Understanding what corrosion is, how it starts and what stops it is the foundation of any effective asset protection strategy. This guide covers the main types of corrosion affecting steel structures, the environmental factors that drive them, and why surface preparation is the most critical variable in preventing coating-protected corrosion.
What Is Corrosion? The Electrochemical Mechanism
Corrosion is not simply “rust.” It is a spontaneous electrochemical process in which a metal returns to its thermodynamically stable oxide state. For steel (iron alloys), the driving force is the natural tendency of iron to oxidise:
Fe → Fe²⁺ + 2e⁻ (anodic reaction — metal dissolves)
O₂ + 2H₂O + 4e⁻ → 4OH⁻ (cathodic reaction — oxygen reduced)
For corrosion to occur, four components must be simultaneously present: an anode (the metal being corroded), a cathode (a more noble surface), an electrolyte (water or moisture containing dissolved ions), and a metallic path connecting anode and cathode. Remove any one of these four elements and corrosion stops. Protective coatings prevent corrosion primarily by acting as a barrier to the electrolyte — which is why coating adhesion to the steel substrate is everything.
The 8 Main Types of Corrosion in Steel Structures
1. Uniform (general) corrosion
The most visible and predictable form — the metal corrodes at roughly the same rate across the entire exposed surface. Produces the characteristic red-brown iron oxide (rust) on carbon steel. Manageable through coating, cathodic protection or allowance for material loss in design. The least dangerous in terms of unpredictability, but the most common cause of long-term mass loss in unprotected or poorly coated structures.
2. Pitting corrosion
Highly localised attack that creates deep pits in the metal surface while leaving the surrounding area relatively intact. Driven by chloride ions, which break down the passive oxide film on the metal locally. Particularly dangerous in pressure vessels, pipelines and offshore structures because pitting can penetrate the full wall thickness without visible surface deterioration. NACE International considers pitting corrosion the most dangerous form for load-bearing steel components.
3. Crevice corrosion
Occurs in geometrically confined spaces — under bolt heads, between overlapping plates, inside socket connections — where oxygen depletion and chloride concentration create an aggressive local chemistry. The confined geometry prevents the cathodic reaction from neutralising the acidic anodic environment, accelerating attack. Common in marine structures, heat exchangers and flanged connections.
4. Galvanic corrosion
When two dissimilar metals are electrically connected in the presence of an electrolyte, the less noble metal (lower in the galvanic series) corrodes preferentially. Common examples: aluminium fittings on steel structures; zinc-coated fasteners in contact with copper; mill scale (magnetite) in contact with bare steel at discontinuities. Mill scale is cathodic to bare steel, making any cracked or damaged mill scale a galvanic corrosion initiation site — one reason mill scale removal before coating is mandatory.
5. Stress corrosion cracking (SCC)
The simultaneous action of tensile stress and a corrosive environment produces cracking that would not occur from either factor alone. Affects high-strength steels, stainless steels in chloride environments and pipelines operating under internal pressure. SCC cracks propagate intergranularly or transgranularly and can lead to sudden brittle fracture with no preceding visible corrosion. Particularly relevant in oil and gas pipeline integrity management.
6. Erosion-corrosion
The combined effect of mechanical wear (from flowing fluid, suspended solids or cavitation) and electrochemical corrosion. The mechanical action continuously removes the protective oxide layer and any coating, exposing fresh metal to the corrosive environment. Prevalent in pipeline bends, pump impellers, heat exchanger tubes and any surface exposed to high-velocity fluid flow.
7. Filiform corrosion
Thread-like corrosion that develops under a coating film, particularly on aluminium and steel with thin organic coatings in humid environments. Starts at a coating defect and propagates as a filament with an active head (anodic) and a corrosion product tail (cathodic). Cosmetically severe and destructive to coating adhesion, though typically not structurally critical unless very widespread.
8. Microbiologically influenced corrosion (MIC)
Corrosion accelerated by the metabolic activity of micro-organisms — sulphate-reducing bacteria (SRB), acid-producing bacteria or metal-oxidising bacteria. Common in buried pipelines, water storage tanks, marine sediments and cooling water systems. MIC can accelerate corrosion rates by 100× compared to purely electrochemical processes and is responsible for a significant proportion of internal pipeline failures.
Environmental Factors That Drive Corrosion Rate
| Factor | Effect on Corrosion Rate | Critical Threshold |
|---|---|---|
| Relative humidity | Corrosion rate increases sharply above 60% RH; electrolyte film forms on steel surface | Critical humidity: ~60% RH. At 80%+ RH, rate increases by order of magnitude |
| Chloride concentration | Chlorides break down passive films, increase electrolyte conductivity, initiate pitting | >5 µg/cm² under a coating accelerates osmotic blistering; >100 µg/cm² on bare steel causes rapid pitting |
| Temperature | Higher temperature increases electrochemical reaction rates; also accelerates oxygen diffusion | Every 10°C rise approximately doubles the corrosion rate in aqueous environments |
| pH | Steel corrodes fastest in acidic environments (pH <4) and is passivated in highly alkaline (pH >12) | Neutral to slightly alkaline environments (pH 7–11) are preferred for cathodic protection systems |
| Oxygen concentration | Oxygen drives the cathodic reaction; both its presence and absence can be corrosive depending on geometry | Differential aeration (high O₂ vs. low O₂ zones) creates concentration cells that drive crevice and pitting attack |
| Dissolved salts / conductivity | Higher ionic conductivity in the electrolyte accelerates all electrochemical reactions | Seawater (~35,000 ppm salinity) is 100× more corrosive than fresh water to bare steel |
Why Surface Preparation Is the Most Important Variable in Corrosion Prevention
The majority of protective coating failures in industrial and marine environments are not caused by defective coating formulations. They are caused by inadequate surface preparation. Research by NACE International and major coating manufacturers consistently shows that 70–80% of premature coating failures can be attributed to one of three surface preparation deficiencies:
Residual mill scale. Mill scale left on the surface is cathodic to bare steel. Any coating applied over mill scale bonds to the scale, not to the steel. When the scale corrodes and spalls, the coating delaminates with it. See our detailed guide on mill scale removal.
Soluble salt contamination. Chlorides, sulphates and nitrates remaining on the surface after preparation create osmotic blisters under the coating as they absorb moisture and build concentration gradients. Even a few µg/cm² of chloride under a high-performance epoxy coating can cause blistering within months in a marine environment.
Insufficient anchor profile. Coatings mechanically interlock with the surface anchor profile. A profile that is too low (typically <40 µm Rz) provides insufficient adhesion area for high-build systems. A profile that is too high leaves uncoated peak tips that become corrosion initiation points. The correct range is specified in the coating manufacturer’s technical data sheet (TDS) — typically 50–100 µm Rz for marine epoxy and zinc silicate systems.
Corrosion Protection: The Prevention Hierarchy
In order of effectiveness for steel structures in industrial and marine environments:
- Correct surface preparation to the specified standard (SSPC-SP10 / Sa 2½ minimum for high-performance systems) — the foundation of all other protective measures
- High-performance protective coatings — high-build epoxy, zinc silicate, polysiloxane applied over a properly prepared surface
- Cathodic protection — sacrificial anodes or impressed current systems; effective complement to coatings but not a substitute for preparation
- Material selection — stainless steel, corrosion-resistant alloys or weathering steel where appropriate
- Design for corrosion resistance — eliminating crevices, ensuring drainage, avoiding bimetallic contacts
For the full technical specification of what SSPC-SP10 requires and how to verify it in the field, see our SSPC-SP10 complete technical guide. For a comparison of surface preparation methods and their suitability for different site conditions, see our article on Bristle Blaster® vs. sandblasting.
Frequently Asked Questions
What is the difference between corrosion and rust?
Rust is a specific form of corrosion — the hydrated iron oxide (Fe₂O₃·nH₂O) that forms on iron and carbon steel in the presence of water and oxygen. Corrosion is the broader electrochemical process that produces rust in iron alloys, and also produces green patina on copper, white powder on aluminium and tarnish on silver. All rust is corrosion, but not all corrosion produces rust.
How quickly does steel corrode?
Uncoated carbon steel in a moderate industrial atmosphere loses approximately 0.05–0.1 mm per year to general corrosion. In marine splash zones or highly acidic industrial environments, rates of 0.5–1.0 mm/year are documented. Pitting corrosion can penetrate far faster — 1–3 mm/year in localised areas. These rates make it clear why even a short gap in coating protection leads to significant material loss.
Can corrosion be reversed?
No. Corrosion is an irreversible chemical process — iron that has converted to iron oxide cannot be spontaneously converted back to iron under service conditions. Corroded metal can only be managed: the corrosion products are removed (surface preparation), the damaged section is replaced, and the surface is re-protected. This is the maintenance cycle that drives the industrial surface preparation market.
What surface preparation standard is required before painting steel?
This depends on the coating system and the service environment. For high-performance coatings in marine, offshore and industrial environments, the minimum standard is typically SSPC-SP10 / ISO 8501-1 Sa 2½ (near-white metal blast). SSPC-SP5 / Sa 3 (white metal) is required for very aggressive environments, immersion service and under thermal spray coatings. SSPC-SP6 (commercial blast) is acceptable for some atmospheric exposure systems with lower durability requirements.
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