Corrosion is the price steel pays for its properties. It is a price that can be deferred for twenty years or paid after five, and the difference between the two is decisions taken before the first member reaches site. This article explains those decisions in the order they are made.
Note: The choice of protection system depends on the site environment classification and the target design life, and must be set out in a written specification for the specific project rather than copied from another one.
1. How does steel actually corrode?
Corrosion is an electrochemical reaction needing three things: metal, oxygen, and moisture. Remove any one and the reaction stops — which is exactly what a protection system does: it isolates the surface from moisture and oxygen.
Known factors accelerate the rate: salts (coastal environments), industrial pollutants and particularly sulphur compounds, permanent damp, and wet-dry cycling. This is why there is no such thing as a "good protection system" in the abstract — only a system appropriate to a particular environment.
One neglected point: corrosion from the inside. Condensation on cold internal surfaces creates a permanently damp environment in places routine visual inspection does not reach — more dangerous than external corrosion precisely because it is not seen until it has advanced.
2. How do you classify the site environment before choosing protection?
This is the first decision, and nothing after it stands if it is wrong. In practical terms:
- Dry, conditioned interior: the lightest requirements; a simple coating system suffices.
- Ordinary rural or urban exterior: a multi-coat system with appropriate surface preparation.
- Industrial: chemical pollutants demand a heavier system and defined chemical resistance.
- Coastal: a saline environment, the most severe. Galvanising or heavy-duty coatings, sometimes both.
State the internal use too: a building storing fertiliser or chemicals has a harsher internal environment than its external one.
3. Why is surface preparation half the life of the protection?
If one line in this article is worth remembering it is this: surface preparation before coating is half the life of the protection. An excellent coating over an untreated surface fails early, and an average coating over a well-prepared one lasts longer.
The reason is simple physics: coatings need a clean surface, rough enough to key into. Mill scale, rust, oils and deposited salts all prevent adhesion — so the coating looks sound and then separates from the steel as a complete sheet.
What belongs in the specification: the required surface preparation grade, the method (blasting, power tool, or manual cleaning), the target surface profile, and a time limit between preparation and coating — because a prepared surface begins to oxidise within hours.
4. Which coating system suits your site?
A coating system is not a product but a build-up of layers, each with a job:
- Primer: adhesion to the steel and electrochemical protection.
- Intermediate coat: thickness and a barrier against moisture and ions.
- Topcoat: UV resistance, appearance and colour.
The measure to compare on: total dry film thickness. Two quotations with the same coats can differ substantially in actual thickness, and thickness is what determines life.
And ask: is thickness actually measured with a gauge and recorded? Thickness measurement is simple and cheap, and its absence from the quality plan tells you enough.
5. When is galvanising better than painting?
Hot-dip galvanising gives protection of a different kind: the zinc layer does not only isolate the surface, it protects it electrochemically — corroding in its place even where a scratch reaches bare steel. Ordinary paint does not offer that.
Against it: higher cost, member size limited by the bath, a finish whose colour can only be chosen by overcoating, and the need for particular design details (drainage and venting holes in hollow sections).
In the most severe environments a duplex system is used: galvanising with a coating over it. More expensive, and substantially longer-lived — worth the calculation on projects where shutting down for maintenance costs more than the system itself.
6. Which points does corrosion start from?
- Site weld locations: heat destroys the protection around them, and every location needs treatment and recoating.
- Scratches during shipping and erection: inevitable; what matters is that they are treated, not denied.
- Faying surfaces and tight gaps: they hold moisture by capillary action and do not dry.
- Column bases at floor level: a zone of repeated wetting and salt deposition.
- Inside hollow sections: unless sealed properly or protected internally.
- Beneath purlins and at fixing points: contact and accumulation zones that a quick inspection does not reach.
This is why a good specification carries dedicated items for site touch-up: the repair material, how the location is prepared, the required thickness, and who inspects it.
7. What does maintaining the protection cost versus neglecting it?
A reasonable programme: an annual visual inspection of the coating and any localised rust; clearing gutters and drainage outlets before the rainy season; checking sealants around penetrations; and treating any scratch that reaches bare metal when it appears, not next season.
And the cost here is sharply asymmetric: treating surface rust across a square metre is negligible, while treating a member that has lost part of its section means structural strengthening or full replacement — with downtime, which is usually the most expensive line in the whole calculation.
That disparity alone justifies the annual inspection, even if it finds nothing in nine years out of ten.
- Corrosion
- Surface preparation
- Coating systems
- Galvanising
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