Technical Info Sheet #4
It is known of many construction metals that they should not contact each other during construction because “contact corrosion” can occur (also called galvanic corrosion or bimetallic corrosion). Contact corrosion can also occur between other construction materials (copper/steel, aluminum/stainless steel). How
Do we prevent contact corrosion when applying hot-dip galvanized steel in combination with other metals?
contact corrosion
Contact corrosion occurs when two different metals are in contact with each other in the presence of a current-conducting liquid (also called electrolyte).
The driving force of this phenomenon is the difference in electrical potential between the two metals with respect to this electrolyte. The electrolyte in most cases is an aqueous solution of pollutants. This can be rainwater, ditch or sea water, process water, but also wet dirt or mud.
Importantly, an electrolyte conducts current and can make metal go into solution. With contact corrosion, we see a (often rapid) dissolution of one of the metals and excessive formation of corrosion products. Proper design requires a good understanding of this phenomenon so that appropriate measures can be taken to prevent it or minimize its likelihood.
Sometimes the design does separate the different materials well and contact corrosion should not be able to occur, yet the phenomenon still occurs. For example, it can happen that water, flowing off a copper roof, contains dissolved copper, which precipitates back onto the zinc of the galvanized steel as metal. At that point, a situation of contact between two different metals is created and the possibility of contact corrosion is real. In this example, holes will be created in the gutter. Furthermore, on the construction site, due to drilling and grinding work or temporary fixings left behind, for example, situations can arise that give rise to contact corrosion.
VOLTAGE RANGE OF METALS
To serve the structural engineer and designer, scientists have constructed tables that indicate, for different situations, which materials are easily corroded (are base) and which are not corroded (are noble) under the same conditions. In such a
table, the materials are arranged in a way that allows one to see which of two metals will experience contact corrosion. An example of such a (simplified) electrochemical series is shown in
Figure 1. Several such sequences can be found on the Internet. We explain the background later.
When one connects a base metal to a nobler metal and the connection is immersed in a conductive salt solution (electrolyte), the base metal (called anode) goes into solution
and the nobler metal (cathode) remains unaffected. Rule of thumb: base = anode = corroded. This rule applies especially to distant metals in the voltage series. Therefore, these should definitely not be connected in a construction.
If the metals in the series are close together, the situation is often much more favorable in practice. However, the size of the contact surfaces of the metals as well as the composition and temperature of the electrolyte certainly play a role. Also, many metals under conditions of use are covered with a layer of oxide, hydroxide or metal salt (patina), which means that the potential difference in reality is different from what the scientific tables for the pure metals show can gradually form.
If the hot-dip galvanized steel is to be painted (duplex system), on the other hand, it is absolutely necessary to remove the white rust in order to allow proper adhesion of the paint layer.
PRECAUTIONS
Designers and fabricators can make useful use of a contact corrosion table, which takes into account as many of the factors discussed above as possible. For hot-dip galvanized steel building components and products, EN ISO 14713 Part 1 includes Table 4.
| METAAL | ATMOSFERISCHE BLOOTSTELLING | ONDER WATER | |||
|---|---|---|---|---|---|
| LANDELIJK | INDUSTRIEEL / STEDELIJK | MARIEN | ZOET WATER | ZEEWATER | |
| ALUMINIUM | A | A-B | A-B | B | B-C |
| MESSING | B | B | A-C | B-C | C-D |
| BRONS | B | B | B-C | B-C | C-D |
| GIETIJZER | B | B | B-C | B-C | C-D |
| KOPER | B | B-C | B-C | B-C | C-D |
| LOOD | A | A-B | A-B | A-C | A-C |
| ROESTVAST STAAL | A-B | A-B | A-B | B | B-C |
| A: De zinken deklaag zal of geen extra corrosie ondervinden, of in het ongunstigste geval, zeer geringe extra corrosie, die in de praktijk doorgaans aanvaardbaar is. B: De zinken deklaag zal geringe of matige extra corrosie ondervinden, die onder bepaalde omstandigheden aanvaardbaar kan zijn. C: De zinken deklaag kan in vrij ernstige mate extra corrosie ondervinden; meestal zijn beschermende maatregelen noodzakelijk. D: De zinken deklaag kan in ernstige mate extra corrosie ondervinden; contact behoort te worden voorkomen. |
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Local conditions that may affect the occurrence of contact corrosion must be taken into account. It matters a great deal whether the contact site is in constant or occasional contact with, for example, seawater, whether condensation will frequently occur at the contact sites, or whether the formation of insulating metal salts or oxides will occur rapidly or slowly. What will still be permissible in a humid atmosphere usually does not apply to contact corrosion in (sea) water. It is unavoidable in certain cases to join two different metals together. Depending on the nature of the corrosion load and the permanent or non-permanent presence of moisture, water or salt solutions, the contact sites must then be insulated. This can be done as follows:
The first two methods give the best results. The latter is really only good for low moisture loads. The insulation should be clearly indicated on drawings and in the specifications.
Illustration of a sluice gate that, due to application of a visible stainless steel strip and underwater connections made of stainless steel, combined with inadequate protection of a coat of paint on the cutting edge of a support, showed rusting within just a few weeks. The supports soon lost their zinc coating on site and the steel had already deteriorated considerably in the meantime.
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
