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Water damage... causes
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<blockquote data-quote="Phil Graham" data-source="post: 48557" data-attributes="member: 430"><p>Re: Water damage... causes</p><p></p><p></p><p></p><p>At some level it comes down to semantics. There is galvanic corrosion, hot corrosion, and a million other terms. You are of course correct than an environment with no ready exchange reaction path (i.e. electrolyte) for mass transport of ions, then then the potential difference between the metals produces no perceivable reaction on normal time scales. The driving force is still there, but it is kinetically limited.</p><p></p><p></p><p></p><p>These reaction systems are <em>always</em> about surface chemistry. You can have a very rapid reversible exchange current reaction (<a href="http://en.wikipedia.org/wiki/Exchange_current_density" target="_blank">Exchange current density - Wikipedia, the free encyclopedia</a>) at the surface, which is the case with dissolved oxygen and iron, but little or no net corrosive reaction. As for the assertion that one of the reactions occurs to completion before the other one kicks in, the reality is both reactions will occur simultaneously with different driving forces and settle on a system wide thermochemical equilibrium. Then, in the real world, kinetics will influence the time domain response of this transition to the equilibrium state.</p><p></p><p></p><p></p><p>A circuit board from 1982 would have most likely have ceramic (SiC) packaging for the ICs and possibly an overall varnish, it surprises me little that it was fine.</p><p></p><p></p><p></p><p>Over the long term, differential aeration tends to drive these reactions on the base metal, with replenishment from the air. Some sort of failed passivation, or sacrificial anode expenditure, probably preceded the differential aeration effects.</p></blockquote><p></p>
[QUOTE="Phil Graham, post: 48557, member: 430"] Re: Water damage... causes At some level it comes down to semantics. There is galvanic corrosion, hot corrosion, and a million other terms. You are of course correct than an environment with no ready exchange reaction path (i.e. electrolyte) for mass transport of ions, then then the potential difference between the metals produces no perceivable reaction on normal time scales. The driving force is still there, but it is kinetically limited. These reaction systems are [I]always[/I] about surface chemistry. You can have a very rapid reversible exchange current reaction ([URL="http://en.wikipedia.org/wiki/Exchange_current_density"]Exchange current density - Wikipedia, the free encyclopedia[/URL]) at the surface, which is the case with dissolved oxygen and iron, but little or no net corrosive reaction. As for the assertion that one of the reactions occurs to completion before the other one kicks in, the reality is both reactions will occur simultaneously with different driving forces and settle on a system wide thermochemical equilibrium. Then, in the real world, kinetics will influence the time domain response of this transition to the equilibrium state. A circuit board from 1982 would have most likely have ceramic (SiC) packaging for the ICs and possibly an overall varnish, it surprises me little that it was fine. Over the long term, differential aeration tends to drive these reactions on the base metal, with replenishment from the air. Some sort of failed passivation, or sacrificial anode expenditure, probably preceded the differential aeration effects. [/QUOTE]
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