Corrosion assessment

Metal structures – especially ones made of steel – are ubiquitous across many different industries. Thanks to its wide availability, its natural specific strength (its strength-to-weight ratio is the highest among all construction elements) and its efficiency, steel is a particularly adaptable material that offers great design flexibility.

On the negative side, when exposed to certain exogenous agents, the resulting corrosion can compromise the strength, efficiency and safety of metal components. For this reason, proper corrosion protection is vital to not only protect a structure, but to preserve all the performance-related features of the metal as well.

Different environments demand different corrosion protection.

How we protect metal structures against corrosion is largely determined by the corrosivity of the chosen material. However, a number of other factors also affect corrosion behavior.

Corrosion protection of products can be assessed more accurately by subjecting specimens to exposure tests in real-world situations. Because atmospheric and environmental factors have the biggest impact on corrosion, it’s crucial to test products for performance in the respective conditions.

 
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 any given application. These include:


-         Temperature and humidity. An increase in temperature can lead to an increase in the rates of chemical reactions that cause corrosion (this is especially true at constant relative humidity levels). On the other hand, where a higher temperature facilitates the drying of wet surfaces, this can slow down corrosion rates. At temperatures below freezing point corrosion is negligible. Therefore, the influence of temperature on corrosion depends heavily on humidity levels – in the absence of humidity, most contaminants have little or no corrosive effect.
-         Chlorides: atmospheric salinity significantly increases corrosion rates. Chlorides have multiple detrimental effects on the corrosion of metals, such as a decrease in saturation humidity, the formation of soluble corrosion products (i.e. metal chlorides) and the destruction of passive films on metals like stainless steel and aluminum.
-         Sulfur dioxide: of all the atmospheric contaminants originating from industrial processes such as fuel combustion and metal smelting, sulfur dioxide is the most important one in terms of concentration and its impact on corrosion rates. When dispersed in the atmosphere, it acidifies the electrolyte on a metal’s 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.
 
 
ISO 9223:2012 corrosion assessment categories
This C1 to C5 classification is based on BS EN ISO 12944-2 and BS EN ISO 9223, providing a standardized system for corrosivity of atmospheres. The reported values are valid for steel, zinc, aluminum and copper, and they connect environmental conditions with corrosion rates.
 
This is the most used classification across the construction and manufacturing industries.

This classification system allows you to estimate the 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).

However, as stated in the ISO 9223:2012 standard, the possible deviation using environmental data and the dose-response function may be as high as 50%. Also, factors such as the 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 a better selection of the an appropriate material.

Corrosion is a natural process influenced by varying environmental factors, which often cannot be foreseen for the entire designed lifetime of a product. It is therefore good practice to take a conservative approach when it comes to the selecting of the most appropriate fastening and installation products.

In our next article, we will talk you through the different corrosion protection measures/methods that are based on the factors we mentioned previously and the classification we just saw.