Hot dip galvanising plant – structure entry conditions

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Chemical composition

Hot-dip galvanising enables coating of structures made of steel with a silicon (Si) content of less than 0.03% or between 0.15% and 0.25% with an anti-corrosion layer of zinc.

Surface condition

The surface of the structure should be free from scale, thick rust, weld spatter, welding slag, silicon scouring agents, paint, grease and oil contamination.

Structure features

The structures should be suitable for suspension by wires or chains under the crosshead.
The structure geometry should allow free inflow of liquid zinc to all spaces and its free outflow.
The proportions of materials of different thickness welded together should not exceed 4 :
Galvanising of structures with large areas of thin unreinforced sheets should be avoided due to the danger of corrugation.

Structure dimensions

The structure overall dimensions must not exceed the working dimensions of the galvanising tank, i.e. 10.2 x 1.4 x 2.7 m (length x width x depth).
When two dimensions are close to the limit, prior consultation is required.

Vent holes

Structures made of closed-section elements should be provided with vent holes to allow free inflow of liquid zinc during immersion and its free outflow during emerging of the structure from the zinc bath.
The table below indicates the size and number of vent holes in the elements of the structure.

Internal dimensions of the closed profile in mm less than:

Minimum diameter of holes in mm at opposite ends of the profile and their number:

round square           rectangular 1 2 3
15 15 20 x 10 8
20 20 30 x 15 10
30 30 40 x 20 12 10
60 60 80 x 40 20 12 10
80 80 80 x 60 20 18 12
100 100 120 x 80 25 20 12
120 120 160 x 80 30 25 16
160 160 200 x 120 40 25 16
200 200 260 x 140 50 30 16

The vent holes can be made as triangular or semicircular cut-outs in the profile walls.
The following diagrams illustrate the layout of vent holes.