Is galvanic corrosion caused by a defective metal?
No. In galvanic corrosion neither metal is defective; the pairing of the two metals is. Galvanic corrosion needs three things at once: two metals of different electrochemical potential, an electrolyte bridging them, and an electrical path between them. Remove any one of the three and the galvanic mechanism stops.
Galvanic series tables rank alloys by measured potential in a given electrolyte, and ASTM G82 is the guide for developing and using such a galvanic series. Galvanic series tables are a screening aid, not a prediction: an alloy’s position in the series indicates the direction of galvanic 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.
What decides how severe galvanic corrosion is?
In galvanic corrosion the damaging variable is usually the area ratio between the two metals, not the choice of alloy. 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 metal loss.
This is why fastener decisions matter in galvanic corrosion. A less noble bolt holding a large, more noble plate is the unfavorable configuration; the same two metals reversed can be tolerable for decades. Damage concentrated near the junction between the two metals and tapering with distance from it is the morphology that points to galvanic attack driven by area ratio.
Can a coating breach make galvanic corrosion worse?
Yes, a breach in a coating can make galvanic corrosion worse by worsening the area ratio between anode and cathode. Coatings are understood as barriers, and on a single metal a small break in the coating 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 the anode’s coating.
The result is deep, isolated penetration at scattered coating defects, which is easily mistaken for random pitting until the galvanic couple is recognized. For this reason, coating practice favors coating the cathode, or both metals.
Why does the coating specification matter in a corrosion dispute?
Where a coating is at issue in a corrosion dispute, the coating specification is evidence in its own right, because it documents a decision. ISO 12944 selects protective paint systems for steel structures against defined atmospheric corrosivity categories, which derive from the classification scheme in ISO 9223. A coating system chosen for a mild corrosivity 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 coating adhesion failures traced to residual mill scale or soluble salts belong to application rather than to the coating product. NACE SP0188 covers holiday testing of new coatings, and whether that holiday testing was performed and recorded is often decisive.
How can you tell whether cathodic protection actually worked?
The records a cathodic protection system generates are what separate a system that existed on paper from one that worked. 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.
Cathodic protection of this kind produces a documentary trail: rectifier readings, test-station potentials, close-interval surveys and anode replacement records. Gaps in that trail, out-of-criteria readings left uncorrected, or interference from a neighboring system separate a cathodic protection system that existed on paper from one that worked.
Can corrosion under deposits or insulation happen without dissimilar metals?
Yes. Under-deposit corrosion needs no dissimilar metals at all. Silt, scale or process solids settling on a metal surface create the same oxygen-depleted, acidified pocket a crevice does, which is why low-flow legs, dead ends and tank floors recur as sites of under-deposit corrosion.
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 catalogs under-deposit corrosion and corrosion under insulation alongside erosion-corrosion and galvanic attack.
How is microbiologically influenced corrosion identified?
Microbiologically influenced corrosion is identified by chemical and biological evidence rather than by the morphology of the damage. 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 localized, often severe, and found in systems whose bulk water chemistry looks unremarkable.
Tubercles over discrete pits, and sulfide-bearing corrosion products where no process sulfur explains them, are indicators of microbiologically influenced corrosion, not proof. What supports the call is sampling the deposit and the pit interior separately, then combining elemental analysis with culture-based or molecular testing, and AMPP field monitoring practice for bacterial growth governs how such samples are taken.
Can hydrotest water or an idle layup period cause corrosion before an asset enters service?
Yes. 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. Some of the most damaging corrosion exposures are brief and undocumented in this way.
Short exposures such as hydrotest water, unpreserved layup and unrecorded wash-downs rarely appear in the design file, so they are reconstructed from commissioning records, water sources, treatment invoices and interviews. Where such exposures exist they reframe a corrosion dispute: the material may have suited the specified duty and never seen it in clean condition.
How do you separate a material specification problem from an operating problem in a corrosion failure?
A material specification problem is separated from an operating problem by distinguishing what was decided from 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.
Corrosion morphology tends to favor 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.