How can galvanic corrosion and pitting corrosion be distinguished?

Confusing galvanic corrosion and pitting corrosion often leads to ineffective corrective actions. In HVAC networks, plumbing systems, building technical equipment, or metal assemblies exposed to moisture, both mechanisms can produce perforations, deposits, and leaks that look visually similar.

Yet their origins are different. Galvanic corrosion is a macro-electrochemical phenomenon: it occurs when a galvanic couple forms between two metals with different potentials in the presence of an electrolyte. One becomes the anode and dissolves preferentially, while the other becomes the cathode. Pitting corrosion, by contrast, is a micro-localized mechanism: it starts with a local breakdown of passivation, often promoted by chlorides, surface heterogeneities, coating defects, or deposits. The attack then concentrates on very small areas, with rapid deep penetration. Replacing a perforated pipe without identifying the mechanism means risking a recurrence within 6 months.

The presence of two different metals electrically connected is the first warning sign. In practice, galvanic corrosion often appears in the immediate vicinity of a fitting, flange, thread, dissimilar weld, insert, or metal accessory added to a more noble or less noble material.

The attack primarily affects the anodic metal and may result in localized loss of thickness, corrosion products concentrated near the interface, or accelerated degradation over the smaller anodic surface coupled to a larger cathodic surface.

The reasoning must take into account the potential difference, the geometry of the surfaces involved, the quality of the electrical contact, and the conductivity of the medium.

Pitting corrosion appears as very localized attacks, often in the form of small craters or narrow openings leading to a deeper cavity. It frequently affects passivable metals when the protective layer is locally destabilized.

Chlorides, deposits, stagnation, metallurgical heterogeneity, a surface defect, or coating damage can initiate the phenomenon.

The external appearance can be misleading: a generally sound surface may conceal rapid deep perforation. It is precisely this discreet nature that makes the pitting mechanism costly when it is underestimated.

In many cases, the decision is made too quickly based on a simple photo, a leak report, or an assumption related to water quality. Yet two different mechanisms can lead to similar symptoms. If pitting corrosion is wrongly identified, there is a risk of changing the water treatment, adding an inhibitor, or replacing only the perforated component, when the real cause is a persistent galvanic couple in the assembly.

Conversely, systematically attributing the defect to metal contact can cause chlorinated contamination, a passivation defect, or a deposit under which corrosion develops to be overlooked. Replacing a perforated pipe without identifying the mechanism means risking a recurrence within 6 months.

Visual appearance alone is not enough to draw a reliable conclusion. A pinpoint perforation may result from active pitting, but also from an anodic area linked to contact between dissimilar metals, stray electrical continuity, or a difference in aeration.

Metallurgical expertise makes it possible to secure the metal corrosion diagnosis by combining surface observations, deposit analysis, micrographic cross-sections, and electrochemical tests.

The goal is to identify the real mechanism, qualify the aggravating factors, and avoid false remedies—for example, treating the water when the main cause is a copper/steel or stainless steel/aluminum contact, or an electrical isolation defect.

This approach is particularly useful for maintenance managers, fluid engineering firms, HVAC installers, and insurance experts dealing with a construction defect assessment.

To confirm this mechanism, the laboratory can carry out a galvanic coupling study and laboratory electrochemical tests.

Measurement of the open-circuit potential (OCV) makes it possible to compare the spontaneous behavior of the materials in the medium considered. The galvanic coupling study quantifies the interactions between two metals. Corrosion rate measurements by LSV and electrochemical impedance (EIS) provide additional information on degradation kinetics and the condition of surfaces or coatings.

In parallel, observing the contact zones and analyzing the deposits helps verify whether the failed area is indeed an active anodic zone.

Identification relies on a multi-scale approach. Examination under optical microscopy and corrosion SEM analysis highlights the morphology of the pits, their density, and their relative depth. Micrographic cross-sections make it possible to observe propagation beneath the surface.

EDX helps characterize the deposits or contaminants present in and around the cavities. Surface chemical analysis can specify the nature of the species responsible for destabilizing passivation. If necessary, elemental analysis in the fluid or deposits look for halogens, oxidizing agents, or trace elements that promote initiation.

To make the decision more reliable, the expertise combines several levels of analysis: targeted visual inspection, optical microscopy, SEM-EDX for morphology and composition, chemical analysis of materials and deposits, grade verification, metallographic cross-sections, and electrochemical tests. This methodology makes it possible to distinguish an attack linked to an anode/cathode zone from an attack resulting from a local breakdown of passivation, to assess the degree of oxidation, to detect precursors such as halogens or contamination, and to guide the correct corrective action: electrical isolation, material change, coating adjustment, control of the environment, or design modification.

An effective approach is to have the failed part, any opposing material, deposits, and the service environment assessed by an expert.

The laboratory can quickly determine the origin of observed corrosion, validate the resistance of materials and processes, or reproduce the behavior in specific environments. Available methods include optical microscopy, MEB-FEG, MEB-EDX, surface chemical analysis, ICP for trace elements, XRD if needed, as well as electrochemical tests OCV, LSV, EIS and study of galvanic coupling.

This approach makes it possible to move from a leak report to a technically sound decision. Have the failed area assessed by an expert. Compare the materials involved. Identify deposits and contaminants. Measure electrochemical behavior. Define the appropriate corrective action.