COATING THICKNESS AND FACTORS INFLUENCING THICKNESS

During the first minute of immersion in the galvanizing bath, zinc-iron alloy layers grow rapidly on the surface of the steels which are most commonly galvanized. The rate of alloy layer growth then diminishes and is finally very slow. When the work is withdrawn from the bath an outer layer of relatively pure zinc is also carried out. The total zinc coating mass applied depends mainly on the mass and thickness of the steel being galvanized.

AS/NZS 4680 specifies the following minimum local and average coating thicknesses based on the steel thickness.

Table 1 – Requirements for coating thickness and mass for articles that are not centrifuged

Steel thickness mm Average coating thickness minimum μm Average coating mass minimum g/m²
≤1.5 45 320
>1.5 to ≤3 55 390
>3 to ≤6 70 500
>6 85 600

Note: 1g/m² coating mass = 0.14μm coating thickness

Table 2 – Requirements for coating thickness and mass for articles that are centrifuged

Thickness of articles (all components including castings) mm Average coating thickness minimum μm Average coating mass minimum g/m²
<8 35 250
≥8 55 390

Notes:

1. For requirements for threaded fasteners see AS/NZS 1214 information in Table 3.
2. 1g/m² coating mass = 0.14μm coating thickness

Table 3 – Requirements for coating thickness and mass for fasteners galvanized to AS/NZS 1214

Steel thickness mm Average batch coating thickness minimum μm Average batch coating mass minimum g/m²
All 50 350

 

As indicated in Table 1, the total coating mass on heavier steel sections normally is a minimum of 600 grams of zinc per square meter of surface area (g/m²) which is equivalent to about 85μm thickness.

Diagram 2 ACA paper 2005

Galvanized coatings are slightly thicker at corners and edges as shown, an important advantage over most organic coatings which thin out in these critical areas.

Commonly, the relatively pure outer zinc layer of the galvanized coating solidifies to give the typical shiny initial finish which can sometimes include a zinc crystal or ‘spangle’ pattern.  Certain steel compositions may result in the zinc-iron alloy layer growing through to the surface of the galvanized coating producing a matte or dull grey finish, as discussed below under ‘Composition of steel‘ below. There is negligible difference between the protective lives provided by each coating structure.

FACTORS INFLUENCING COATING THICKNESS

The thickness, alloy structure and finish of galvanized caotings are influenced by:

  1. Steel thickness
  2. Composition of the steel
  3. Surface condition of the steel

Increasing the period of immersion in the galvanizing bath will not have a significant increase on coating thickness except in the case of silicon steels.

A common misconception is that ‘double dipping’ refers to dipping the steel into the molten zinc a second time to increase the thickness of a galvanized coating, which is not the case. The terms ‘double dipping’ and ‘double-end dipping’ refer to the same thing, though are sometimes confused. Both are a method of galvanizing articles too long, wide or deep for the available bath by immersing one end of the work at a time. More information about ‘double dipping’ and design considerations can be found here.

SURFACE CONDITION OF STEEL

Grit blasting steel before galvanizing roughens the surface and increases its surface area, producing thicker coatings, although in some cases can be at the expense of a rough surface finish and/or dull appearance.

Applying this method to achieve thicker coatings is generally limited by practical and economic considerations, and may be required when galvanizing aluminium killed steels with very low silicon levels to achieve the minimum coating thickness required. Where increased service life or reduced maintenance is required the use of duplex (galvanizing-plus-paint) systems is a preferable alternative.

COMPOSITION OF STEEL

Both silicon and phosphorous contents can have major effects on the structure, appearance and properties of galvanized coatings.

As a guide to the influence of silicon and phosphorous content in steels on galvanizing, the following criteria should be applied if aesthetics is the critical consideration:

% Si < 0.04% and %P <0.025%

and

% Si + (2.5 x % P) <0.09%

Galvanized coatings on silicon killed steels can be dull grey or patchy grey in colour with a rough finish, and may be brittle if there is excessive growth. Coating service life is proportional to the coating thickness and is unaffected by appearance, provided the coating is sound and continuous. The thickness, adherence and appearance of galvanized coatings are outside the control of the galvanizer. For more information on appearance and factors influencing appearance go to Factors that Influence Appearance or the FAQ.