What does the corrosion product on a failed component reveal?
The corrosion product on a failed component reveals why the metal was lost: metal loss tells you how much was removed, but what that metal turned into tells you why. Corrosion products are compounds formed from the metal and the environment, so their composition reflects the species present at the corroding surface — not the species present in a bulk sample taken elsewhere and much later.
Scanning electron microscopy with energy-dispersive spectroscopy is the workhorse technique for analyzing corrosion product, imaging pit morphology while identifying the elements in the scale, the deposit and the pit interior separately. X-ray diffraction goes further and identifies the corrosion compounds themselves rather than only the elements present.
What can the chemistry of corrosion product actually tell an investigator?
Corrosion product chemistry can point toward microbiological involvement, local passive-film breakdown or a galvanic path, and several findings do real diagnostic work. Sulfur-bearing corrosion products where no process sulfur explains them raise the possibility of microbiological involvement. Chloride concentrated inside a pit but not on the surrounding surface supports local passive-film breakdown rather than general attack. Elements from a coupled dissimilar metal at the attack site indicate a galvanic path.
None of these corrosion product findings survive cleaning. Chloride enrichment within an occluded pit is measured in the pit, and rinsing removes exactly the soluble species that carry the signal.
Can layers of corrosion scale show the sequence of what happened?
Layers of corrosion scale can show sequence when the scale is left intact. Corrosion scale is often stratified, with the innermost layer formed earliest against the metal and later products deposited over it, so cross-sectioning through an intact deposit and mapping composition across those layers can show that the chemistry changed during service — a treatment change, a contamination event, an upset.
Layer analysis is one of the few ways a corrosion investigation reaches sequence rather than only state. The layered record in corrosion scale is also the first thing lost to a wire brush, which mixes and removes the layers in one pass.
Should a corroded component be cleaned on site or in the laboratory?
A corroded component should be cleaned in the laboratory, not on site. Corroded specimens do eventually get cleaned: measuring mass loss or examining the metal beneath the corrosion product requires it, and ASTM G1 sets out the accepted cleaning procedures along with the repeated-cleaning approach that distinguishes product removal from attack on the base metal.
The difference is that laboratory cleaning of a corroded specimen happens after documentation and after the removed corrosion product has been sampled and analyzed, using a method selected for the alloy and recorded in the report. Site cleaning happens before anyone knows what was there, with no record of what was removed.
What does each cleaning method destroy on a corroded surface?
Abrasive blasting, wire-brushing, grinding and acid descaling each destroy corrosion evidence in a different way. Abrasive blasting removes the deposit entirely and works the surface underneath, rounding pit mouths and imposing a texture that obscures the original morphology. Wire-brushing and grinding do the same more selectively and can smear metal across pit openings, making pit depth and shape unreadable.
Acid descaling is the most damaging cleaning method because it appears the gentlest. Uninhibited acid dissolves corrosion product and base metal together, deepening the very features being measured, and it removes any chance of identifying what it dissolved. Re-coating a failed section seals whatever corrosion evidence remains under a new layer and adds a fresh contamination source to every later analysis.
What should be documented before anyone touches a corroded component?
Before any intervention, a corroded component should be photographed in place: the sequence that preserves a corrosion case is photograph, sample, then remove. In-place photography with scale and orientation, showing the deposit and the surrounding condition before anything is disturbed, is frequently the only record of the as-found state that ever exists.
ASTM E860 addresses examining and testing items that are or may become involved in litigation, and ASTM E1188 addresses collection and preservation of information and physical items by a technical investigator. ASTM E860 and ASTM E1188 point the same way: fix the condition in the record before altering it, and document the alteration when it happens.
What else should be collected after a corrosion failure besides the component?
Beyond the failed component itself, a corrosion investigation needs a sample of the deposit or scale taken dry and undisturbed; the process fluid, water or soil, sampled promptly because chemistry changes once the system is opened; coating or lining remnants; and any insulation in contact with the corroded surface.
The paper record of a corrosion failure is perishable in its own way. Water treatment logs, thickness survey histories, cathodic protection readings, coating submittals and maintenance work orders all sit under retention schedules, and requesting them early costs nothing.
How should a corroded section be cut out without wrecking the evidence?
A corroded section should be cut out well clear of the damage, preferably by cold cutting, with its orientation marked so it survives removal. Removal is itself an intervention. Cold cutting is preferred because flame cutting introduces heat that alters microstructure near the cut and can transform corrosion products across a wider area than expected.
Marking flow direction, the twelve o’clock position and which face was internal preserves the link between the damage location on a corroded section and its service condition. Without orientation marks, a section showing exactly what happened can no longer show where.
How do you balance restoring service against preserving corrosion evidence?
Restoring service and preserving corrosion evidence can usually be separated by deliberate sequencing; the distinction worth drawing is between restoring service and destroying the record. The pressure to clean up is legitimate. A leaking line has to be isolated and repaired, and an operator running an outage has obligations of its own.
Where a corroded surface has already been cleaned, the analysis is narrowed rather than ended. Adjacent uncleaned areas, the removed material if it was captured, mating parts and residues in low points may still hold corrosion product. Aggressive cleaning also erases the secondary cracking that would otherwise flag stress-corrosion cracking as a separate mechanism. A report stating plainly what was cleaned, by what method and when is worth far more than one proceeding as though the surface were as found.
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.