1.1 – zintek® – 7. Corrosion resistance 40 Compatibility between metals When two metals come into contact, one metal and its compound or one base metal and some metal impurities with different electrochemical potential, a potential difference is generated; that is, a short-circuited battery in which the metals act as electrodes, and the one with least potential acts as an anode and corrodes. When choosing construction materials, it is therefore indispensable to pay utmost attention to any possible chemical contamination by the substrate or adjacent materials. This applies above all for the application of sheets on humid or fresh concrete, for exposed bituminous waterproofing, for green roofs and for wooden roof and façade claddings. The environment mainly causes chemical or galvanic type corrosion due to the pH and humidity respectively. In environments with high resistivity, corrosion is limited to the anodic zone close to the connection with the cathodic area. For this reason, this type of corrosion is extremely serious in the presence of sea water but not fresh water, that has a conductivity of at least 2 orders of magnitude less. Galvanic corrosion, linked to the relative nobility of two metals, will be greater the further the elements are from each other in the standard reduction potentials scale or galvanic scale. Instead, two materials can be called “galvanically compatible” if they are close to each other in the galvanic series. Electrochemical corrosion occurs when two metals with different voltage potential come into contact and the rainwater acts as an electrolyte. The entity of the corrosion depends on the voltage potential difference between adjacent metals and their surface exposure. The flow of rainwater also affects the corrosion process even although this factor is often underestimated in practice: the electrolytes in the water create a galvanic element and the current flows from the anode to the cathode, corroding the less noble metal. 1.1. 7. CORROSION RESISTANCE Voltage potential scale of the materials EXCERPT
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