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Corrosion is the electrochemical degradation of a metal in its environment, and it kills structures in several distinct ways. This topic maps the main failure modes from uniform attack to stress-corrosion cracking, with the Silver Bridge collapse as a forensic case study.
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Metal corrodes. Everyone knows that. What most people do not appreciate is that corrosion is not one thing , it is a family of distinctly different attack mechanisms, each governed by a different set of conditions, each leaving a different signature on the fracture surface, and each demanding a different investigation strategy when a structure collapses.
The 1967 Silver Bridge disaster over the Ohio River is the clearest illustration of why the distinction matters. Forty-six people died not because steel is weak, but because stress-corrosion cracking , a specific, mechanism-driven failure mode , had been slowly eating through an eyebar link for years without any visible surface evidence. By the time it reached critical crack length, the fracture took seconds. A post-mortem fractographic examination of the recovered link showed the characteristic intergranular, branching crack path of SCC in high-strength steel, along with a final brittle overload zone. The whole story was written on the broken metal.
This topic covers the main corrosion failure modes a forensic engineer will encounter: uniform, galvanic, pitting, crevice, intergranular, stress-corrosion cracking, corrosion fatigue, and selective-phase attack. For each, you need to understand the driving electrochemistry, the physical appearance, and the conditions that switch one mode into another. That knowledge is what lets you stand in front of a recovered structural element and read what happened to it.
The modes you can see and measure , and the one that surprises you at a joint.
Uniform corrosion is the electrochemical dissolution of metal more or less evenly across the exposed surface. It is the least deceptive mode: you can measure the section loss with a calliper or an ultrasonic gauge, you can estimate the corrosion rate in millimetres per year, and you can predict residual life. Failures attributed purely to uniform corrosion are usually management failures , the calculated service life was exceeded, or protective coatings broke down and were not repaired.
Galvanic corrosion introduces a spatial twist. When two metals with different positions on the galvanic series are in electrical contact in an electrolyte , seawater, condensed moisture, even high-humidity air , a current flows. The more active metal (the anode) corrodes preferentially. The practical importance is the area ratio: a large cathode coupled to a small anode drives severe, concentrated attack on the anodic metal. A bolt of a more active alloy in a large stainless plate can corrode through in a fraction of the time the uniform rate would predict.
The hidden geometry that drives catastrophic localised attack.
Pitting is dangerous precisely because it is invisible from the outside for most of its life. The passive film on stainless steel or aluminium breaks down at a point , triggered by a chloride ion breaching the oxide, a surface inclusion, or a surface defect. Once a pit forms, its interior chemistry becomes self-sustaining: oxygen is depleted inside the pit while the cathodic reaction runs on the large surrounding surface, the pit environment acidifies, and local chloride concentration rises. The pit grows inward far faster than the general surface loss would suggest.
Crevice corrosion follows the same chemistry but is driven by geometry. Any tight gap , under a gasket, between overlapping plates, inside a threaded connection , restricts oxygen replenishment. The differential aeration between the crevice interior and the open surface establishes the same driven micro-cell. Stainless steels that are immune to pitting in open seawater can suffer severe crevice attack at flanges.
| Feature | Pitting | Crevice corrosion |
|---|---|---|
| Initiation site | Passive-film breakdown point on open surface | Confined geometry with restricted O2 access |
| Driving chemistry | Differential aeration + acidification inside pit | Differential aeration between crevice and bulk |
| Typical materials at risk | Stainless steel, aluminium, titanium in Cl- media | Same materials wherever bolted or gasketed joints exist |
| Inspection challenge | Invisible until pit penetrates or causes fracture | Hidden under gaskets or inside joints unless disassembled |
| ASTM test method | ASTM G48 (pitting/crevice in stainless/Ni alloys) | ASTM G48 Method B (crevice testing with washers) |
In a failure investigation, pits matter as fatigue crack initiators even when they have not themselves penetrated the section. A pit concentrates stress by roughly a factor of three at its base, enough to start a fatigue crack in a component that would otherwise have survived indefinitely. Finding a pit at the origin of a fatigue fracture is one of the cleaner causal chains in metallurgical failure analysis.
A sustained load plus a hostile environment equals a crack that grows silently to failure.
Stress-corrosion cracking occurs when three things coincide: a susceptible material, a specific corrosive environment for that alloy system, and a sustained tensile stress above a threshold. Remove any one leg and SCC stops. That is why the forensic task is to prove all three were present and acting together, not just one.
Forty-six deaths from a crack that measured 2.5 mm deep in a link that had never been inspected.
The Silver Bridge at Point Pleasant, West Virginia, was an eyebar-chain suspension bridge built in 1928. Its design was unusual: instead of wire cables with many parallel load paths, the suspension system used chains of individual eyebars, each link carrying the full load. There was no redundancy. A single failed link brought everything down.
The National Bureau of Standards investigation, published in 1971, identified the failure in eyebar C13N. A stress-corrosion crack had grown from a corrosion pit on the inside of the eye head, where a finger of corrosive solution could penetrate the tight eyebar-pin contact. The steel was a high-strength carbon steel (ASTM A7 equivalent), susceptible to SCC in corrosive environments. The residual stress at the eye head, combined with the sustained bridge load, provided the tensile component. The Ohio Valley atmosphere , humid, mildly acidic from industrial pollution , was the corrosive environment.
The recovered fracture surface showed an approximately 2.5 mm deep SCC crack front, followed by a brittle fast-fracture zone that covered the rest of the section. There was no evidence of prior visual cracking or significant plastic deformation , the fracture was sudden once the crack reached the critical size for K_I to equal K_IC of the material. The investigation concluded that the bridge had never been inspected for internal crack development at the eye heads, a geometrically inaccessible location. The collapse led directly to federal bridge inspection standards in the US and elevated SCC to a first-order concern in high-strength steel infrastructure.
Cyclic loading meets a hostile environment, and the fatigue limit disappears.
Corrosion fatigue is distinct from SCC in one important way: it does not require a specific environment-material pairing. Any corrosive medium that degrades the passive film, lowers the surface energy for crack initiation, or anodically dissolves crack-tip material can accelerate fatigue. The S-N curve shifts downward , meaning failure at stress amplitudes that would be safe in air. More significantly, the fatigue limit (the stress below which failure does not occur in air) effectively disappears in a corrosive medium: cracks can initiate and grow at arbitrarily low cyclic stress if the environment is aggressive enough and time is long enough.
Selective-phase corrosion attacks the microstructure itself rather than the surface uniformly. Dezincification removes zinc from brass, leaving a porous copper skeleton that looks intact but has lost most of its strength and ductility. Graphitisation of grey cast iron dissolves the iron matrix, leaving a graphite network. In both cases, cross-section measurements can be misleading: the section dimensions are unchanged but the material that remains is structurally compromised. Cutting a section and doing a microhardness traverse or an energy-dispersive X-ray analysis quickly reveals the depleted zone.
When heat treatment turns the grain boundaries into corrosion highways.
Intergranular corrosion (IGC) preferentially attacks grain boundary regions rather than the grain interior. In austenitic stainless steels, the classical trigger is sensitisation: heating in the range 425 to 815 degrees Celsius , the welding heat-affected zone temperatures , causes chromium carbide to precipitate at grain boundaries, depleting the adjacent metal of chromium below the 12% threshold needed to maintain passivity. The boundaries become active relative to the grain interiors, and selective attack follows.
In a failure context, weld heat-affected zones are the first place to look for sensitisation in austenitic stainless. The Strauss test (ASTM A262 Practice E, boiling copper sulfate/sulfuric acid) and the Huey test (ASTM A262 Practice C, boiling nitric acid) are the laboratory evaluation methods. If the failure is in a sensitised zone, the next question is whether the material was specified correctly (low-carbon L-grade or stabilised Ti/Nb grade would have been resistant), whether post-weld solution annealing was carried out and verified, or whether there was a process upset that unexpectedly held the component in the sensitisation range.
What three conditions must simultaneously be present for stress-corrosion cracking to occur?
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