Corrosion Failure Analysis Explained: Causes, Types, and Methods
What Is Corrosion Failure Analysis?
Corrosion failure analysis is the systematic investigation of a metal component that has degraded or failed due to a corrosion mechanism. The objective is to identify the corrosion type, determine the root cause, and provide findings specific enough to prevent recurrence. It is distinct from routine corrosion testing – the component has already failed, and the analysis works backward from the damage to the conditions that produced it.
The output is a root cause report documenting the corrosion mechanism, contributing factors (material selection, surface condition, environment, protective system breakdown), and corrective action recommendations. Industries from oil and gas to aerospace use corrosion failure analysis to reduce unplanned downtime, support warranty investigations, and satisfy regulatory reporting requirements after in-service failures.
Corrosion Mechanisms
Uniform Corrosion
Uniform corrosion removes material evenly across an exposed surface. It is the most predictable form – measured as mass loss or thickness reduction per unit time – and is addressed through coating selection, cathodic protection, or material substitution. ASTM G31 covers laboratory immersion testing to evaluate uniform corrosion rates.
Pitting Corrosion
Pitting initiates at surface defects or breakdown sites in a passive film and propagates as a narrow cavity with a passive exterior. It is particularly dangerous because small pits cause disproportionate stress concentration. Stainless steels and aluminum alloys in chloride environments are the most common cases. ASTM G48 is the standard test method for pitting susceptibility in stainless alloys.
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals in electrical contact are exposed to an electrolyte. The more active metal (anode) corrodes preferentially. The severity depends on the potential difference between the metals, the area ratio of anode to cathode, and the conductivity of the electrolyte. ASTM G82 and ASTM G71 cover galvanic testing and ranking.
Stress Corrosion Cracking (SCC)
SCC requires the simultaneous presence of tensile stress, a susceptible material, and a specific corrosive environment. Cracks propagate at stress levels well below the material’s yield strength and can cause sudden, brittle-appearing fracture in otherwise ductile alloys. High-strength steels in H2S (sulfide stress cracking), stainless steels in chlorides, and brass in ammonia are classic SCC systems. ASTM G36, G44, and G47 cover common SCC test methods.
Also Read – Corrosion Analysis & Observation: Techniques, Tools & Lab Methods
Crevice and Intergranular Corrosion
Crevice corrosion develops in occluded spaces – under gaskets, at flanged joints, beneath deposits – where oxygen depletion shifts the local chemistry toward more aggressive conditions. Intergranular corrosion follows grain boundaries sensitized by heat treatment or welding, most commonly in austenitic stainless steels. ASTM A262 covers intergranular corrosion susceptibility testing.
How Corrosion Failure Analysis Works
A structured corrosion failure analysis follows a defined sequence: visual examination and documentation of the failure site; optical and scanning electron microscopy (SEM) of the corroded surface and fracture morphology; energy dispersive X-ray spectroscopy (EDS/EDX) for elemental mapping of corrosion products; metallographic cross-section to examine pit depth, crack path, and microstructural features; and chemical analysis of the base metal and any deposits or scale.
Environmental data review – service fluid chemistry, temperature excursions, cathodic protection records, coating inspection history – runs in parallel with the laboratory work. The final report correlates the observed damage morphology with the operating history to establish mechanism and contributing cause. ASTM G161 and ASTM E1351 provide guidance on failure examination procedures.
Industry Specifications
- Oil and Gas: NACE MR0175/ISO 15156 (SSC/SCC in H2S), NACE SP0169 (cathodic protection), API 571 (damage mechanisms)
- Aerospace: AMS 2770, ASTM G47 (SCC in aluminum), ASTM G64 (SCC ratings for titanium)
- Power Generation: ASTM G36 (SCC in boiling MgCl2), ASTM A262 (intergranular corrosion)
- Marine and Offshore: ASTM G71 (galvanic corrosion), ASTM B117 (salt spray), ISO 9226
- Automotive: GMW14872 (cyclic corrosion), ASTM G85 (modified salt spray), SAE J2334
- General Laboratory Methods: ASTM G31 (immersion), ASTM G48 (pitting), ASTM G82 (galvanic), ASTM G161 (failure analysis)
Also Read – Copper-Accelerated Acetic Acid Salt Spray (CASS) Test: Method & Guide
Conclusion
Corrosion failure analysis identifies not just what corroded but why – which mechanism was active, what environmental and material conditions enabled it, and what changed to trigger the failure. Without that specificity, corrective actions address symptoms rather than causes. Whether the failure is a pitted pipeline fitting, a stress-corrosion crack in a structural fastener, or galvanic attack at a dissimilar metal joint, the analytical path is the same: characterize the damage, reconstruct the conditions, and close the gap.
What is the difference between corrosion testing and corrosion failure analysis? Corrosion testing is prospective - it exposes a material to a controlled environment to measure its corrosion rate or susceptibility before service. Failure analysis is retrospective - it examines a component that has already degraded or failed to determine mechanism and cause. The tools overlap (SEM, EDS, metallography) but the purpose is different. Testing qualifies materials; failure analysis explains why a qualified material still failed in service.
How is SCC distinguished from hydrogen embrittlement in a failure? Both produce brittle-appearing fractures in ductile alloys at low stress, and their crack morphologies can look similar. The distinction comes from the specific environment and fracture path. SCC typically produces intergranular or transgranular cracking with corrosion product inside the crack. Hydrogen embrittlement produces similar crack paths but with minimal corrosion product and is confirmed by charging tests or by correlating failure with a hydrogen source - acid pickling, electroplating, or cathodic overprotection.
Can a component be analyzed for corrosion mechanism without knowing its service history? Yes, though the analysis takes longer. The corrosion morphology itself - pit shape, crack path, deposit chemistry, corrosion product composition - constrains the mechanism even without service records. EDS analysis of corrosion products identifies the aggressive species (chlorides, sulfides, oxides). Crack path and fracture surface features distinguish SCC from fatigue from pitting. Service history accelerates the root cause step, but the lab work can stand alone.
What sample condition is needed for corrosion failure analysis? The failed component should be submitted as-received without cleaning or sectioning. Corrosion products, deposits, and scale on the surface carry chemical information that is destroyed by cleaning. If the component must be shipped, wrap it in clean, dry cloth or paper - not plastic, which can trap moisture and continue the corrosion reaction. Photographs documenting the failure location in its assembly context are valuable before disassembly.
How long does a corrosion failure analysis take? A standard failure analysis with visual examination, SEM/EDS, metallography, and report typically runs two to four weeks from sample receipt. Cases requiring chemical analysis of the base material, environmental testing, or litigation-quality chain-of-custody documentation take longer. Expedited turnaround is available for critical path situations. The report timeline depends on the scope agreed at project initiation, not on the severity of the failure.
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