Temperature-adjusted tank bottom corrosion rate estimation and similar-service assessment checklist (API RP 575, §7.4, Annex B)
◉ API RP 575 (5TH ED., 2024) §7.4, ANNEX B STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only. Similar service is an accepted method for estimating tank bottom corrosion rates from historical data on comparable tanks, in lieu of or in addition to direct measurement — per API 653, API 510, and API 570. It may not be permitted by regulation for tanks under certain jurisdictions; confirm applicability before relying on it. Similar service assessment guidelines live in API 653—Annex H; this annex presents one method, and other methods are allowed. Product-side and soil-side corrosion should always be assessed independently — their mechanisms and rates are unrelated.
Identification
Research entries stay private to you for practice/testing. Switch to Official once this is a real design your company should see.
Temperature-Adjusted Corrosion Rate (§7.4.2)
Reaction/corrosion rates typically double for every 18°F (10°C) temperature increase. CRnew = CRref × 2(ΔT/18°F or /10°C). Per the standard's own worked example (Table B.2), the temperature term generally isn't significant unless the difference exceeds 10°F (5.5°C).
Aviation gas (product-side)2–3× finished fuel rate
Crude oils (product-side)Highly variable — no fixed range given
Soil-side corrosionNegligible up to ~20 mil/year (~0.51 mm/year)
Double-bottom elevation above old bottom≥4 in. (10 cm)
Typical internal lining coverage from bottom2–3 ft (61–91 cm) up the shell
Similar Service Assessment Checklist (Table B.1)
Selected considerations when comparing an existing tank's corrosion history to a new/different tank for similar-service purposes. Assess product-side and soil-side factors independently.
Factor
Typical Range
Stock
Nil to very high rates; pitting or general thinning or both. Apply similar service to similar (preferably the same) stock.
Temperature
Ambient tanks vs. heated (typically <500°F/260°C; asphalt ~500°F/260°C) vs. refrigerated (ambient down to cryogenic).
Water bottoms
None (e.g. asphalt, lube oil) to very aggressive (e.g. crude oil). Controlled largely by water chemistry/pH.
Bottom design
Flat, cone-up, cone-down, single slope — affects ability to fully remove water/debris.
Internal lining
Unlined to fully lined (thin-film, thick-film, reinforced thick-film).
Internal cathodic protection
Typically only water storage tanks without internal linings.
Foundation material
Concrete ringwall, concrete slab, engineered fill, native soil. Concrete is alkaline and reduces corrosion vs. typical soil.
Release prevention barrier (RPB)
Double bottoms, plastic liner, lined secondary containment. RPBs/liners can trap water and increase corrosion.
Drainage
Poor/stagnant to well drained.
Isolation from old bottom
Electrical insulation of old to new bottom in double-bottom tanks. New steel is anodic (more corrosive) than old steel if not isolated.
Cathodic protection (soil-side)
Galvanic (finite life, no maintenance) or impressed current (prolonged life, requires maintenance). Compare only within the same kind.