In the field, discovering signs of deterioration on metal parts is always bad news. But to treat the problem, you first have to identify it. Two corrosion phenomena come up often: galvanic corrosion and pitting corrosion.
Although both lead to material loss, their causes and appearances are radically different. Here are the key points to help you tell them apart for good.
Galvanic corrosion: the toxic match
Galvanic corrosion occurs when two metals of different types are brought into (electrical) contact in a conductive environment (moisture, salt water).
The mechanism: it is the principle of a battery. The less noble metal (the anode) sacrifices itself and corrodes to protect the more noble metal (the cathode).
The appearance: it is precisely localized at the junction between the two metals. Significant mass loss is observed on one of the components, while the other remains intact.
Typical example: a stainless steel screw on an aluminum plate in a marine environment. The aluminum (less noble) will literally “melt” around the screw.
Pitting corrosion: the invisible enemy
Pitting corrosion (or pitting) is one of the most insidious forms of degradation because it is extremely localized and can penetrate through a metal thickness in record time.
The mechanism: it often occurs on “passivated” metals (such as stainless steel or aluminum). A local break in the protective layer, often caused by chloride ions (salt), creates a corrosion site that sustains itself.
The appearance: it shows up as small holes or cavities on the surface, sometimes almost invisible to the naked eye, while the rest of the surface appears perfectly sound.
The danger: it can cause a tank or pipeline to perforate without any warning signs of widespread corrosion.
Analytical techniques: from the field to the laboratory
When visual inspection reaches its limits, the laboratory takes over to confirm the corrosion mechanism and assess how dangerous the defects are.
Metallography (microstructural section)
This is the reference test for pitting corrosion.
The principle: a sample is taken, polished to a mirror finish, then examined under an optical microscope or an electron microscope.
What we look for: the morphology of the pit. Is it narrow and deep (a "well" type) or wide and shallow? Metallography makes it possible to measure the actual depth of penetration, often far greater than the surface opening suggests.
SEM-EDX
For galvanic corrosion, it is essential to identify the chemical elements present in the corrosion products.
The principle: the SEM makes it possible to observe microcracks with extreme resolution, while the EDX probe analyzes the elemental chemical composition.
What we look for: the presence of chlorine (confirming pitting attack related to salts) or the transfer of ions from one metal to another (confirming a galvanic couple).
Electrochemical tests
Electrochemical tests are used to predict the behavior of an alloy in a specific environment or to validate cathodic protection.
The principle: polarization curves are plotted to determine the pitting potential (Epit).
The advantage: if the operating potential of your installation exceeds this Epit threshold, pitting corrosion will inevitably start. It is the ultimate tool for choosing the right material before construction even begins.
Salt spray test
The salt spray test is ideal for simulating severe exposure in accelerated conditions.
The principle: assemblies (for example, two metals screwed together) are placed in a chamber saturated with saline vapor.
The benefit: it is the gold-standard method for checking whether insulation between two metals (surface treatment, Teflon gasket) delivers on its promises against galvanic corrosion over the long term.
Conclusion: mastering corrosion mechanisms, an industrial imperative
Whether it is galvanic (resulting from a poor pairing of materials) or pitting (resulting from an invisible localized attack), corrosion is never inevitable. The key to a long-lasting infrastructure lies in the ability to move beyond simple visual observation toward a genuine scientific understanding of the phenomenon.
Knowing how to distinguish these two mechanisms is the first step toward choosing the right maintenance strategy:
For galvanic corrosion: contacts can be isolated or the hierarchy of potentials can be adjusted.
For pitting: more resistant alloys can be selected, or the chemical environment can be rigorously controlled.
When in doubt, an expert’s eye and laboratory analyses remain your best allies. A metallographic study or electrochemical test today will spare you costly production downtime or critical structural failures tomorrow.
Frequently asked questions
What is the difference between galvanic corrosion and pitting corrosion?
Galvanic corrosion results from the electrical coupling of materials with different electrochemical behaviors in a conductive environment. Pitting corrosion is a localized form of attack, often associated with a breakdown of the passive layer. The assembly context, environment, and morphology of the defects help guide the diagnosis, which may require several complementary examinations.
What signs should be looked for when diagnosing galvanic corrosion?
The presence of dissimilar metals in electrical contact, an electrolyte, and preferential degradation of one of the components together form a set of telltale signs. The examination also considers the geometry and relative surface area of the exposed materials. Observations and analysis must be considered in light of the service conditions to distinguish galvanic coupling from other corrosion mechanisms.
Why perform a metallographic cross-section on a part showing pitting?
A metallographic cross-section makes it possible to examine the shape of the cavities and their depth within the material. A limited opening at the surface may conceal more extensive penetration. This examination complements visual inspection and helps characterize the extent of the attack, taking into account the representativeness of the sampled areas and the specimen preparation.
Is the presence of chlorine detected by SEM-EDX sufficient to confirm pitting corrosion?
The detection of chlorine may support the hypothesis of a chloride-containing environment, but it does not, on its own, prove the corrosion mechanism. SEM-EDX provides information on the morphology and elemental composition of the areas examined. The diagnosis combines these results with metallography, the materials present, and the part’s exposure conditions.
Can an electrochemical test or salt spray test accurately predict the service life of an assembly?
These tests make it possible to compare performance under controlled conditions and assess certain protective measures. Their interpretation depends on the test protocol, the material, and the environment. They do not translate directly into service life: the results must be considered in relation to actual exposure, the assembly design, and the selected acceptance criteria.