Austenitic stainless steels are extensively used in hydrotreating 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 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) Polythionic Acid Formation  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. There is always the risk of polythionic acid attack 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. Reactor Section of Hydroprocessing Unit susceptible to Polythionic Acid Attack   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

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2. Avoiding Polythionic Acid Attack in
hydroprocessing Units
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polythionic-acid-attack-in-hydroprocessing-units/

3. Introduction
Austenitic stainless steels are extensively used in
hydrotreating 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 cracking is intergranular and occurs
more readily in sensitized stainless steel.
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4. Formation of PTA
• 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.
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5. Reaction of PTA
• 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)
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6. Sensitization of Austenitic Stainless Steel
• 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
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7. Equipment Made of Austenitic Stainless Steel
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. There is
always the risk of polythionic acid attack 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.
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9. 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.
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10. Techniques to Avoid Polythionic Acid Attack
• 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. For further details view the blog;
• https://thepetrosolutions.com/avoiding-polythionic-acid-
attack-in-hydroprocessing-units/
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11. Avoiding Polythionic Acid
Attack in hydroprocessing
Units
https://thepetrosolutions.com/avoi
ding-polythionic-acid-attack-in-
hydroprocessing-units/

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Nasir Hussain
Refinery Professional/ Consultant/ Trainer/Blog
Writer
Site Administrator
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