Introduction

Flame-cutting, laser-cutting, and plasma-cutting change the steel composition and microstructure in the zone on and around the cut surface. This influences the formation of a hot dip galvanized coating which sometimes makes the minimum coating thickness more difficult to obtain and may cause decreased cohesion/adhesion of the coating to the steel substrate. Clause 9.6 of AS/NZS 2312.2 and Appendix D1.3 of AS/NZS 4680 also describe the influence of the thermal cutting processes on a hot dip galvanized coating.

Thinner Coatings

Cutting using high temperature techniques makes the steel surface of the cut edge less reactive due to the depletion of alloying elements such as silicon in these areas.  The formation of the zinc-iron alloy layers during the galvanizing process is consequently reduced and upon withdrawal from the zinc bath these areas will have a somewhat thinner coating compared to the other surfaces of the article.

The thinning effect is illustrated in the two micrographs seen in Figures 1 and 2 showing the zinc coating on different surfaces of an 8 mm thick steel part. The thickness of the coating at the edge of the part (Figure 1) is approximately 60 μm whereas the galvanized coating thickness 10 mm from the cut edge (Figure 2) is approximately 100 μm.

Figure 1: Micrograph of coating on flame-cut edge (200x)

Figure 2: Micrograph of coating on uncut areas (200x)

Figures 3 and 4 show a 50 mm base plate of a fabricated article with flame-cut edges. As well as a visible thickness variation on the cut edge itself, Figure 4 shows the measurement of the coating thickness on different areas of the plate with a significantly thinner coating on the thermally cut edge.

Figure 3: Base plate, 50mm thick, with flame-cut edges

Figure 4: Measurements showing significant variation in coating thickness on flame-cut edge

Decreased Cohesion/Adhesion

Coatings formed on thermal cut surfaces are known to be more susceptible to flaking at edges during transport and handling. Thermal cutting processes can produce slag and oxide inclusions on the steel surface which generally are unable to be removed in the normal galvanizing pre-treatment process. If these slag and oxide inclusions are not removed prior to galvanizing, a rough coating with poor adherence will result.

Figure 5: Flaking of coating at a laser cut edge

Figure 6: Micrograph of flame-cut surface showing retained slag/oxide

Fabrication to AS/NZS 5131

Clause 9.8.5 of AS/NZS 5131 recognises the issue of thermal processing and requires flame cut, laser cut, and plasma cut surfaces to be ground off and the sharp edges associated with these surfaces to be treated to the specified AS/NZS 5131 treatment grade. However, internal faces, bolt holes, and venting and draining provisions are generally not required to be ground off unless they create sharp edges likely to be damaged in transport or handling.

Summary

To ensure adequate cohesion and adhesion of the HDG coating, flame-cut, laser-cut, and plasma-cut surfaces should be ground off by the fabricator and sharp edges should be removed prior to galvanizing.  If the fabrication specification includes a requirement to meet AS/NZS 5131, then thermal cut surfaces must be treated to the requirements of the Standard.

Grinding exposed thermal cut faces will lead to a more reliable hot dip galvanized coating that is less susceptible to transport and handling damage and which also meets the requirements of AS/NZS 4680.

References

Pasminco. (1990). Report TSP No TP06: The Uneven Galvanizing of Flame-Cut Edges. Port Pirie: Pasminco Research Centre.

Standards Australia / Standards New Zealand. (2006). AS/NZS 4680 Hot-dip galvanized (zinc) coatings on fabricated ferrous articles. Sydney and Wellington: Standards Australia & Standards New Zealand.

Standards Australia / Standards New Zealand. (2014). AS/NZS 2312.2 Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings – Part 2: Hot dip galvanizing. Sydney and Wellington: Standards Australia & Standards New Zealand.

Standards Australia / Standards New Zealand. (2016). AS/NZS 5131 Structural steelwork – Fabrication and erection. Sydney & Wellington: Standards Australia and Standards New Zealand.