Corrosion Assessment

Metal structures – especially the ones made of steel – are becoming more and more popular in different, numerous industries. This is mainly related to the high freedom in design steel gives to specifiers, thanks to its natural specific strength (strength-to-weight ratio, which is the highest among all construction elements), availability and efficiency. However, when exposed to exogenous agents, all the characteristics mentioned above are put at risk due to corrosion. In fact, a proper corrosion protection is vital to not only protect the structure itself but to preserve all the performance-related features of the metal itself, too.

Different environments require different corrosion protection. How we do protect metal structures against corrosion is mainly determined by the corrosivity of the material; however, a larger number of factors affect corrosion behavior. Corrosion protection of products can be, indeed, assessed most accurately by exposure tests of specimens and products in real atmospheric environmental conditions, as the surrounding environment of the structure itself has the biggest impact on corrosion. It is therefore necessary to test the products in the condition they will have in the application for which they are to be used to be able to properly protect the application against corrosion.

How to assess corrosion in a specific environment/application

Under specific circumstances, the corrosivity and, respectively, the corrosion rates of zinc and steel products can be estimated when typical atmospheric parameters are known for a certain application:

Temperature and humidity: an increase in temperature leads to an increase of the rate of chemical reaction, increasing the corrosion rate, too (this is especially true at constant relative humidity levels). On the other hand, increasing temperature facilitates the drying of wet surfaces and can slow down corrosion rates. At temperatures below the freezing point corrosion is negligible. The influence of the temperature on corrosion can, therefore, go in both directions depending on the level of humidity, too - in the absence of humidity, in fact, most contaminants would have little or no corrosive effect.

Chlorides: atmospheric salinity distinctly increases corrosion rates. Chlorides have multiple detrimental effects on the corrosion of metals, such as decrease in the saturation humidity, formation of soluble corrosion products (i.e. metal chlorides), destruction of passive films on metals like stainless steel and aluminum.

Sulphur dioxide: of all the atmospheric contaminants originating from industrial processes such as fuel combustion and metal smelting, sulphur dioxide is the most important one in terms of concentration and its effect on corrosion rates. When dispersed in the atmosphere, it acidifies the electrolyte on the surface and leads to the formation of soluble corrosion products. Corrosion rates are thus increased on many metals, e.g. zinc, steel, aluminum and stainless steel.

The outcome of this approach is an estimated rate of corrosion of zinc or steel in a given environment. The resulting corrosion rates define the prevalent corrosivity category (C-class, see table 3). As stated in the standard, the possible deviation using environmental data and the dose-response function may be up to 50 %.

Factors such as accumulation of corrosive substances or galvanic corrosion, which can have a significant effect on the corrosion rate, are not taken into account in this approach. Nevertheless, as long as these other potential sources of corrosion are not present, the results of this calculation are usually accurate enough to allow selection of the right material.

Corrosion is a natural process influenced by varying environmental factors which cannot be foreseen for the entire designed lifetime. A conservative approach is therefore always advised when it comes to the use of fastening and installation products. In the next article, we will go through the different corrosion protection measures/methods based on the factors mentioned previously and the classification we just saw.