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Choosing the right protective cap

The purpose of protective caps is to prevent corrosion on bolted connections or to slow down such damaging processes. Since there are a large number of different products on the market, the selection of a suitable protective cap is of crucial importance.

Not all protective caps are the same.

They may appear similar at a cursory glance but these protective systems can vary greatly. What goes on inside a cap and its structural composition play a great role here. The choice of the wrong protective cap not only does little to counteract corrosion processes but can in the worse cases even accelerate such processes. Experts divide corrosion into different categories. A particularly aggressive type of corrosion is the so-called crevice corrosion which occurs in areas where two surfaces are in close contact with each other and where humidity can accumulate. The crevice formed between the two surfaces has a very small volume and the process of crevice corrosion is triggered in such a crevice even at extremely low levels of corrosion. An acidic, low-oxygen medium which is many times more aggressive than the processes in surface corrosion is formed very quickly in the crevice. Unlike other types of corrosion, crevice corrosion is also triggered at lower temperatures and in a less acidic medium. The purpose of the following example, in which the effect of crevice corrosion was not taken into account, is to show the consequences of choosing an unsuitable protective cap.

As can be seen in the illustration, in this type of protective cap the cap itself is in contact with every surface of the nut, thereby creating a very favourable environment for crevice corrosion over the whole area of the nut. The processes taking place in the crevice and their interaction with the crevice-free outer area reduce the pH within the crevice. Oxygen depletion also occurs in the crevice, preventing the formation of a protective oxide layer. The associated difference in concentration between the crevice and the outer area leads to electrochemical corrosion of the crevice. This process results in a significant acceleration of the corrosion onset. The width and depth of the crevice as well as their interrelationship play an additional role. The narrower or deeper a crevice is, the more aggressive the process of crevice corrosion. A typical sign is the two-dimensional wear in the crevice area undergone by the material, which looks as though it has been etched. This wear is recognisable often only after opening, although the corrosion process is already then at a well advanced stage. Using such a protective cap therefore promotes rather than prevents corrosion. Nuts and threaded bolts are effected even more quickly than without the use of this protective system. The need to prevent the formation of such corrosion-promoting crevices should therefore be taken into account at all costs when choosing a suitable protective cap. Failure to take these important factors into account can lead to considerable damage to the metallic materials which are to be protected. A crevice should be at least 0.5 mm in size to meet the requirements of DIN EN 12502-4 “Protection of metallic materials against corrosion – influencing factors for stainless steels”. The width of the crevice must be adjusted by 1 mm or more depending on its depth.
All RADOLID protective caps have a circular internal cross section which prevents the formation of a crevice asisst capillary action when used for a hex fitting.

  
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