Technical Sheet #20
Damages are for almost all forms of corrosion protection a weak link. In everyday practice, it appears that after the materials are neatly provided with the conservation system,
various operations still occur that cause minor damage. These include contact with forklift scoops, loading onto trucks moving during transport and mounting objects on
the project site. Small scratches and scrapes give rise to start rust/corrosion. Sharp edges are also often the starting point of iron rust. With hot-dip galvanizing, this is completely different: damages as described above nor the sharp edges will give rise to rust formation. In this Infosheet, we explain Why that is.
CATHODIC PROTECTION
All metals have a so-called normal potential, which characterizes the tendency to oxidize – and thereby to give off positive ions. In other words, every metal has a certain electrochemical voltage. Since this voltage is different in different metals, it can be represented in an electrochemical voltage series (see Figure 1).
In this table, noble metals (gold, silver) with their positive potentials are at the top and the relatively base metals magnesium, aluminum and zinc with their negative potentials are at the bottom. This table makes it clear that, electrochemically, zinc is more base than iron. However, this property of zinc is revealed in a very positive way. Namely, if the zinc layer on steel objects is damaged to the point that the zinc layer is locally gone down to the steel, then in the presence of rain/moisture (= an electrolyte) a galvanic element is formed (see Figure 2).
A current flows between the different metals, so to speak. The material combination iron-zinc, as in the case of hot-dip galvanized steel, causes the formation of cathodic and anodic regions when the surface is damaged. Zinc thereby becomes, under normal atmospheric conditions, anodic and steel cathodic. Because of the different potentials, the negative zinc as an anode constantly gives off electrons, which the nobler cathode (the steel) picks up. As a result, the zinc will slowly go into solution and nothing happens on the side of the steel. This is precisely what prevents rust formation. Just this electrochemical reaction leads to rust not having a chance with scratches or scrapes. So if the zinc layer is damaged, the zinc in the area of the scratch provides corrosion protection, basically a free extra.
There is one exception, however. As mentioned, under normal circumstances there will be cathodic protection of the steel by the anodic zinc. In very rare and special circumstances, however, the steel may begin to function as an anode. This occurs, for example, in those cases where there is a higher temperature (e.g., when pumping thermal oil, steam pipes, hot water pipes).
In short: if conditions are different from normal, there may be a reversal of polarity. Steel “sacrifices” in that case and not the zinc.
EFFECT OF CATHODIC PROTECTION
As with all free benefits, there is a limitation to this. Depending on environmental conditions, humidity and the conductivity of the electrolyte, the effect of this so-called cathodic protection varies greatly. In practice, the range is rarely more than about 2-3 mm. The length of the scratch does not matter in such a case, but its width had better not exceed the mentioned value. In theory, it should be possible to achieve 2-3 cm with cathodic protection. However, then there must be a good conductive electrolyte of up to several mm thickness. In practice, this is usually not the case.
Although a brown discoloration of the damaged area indicates an electrochemical reaction inhibited from time to time (e.g. due to an insufficient amount of electrolyte), this phenomenon is relatively insignificant. Larger damages should be repaired by conventional means (see also Technical Data Sheet 2: Procedure for touch-up).
WHY IS ROUNDING OF EDGES TO BE CONSIDERED?
Corrosion-wise, (sharp) edges are more susceptible to corrosion than the flat parts of a structure. This is especially true for paint systems. This is caused by a physical effect: due to their surface tension, liquids always withdraw from the edge of the part in order to assume the droplet shape as much as possible. As a result, a paint layer after drying on the edges of parts is always thinner than on adjacent surfaces. Since the effectiveness of a paint layer basically depends precisely on the available layer thickness, a problem can easily arise from this. In paint systems, the rounding or chamfering of sharp edges is therefore very important and necessary. This is also stipulated in international standards. In the case of hot-dip galvanizing, alloy layers are formed from the entire surface. At edges, this zinc-iron alloy layer fans out. In the process, the gap formed is filled by metallic zinc. The effect of this is that the zinc layers on corners and edges are not thinner than on smooth surfaces (see Figures 3 and 4) but actually a little thicker. Rounding the edges, prior to hot-dip galvanizing, can thus lead to a nicer final result. In the case of hot-dip galvanized objects that also have an aesthetic purpose, rounding off the sharp edges is recommended. The excess liquid zinc flows away more easily than with a sharp edge.
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
EN ISO 12944-3
Paints and varnishes – Protection of steel structures against corrosion by protective paint systems – Part 3: Design criteria
TECHNICAL SHEET 2
Procedure for updating
TECHNICAL SHEET 10
Corrosion resistance of hot-dip galvanized steel
TECHNICAL SHEET 12
Mechanical properties of hot-dip galvanized steel
