Corrosivity categories C1 to CX: classifying the environment before choosing the material

A corrosivity category describes the place, not the product. EN ISO 9223 sorts atmospheres into six classes from C1 to CX, and EN ISO 12944 uses that classification to decide how much protection a steel structure needs and how long that protection should last. Naming the category at the enquiry stage costs nothing. Getting it wrong shows up three winters later as rust on a structure that was specified correctly for somewhere else.

Two standards with two different jobs

The categories are often called the ISO 12944 classes, which is only half right. One standard defines them and another puts them to work.

The practical value is that everyone uses the same words. When a galvanizer, a paint supplier and a steel stockholder all say C4, they mean one defined environment rather than three different impressions of the word severe.

The six atmospheric categories

Each category is described twice, once for outdoor exposure and once for interiors, because a heated office and an unheated warehouse are not the same environment even inside the same building.

Distance from the sea is the single most useful outdoor indicator. As a guide, sites roughly 1 to 30 km inland usually sit in C3, while the first kilometre from open water is normally treated as C4. Prevailing wind matters more than the map: a windward coastal site collects more airborne chloride than a sheltered one at the same distance.

Behind the descriptions there is measurement. EN ISO 9223 defines each class by the first year loss of standard specimens of carbon steel, zinc, copper and aluminium exposed on site, which is why one classification can serve several materials. What that zinc loss means in years of service is set out in our guide to galvanised steel corrosion protection.

Water and soil are classified separately

The C categories cover atmospheric exposure only. Anything buried or immersed sits in its own set of classes.

One structure often spans two classes. A fence post concreted into the ground has a buried length in Im3 and a length above ground in C3 or C4, and the transition at ground level is where posts actually fail. The same split applies to ground mounted solar structures, where the pile sits in soil while the frame it carries sits in air.

How to work out your category without test panels

EN ISO 9223 offers two routes. One is measurement: expose standard specimens for a year and read the loss. The other is estimation from three environmental parameters, namely time of wetness, sulphur dioxide deposition and airborne chloride deposition.

Neither is available when a quotation is due on Thursday. In practice four questions settle it well enough to specify against.

• How far from open water, and on which side? Under a kilometre with an onshore wind moves the answer to C4 on its own.
• Is there heavy industry, a busy road or de-icing salt nearby? Traffic spray carries chloride well beyond the carriageway, and further in wind.
• Is the steel outdoors, in an unheated building, or in a heated one? That single distinction separates C1 from C3 without any measurement at all.
• Does rain wash the surface, or does it only get wet? Washed surfaces shed the salt that unwashed surfaces accumulate.

Where two answers disagree, take the higher category. The cost difference between C3 and C4 protection is far smaller than the cost of replacing a structure that was specified a class too low.

The microclimate beats the map

A category applies to a location, but corrosion happens on a surface. The two diverge more often than most specifications allow for.

Sheltered surfaces are the classic case. The underside of a canopy, the back of a fascia, the inner face of a channel: none of them are rained on, so the chloride and sulphates that land there stay there and are never rinsed away. Such details behave one category worse than the exposed steel a metre away, on the same structure and in the same air.

Crevices are the second case. Bolted laps, back to back angles and any gap that holds water by capillary action stay wet long after the frame around them has dried, and time of wetness is one of the three parameters the whole classification is built on. Detailing that drains, and connections that can be inspected, are worth more than an upgrade of the coating.

The third case is storage before installation. Material stacked wet and tight corrodes in the yard at a rate that has nothing to do with the category of its final home, which is covered in the guide to storing steel products.

Durability is a range, not a guarantee

A category on its own is half a specification. The other half is how long the protection has to last, and EN ISO 12944-5 gives that a defined vocabulary.

The standard is explicit that this is not a guarantee period. It is a planning parameter that helps an owner budget maintenance, while the contractual guarantee is a separate matter and is normally shorter. Writing C4 alone leaves the supplier to guess. Writing C4 high says what is actually wanted.

Galvanising thickness does not rise with the category

This is the point that surprises buyers most often, and knowing it saves a lot of pointless argument with galvanizers.

Under EN ISO 1461 the minimum coating thickness follows the thickness of the steel being dipped, not the environment it will stand in. Steel over 6 mm takes a minimum mean coating of 85 micrometres, steel from 3 to 6 mm takes 70, steel from 1.5 to 3 mm takes 55, and thinner material takes 45. A C5 site does not change any of those numbers, because the reaction that forms the coating knows nothing about where the part is going.

A harsher category is therefore answered by changing the system rather than by asking for thicker zinc. The usual moves are a duplex system of galvanising plus paint, a zinc magnesium coated coil product, a heavier section, or a different material entirely. The process differences behind those choices are in the comparison of hot-dip and electro-galvanised coatings, and the coil coating classes are in the guide to DX51D and galvanised steel.

What the category changes, material by material

The classification is shared across materials, but each material answers it in a different line of the specification.

Aluminium deserves a note of its own, because the category question is often asked the wrong way round for it. The metal carries its own oxide film and performs well in C4 and C5 air, so the risk in an aggressive category is rarely general corrosion. It is pitting in chlorides and galvanic attack where aluminium meets another metal, both of which are set out in does aluminium corrode. Where powder coating goes near the coast, the difference that decides the outcome is a deeper pretreatment etch, roughly twice the standard amount of aluminium removed before coating, together with filiform corrosion testing that the base coating standard does not require. The finishes themselves are compared in the guide to anodising and powder coating.

Choosing a stainless grade from the environment

Stainless steel has no coating to consume, so it is not classified the way a paint system is. It has a condition instead: the passive film survives, or chlorides break it locally and it does not heal.

EN 1993-1-4 Annex A turns that into a procedure. Three factors are scored: exposure to chlorides from sea air or de-icing salt, exposure to sulphur dioxide, and the cleaning regime including whether rain washes the surface. Together they give a corrosion resistance factor, which maps to a corrosion resistance class, which in turn lists the grades suitable for the job.

Two consequences are worth carrying into an enquiry. Washing counts as heavily as location, so two balustrades in the same town can need different grades if one is rinsed by rain and the other sits under a canopy. And the standard treats grade selection as a structural decision rather than a preference, which is why 304 and 316 are chosen by exposure, and why duplex 2205 appears where strength and chloride resistance are both required.

What to put in the enquiry

Five lines turn a corrosion question into a specification that can be quoted and checked:

• The category and the reason for it, for example C4 because the site is 600 m from the coast.
• Any part that is buried or immersed, with its Im class, because the C categories do not cover it.
• The durability range required, low, medium, high or very high.
• The protection standard, whether that is EN ISO 1461, a zinc class to EN 10346, a paint system to EN ISO 12944-5, or a stainless grade.
• Whether a 3.1 inspection certificate under EN 10204 is required, as explained in the guide to material certificates.

The second line is the one most often missing, and buried transitions are where structures fail first.

Material for the environment, from ULAMEX

ULAMEX supplies galvanised steel, stainless steel, black steel and aluminium from stock in Zawada, Poland, to customers across continental Europe. The desk is used to enquiries that begin with the environment rather than with a product code, so tell us where the structure stands and what it has to survive, and the recommendation can start from the category instead of a catalogue page.

Welded load-bearing structures fabricated by ULAMEX carry CE marking to EN 1090-1 EXC2, with welding under an ISO 3834-2 quality system. A 3.1 inspection certificate under EN 10204 is available on request. The company has traded in steel and aluminium since 1988, that is 38 years.

There is no minimum order on stocked SKUs, and delivery within continental Europe usually takes 2 to 7 working days. Send the application, the location and the quantity through our contact page, or read what the supply desk covers on the page of capabilities, and we will come back with a non-binding quotation.

Frequently asked questions