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CONSTRUCTION FIXINGS Corrosion properties of drill screws


The choice of the steel type in the case of corrosion-resistant steel is defined by the requirements of corrosion protection. Below Etanco GmbH Bauverbindungssysteme describes the principle damage mechanisms of various types of corrosion and the countermeasures that can be taken during the production of the steel.


Erosion corrosion By erosion corrosion we mean evenly


or nearly evenly distributed erosion of the material surface under the influence of a corrosive medium, usually involving liquids containing chloride or sulphur dioxide. This type of corrosion is described as rust for in the case of non and low alloy steels. If the erosion rate is less than 0.1mm/y, the material is viewed as being adequately resistant against corrosion.


Countermeasures: corrosion- resistant steel, stainless steel The chemical element chromium (Cr)


is added to the steel to achieve a general improvement of resistance against erosion corrosion. As of a proportion of approx. 12 M-% Cr, one refers to stainless steel. Stainless steel acquires its resistance to corrosion due to the formation of the so- called passive layer on its surface, which consists of a compound of chromium and oxygen. In contrast to an oxide layer, in order to retain this passive layer, the steel must be in constant contact with oxygen. The addition of molybdenum (Mo)


further improves corrosion resistance. The pitting index PI = %Cr + 3.3. % Mo[ + (16 ÷ 30). %N] describes this influence of increasing corrosion resistance in a relationship with the increasing of the alloy elements. Steel is resistant against seawater starting at and above a pitting index of 33. The commonly used names V2A and V4A for corrosion resistant steel are not standardised and are only differentiated by the question of whether they contain the material Mo (V4A) or not (V2A). Accordingly, the material V4A is characterised by the fact that, due to its Mo content, it has a higher pitting index and thus demonstrates significantly greater general resistance to corrosion. However, because the addition of Cr


and Mo also significantly reduces the ductile values of the steel, nickel (Ni) is added to improve these. The resulting iron-chromium-nickel


alloy with a minor proportion of carbon (C) is imbued with an austenitic structure due to the addition of nickel (approx. 10 M-%), which is why one also refers to these stainless steels as austenite. The


82 Fastener + Fixing Magazine • Issue 71 September 2011


composition of the material X7CrNi18 10 (1.4301) approximately corresponds to that of the first austenitic steel manufactured. According to its description, this steel contains approx. 0.07 M-% C, 18 M-% Cr and 10 M-% Ni.


Pitting corrosion This starts with small, often only


pinprick sized holes on the surface of the


Schematic diagram of various corrosion types:


a) intercrystalline corrosion


material, which result from the fact that the passive layer required for corrosion resistance has, for example, been locally destroyed by deposits, foreign rust or tarnishing. Mostly resulting from aggressive chlorides, large cavities extend out from these small holes into the interior of the material. This corrosion is very often the cause of all kinds of damage to stainless steel.


Crevice corrosion This occurs in crevices, because the


surface of the material is not supplied with enough oxygen to form a passive layer. The corrosion process for crevice corrosion is very similar to that of pitting corrosion.


Countermeasures: increasing the pitting index The best way to prevent pitting or


b) erosion corrosion


crevice corrosion is to increase the pitting index (alloying of Mo), by using a material with Mo (V4A, e.g. 1.4404). This prevents almost all damage that occurs when using a material without Mo (V2A, e.g. 1.4301) with a corresponding corrosion stress.


c) pitting corrosion


Intercrystalline corrosion (IC), intergranular attack IC is a form of corrosion that occurs


d) crevice corrosion


when chromium carbide (compound of Cr and C) forms at the grain boundaries of the material. Because this also takes place on the surface of the material, there is no longer enough Cr available at the interfaces of the grain boundaries with the surface to form the passive layer. In the event of an attack by a special medium, the material corrodes from these points along the grain boundaries. Because the material grains are effectively separated from the material, one also refers to this type of corrosion as intergranular attack.


e) stress corrosion


Countermeasures: stabilisation, low-carbon steels / extra low carbon IC is avoided by preventing the


formation of chromium carbide. This is achieved either by removing carbon from the material (LC, ELC steel) or by adding an element having a greater affinity to carbon than chromium, such as titanium.


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