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 When the work is withdrawn from the bath an outer layer of relatively pure zinc is also carried out.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 average coating thickness.

 

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

 

Steel thickness mm Local coating thickness minimum μm Average coating thickness minimum μm Average coating mass minimum g/m²
≤1.5 35 45 320
>1.5≤3 45 55 390
>3≤6 55 70 500
>6 70 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 Local coating thickness minimum μm Average coating thickness minimum μm Average coating mass minimum g/m²
<8 25 35 250
≥8 40 55 390

Notes: 1. For requirements for threaded fasteners refer to AS 1214
          2. 1g/m² coating mass = 0.14μm coating thickness

 

As indicated the total coating mass on heavier steel sections normally contains a minimum of 600 grams of zinc per square metre of surface area (g/m²) equivalent to about 85μm thickness.  As illustrated below, coating thickness is slightly greater at corners.

 

 

 

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.








On most commonly galvanized steels, the relatively pure outer zinc layer of the galvanized coating solidifies to give the typical bright zinc crystal or 'spangle' finish.  Certain steel compositions may cause the zinc-iron alloy layer to grow through to the surface of the galvanized coating producing a matt grey finish sometimes known as 'grey bar', as discussed below under 'Composition of steel'. There is negligible difference between the protective lives provided by each coating.

Factors influencing coating thickness

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

1. Surface conditions of the steel
2. Composition of the steel

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

 

Surface condition of steel

Grit blasting steel before galvanizing roughens the surface and increases its surface area, resulting in higher reactiveness to molten zinc. Greater zinc-iron alloy growth occurs during galvanizing, producing thicker coatings, though at the expense of rougher surface and poorer appearance.

Application of this method of achieving thicker coatings is generally limited by practical and economic considerations.  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.

 

Suitability of silicon/phosphorous steels for galvanizing

As a guide to the suitability of silicon and phosphorous containing steels for galvanizing, the following criteria chould be applied if aesthetics is the critical consideration:

% Si < 0.04%

and

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

Galvanized coatings on silicon 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 increased thickness and is unaffected by appearance, provided the coating is sound and continuous.  The thickness, adherence and appearance of galvanized coatings on silicon and phosphorous steels are outside the control of the galvanizer.  For more information on Steel Composition go to FAQ.

 

Double dipping or galvanizing a second time will not increase the thickness of a galvanized coating for reasons discussed above, and may adversely affect coating appearance.

The terms 'double dipping' and 'double-end dipping' are sometimes confused.  Double-end dipping is a method of galvanizing articles too long for the available bath by immersing one end of the work at a time.  Explained more here.

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