involved: The concentration of un-dissociated acetic acid at the bottom of the line is similar to the concentration of total acetate in the condensed liquid.
3. Successful effort to model the processes and enable predictions: TLC can occur only in stratified or wavy stratified flow.
4. H₂S (up to a certain extent) has a beneficial effect on sweet TLC.

TLC stabilization (i.e., corrosion rate reduction rather than complete stoppage) is influenced by:

1. Interaction between the FeCO₃ scale and the bottom of the pit
2. Change in chemistry close to the steel surface (acetic acid depletion)
3. Gradient of temperature between the bottom of the pit and surrounding steel
4. Hydrocarbon wetting (gravity, turbulence and entrainment)

1. Localized corrosion behavior (pit initiation, propagation, stabilization)
2. Sour TLC (mechanism is complex as the condensed fluid chemistry is likely to be more corrosive than the bulk fluid, it intensifies in the presence of volatile acids and is very difficult to inhibit; inhibition may not happen under low flow conditions.)
3. Hydrocarbon co-condensation
4. Droplet transport