By Kendall Smith
Galvanized steel can be very tricky to paint! Just look at any stop light at any intersection or any galvanized gutter with paint peeling. Properly assessing the condition of galvanizing, properly specifying the cleaning/surface preparation and using the proper coating or paint CAN be difficult! You need to assess the condition of the galvanizing accurately, which most people never accomplish, partly because there are no readily available visual guides to show “before and after” photographs of new and weathered galvanizing. The first step is to define the stages of Hot Dipped Galvanized steel’s (HDG) life.
- Brand New HDG — Five layers with a surface layer of 100% zinc. Spangled and shiny zinc metal color.
- Second Stage HDG — Pure zinc layer reacts with atmospheric oxygen (O2) and forms a zinc oxide (OH2) layer. This second stage is also unstable, as is the first stage, and will normally occur in the first 48 hours.
- Third Stage HDG — The zinc oxide layer reacts with moisture in the atmosphere and forms zinc hydroxide, which is also an unstable form of HDG requiring special procedures to properly coat. This third stage of HDG can take from 48 hours to six months to occur.
- Fourth Stage HDG — Zinc hydroxide reacts with carbon dioxide in the atmosphere to form zinc carbamate, which is the first stage of “weathered galvanized” in that it is a stable form of zinc for coatings purposes. This stage can take from six months to two years to occur.
- Fifth Stage HDG (2nd stage of aged galvanized) — This A1 stage can take a short time to many decades to occur. It can be described as fully weathered galvanized HDG steel that has not yet begun to corrode, or has very little zinc/iron alloy layer showing with little to no staining from exposed layers of zinc/iron alloy exposure.
- Sixth Stage HDG (3rd stage of weathering) — <10% of the surface area showing signs of corrosion and/or zinc/iron alloy staining.
- Seventh Stage HDG (4th stage of weathering) — 10-50% of the surface area showing signs of corrosion and/or zinc/iron alloy staining.
- Eighth Stage HDG (5th stage of weathering) — 50-90% of the surface area showing signs of corrosion and/or zinc/iron alloy staining, and some pitting corrosion as well.
- Ninth Stage HDG (6th stage of weathering) — 100% of the surface area corroded, and little to zero galvanized remaining. Condition is closer to pitted corroded carbon steel, as no galvanized remains.
Founded in 1947, Induron manufactures high performance coatings that serve a range of industrial applications, including the wastewater, transmission and distribution and groundwater storage industries. Learn more about us at www.Induron.com.