Galvanized coatings can take on a number of different appearances. Commonly, steel chemistries with atypical levels of silicon, phosphorus, manganese & carbon tend to produce galvanized coatings made up primarily of zinc-iron intermetallic layers, with little or no free zinc layer. When no free zinc layer is present, a darker, matte gray appearance is common. Below you will see examples of different coating appearances, each of these appearances were present right after galvanizing.
Visual – the coating can be bright and shiny, spangled, matte gray, or combination of all of these (see photos below). The matte gray appearance may be due to the absence of the free zinc layer or could be a difference in cooling time.
Adherence – Atypical coatings (dark gray) tend to be thicker than typical galvanized coatings. While it is true the thicker the coating, the longer the service life, excessively thick coatings that may result from atypical steel chemistries can be prone to adhesion problems.
Corrosion Resistance – Galvanizing is specified for corrosion prevention. While a matte appearance may occur, differences in appearance do not affect the corrosion protection provided. Dark gray coatings’ corrosion resistance is equal to the corrosion resistance of bright and shiny coatings.
When possible, galvanizers should be advised of the grade of steel selected in order to determine whether to utilize special galvanizing techniques that mitigate the effect of atypical chemistries.
Weathering of Galvanized Steel
Regardless of the coating appearance immediately after galvanizing, over time all galvanized coatings take on a uniform matte gray appearance. The photos below illustrate how galvanized pieces with different appearances will weather and blend in with surrounding galvanized steel upon installation. The dull-gray section of guardrail on the left side of the top picture was taken from the interior of a stack of hot-dip galvanized guardrail exposed to moisture (humidity or rain) but not exposed to freely-flowing air. As a result, these sections of guardrail formed a zinc corrosion product film (wet storage stain) over a period of one week. The shiny section of guardrail on the right side of the top picture was galvanized and stored under cover while exposed to freely-flowing air. Both were galvanized in the same batch.
The lower photo depicts the same guardrail sections 3½ months after they were first attached to the concrete barrier. The left section of the guardrail now matches the right piece of guardrail that was originally bright and shiny. When hot-dip galvanized steel with light to moderate wet storage stain is ultimately exposed to freely-flowing air, the zinc corrosion products react with carbon dioxide to form the matte gray zinc carbonate film that we recognize as the stable patina that gives hot-dip galvanized steel its incredible resistance to corrosion. The section on the right progressed in a normal fashion to the zinc carbonate stage, and the corrosion protection provided by each section is identical.