Durability of hot dip galvanized coatings

Hot dip galvanizing has proven to be more serviceable and predictable than all other steel protective coatings in the atmosphere. Its excellent performance is due to its inherent corrosion resistance, high tolerance to mechanical damage and inertness to UV.

The corrosivity of particular environments have been widely researched and the corrosion rates of both steel and zinc are classified in regional and International Standards as a function of the temperature, relative humidity, the amount of airborne salinity and the amount of airborne pollution present.

The life of a hot dip galvanized coating is (to a first approximation) proportional to its thickness, which is normally a function of the article’s steel thickness.

The table shows that fabricated articles more than 6mm thick in a C4 corrosivity zone will be protected from rust for a minimum of 20 – 40 years where the lower life is when the article is closer to the more corrosive area.

Corrosivity categories and corrosion rates of steel and zinc as per ISO 9223 Expected service life for hot dip galvanized articles as per ISO 1461 (years)
Corrosion rate for the first year of exposure  (µm/y) Fabricated Article Thickness (mm)
Corrosion Category Typical environment Steel HDG < 1.5 ≥ 1.5 to ≤ 3.0 > 3.0 to ≤ 6.0 > 6.0
C1 Dry indoors ≤1.3 ≤0.1 100+ 100+ 100+ 100+
C2 Arid/Urban inland >1.3 to ≤25 >0.1 to ≤0.7 64 to 100+ 78 to 100+ 100+ 100+
C3 Coastal or industrial >25 to ≤50 >0.7 to ≤2.1 21 to 64 26 to 78 33 to 100 40 to 100+
C4 Calm sea-shore >50 to ≤80 >2.1 to ≤4.2 11 to 21 13 to 26 17 to 33 20 to 40
C5 Surf sea-shore >80 to ≤200 >4.2 to ≤8.4 5 to 11 7 to 13 8 to 17 10 to 20
CX Off-shore >200 to ≤700 >8.4 to ≤25 2 to 5 2 to 7 3 to 8 3 to 10

Many countries have local Standards that provide more information on the durability of hot dip galvanizing in their region.

Painting over hot dip galvanizing

Painting of hot dip galvanized steel is sometimes required for decorative purposes, to provide an identifying colour or to enhance the service life of the article.

Guidelines can be provided which direct specifiers and applicators to recommended paint systems and advice on surface preparation and application practices that will provide a durable paint finish over galvanizing for a particular application. Where a paint or similar type coating is to be applied to the galvanized coating, the galvanizer should be advised at the time of order.