This polythionic acid is highly corrosive, specifically to the grain boundaries of susceptible alloys.
The 2021 edition of NACE SP0170 PDF has undergone significant updates, reflecting advances in technology, changes in industry practices, and lessons learned from recent corrosion failures. Some key updates include:
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Using dehumidified air to ensure no liquid water can condense, as water is a required "ingredient" for the acid.
Dry nitrogen with low oxygen content (typically
This approach utilizes pH management to neutralize acidic environments, preventing the initiation of stress corrosion cracking. 3. Thermal Management
| Parameter | Requirement (2021) | |-----------|--------------------| | | ≥ 99.5% Zn (per ASTM B833) | | Thickness (typical) | 250–500 µm (10–20 mils) | | Bond strength | ≥ 1.5 MPa (pull-off test) | | Porosity | ≤ 5% (image analysis or ferroxyl test) | | Electrical continuity | < 2 ohms between segments | | Current density | 5–20 mA/m² of anode surface |
The solution must be allowed to soak, and in some cases, a subsequent wash with deionized water is recommended to prevent chloride stress cracking. B. Dry Air Purge (Dehumidification)
Sensitized Alloy + Sulfide Scale + Oxygen & Water = Rapid PTA SCC
: Choosing corrosion-resistant materials or employing specific fabrication practices to reduce susceptibility to sensitization. Inspectioneering Evolution of the Standard
Primary objectives:
The active baseline text detailing advanced protection for external heater tubes and refined risk-analysis provisions.
. This cracking typically occurs during refinery equipment shutdowns when sulfide corrosion products react with oxygen and water to form polythionic acids.
While the standard was technically last updated in 2018, it is commonly referred to in modern updates, sometimes as NACE SP0170-2018 or via the AMPP store .
