Technical Info Sheet #23
To provide hollow structures with a hot-dip galvanized protective coating, it is necessary to allow zinc to flow into profiles and other hollow spaces. It is also important to have holes where air and residue can escape from the galvanizing process.
Hanging options should also be provided. In a number of cases, countersink holes are applied without the holes being visible on the outside of the workpiece. In practice, this is sometimes called “blind holes” or “internally drilled”. Standard texts refer to “internally vented enclosed spaces.”
Visual entrance control is then hardly possible by the galvanizing plant. As a precaution, the workpiece will not be handled. If no galvanization holes have been made, there is a risk of explosion. There have unfortunately been examples of this in the past, sometimes with dramatic consequences.
HOT-DIP GALVANIZING OF HOLLOW PROFILES AND WORKPIECES
Products that are going to be hot-dip galvanized should at least have suspension holes. To these suspension holes, the workpiece is connected with iron wire/chains/rods to a traverse or other auxiliary tool with which transport through the galvanizing process takes place. However, should the workpiece consist of hollow profiles, such as tubes and pipes, or be hollow in itself, e.g. tanks for manure transport etc., additional provisions will be made to allow galvanizing of the inside as well. The countersink holes are intended to fill the workpiece with liquid zinc upon immersion. These holes also serve to allow air and residues from the pretreatment process to escape to the zinc bath surface during galvanizing. The exact placement of the countersink holes is in relation to the suspension holes, these should be placed so that during the pretreatment of the workpiece and the countersinking itself, liquid and then the zinc can get into all the nooks and crannies.
For aesthetic reasons, these holes are sometimes considered less attractive or the holes are undesirable because of their intended use. Then the holes are sometimes provided in a way that they are not or hardly visible after assembling the materials into a workpiece, as an example a fence consisting of a top and bottom tube with a number of tube posts in between. These tube arrows should be provided with holes where one has two choices. One is to pre-drill the holes in the upper and lower tubes and then weld the bars in between and the other is to provide the bars themselves with openings at the location of the upper and lower tubes. In the former manner (photo 3), the zinc will enter the bars through the lower sleeve while air and residue can escape through the upper sleeve. However, these holes are no longer visible when the work pieces are delivered to the galvanizing plant. Sometimes one can still look into the top and bottom chutes for a bit and judge some of the bars, but most of them are not.
Assembling a workpiece in the above manner is undesirable for safety reasons and should be avoided. If such internal venting cannot take place in any other way, this will have to be discussed in advance with the galvanizing plant [This is an obligation imposed on the client, see also ISO 1461, A.2, e]. Thereby, in accordance with ISO 14713-2: 2019, it will have to be ensured that:
BUOYANCY AND EXPLOSION HAZARD
If a workpiece is not provided with (sufficient) galvanizing holes, there is a danger that the product to be galvanized will not completely submerge in the liquid zinc. This is because an object immersed in a liquid experiences an upward force equal to the weight of the displaced liquid. Should the object, despite having insufficient holes provided, still submerge due to a high own weight, there is a danger that the sealed and unvented space will lead to an explosion. That explosion is caused because moisture that has entered the enclosed space (pretreatment liquid) cannot escape during galvanizing, but does rise in temperature. Steam is created and this results in a large increasing pressure, which will lead to the explosion of the part in question. Needless to say, this creates a very dangerous situation for the workers at the site of the zinc bath. In addition, enormous damage is caused by the liquid zinc flying away. There is also great danger to the integrity of the zinc bath and the construction of the furnace plant. Indeed, the zinc bath may deform from the explosion and possibly the furnace plant as well, which may require accelerated zinc bath replacement and other repairs.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 1
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 1: General design principles and corrosion resistance.
EN ISO 14713 part 2
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 2: Hot dip galvanizing
