Technical Info Sheet #28
People often talk about “galvanizing.” However, there are different zinc application methods, each with its pros and cons. It is therefore important to understand what these differences can lead to. See also ‘[Ver-zin-ken] Different techniques for galvanizing’ and Technical Information Sheet 11: Zinc Application Methods.
WHAT YOU SEE IS WHAT YOU GET!
Nothing offers more security than a fair system. For more than 150 years, hot-dip galvanizing has been the champion of corrosion protection. That zinc as a base metal can protect something like steel has long been a mystery. Alchemists must have drawn hope from it to search further for a method of making gold. Since the early 18th century, it has been known that a layer of zinc protects steel from rusting. Yet it wasn’t until the 19th century that people managed to find a method to do this production-wise, namely the invention of pickling. Again only after 100 years, through experimentation and laboratory research, insight is gained into how and why. Everyday practice shows us again and again the enormously wide application of hot-dip galvanized steel in structures, tools and design. Just a one-time investment and no more maintenance is required. No wonder that many, almost 300 years after its invention, still choose hot-dip galvanized steel. To make hot-dip galvanizing possible, galvanizing holes must be made in an object. In this technical info sheet, we will explain the why and how.
Hot-dip galvanizing is a metallurgical process that occurs when we immerse steel in molten zinc. To have the zinc liquid to allow immersion, a temperature above that of the melting point of zinc is required: 419 °Celsius. Almost always a temperature of 450 °Celsius is maintained in the zinc bath. When immersing the steel object to be treated, a chemical process takes place between iron and the liquid zinc once the object has reached the temperature of the zinc bath. Several zinc-iron alloy layers are formed in quick succession, which are covered by a layer of pure zinc due to solidification of the zinc when the object is hoisted out of the zinc bath.
The international standard EN-ISO 14713-2 contains guidelines and recommendations in the field of galvanizing holes. If the advice given in the standard is followed in the design of the object to be galvanized, it can be expected that galvanizing can be done safely and properly resulting in a completely closed zinc layer free of ungalvanized spots and residues/residues from the galvanization pretreatment.
About the why of sinkholes, we can state an obvious reason. After all, in order to submerge, there will have to be sink holes because otherwise the object will float. This is because the object to be immersed experiences an upward force equal to the weight of the displaced liquid. Zinc is over 7 times heavier than water and the buoyancy force is therefore 7 times greater. It is also important that nooks, crannies and hollow profiles or objects be completely sealed with a fully sealed zinc coating. The zinc must be able to enter an object anywhere. Therefore, holes are needed to allow air to escape that is displaced by the liquid zinc during dipping. These vent holes also serve the purpose of allowing the residues of pretreatment fluids to escape. The holes should therefore be chosen larger than would be necessary for air alone. The ashes of pretreatment fluids are voluminous and light so they can easily be left behind in, for example, a tube, recess or even on a sharp edge.
In addition to the zinc inflow holes and vent holes, holes are needed to suspend items from an auxiliary support beam (traverse) of the galvanizing line using iron wire. Often vent holes can serve as suspension holes but this is not possible in all cases.
The size of the countersink holes, especially those for venting and zinc inflow, should be chosen as large as possible. The larger the holes the faster an object can be immersed in the zinc bath and the faster it can be lifted out. This speed is not necessary because one is in a hurry or wants to increase production at the galvanizing plant, but this is necessary to obtain an optimal galvanizing result. After all, there is a chance that an object, in the heat of the zinc bath under the influence of tensions in the object, may deform. In addition to design details and method of welding, the difference in temperature is also a possible cause. When an object, because it has small openings, can be dipped only slowly, a large temperature difference arises between the part in the liquid zinc and the part above it. Heating causes steel to expand which can lead to differences in expansion, which together with existing stresses can lead to deformation.
In addition to deformation, a greater dipping and hardening velocity is required for the aforementioned residues of pretreatment liquid to easily disappear from the object toward the zinc bath surface. The flow rate of the liquid plays a role in this process. The higher the velocity, the easier these ash residues are released from the steel surface.
In addition to the function of the countersink holes and their size, the correctly chosen location of the hole is of enormous importance. These holes should be placed as far as possible in corners to avoid zinc accumulation, but also to avoid the possibility of ungalvanized areas. So it is important to apply the holes before the object is assembled. Afterwards, it is usually not possible to drill sufficiently far into the corners. After all, often there is a weld in the desired place where it is better not to drill or the ideal spot is simply no longer reachable with a drill. For the location and size of countersink holes there is a poster “Checklist good and safe countersinking”. This can be found on our website and can also be requested to hang up in your company.
The client who presents the object for treatment to a galvanizing plant is partly responsible for the final result. In this publication we have explained the how-and-why of galvanizing holes. It is up to the client to make sure that the holes are positioned correctly so that proper and safe galvanizing is possible. Whether an object has a completely closed zinc layer without imperfections is almost always due to the design of the object and only to a limited extent due to carelessness at a galvanizing plant. Should the galvanizing plant have doubts about the safe galvanizing of an object, the processing of the object will be delayed until there is certainty (see Technical Information Sheet 23 – Invisible galvanizing holes). The latter can be done by the client adjusting the object or demonstrating that the holes are present in the structure. Not properly installed countersink holes in an object can lead to an explosion in the zinc bath. This is a life-threatening situation that should not be taken lightly.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
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
TECHNICAL DATA SHEET 23
Non-visible countersinks/blind holes
