H2S partial pressure calculation and SSC environmental severity region lookup for carbon/low-alloy steel material selection (NACE MR0175/ISO 15156-1 Annex C, -2 §7.2.1)
◉ NACE MR0175/ISO 15156-1:2001, -2:2003 STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only. The SSC Region boundaries (Figure 1) were digitized by carefully reading gridline intersections on the source chart image — every intersection point matched the chart's own gridlines exactly in our verification, but this is still a visual digitization, not a formula published in the standard's text. Sour service material selection is safety-critical: verify against the original Figure 1 before using this for procurement or design decisions. This tool covers Part 1 (general principles, H2S partial pressure) and Part 2 (carbon/low-alloy steel SSC regions) only \u2014 Part 3 (corrosion-resistant alloys) was not present in the uploaded document.
Identification
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H2S Partial Pressure (Annex C.1)
pH2S = p × (xH2S/100). For gas-free liquid systems, Annex C.2 gives a separate bubble-point-pressure-based method not covered by this simple tool.
Result
H2S partial pressure—
H2S partial pressure (kPa, for region lookup below)—
SSC Environmental Severity Region (Figure 1)
Valid chart range: 0.1–1000 kPa H2S, pH 2.5–6.5. The chart's own Note 1: discontinuities below 0.3 kPa and above 1 MPa (150 psi) reflect measurement uncertainty at those extremes.
Result Not evaluated
SSC Region—
Material selection guidance—
Material selection by region (§7.2.1.2): Region 1 steels may be selected using Annex A.2, A.3, or A.4. Region 2 steels may be selected using A.2 or A.3. Region 3 steels may be selected using A.2 only. In the absence of suitable choices from Annex A, steels may be tested/qualified per Annex B, or documented field experience may be used per Part 1.
H2S (kPa)
0.3
1
10
100
1000
Region 1/2 boundary (pH)
—
4.5
5.5*
6.5
6.5
Region 2/3 boundary (pH)
3.5
3.5
4.5*
5.5
5.5
*Interpolated (log-linear between 1 and 100 kPa); other values read directly from chart gridlines. Below 0.3 kPa: Region 0 (regardless of pH).