Polythionic Acid Stress Corrosion Cracking

The chemical reaction of sulfur compounds, oxygen, and water at elevated temperatures, which typically occurs during process unit shutdowns, can lead to the formation of oxoacids known as polythionic acids (PTAs). These acids can destroy the passive oxide layer of austenitic steels and alloys. Consequently, if the austenite stainless steel is sensitized and subjected to stress, it can lead to an intergranular cracking phenomenon known as Polythionic Acid Stress Corrosion Cracking (PTASCC). This mechanism can occur in any unit that using sensitized austenitic material in high-temperature sulfur containing environments. PTASCC is typically seen in FCC regenerators and heater/furnace tubes in crude and hydroprocessing units.

General Information

Polythionic Acid Stress Corrosion Cracking (PTASCC) is a form of intergranular cracking that requires coexistence of three elements: a susceptible material (e.g. sensitized austenitic stainless steels and some Ni alloys), presence of polythionic acids and stress.1 2 Therefore, PTASCC occurs commonly in areas typically operating in range of 370-843°C (700-1550°F) where sensitization of austenitic materials will progress.2 3 4

Polythionic acids, the second element, are typically formed during shutdown or process upset events when oxygen and water/moisture ingress may take place. Stress, the third necessary element for PTASCC and typically arises during cold and/or hot mechanical operations such as welding and bending. The interaction of these three elements is schematically shown in Figure 1.

PTASCC mechanism general concept.
Figure 1: PTASCC mechanism general concept.

Polythionic Acid Stress Corrosion Cracking is governed by a combination of several factors like chemical species, temperature and materials regimes.

To find out more information about PTASCC and impact of various parameters register for free or buy a subscription.

References

    Paid Subscribers can have access to the list of references.