Technical Info Sheet #5
It is important to take into account on the drawing board already that the object will be galvanized. Keep in mind to design with bolted connections as much as possible so that the workpiece is flat and loose assemblies or, better yet, consists of straight, loose profiles. Avoid welding on the structure after galvanizing.
QUESTIONS
The following points of interest are very important for optimal results:
CLEANLINESS OF THE AREA AROUND THE WELDS
The cleanliness of the area around the welds helps determine the quality
and appearance of the galvanizing layer on and near the welds. Avoid as much as possible the use of welding sprays. If used anyway, the sprays should not contain silicones or grease and should be applied as thinly as possible. This is because sprays containing grease and/or silicone, as well as sprays applied too thickly, are not removed during the chemical pretreatment for the hot-dip galvanizing process. The result is that locally there is no or insufficient interaction between the steel and the zinc. This results in black-colored spots where no zinc layer is present. Residues of the welding spray are hardly visible until the moment of galvanization, and therefore they must be removed immediately after welding (for example, by blasting). This also applies to weld slag on and immediately adjacent to the weld seam. The chemical pretreatment of the steel in the galvanizing plant does not remove them. That must be done mechanically (steel brush, blasting) immediately after welding. The setting of the welding machine is very important as is the welding speed. It must be prevented that residues of welding (the welding slag) are left behind by burn-in. Wherever slag is visible after welding, this means an admittedly small but still ungalvanized spot in the final result.
COMPOSITION OF THE WELDING ELECTRODE OR WIRE
The effect of the Si content of steel on the coating thickness and appearance of the zinc coating is well known (see Technical Data Sheet 18). If the chemical composition of the welding electrode or welding wire differs greatly from the composition of the steel to be welded, marked differences may occur after galvanizing both visually and in the zinc coating thickness at the weld site. Some welding electrodes have a high Si content and may give rise to relatively thick, dull gray and often poorly adhering zinc coating on the welds. Such cases are referred to as “grown through” or “risen” welds (see also Fig. 2). To prevent the latter, as much as possible, it is best to use welding electrodes or welding wire containing no more than 0.7% Si. Even 0.7% Si is actually far too high a content. However, due to the dilution effect that occurs during welding, the Si content at the weld site is sufficiently limited (≤ 0.25%Si).
PREVENTION OF RUST WATER STAINS (THE “WELD LEAK”)
If small craters form in the weld, it is the case that no zinc layer will form in those craters when immersed in the zinc bath. As a result, after galvanizing under the influence of moisture and rain, small rust spots will form and often a rust brown streak across the galvanized surface. This unsightly brown discoloration has no adverse effect on the service life. After all, because of the cathodic protection, after a short time the rusting process will stop and the ungalvanized tip will be protected by the surrounding zinc.
Avoid the use of fully enclosed welds when joining large steel surfaces because this can create large overlaps and gaps in which no zinc layer is formed. After all, it cannot be ruled out that small seams and pores are insufficiently sealed. As a result, unsightly rust water spots and streaks may develop at a later stage in and around these areas. Incidentally, these deviations do not affect the lifespan of the corrosion protection of the zinc coating. Excessive overlaps can lead to an explosion or deformation of the workpiece. For this reason, overlaps should not exceed 100 cm².
APPLYING CHAIN WELDING
Following on from the previous explanation of rust water run-off, the use of chain welding deserves extra attention in this respect. If two profiles are joined by means of chain welding and no allowance is made for a gap, this means that no (or hardly any) zinc layer will form between these two surfaces, resulting in rust water run-off. It is therefore important to leave a gap of at least 2.5 mm between the two surfaces to be joined. In this way, all pre-treatment liquids from the galvanizing process as well as the zinc can penetrate between these surfaces and form a zinc layer.
Prevent partially welding and partially not welding. This may create enclosed spaces and again will not form a completely closed zinc layer. Even if the previously mentioned gap is maintained.
MINIMUM WELDING STRESSES
The stresses caused by welding can deform steel structures
during the heating and cooling associated with the process of hot-dip galvanizing (see also Technical Data Sheet 3 – Thermal Deformation by Galvanizing). During welding, a large amount of heat is introduced locally into the steel. This local heating and subsequent cooling produces a number of interactions whose effects are shrinkage stresses. We distinguish between; longitudinal shrinkage, transverse shrinkage and thickness shrinkage. In fact, bond welds and their direction can cause shrinkage. There is also a distinction between free lying welding and clamped (or tensioned) welding. In the case of clamped welding, there is little shrinkage but a lot of tension. In the case of freestanding welding, it is the other way around: a lot of shrinkage and little stress.
Below are some points of interest to minimize stresses caused by welding:
Information on welding and standards can be found on the site of the Dutch Institute for Welding Technology (NIL) and the Belgian Institute for Welding Technology (BIL)….
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
