(Quoted from Marcus Thielen and Signweb.com. Full article here.)

This month, I'll deal with corrosion - especially the electrolytic effects in LED Signs, because humidity is one of the worst wintertime enemies of electric signage. Corrosion, from the Latin corrodere, which means “eating away,” describes how some chemical agents (like acids) can change an object’s constitution and finally dissolve it. I’ll confine myself here to metal corrosion, because it’s the most prevalent type of corrosion in signs. 
Rusting iron is the most common form of metal corrosion. However, iron won’t rust automatically; kept dry or in absolutely pure water, it’ll stay shiny forever. But in a humid environment, iron transforms into a brittle, brown-grey substance: rust, chemically called iron oxide (more precisely: a mixture of different iron oxides, oxide-hydrates and/or iron carbonates). To understand corrosion – and prevent it – we need insights into its surface chemistry.

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