Pitting Corrosion
General / GeneralDefinition
Pitting corrosion is a localized form of corrosion that creates small, deep holes (pits) in a metal surface while the surrounding area remains largely unaffected. It is one of the most dangerous corrosion mechanisms because a pit can penetrate a pipe or vessel wall with very little overall metal loss, causing leaks or perforation that visual inspection may miss. Chloride-induced pitting of stainless steel is the most common industrial example, and it can cause through-wall penetration in weeks or months under the right (wrong) conditions.
Working Principle
Pitting initiates when a localized breakdown of the passive film occurs, typically at a surface defect, inclusion, or crevice where chlorides concentrate. The small anodic area inside the pit dissolves rapidly, while the large surrounding cathodic area (the intact passive surface) supports the electrochemical reaction. As the pit deepens, the environment inside becomes increasingly acidic (pH 0-2) and chloride-concentrated — a self-sustaining condition that accelerates penetration. The pit may be covered by a crust of corrosion products, making it invisible to casual inspection. The pit-to-surface area ratio means that metal loss appears negligible even as the pit approaches through-wall penetration.
Types & Variants
| Factor | Effect on Pitting | Control Method |
|---|---|---|
| Chloride concentration | Higher Cl- increases initiation rate | Material upgrade, deaeration, chloride removal |
| Temperature | Higher temp increases rate exponentially | Stay below Critical Pitting Temperature (CPT) |
| Oxygen | Oxidizing conditions accelerate pitting | Deaeration, oxygen scavengers |
| pH | Low pH promotes pit initiation | pH control, neutralization |
| Stagnant flow | Allows chloride concentration at surface | Maintain flow velocity > 1.5 m/s |
| Surface finish | Rough surfaces trap chlorides | Polish to Ra 0.4 um or better |
Materials
Pitting Resistance Equivalent (PRE = %Cr + 3.3 x %Mo + 16 x %N) ranks alloys: 304SS (PRE ~18) — poor pitting resistance. 316SS (PRE ~24) — moderate, suitable for <500 ppm Cl- at ambient temperature. 2205 Duplex (PRE ~34) — good, handles seawater at moderate temperatures. 2507 Super Duplex (PRE ~42) — excellent, high-chloride service. 6% Mo super-austenitic (PRE ~43) — excellent, for desalination and offshore. Titanium — virtually immune to chloride pitting at any temperature below 200F in most environments.
Applicable Standards
ASTM G48 (Pitting and Crevice Corrosion Resistance of Stainless Steels) — the standard ferric chloride test for ranking alloys. ASTM G61 (Cyclic Potentiodynamic Polarization) measures pitting potential electrochemically. NACE MR0175/ISO 15156 addresses sulfide stress cracking but also considers pitting in sour service. ASME B31.3 requires evaluation of pitting susceptibility for stainless steel piping in chloride-containing services.
Applications
Seawater cooling systems: 316SS heat exchanger tubes pit and perforate within 1-3 years — 2205 duplex or 6% Mo alloys are the standard upgrade. Desalination plants: high-chloride, high-temperature brine requires super-duplex or titanium. Offshore platforms: splash zone piping and structural members face chloride deposition and wet-dry cycling. Chemical processing: chlorinated solvents and hydrochloric acid services where 316SS is borderline. Pulp and paper: bleach plant equipment in chloride-containing wash liquors. Any application where stainless steel meets chlorides at elevated temperature is a pitting risk.
Advantages
- Can be predicted using PRE number and CPT data for material selection
- Laboratory testing (ASTM G48) ranks alloys before plant installation
- Flow velocity control reduces initiation risk
- Cathodic protection can suppress pitting in immersed structures
- Regular inspection (ultrasonic, eddy current) detects pits before penetration
Disadvantages
- Highly localized — difficult to detect by visual or thickness measurement
- Penetration can