Rothoblaas - Wood screws and connectors Page 252
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CORROSION TYPES AND TRIGGERING FACTORS The corrosion phenomenon involves an electrochemical interaction between a metal and its environment and can lead to degradation of the material and its properties. Although associated with material degradation, corrosion is not in itself negative. In some cases, for example, it enables metals to form a patina to protect them from further corrosion. This is the case with stainless steel or COR-TEN steel. GENERALISED CORROSION This is the corrosion that affects most metal surfaces exposed to an aggressive environment. A distinction must be made between uniform and uneven generalised corrosion: in the first case, penetration is the same over the entire surface while, in the second, the profile may be more or less regular. LOCALISED CORROSION - PITTING Pitting corrosion occurs with extremely localised attacks, called pits; from the surface, such pitting penetrates the thickness of the metal very quickly. Pit sizes range from a few tens of microns to a few millimetres; they are triggered and propagate at individual points, leaving most of the metal surface exposed to the environment unaltered. LOCALISED - CREVICE CORROSION In general, the presence of crevices or surface parts not openly exposed to the environment is always an aggravating factor for corrosion; the expression "crevice corrosion" highlights the contribution the geometric component — in the form of crevices or, more generally, of shielded areas — makes to corrosion. Corrosion is caused in cracks — those interstices that allow the aggressive environment to enter and yet are so narrow as to render negligible any diffusive, convective motion between the inside and outside. Openings ranging from a few hundredths to a few tenths of a millimetre prove critical. 252 | CORROSION | OUTDOOR In the analysis of corrosion, as for any chemical reaction, the speed of the reaction must also be taken into account. In fact, it is not only important to understand whether or not there may be corrosion, but also how long it takes for it to produce significant degradation of the material.
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