Caustic service chart zone lookup and sulfuric acid corrosion critical-factor screening (API RP 571, §3.14–3.15, §3.62)
◉ API RP 571 (3RD ED., 2020) §3.14–3.15, §3.62 STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only, and built from a partial excerpt of API 571. The uploaded file contained full body content for only 11 of the standard's 67 damage mechanisms (the rest were table-of-contents entries only, with no section text). This tool covers the two mechanisms with the richest verified quantitative content: Caustic Corrosion/Caustic SCC (§3.14–3.15) and Sulfuric Acid Corrosion (§3.62). The caustic service chart's curve values were digitized by reading gridline positions on the source figure — treat them as approximate, and verify against the original NACE SP0403 chart (the standard's own cited source) for any safety-critical decision.
Identification
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Caustic Service Chart Zone Lookup (Figure 3-15-1)
Source: NACE SP0403-2015 (formerly RP0403), as reproduced in API 571 Figure 3-15-1. Per the standard's own footnote, most users don't require PWHT below 2% NaOH regardless of temperature; some use 5% as their threshold.
Result Not evaluated
Zone—
Guidance—
Lower boundary (Area A/B) at this concentration—
Upper boundary (Area B/C) at this concentration—
Additional reference points (§3.15.3–.6): Cracking can occur at low caustic levels (as little as 50–100 ppm) if a concentrating mechanism is present, even in nominally "safe" Area A conditions. Effective PWHT: 1150°F (620°C) minimum, 1 hr minimum hold time. Molten caustic cracking of nickel-based alloys has been observed at 604°F (318°C) at atmospheric pressure, in the absence of free water. 300 series SS offers little advantage over carbon steel for caustic SCC resistance.
Thresholds as stated in the standard: corrosion increases significantly above ~2–3 fps (0.6–0.9 m/s) or below 65% concentration (protective ferrous sulfate scale less stable). Below ~0.5 fps (0.15 m/s), hydrogen grooving (an erosion-corrosion mechanism) can occur instead. SS performance approaches carbon steel's, especially below 92% concentration or at elevated temperature.