Two positions are available in almost every corrosion dispute, and both are usually arguable on the same facts. One is that the wrong material was specified for the service. The other is that the material suited the service described, and the service it actually saw was different. Galvanic couples, coating breaches, deposits and biofilms sit between those positions, because each accelerates attack locally while leaving the bulk material blameless. Telling them apart is a matter of where the damage sits and what the record shows about how the asset was run.
The couple, not the metal
Galvanic corrosion needs three things at once: two metals of different electrochemical potential, an electrolyte bridging them, and an electrical path between them. Remove one and the mechanism stops. Neither metal is defective; the pairing is.
Galvanic series tables rank alloys by measured potential in a given electrolyte, and ASTM G82 is the guide for developing and using such a series. They are a screening aid, not a prediction — position indicates the direction of attack, not its magnitude, and the ordering shifts with temperature and aeration. ASTM G71 covers galvanic testing where the actual couple and electrolyte need reproducing.
Area ratio decides severity
The damaging variable is usually area, not alloy choice. Corrosion current spreads across the anodic surface, so a large cathode coupled to a small anode concentrates that entire current into a small area and drives rapid, deep local loss.
This is why fastener decisions matter. A less noble bolt holding a large, more noble plate is the unfavourable configuration; the same metals reversed can be tolerable for decades. Damage concentrated near the junction and tapering with distance is the morphology pointing here.
A coating breach can make the ratio worse
Coatings are understood as barriers, and on a single metal a small break exposes a small area. On a galvanic couple the arithmetic inverts: coating the anode and leaving the cathode bare concentrates the whole cathodic demand onto whatever pinholes and holidays exist in that coating.
The result is deep, isolated penetration at scattered coating defects — easily mistaken for random pitting until the couple is recognised. Coating practice favours coating the cathode, or both, for this reason.
Coating specification is a documented decision
Where a coating is at issue, the specification is evidence in its own right. ISO 12944 selects protective paint systems for steel structures against defined atmospheric corrosivity categories, which derive from the classification scheme in ISO 9223. A system chosen for a mild category and installed in a marine one is a specification question.
Surface preparation carries similar weight. The joint SSPC/NACE standards, now under AMPP, define blast cleanliness grades and profile, and adhesion failures traced to residual mill scale or soluble salts belong to application rather than product. NACE SP0188 covers holiday testing of new coatings, and whether it was performed and recorded is often decisive.
Cathodic protection as designed and as operated
Buried and submerged steel is normally protected electrochemically as well as by coating, and NACE SP0169 sets out the practice for controlling external corrosion on underground or submerged metallic piping, including protection criteria and monitoring.
That produces a documentary trail: rectifier readings, test-station potentials, close-interval surveys, anode replacement records. Gaps in the trail, out-of-criteria readings left uncorrected, or interference from a neighbouring system separate a protection system that existed on paper from one that worked.
Deposits, insulation and the stagnant zone
Under-deposit corrosion needs no dissimilar metals at all. Silt, scale or process solids settling on a surface create the same oxygen-depleted, acidified pocket a crevice does, which is why low-flow legs, dead ends and tank floors recur.
Corrosion under insulation follows the same logic, with water trapped against the steel by a covering meant to keep it out. API RP 583 addresses corrosion under insulation and fireproofing directly, and API RP 571 catalogues these mechanisms alongside erosion-corrosion and galvanic attack.
When the environment is alive
Microbiologically influenced corrosion is a modifier rather than a mechanism of its own: biofilms change the local chemistry at the metal surface, and the resulting attack is localised, often severe, and found in systems whose bulk water chemistry looks unremarkable.
The evidence is chemical and biological rather than morphological. Tubercles over discrete pits, and sulfide-bearing corrosion products where no process sulfur explains them, are indicators, not proof. Sampling the deposit and the pit interior separately, then combining elemental analysis with culture-based or molecular testing, is what supports the call — and AMPP field monitoring practice for bacterial growth governs how such samples are taken.
Hydrotest water, layup and other short exposures
Some of the most damaging exposures are brief and undocumented. Untreated water left in a system after hydrotesting, an idle period without preservation, or an unrecorded wash-down can establish pitting or seed a biofilm before the asset enters service.
These episodes rarely appear in the design file, so they are reconstructed from commissioning records, water sources, treatment invoices and interviews. Where they exist they reframe the case: the material may have suited the specified duty and never seen it in clean condition.
Separating specification from operation
The distinction that organises the analysis is between what was decided and what was done. Alloy selection, coating system, joint design and protection scheme are decided once and traceable to drawings and approvals. Water treatment, flow regime, layup and inspection are operational and traceable to logs.
Morphology tends to favour one side. Attack concentrated at junctions, at coating defects or beneath deposits argues for a configuration or housekeeping problem; broadly distributed attack on properly coated and protected surfaces argues the environment exceeded what the specification contemplated. Both conclusions need the documents, not only the metal.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.