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ASTM F436 Type 3 Corrosion Index (CRI) 6 Selection Guide
ASTM F436 Section 2.1 allows Type 3 weathering steel washers to be supplied either to a fixed chemistry envelope (Type 3A in F436 Table 1) OR to a Corrosion Index (CRI) of 6 or higher computed from the Legault-Leckie formula in ASTM Guide G101. The CRI option gives the supplier flexibility to vary chemistry within a wider band as long as the calculated corrosion-resistance index meets the 6.0 threshold. This page documents the Legault-Leckie formula, the CRI 6 cut-off, and when to specify Type 3A vs Type 3 CRI on a project requisition.
The Legault-Leckie corrosion index formula
ASTM Guide G101 publishes the Legault-Leckie empirical formula for atmospheric corrosion resistance of low-alloy structural steels, derived from extensive field exposure tests in industrial, marine and rural atmospheres. The formula is:
I = 26.01 (%Cu) + 3.88 (%Ni) + 1.20 (%Cr) + 1.49 (%Si) + 17.28 (%P) - 7.29 (%Cu)(%Ni) - 9.10 (%Ni)(%P) - 33.39 (%Cu)^2
Each element appears with its percentage by weight. The chromium, nickel, copper, silicon and phosphorus coefficients are positive (they improve corrosion resistance); the Cu-Ni, Ni-P, and Cu^2 cross-terms are negative (interactions between high-content elements reduce the marginal benefit).
CRI 6 worked example — F436 Type 3A chemistry
Take a Type 3A washer at mid-band chemistry: Si 0.25, Cr 0.55, Ni 0.35, Cu 0.35, P 0.030. Compute Legault-Leckie:
| Term | Value | Contribution |
|---|---|---|
| 26.01 x 0.35 (Cu) | +9.10 | large positive |
| 3.88 x 0.35 (Ni) | +1.36 | moderate positive |
| 1.20 x 0.55 (Cr) | +0.66 | small positive |
| 1.49 x 0.25 (Si) | +0.37 | small positive |
| 17.28 x 0.030 (P) | +0.52 | small positive |
| -7.29 x 0.35 x 0.35 (Cu-Ni) | -0.89 | moderate negative |
| -9.10 x 0.35 x 0.030 (Ni-P) | -0.10 | small negative |
| -33.39 x 0.35^2 (Cu^2) | -4.09 | large negative |
| I (total) | +6.93 | passes CRI 6 cut-off |
Type 3A mid-band chemistry computes to I = 6.93, comfortably above the 6.0 threshold. The Cu^2 quadratic penalty at 0.35 percent Cu is roughly half the linear Cu credit, which is why Type 3A caps Cu at 0.45 — pushing higher does not improve corrosion resistance proportionately.
Type 3A vs Type 3 CRI 6 — when to pick which
| Decision factor | Type 3A (fixed chemistry) | Type 3 CRI 6 (formula) |
|---|---|---|
| Project spec citation | "F436 Type 3A" or default Type 3 | "F436 Type 3 to CRI 6 minimum" |
| Chemistry verification | Single chemistry envelope check against Table 1 | Heat chemistry + formula computation on cert |
| Supplier flexibility | Lower (must hit narrow band) | Higher (wider chemistry as long as I >= 6) |
| Multi-source compatibility | Easier (everyone hits same band) | Requires CRI cert per heat from each source |
| Inspection workflow | Read chemistry table directly | Run Legault-Leckie computation; AS101 online calculator |
| Cost | Slightly higher (tighter band) | Slightly lower (wider chemistry options) |
When the CRI option matters
For most refinery, bridge and transmission-tower projects, Type 3A fixed chemistry is the clearer call — every mill cert reads the same envelope and the project quality plan checks a single table. The CRI 6 option matters when (a) the project sources from multiple weathering-steel mills with slightly different chemistry recipes, (b) the project uses a non-standard weathering steel listed in A588 Table 2 (which F436 Note B permits), or (c) the design pre-existed and used the Legault-Leckie formula as its corrosion-resistance basis. In those cases, specifying "F436 Type 3 to CRI 6 or higher" gives the engineer flexibility without sacrificing the corrosion guarantee.
Cross-references
For full F436 Type 3 chemistry and the parent specification see F436 Type 3 washers. The hot-dip galvanizing alternative to weathering chemistry is on F436 hot-dip galvanizing F2329; choose between Type 3 weathering and HDG Type 1 based on whether direct atmospheric patina exposure is acceptable. For complete material specification background see F436 washer specifications.
Frequently asked questions
What is the F436 corrosion index CRI 6?
CRI is the Corrosion Resistance Index computed from the Legault-Leckie empirical formula published in ASTM Guide G101. F436 Section 2.1 allows Type 3 weathering steel washers to be supplied to a CRI of 6 or higher as an alternative to the fixed Type 3A chemistry envelope in Table 1. The formula combines weighted contributions from Cu, Ni, Cr, Si and P with quadratic and cross-term penalties; a CRI value of 6 corresponds to approximately twice the atmospheric corrosion resistance of plain carbon steel under standard industrial-atmosphere exposure conditions.
How is the F436 CRI calculated?
Apply the Legault-Leckie formula I = 26.01(Cu) + 3.88(Ni) + 1.20(Cr) + 1.49(Si) + 17.28(P) - 7.29(Cu)(Ni) - 9.10(Ni)(P) - 33.39(Cu)^2 with each element percentage by weight from the heat-analysis chemistry. ASTM hosts an online spreadsheet calculator at G101 Calculator that performs the math. The result must be at least 6.0 for the heat to qualify as F436 Type 3 to CRI 6. TorqBolt mill certs list the heat chemistry, the computed CRI, and the calculator URL alongside the standard test results.
Is Type 3A more conservative than Type 3 CRI 6?
Type 3A and Type 3 CRI 6 produce similar mid-band CRI values (around 6.9 for Type 3A mid-band chemistry). Type 3A is the more conservative choice in the sense that every mill cert reads the same chemistry envelope, making project QA simpler. Type 3 CRI 6 is more flexible: a heat can vary chemistry within a wider band as long as the computed CRI clears 6.0. Both are equally compliant with F436; the choice is a project-management trade-off between flexibility and inspection simplicity.
Can I use F436 Type 3 with a non-A588 weathering steel?
Yes, but only if the resulting CRI is 6.0 or higher. F436 Note B permits Type 3 washers manufactured from any of the steels listed in A588 / A588M Table 2 and ASTM F3125, OR from any other steel chemistry that computes to a Legault-Leckie CRI of 6 or higher. This second clause lets a supplier use a non-standard alloy as long as the CRI computation is documented on the heat cert. The clause is rarely invoked because A588 chemistry is widely available; it exists for edge cases like recycled or salvaged-heat material.
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