Corrosion is not an event but a process with a known rate. It cannot be “prevented” once and for all — it can only be slowed to a rate at which the structure survives its design life. Everything else in corrosion protection follows from this idea. Below we look at what actually destroys steel in our climate, what protection methods exist and how the right one is chosen.

What corrosion really costs

Direct corrosion losses worldwide are estimated at 3–4 % of gross product. For an asset owner, however, a different figure matters more: re-protection almost always costs more than the original protection. The reason is simple — the first time, clean steel is coated at ground level or in a shop; the second time, production must be stopped, scaffolding erected, the old coating and corrosion products removed, and only then can painting begin.

A typical ratio in our experience: surface preparation and access account for most of the budget, the material itself for far less. This is precisely why saving on the material rarely pays off — you save a small share of the cost and put the rest at risk.

Why a standard specification from elsewhere does not transfer to Uzbekistan

Corrosion rate is not driven by “aggressive air” in general, but by three factors together: time of wetness, the concentration of sulphur compounds in the air, and the concentration of chlorides.

Our climate is sharply continental and dry. Time of wetness — the number of hours per year when a film of moisture sits on the steel — is markedly lower than in coastal or northern regions. By this measure our conditions are milder.

Two circumstances cancel that advantage out:

Daily temperature swings. Steel heats up during the day and cools sharply at night. Condensation forms on the surface — exactly the wetting that “should not” occur given the climate. For a storage tank or an uninsulated structure this can be the primary source of moisture.
Extreme ultraviolet. And here is where the most common mistake lies. Ultraviolet does not cause corrosion — it destroys the coating that prevents it. Once the coating has degraded, the steel beneath starts rusting by the ordinary rules.

The difference between “the climate is mild” and “the coating degrades faster” is invisible on site until the third or fourth year. A separate article in this series is devoted to that subject.

Corrosivity categories: the language specifiers use

ISO 12944 is the principal international standard for the protection of steel structures by protective paint systems. It divides environments into categories, and all further reasoning starts from them:

C1 — very low. Heated buildings with clean air.
C2 — low. Unheated buildings; rural areas with clean atmosphere.
C3 — medium. Urban and industrial atmospheres, moderate sulphur dioxide pollution; production areas with high humidity.
C4 — high. Industrial areas, coastal areas with moderate salinity, chemical plants.
C5 — very high. Industrial areas with high humidity and aggressive atmosphere; coastal areas with high salinity.
CX — extreme. Offshore structures, areas with near-permanent exposure to salt and high humidity.

The standard also defines immersion categories: Im1 (fresh water), Im2 (sea and brackish water), Im3 (soil). The inside of a tank holding an aggressive medium is no longer an atmospheric problem, and it is approached differently. A separate article covers it.

For most facilities in Uzbekistan the governing category proves to be C3; for chemical and oil-and-gas sites, C4, occasionally C5. But a category is not “assigned by city”: it is determined by the actual conditions at a specific site, and neighbouring facilities may well fall into different categories.

Durability — the other half of the specification

The corrosivity category alone is not enough. The second mandatory input is the design durability of the coating to first major maintenance. ISO 12944 divides it into ranges: low (up to 7 years), medium (7–15 years), high (15–25 years) and very high (over 25 years).

It is the pair “environment category + required durability” that defines the coating system: how many coats, of what type, and to what total thickness. Without that pair, any discussion of materials is meaningless. When a contractor is told to “paint it properly”, there is no objective criterion, and any quotation is guesswork.

An illustration: one and the same epoxy primer-finish coating may have a working dry film thickness range from 75 to 275 µm. That is a nearly fourfold spread, and it is governed by service conditions and required durability — not by the applicator’s preference.

Four ways to fight corrosion

1. Protective paint systems

A barrier principle: the coating isolates the steel from moisture and oxygen. The most widespread, most flexible and most maintainable method. A typical system for C4–C5 consists of three coats: a zinc-rich or epoxy primer, an intermediate epoxy coat that builds most of the thickness, and a polyurethane topcoat responsible for UV resistance and appearance.

The point most often ignored in our region: epoxy on its own does not withstand sunlight. Outdoors it chalks, loses gloss and gradually degrades. It is excellent as a build coat, but a UV-resistant topcoat above it is mandatory.

2. Metallic coatings: galvanizing and thermal spray

Hot-dip galvanizing provides sacrificial protection — the zinc corrodes first, protecting the steel electrochemically even at scratches. It is durable, but limited to what fits in the bath and unsuitable for erected structures. Thermal spraying of zinc or aluminium removes the size limit but demands equipment and skill.

In practice galvanizing and painting are often combined into a duplex system, whose service life exceeds the sum of the two methods taken separately.

3. Cathodic protection

Sacrificial anodes or impressed current are a working solution for buried pipelines, tank bottoms and immersed structures. It does not replace coating: in practice the two are used together, and the coating sharply reduces the protective current required.

4. Choice of construction material

Stainless and low-alloy steels solve the problem at design stage, but cost significantly more and are justified where access for maintenance is impossible or where painting is ruled out by the process.

Where service life is actually lost

Reviewing the facilities we are called in to rework, we see the same short list every time:

Surface preparation. The leading cause of failure, by a wide margin. Any system applied to poorly prepared steel will delaminate regardless of what the material cost. A separate article covers this.
Thickness by eye. The thickness stated in the specification and the actual thickness on site are different quantities unless someone has used a gauge. A shortfall on the intermediate coat eats years of service life.
Epoxy without a topcoat outdoors. See above on ultraviolet.
Incompatible coats from different suppliers. A coating system is specified as a whole, not assembled from whatever is in the store.
Welds, edges and bolted connections. Coating on an edge is always thinner than on a flat surface, and that is where rusting starts. The remedy is stripe coating — a separate brush-applied pass before the main coats.

In brief

Corrosion protection is not “a more expensive paint” — it is a calculation. It starts with two figures: the corrosivity category of the environment and the required durability. From them follows the coating system; from the system follow the surface preparation requirements and the thickness checks on site. Omit any link and the rest loses its value.

Further articles in this series cover, separately: internal protection of tanks holding aggressive media; surface preparation and abrasive blast cleaning; coating behaviour under our ultraviolet; and, separately, tanks for aviation fuel storage.

Corrosion of steel structures: consequences and protection methods