Tensile stresses from welding, cold working, or thermal expansion create the mechanical driving force for crack propagation.
This method is a specialized form of purging that uses dry air to prevent the formation of liquid water. While seemingly contradictory to nitrogen purging, which uses an inert gas, dry air purging focuses on the second critical condition for SCC: the presence of a liquid water phase. This method relies on maintaining the relative humidity within the equipment below a certain threshold so that free water cannot condense on the metal surfaces. This denies the electrolyte necessary for the electrochemical corrosion reaction to proceed .
Using "L-grade" stainless steels (e.g., 304L, 316L) minimizes carbide precipitation because there is less carbon available to bind with chromium.
When austenitic stainless steels (such as 304, 304H, 316, or 316H) are exposed to temperatures between 425°C and 815°C (800°F to 1500°F) during operation, chromium carbides precipitate along the grain boundaries. This process, known as sensitization , leaves the zones immediately adjacent to the grain boundaries depleted of chromium, dropping below the 12% threshold required for passivation. nace sp0170 pdf
), or specialized commercial alkaline cleaners, immediately after opening the vessel. NACE SP0170-2018 Standard Sections The standard is organized to ensure robust protection: Defines which equipment and alloys are covered.
Incorrect chemical concentrations for neutralizing solutions.
One of the primary methods for preventing PTA SCC is to eliminate oxygen from the equipment environment. Equipment is purged with dry nitrogen ( N2cap N sub 2 ) to remove air. Tensile stresses from welding, cold working, or thermal
When austenitic stainless steels are exposed to high operational temperatures ( 425∘C425 raised to the composed with power C 815∘C815 raised to the composed with power C 800∘F800 raised to the composed with power F 1500∘F1500 raised to the composed with power F
The standard outlines several acceptable protection methodologies depending on whether the equipment needs to be opened for internal inspection/maintenance or kept closed during the turnaround. 1. Exclusion of Oxygen (Nitrogen Purging)
While primarily aimed at 300-series stainless steel (e.g., 304, 316), the standard also covers other austenitic alloys prone to sensitization. This method relies on maintaining the relative humidity
A solution of sodium carbonate (soda ash, Na₂CO₃) dissolved in potable water, typically ranging from 1% to 2% by weight.
Identify all assets constructed from 300-series stainless steels operating within the sensitization temperature window.
It is available in PDF format for immediate download. Summary of Changes in NACE SP0170-2018
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