Austenitic stainless steels are extensively used in hydroprocessing units of oil refineries because of their resistance to H2S+H2 attacks at high temperatures and pressures. However, during the shutdown period presence of metal sulfides leads to polythionic acid cracking. Polythionic Acid (PTA), (H2SXO6) is a complex acid formed by the reaction of sulfide of metals with water and oxygen during shutdown when the system is cooled, and oxygen and water can enter the system. The Polythionic Acid corrosion cracking is intergranular and occurs more readily in sensitized stainless steel.
In hydroprocessing units, the Polythionic Acid Stress Corrosion Cracking often occurs sometimes during shutdowns or turnaround. The reason is that during normal operation of the hydroprocessing unit, Iron or Chromium Sulfide layers are formed on austenitic stainless steel. These sulfide scales react with O2 and liquid water forming the polythionic acid that ultimately cracks the metals. The cracking initiates during the shutdown but then propagates to failure during startup or operation. The reaction of moisture and oxygen in the air with the sulfide scale form polythionic acid (H2SxO6), which then attacks the metal. Polythionic Acid can be formed in the presence of FeS through this reaction;
4FeS + 5.5O2 + H2O → 2Fe2O3 + H2S4O6 (Tetrathionic acid)
There are higher chances of Polythionic Acid attack if the austenitic material is sensitized. Sensitization of austenitic stainless steel means the precipitation of chromium carbide at the grain boundaries when they are in operation at high temperatures for a long duration. This precipitation happens because the carbides are insoluble at these temperatures. Carbide precipitation takes chromium from the surrounding metal and creates a chromium-depleted zone around the grain boundaries. The sensitized alloy is highly susceptible to polythionic acid attack due to weak grain boundaries and loss of metal strength. The temperature ranges of sensitization are given in Table 1.0
For an on-stream hydrotreating unit, all equipment and piping made of austenitic stainless steel should be considered to contain a layer of iron sulfide scale. In the sensitized austenitic stainless steel, there is always the risk of polythionic acid corrosion even if the sulfur content in the feed is low and the layer of metal sulfide scale is very thin. The stainless steels are at high risk of Polythionic Acid attack, especially in the areas of residual tensile stresses, heat-affected zones adjacent to welds, and sensitized steels.
Techniques to Avoid Polythionic Acid Attack
Polythioninc Acid Attack can be avoided by removing one of the items in the triangle, the Iron Sulfide Scale, Oxygen, and Water. Elimination of water or oxygen will prevent the formation of Polythionic Acid.
Various techniques are followed to avoid PTA as follows;
- Removal of water or oxygen by applying dry nitrogen purge. Nitrogen is used for the preservation of equipment, catalyst, and piping. Nitrogen used for purging and preservation should be dry and contain oxygen less than 1000 mol ppm, otherwise, it will provide the source of oxygen.
- When the Austenitic Stainless Steel system has to depressurize and the equipment is opened to air for maintenance activity or blinding then maintain positive Nitrogen pressure or Nitrogen purge inside the system to avoid air ingress.
- Use of dry air with a dew point less than -15 oC or 22 oC lower than the internal surface metal temperature. Normally, Instrument Air is used for the external surface of the heater but avoid using air for catalyst, equipment, and piping.
- Aqueous soda ash solutions (Na2CO3, Sodium Carbonate) in the range of 2~5 % are used for neutralization of sulfide layers present on the steel. A 1.5~2 % concentered solution is recommended for sufficient alkalinity on the metal surface. Soda ash solutions neutralize acidic layers and after draining and dry-out leave a thin alkaline film on the surface of the metal that can neutralize any additional acid formation. Further, it is vital that this alkaline film is not washed off by water or steam.
- The equipment or pipe made of stainless steel is filled completely with the solution and allowed to soak for a minimum of 02 hours. After that, it is drained and allowed to dry out so that an alkaline layer is formed.
- If there are pockets and deposits or sludges present in the system then, the solution should be vigorously circulated for a minimum of two hours.
- For large vessels or reactors where soda ash filling is difficult then it should be sprayed on the walls by means of a high-pressure nozzle.
- Hydrotesting and hydro jetting of the equipment made of Austenitic Stainless Steel should be done with the soda ash solution to avoid the removal of the layer when the water is used. After the activity, the equipment should be kept dry and out of the weather.
- If the maintenance job is on one section of the unit, then it is better to isolate this part from the system. Neutralize the part where the job is planned to perform while preserving the remaining system with a positive pressure of Nitrogen.
- Protection of the external surface of heater tubes made of austenitic stainless steel is considered when sulfur-containing fuels have been used for furnace firing. Austenitic stainless-steel tubes of the Fired heater can be externally protected by maintaining, the firebox temperature above the dew point ~ 200 °C. This is done by maintaining small fires by keeping alive pilot burners.
- The Chloride concentration in the neutralization solution should be limited to 250 ppm. Corrosion Inhibitor (0.4 wt % NaNO3) can be added to the solution to avoid stress cession cracking due to Chloride.
- If a temperature above the dew point cannot be maintained or the pilots’ burners have to be shut off for maintenance, then the firebox should be positively pressurized with Nitrogen or instrument air.
- Further, if the firebox has to be opened for inspection or repair of tubes during turnaround, then the tubes’ external surface is sprayed with soda ash solution. Hand Pump Sprayer Bottles can be used where the approach is difficult.
- For internal surface protection, the heater is isolated by blinding from another network, and soda ash solution is filled through the nozzles provided at the inlet and outlet of the heater. After the completion of soaking time, the solution can be removed by providing dry Nitrogen or Instrument Air purge through the tubes.
- Whenever a hydrotreating reactor is opened, a sufficient nitrogen purge should be maintained. Catalyst removal is carried out under a Nitrogen blanket from the reactor outlet to the top manway of the reactor. When the top manway along with the inlet diffuser is removed it should be immediately dipped in the soda ash solution already prepared in a tank. Short-time exposure of the inlet cap along with the diffuser is not considered harmful, but the actions should be immediate to avoid water condensation.
- After the catalyst has been removed from the reactor, the reactor is filled with soda ash solution and soaked for a minimum of two hours. During this time Nitrogen purging should be continued until the reactor has been filled with soda ash solution. During maintenance, use only soda ash solution if required for flushing of the reactor internals. Further, save the reactor internals from water and rainwater.
- In the same way, heat exchangers made of austenitic stainless steel should be isolated from another system and neutralized with the soda ash solution. Hydro jetting and hydro-testing should be done only with the soda ash solution.