Why corrosion is steel’s main enemy

Unprotected steel exposed to air and moisture starts to rust within a few weeks. Corrosion reduces the cross-section of members, weakens connections and, over time, can affect the load-bearing capacity of the structure. That is why corrosion protection is not a cosmetic detail but an essential part of the design, as important as structural sizing.

Step one: the corrosivity category of the environment

The international standards of the ISO 12944 series divide environments into corrosivity categories, from C1 (heated, dry interiors) to C5 (very aggressive industrial and marine environments) and CX (extreme). Some indicative examples:

  • C2 – unheated warehouses, rural areas with low pollution.
  • C3 – urban or moderately industrial environments, production buildings with moderate humidity.
  • C4 – industrial areas, livestock farms, swimming pools, some food plants.
  • C5 – chemical industry, wastewater treatment plants, permanently high humidity.

The second decision is the desired durability of the protection: low, medium, high or very high, meaning up to more than 25 years before the first major maintenance.

Hot-dip galvanizing

After pickling, the steel element is immersed in a bath of molten zinc at approximately 450°C. This forms a layer of iron-zinc alloys metallurgically bonded to the steel, covered with pure zinc.

  • Advantages: protection on all surfaces, including inside closed sections; cathodic protection – the zinc “sacrifices” itself even at small scratches; very high durability in C2–C3 environments; minimal maintenance.
  • Limitations: element size is limited by the size of the bath; possible distortion of thin, long parts; gray appearance, not always desired; on-site repairs are made with zinc-rich paints.

Multi-coat paint systems

An industrial paint system includes surface preparation by blast cleaning, an anti-corrosion primer, often zinc-rich or epoxy, one or more intermediate coats and a topcoat, usually a UV-resistant polyurethane.

  • Advantages: any color in the range; no limit on element size; the system can be matched precisely to the corrosivity category; can be combined with intumescent paints for fire protection.
  • Limitations: quality depends heavily on surface preparation and application conditions – temperature, humidity, thickness; requires periodic renewal.

Surface preparation – the key to durable painting

A large share of paint defects stem from inadequate preparation. Blast cleaning to the appropriate cleanliness grade and controlling surface roughness ensure adhesion. Measuring dry film thickness with dedicated gauges should be mandatory practice.

The duplex system

For aggressive environments or a very long service life, a duplex system is used: hot-dip galvanizing followed by painting. The combined effect is greater than the sum of the two: the paint protects the zinc, and the zinc prevents rust from forming under the paint. It is a common choice for wastewater treatment plants, livestock farms and exposed outdoor structures.

Rule of thumb: galvanizing is often the most cost-effective long-term option for medium-sized elements in moderate environments; painting offers flexibility in color and size; duplex is the choice for aggressive environments.

Design details that matter

  • Avoiding areas where water and dirt accumulate.
  • Vent and drain holes for elements to be galvanized.
  • Rounded edges, where paint adheres better.
  • Access for inspection and later repairs.

Maintenance

Regardless of the system, the structure must be inspected periodically and local damage repaired promptly. The cost of a local repair is small compared to a full rehabilitation.

At the S.A. COLASS production plant, corrosion protection is built into the fabrication workflow, and the system is chosen based on the service environment of each project. For a specific recommendation, call +373 78 30 40 40.