Failure mechanism
Corrosion and structural deterioration of steel
Corrosion removes section, and a reduced section carries less load. Whether the loss is uniform wastage or localised pitting, crevice or galvanic attack decides whether it can be measured and assessed or whether it stays hidden under a coating that still looks sound. The controlling variable in service is seldom the paint brand: it is the surface preparation beneath it and the corrosivity of the environment the system was selected for.
How this failure works
Uniform corrosion removes metal at a roughly even rate across an exposed surface, and it is the most forgiving form because the loss can be measured and the remaining section assessed against it. Pitting and crevice attack are not forgiving. They remove metal in small, deep locations, frequently beneath a coating that is intact everywhere except at the pit, and the penetration rate at a pit can exceed the general rate by a wide margin. Galvanic corrosion follows from two dissimilar metals coupled through an electrolyte, so a stainless fastener in a carbon steel member, or a copper bracket on galvanised steel, corrodes the less noble of the two — fastest where the area ratio is unfavourable, which is to say a small anodic area joined to a large cathodic one.
Coating selection begins with the corrosivity of the environment rather than with a product range. ISO 9223 sets the category from time of wetness, chloride deposition and sulphur pollution, and the category decides the protection system. A plant building with washdown, condensate, process fumes and airborne chloride can sit several categories above the same steel under a dry canopy a short distance inland, and a system specified for the lower one will fail from the outside in — most often at edges and fixings, where the film is thinnest and the geometry is worst. Getting the category wrong makes the coating choice wrong before any product is named.
For a given system the surface preparation grade governs service life far more than the product does. Mill scale, rust and soluble salts beneath a coating are the seed of its failure: the film bonds to them instead of to the steel, and corrosion proceeds under the coating until it lifts it off. ISO 12944 treats the preparation grade and the dry film thickness as the controlling parameters, and a blast to the specified grade at the specified thickness will outlast a premium product applied over a wire-brushed surface. Sharp edges, weld spatter, rough or undercut welds and skip-welded seams are the same problem relocated: a coating thins over an edge, pulls away from a sharp corner, and cannot be applied behind a skip weld at all.
The structural consequence is a change of section, and capacity does not fall in proportion to the metal lost. Loss on a compression flange matters more than the same loss on a web; loss on a slender member matters more than on a stocky one; loss at a connection, where the load is already concentrated, matters more than loss at mid-span. Effective section loss is also routinely underestimated because it is measured on the faces that can be reached and the damage is on the faces that cannot. That is why the practical control is to protect the section rather than the surface: the details where water stands, where two plates meet, where a hollow section is sealed without being dry inside, and where a member sits on a support that is permanently damp, are the details where the structure actually loses its capacity.
Illustration
Conceptual illustration of a section in cross-section showing uniform wastage, a pit through the wall, and a crevice between two back-to-back plates holding water.
Typical causes
- A coating system chosen for a lower corrosivity category than the environment actually imposes.
- Surface preparation below the specified grade, so the film is bonded to mill scale, rust or soluble salts.
- Coating damage at edges, in transit or at site connections that is never repaired.
- Details that hold water — two members back to back, a plate on a plate, a hollow section sealed damp, a member standing on a wet support.
- Dissimilar metals in contact under an electrolyte, with a small anodic area coupled to a large cathodic one.
- Washdown, condensate, a leaking gutter or a process spill keeping the surface wet for long periods and carrying chlorides in.
- Inspection that reads the accessible faces and never the underside, the connection or the base of the column.
Warning signs
- Rust staining at an edge, a bolt or a weld — the first visible indication that a coating has already failed there.
- Blistering, flaking, or a chalked surface that rubs off on the hand, which is an aged film rather than a sound one.
- A hollow or drum-like sound when a plate or a section is tapped, suggesting under-film or internal corrosion.
- Water standing in a section, around a base plate, or on a horizontal surface that was never detailed to drain.
- Localised pitting, or one small patch gone thin while the steel around it is still full section.
- Deflection or vibration in a member that was not there before, which is what a loss of section looks like from outside.
Inspection methods
- 01Record the environment as a corrosivity category, and compare the coating system in place against what that category requires.
- 02Assess the coating condition systematically by area and by detail, including undersides, connections, bearing points and column bases.
- 03Measure remaining section where it governs — compression flanges, slender members and connection regions — by direct measurement or thickness gauging.
- 04Examine crevices, back-to-back contacts, hollow section interiors and the interface at a support, which is where a surface walkover misses the real loss.
- 05Look for the cause alongside the effect — a leaking gutter, a failed sealant line, a condensate drip — because recoating without curing the source repeats the failure.
- 06Record findings with the section loss quantified where it governs, so that a structural assessment has a measured number rather than a description.
Preventive measures
- Classify the environment first, then specify the coating system and dry film thickness that the category requires.
- Specify and verify the surface preparation grade and the soluble salt level, and do not let a thick system hide a poorly prepared surface.
- Detail for drainage and access: no water traps, no unsealed back-to-back flats, hollow sections closed dry, and every surface reachable for maintenance.
- Isolate dissimilar metals, and select the coating and the fasteners as one system rather than as two purchases.
- Repair coating damage when it occurs, and treat site connections and touch-up as a scheduled operation rather than an afterthought.
- Inspect on a defined cycle with a written record, so that section loss is measured against a baseline instead of estimated from memory.
- Assess the structure on the section that remains rather than the section it was designed with, once corrosion has been allowed to progress.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- ISO 12944
- ISO 9223
- IS 800
- IS 3177
- IS 4759
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.