Protection of Austenitic Stainless Steel

1. Purpose

This procedure addresses protection of austenitic stainless steel equipment and piping from polythionic acid stress corrosion cracking (SCC) during shutdown of refinery equipment.

2. General

Since polythionic acid SCC of austenitic stainless steel may lead to failure of the equipment and piping involved, it is of the utmost importance to protect this equipment. All operating personnel, especially supervisory personnel, shall know the procedures used to protect equipment; know the locations of piping and equipment fabricated from austenitic stainless steel; and recognize the need for special handling of these sections of the unit during startup, shutdown, flushing, cleaning, maintenance and inspection.

This Procedure makes frequent reference to NACE Standard Practice SP0170. The user of this Procedure (IPE-TM-700-05) is encouraged to obtain a copy of the latest edition of SP0170 and become knowledgeable of its requirements.

3. Austenitic Stainless Steels

3.1 Description

Austenitic stainless steels are those of the "300 series," the compositions of which are nominally 18% chromium and 8% nickel. The most common types used in the petroleum refining industry are Types 304, 316, 321 and 347. Because of their inherent high temperature strength properties and high corrosion resistance, they are used in hydrocracking, hydrodesulfurization, and hydrodealkylation units in areas of moderate/high temperature and where substantial resistance to sulfur and hydrogen sulfide corrosion is required (e.g., heater tubes, reactors, reactor effluent exchangers, and piping).

3.2 Chloride SCC

The presence of halides (chlorides are usually the most serious offenders) along with an aqueous water phase and tensile stresses may result in SCC of austenitic stainless steels. This type of cracking is predominantly transgranular and is somewhat dependent on time, temperature and chloride concentration. Take precautions to minimize the amount of chloride in the process material that comes in contact with austenitic stainless steel equipment. Under normal shutdown period conditions, chloride cracking is not likely to be a problem as long as chlorides are not allowed to accumulate and concentrate in hot equipment and the chloride content is limited to low levels in any flushing, purging or neutralizing agents used in the system.

The reader is directed to Section 4, paragraph 4.3 of NACE “Standard Practice SP0170” for additional important information.

3.3 Polythionic Acid SCC

There are five factors that need to be present simultaneously for polythionic acid SCC to occur. These are:

• Sensitized microstructure
• Sulfide scale on the metal
• Liquid water
• Oxygen
• Tensile stress (residual or applied).

Polythionic acid is formed when water condenses on the surface of a metal that has a sulfide scale. The water reacts with the scale to form an acid of the type H2SXO6. This acid then attacks the grain boundaries of a sensitized austenitic stainless steel, and tensile stress causes intergranular cracking.

Elimination of any one of the five factors will mitigate the occurrence of polythionic acid SCC. In practice, however, it is usually not possible to prevent sensitization (due to long-term, elevated temperature exposure), sulfide scale (due to the process environment), or tensile stress (residual stress, piping stresses, etc.). In some cases it may be possible to eliminate liquid water (see Sections 4.3, 6.1a, 6.1b, 6.1d, and 6.2d), or oxygen (see 4.1, 4.2).

If one (or more) of the five factors cannot be eliminates, then downtime neutralization with soda ash solution is necessary.

3.4 Protection Against Polythionic Acid SCC

Protection against polythionic acid SCC can be achieved by preventing the corrosive environment from forming or providing an agent that will neutralize any corrosive acids in-situ as they are formed.

The reader is directed to Section 2 of NACE Standard Practice SP0170 for additional important information.

a. Preventing the Formation of Polythionic Acid

The formation of polythionic acid can be prevented by precluding the condensation of water (heat, N2, dry air), or excluding oxygen/air (N2, ammoniated N2).

Other than a startup immediately following a catalyst regeneration (where there may be significant amounts of oxygen present before purging), there is essentially no oxygen present in the system during normal operation. During shutdown periods, however, oxygen will enter when the system is depressured and the equipment is open and exposed to air. Maintaining a nitrogen purge throughout the exposed equipment and piping will prevent the ingress of oxygen (air) and thus prevent the formation of polythionic acid. If possible, blind or blank-off the equipment during this period and keep it under a slight positive pressure of nitrogen. Refer to sections 4.1 and 4.2 of this procedure

b. Neutralization

If it is not possible to maintain temperatures above the dew point of water or to provide an adequate nitrogen purge, establish a protective neutralizing environment in the austenitic equipment and piping by purging with and maintaining an ammoniated nitrogen blanket or by washing with a dilute soda ash solution prior to exposure to air.

4. Purging and Neutralizing

4.1 Nitrogen

Use good quality commercial grade nitrogen for purging and protection of austenitic stainless steels. Nitrogen is to be dry with a maximum oxygen content of 1000 ppm. If the nitrogen available has an oxygen content greater than 1000 ppm, or if the oxygen content is unknown, use ammoniated nitrogen.

The reader is directed to Section 3 of NACE Practice SP0170 for additional important information.

4.2 Ammoniated Nitrogen

When ammonia is added to the reactor system, check the ammonia content of the recycle gas. The catalyst is expected to absorb a considerable amount of ammonia, and additional ammonia is required to maintain 5000 ppm concentration until the system reaches equilibrium.

When the system is at high pressure, a high pressure "blow case" may be used. Liquid ammonia is pressurized into the blow case at low pressure from the ammonia cylinder. The blow case is isolated. Use high pressure gas from the discharge of the recycle gas compressor to pressure up the blow case and to force the ammonia into the system at a location of lower pressure.

To preserve catalyst activity in the reactors, do not pass ammonia over the catalyst when it is in its oxidized form (e.g., whenever the catalyst is either fresh or freshly regenerated). When dealing with platinum-type catalyst, avoid ammonia regardless of the state of the catalyst.

Brass and most other copper alloys are subject to SCC from ammonia. Make arrangements to isolate equipment and piping from the system before admitting any ammonia.

Be familiar with the toxic nature of ammonia, and follow appropriate safety precautions. For example; equip workers when opening flanges or manways with fresh air masks or other oxygen breathing equipment.

4.3 Dry Air

The use of dry (dehumidified) air may be a practical approach to prevent the condensation of water and thus mitigate the potential for polythionic acid SCC. The reader is directed to Section 5 of NACE Standard Practice SP0170 for additional important information.

4.4 Soda Ash Solutions

a. Composition

Prepare aqueous neutralizing solutions of soda ash (Na2C03) in the range of 2% to 5% by weight. Preheat the water to about 120F (49ºC) to dissolve the soda ash and promote penetration of coke, scale, or oil films. A 2% solution usually provides a sufficiently high alkaline level to neutralize any reasonable amount of polythionic acids that may be formed. Limit the chloride content in the freshly mixed soda ash solution. As added protection against the small amount of chloride present in the neutralizing solution, add 0.5% by weight of sodium nitrate to the soda ash solution.

The addition of an alkaline surfactant to the wash solution at a 0.2 wt % concentration is recommended to promote penetration of deposits (e.g., coke, scale or oil films).

The reader is directed to Section 4 of NACE Standard Practice SP0170 for additional important information.

b. Neutralization Techniques

When a soda ash solution is used for neutralizing and protecting austenitic stainless steel, fill the piping or piece of equipment involved completely full with the solution. Allow the equipment and piping to soak for a minimum of two hours before completely draining the soda ash solution and exposing the equipment and piping to the air. If any pockets or unvented high areas in the equipment and piping are unreachable with the soda ash solution, vigorously circulate the solution through the equipment to ensure thorough contact of all austenitic stainless steel surfaces. Continue this circulation for a minimum period of two hours before draining and exposing the equipment and piping to air.

For extremely large surfaces, such as reactor or large vessel walls and internals, where filling with soda ash solution is not only impractical but in some cases impossible because of foundation load limitations, wash the areas thoroughly by means of high pressure hose equipped with a spray nozzle. This type of washing is done after the vessel has been opened to allow entry. When neutralization is accomplished via spraying, the spraying must be done as soon after the equipment is opened as practical.

c. Protective Film

In all cases of flushing or washing with soda ash solution, allow surfaces to dry until a film or fine deposit of soda ash remains. Since all equipment and piping is not free draining, it is important to thoroughly dry all equipment and piping so no pockets of soda ash solution remain. Do not rinse the system with steam or water. The small amount of soda ash remaining on the surfaces does not have a significant effect on the activity of any but the platinum-type catalysts.

d. Precautions with Platinum Catalysts

To prevent sodium contamination of a platinum-type catalyst, remove any soda ash film just prior to reloading the catalyst or just prior to connecting the austenitic stainless steel equipment back into the reactor circuit. Before removing the film, check that the equipment involved is isolated from the reactor to prevent any contact of the catalyst by soda ash. Purge the system with nitrogen to remove all oxygen. While maintaining a nitrogen blanket on the system, flush the system thoroughly with clean, deaerated condensate to remove the soda ash deposits. Continue to maintain a nitrogen blanket on the system and drain the condensate. Purge with nitrogen at maximum rate to remove any remaining pockets of water. Where austenitic stainless steel heater tubes are involved, light small fires as soon as the nitrogen purging is started and adjust for a nominal 400F (204C) firebox temperatures to thoroughly dry the tubes. While maintaining sufficient nitrogen purges from both sides of the (closed) flanges, make all necessary connections to prepare the unit for startup and normal operation.

5. Hydrotesting

5.1 New Austenitic Stainless Steel

To reduce the possibility of concentrating chlorides in pockets or dead areas of the system, use water with a chloride content not exceeding 50 ppm by weight to conduct hydrostatic tests on new equipment and piping. If the only water available has a chloride content in excess of 50 ppm up to a maximum of 250 ppm, add 0.5% of sodium nitrate.

5.2 Used Austenitic Stainless Steel

When a piece of equipment and piping is used for the processing of hydrocarbons in hydrocracking, hydrodesulfurization or hydrodealkylation service, assume that some degree of sulfide scale may be present, even though the equipment and piping may have been cleaned by mechanical means, burning, or acidizing. Even if the sulfide scale is so slight that it is difficult to detect, possible polythionic acid formation may result in intergranular SCC. Conduct any hydrostatic testing operations (and any cleaning by hydroblasting) on used equipment and piping with the dilute soda ash solution specified for neutralizing this equipment and piping. Allow a protective film of dried soda ash to remain on the surfaces of the equipment and piping while it is exposed to the air.

6. Special Procedures

6.1 Reactor Charge Heater Tubes

a. Maintaining Small Fires

To protect the austenitic stainless steel tubes in a reactor charge heater, maintain a balanced set of small fires (or pilots, as applicable) in the heater box at all times, even when there is no circulation of process material through the tubes. Adjust these small fires to keep the tubes warm and dry, to maintain the environment inside the tubes above the dewpoint of water. Fires at about 400F (204C) (as measured by thermocouples placed in the hip sections of the heater, placed directly below any convection coils that may exist) are usually sufficient for this purpose. However, determine the dew point for each specific condition involved and adjust the temperature as necessary. Use only fuel gas firing for this operation because of the difficulty in controlling and maintaining sufficiently small flames when burning fuel oil.

Keep the heater firing under strict control and establish a proper firing pattern to provide good heat distribution. Install sufficient thermocouples throughout the hip sections of the heater to provide a good measurement of the firebox temperatures and to monitor the distribution of heat in the firebox. Locate these thermocouples below any convection bank in the heater, and connect them to a continuous recorder provided with high and low alarm points. Set the low alarm point at about 300F (149ºC) and the high alarm point at about 450F (232ºC). Never use stack temperatures to control firebox temperatures.

b. To Shut Down Fires

Shut down the fires in a charge heater containing austenitic stainless steel tubes only when absolutely certain that the tubes do not contain both oxygen and water/water vapor. An equilibrium amount of water is usually present as a result of the operation of the reactor effluent wash facilities, and during or after a period of in-situ catalyst regeneration. If the heater fires must be shut down, do so during a period of normal operation, when oxygen is usually not present. As the heater tubes cool the small amounts of water condensing inside the tubes will not be harmful.

If trace quantities of oxygen are suspected, depressurize the system completely before cooling the heater. Do not evacuate the system because air may enter. Continue to maintain the 400F (204ºC) heater box temperatures while depressurizing and purging. After the system is depressurized, pressure with nitrogen to any convenient pressure level. Repeat this depressurizing/pressurizing procedure as many times as required to reduce the oxygen concentration, by dilution, to as much below 100 mol-ppm as is possible and reasonable. Shut down the heater and allow the fires to cool.

c. Neutralization

If neutralization is necessary (e.g., a tube or tubes are cut out of the coil; exposure to air at temperatures below the dew point of water cannot be avoided), fill the tubes with soda ash solution and allow them to soak for a minimum of two hours.

It is not possible to completely fill the unvented upper return bends of vertical coils, and in such case it is necessary to vigorously circulate the soda ash solution through the tubes for a minimum of two hours to ensure contact of all surfaces. After draining the soda ash solution, do not flush with steam or water but allow a film of protective soda ash to remain in the tubes.

After completion of soda ash washing, all of the wash solution must be drained from low points (e.g. bottom return bends) before returning the equipment to service. Failure to do so can lead to concentration of carbonate and chloride salts by evaporation, causing SCC.

d. Exterior Surfaces

When heater fires are shut down and the tubes are allowed to cool, protection of the exterior tube surfaces, especially in heaters where fuel oil or high sulfur content fuel gas is employed, may be necessary. The reader is directed to Section 1, paragraph 1.8 of NACE Standard Practice SP0170 for additional important information. Two procedures may be followed to prevent this.

First, prevent moisture from condensing by purging the firebox with copious amounts of dry air. Normal instrument air is prepared by processing it through a set of driers where the dew point is reduced to a sufficiently low level to prevent condensation from occurring at ambient conditions. Use instrument air to maintain a dry air blanket in the heater box both during cooling and throughout the entire period the fires are out. In order to minimize the consumption of instrument air and to prevent moist air from entering the heater box, close the stack damper, all burner air, registers, and all doors and ports in the heater box.

An alternate method of protecting the exterior tube surfaces is to cover the exterior tube surfaces with a protective film of soda ash of the same dilute solution recommended for general neutralization. Apply the soda ash to the tube surfaces as soon as the heater box has cooled sufficiently to prevent vaporizing the soda ash solution, and preferably before any moisture has begun to condense out on the tube surfaces. A fairly efficient and effective method of applying the soda ash solution is to use a vat or tank with a small portable pump and spray nozzle that produces a fairly fine mist. A low pressure spray is advisable as high pressure may erode the refractory. Small diameter pipe extensions may be fitted to the hose to allow reaching up to the tube areas at the top of the heater box. This type of spray equipment minimizes the soda ash consumption and provides a reasonable means to reach all tube surfaces that are exposed to the heater flames. Once the soda ash solution has been applied, allow it to dry to form a protective film on the surfaces of the tubes. Do not wash off this protective film.

If the exterior tube surfaces are heavily coated with an oxide or carbonaceous material, remove it by wire brushing or sand blasting. This cleaning also removes any protective soda ash film. Apply another film of soda ash immediately.

6.2 Fractionator Heater Tube

When fractionator heater tubes are made of austenitic stainless steel and it is necessary to work on the fractionator section, prepare for opening the fractionator column in the following manner:

a. Pump or pressure all oil from the flash drum or fractionator feed drum into the fractionator and out to storage or slop. Fire the heater to maintain a firebox temperature (as measured by thermocouples placed in the hip section of the heater) at about 400-600 F (204-316ºC) to keep the tubes warm and dry and to ensure that the steam used to purge the system does not condense in the tubes. Get operator and supervisor input at this time.

b. Drain all condensate from the steam lines. Open the steam valves at the inlet of the fractionator heater on all heater passes. Use superheated steam. Simultaneously increase the heater firing to heat the steam above its saturation temperature, preventing condensation in the tubes. A heater transfer temperature of about 600F (316ºC) is preferred as long as the temperature limitations of the heater or the fractionator system are not exceeded.

c. After the system has been thoroughly purged with steam, stop the steam and immediately start a nitrogen purge through the coils and into the column to sweep out any remaining steam. The nitrogen flow is usually less than the steam flow, and as a result a corresponding reduction in heater firing will be necessary.

d. If the heater is not to be entered, reduce the heat only to the point where a 400F (204ºC) firebox temperature is maintained. If the heater is entered, cut the fires and maintain a continuous nitrogen purge through the tubes. If necessary protect the exterior tube surfaces from polythionic acid attack whenever the heater fires are shut down, in accordance with the recommendations in Section 6.1.d.

e. If the fractionator column is opened, install blinds at the column to isolate the heater coils and maintain a positive pressure of nitrogen in the coils. If heater tubes are cut or the insides of the tubes are exposed to air, thoroughly soak/flush the tubes with soda ash solution, as discussed in Section 6.1.c.

6.3 Heat Exchangers

If lines leading to or from heat exchangers containing austenitic stainless steel are opened, rapidly insert blinds to isolate the exchanger. Maintain a nitrogen blanket or continuous nitrogen purge in the exchanger throughout this maintenance period.

If shell and tube exchangers containing austenitic stainless steel are opened and inspected, or if the tube bundles are pulled, flood both shell and tube sides with soda 1ash solution and soak for a minimum of two hours before exposing this equipment to air. If there are any pockets or high areas that cannot be reached with the soda ash solution, vigorously circulate the solution through the exchanger for a minimum of two hours. Do not rinse with water, but instead allow a film of soda to remain on the surfaces.

Protect shell openings from ingress of water. Neutralized bundles, if removed, shall be kept dry and protected from the weather.

If tube bundles of austenitic stainless steel are cleaned by hydroblasting, use soda ash solution for this purpose instead of water alone.

6.4 Reactor Internals

When a reactor is opened, maintain a sufficient nitrogen purge to prevent the entry of air into any part of the system. Isolate the charge heater coils and the reactor effluent system with blinds. Maintain a blanket of nitrogen in the reactor, especially if it contains unregenerated catalyst. A slight amount of air coming in contact with the reactor internals for relatively short periods of time is normally not considered to be harmful to the metal; however, with air present take special precautions to prevent contact with water or moisture. Purge out any exposed areas with nitrogen as soon as possible.

When the reactor internals are exposed to air for a prolonged period of time, such as during a catalyst change, wash the reactor walls and internals promptly and thoroughly with a high pressure hose, using copious amounts of soda ash solution. A portable pump and a vat of soda ash solution on skids is advisable for this operation. A worker equipped with a fresh air mask and following safety precautions may have to enter the vessel to ensure that all surfaces, including the underside of the top head, are thoroughly wetted. Pay attention when washing to welded areas, especially welds normally required to support heavy loads, such as those on support beams, grids and trays.

When the reactor contains trays which would make wetting all surfaces with soda ash solution difficult, spray a sufficient amount of soda ash solution around the top of the reactor and allow it to rain down through the reactor to wet as much of the surfaces as possible. Be sure to thoroughly soak and keep wetted any used catalyst remaining in the reactor. Air may then be drawn through the reactor so that personnel may enter. During this time maintain a small flow of soda ash solution to the reactor. As each tray manway is removed, wash with soda ash solution the vessel area beneath that tray and the underside of the tray.

Whenever spent, unregenerated catalyst is unloaded from a reactor, some amounts of catalyst inadvertently remain on the trays and in the bottom of the reactor. Keep this catalyst wet to prevent ignition of sulfide scale when air is admitted.

After washing with soda ash solution, allow the surfaces to dry with a fine deposit of soda ash.

6.5 Cooling Catalyst After Regeneration

When it is necessary to reduce the temperature in an austenitic stainless steel charge heater coils below the dew point of water when oxygen is present (e.g., when cooling the catalyst bed to a temperature which allows entry into reactor following a catalyst regeneration), first reduce the oxygen to an acceptable level. Maintain the final reactor temperature used in the regeneration. Continue gas circulation and depressure to the minimum allowable for recycle compressor operation. Pressure with nitrogen. Conduct this procedure at least three times, or as necessary to reduce the oxygen concentration in the circulating gas by dilution to as much below 100 mol-ppm as is possible and reasonable.

Maintain reactor temperatures and gas circulation, after shutting down and draining the caustic and water systems, until the reactor system is dry and no more water collects in the separator. The system may then be cooled to about 150F (66ºC) at the reactor outlet. Shut down the recycle gas compressor, but maintain heater fires at about 400F (204ºC) firebox temperatures throughout the shutdown period.

7. References

For additional information on the subject of the protection of austenitic stainless steel, see the NACE International Standard Practice SP0170, entitled "Recommended Practice, Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment." Obtain copies from NACE International, , or at .