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Water damage... causes
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<blockquote data-quote="Phil Graham" data-source="post: 48544" data-attributes="member: 430"><p>Re: Water damage... causes</p><p></p><p></p><p></p><p>Jay,</p><p></p><p>A few (chemistry filled) thoughts: <ol> <li data-xf-list-type="ol">There are dissimilar metals in this case, as the soft iron core of the driver magnetics system is protected from corrosion by a surface coating of unknown composition. The coating will most likely also offer some measure of passivation and/or cathodic protection.</li> <li data-xf-list-type="ol">If the local galvanic couple is such that the sacrificial anode is depleted, or the passivation porous, then we enter the zone where O2 potentially matters.</li> <li data-xf-list-type="ol">Once we are in the zone where O2 matters, the Ksp of O2 in the system has overall little relevance on the corrosion, as there are multiple points of flux for oxygen bearing species, and the local chemical potentials can be far from equilibrium. O2 flows in to the system from the air, and out of the system as a corrosion product.</li> <li data-xf-list-type="ol">Often, diffusion of O2 through an oxide product layer is the rate limiting step of corrosion process. This is essence of much passivation behavior.</li> <li data-xf-list-type="ol">For iron or steel, magnetite (FeO*Fe2O3) forms a stable passivation layer in the absence of dissolved oxygen. Dissolved oxygen attacks that layer.</li> <li data-xf-list-type="ol">Concentrations of ions and neutral chemical species can vary wildly inside localized electrolyte regions on the surface.</li> <li data-xf-list-type="ol">Differential amounts of aeration of the liquid produce an electrochemical cell between a metal that nominally has the same electrochemical potential everywhere. On the aerated side of the cell, the cathodic reduction reaction is: O2 + 2H2O + 4e- <--> 4OH-</li> <li data-xf-list-type="ol">The electrochemical potential of this differential aeration cell will drive corrosion of both the new cathode and anode. Over time the chemical reaction in #7 raises the pH and passivates the cathode, resulting in the de-aerated anode corroding faster with time. Again, the base metal is nominally the same everywhere.</li> </ol><p>See <em>Principles and Prevention of Corrosion, 2nd Ed</em> by Jones</p><p>Section 6.5, starting on pp. 191</p><p>Section 11.1, starting on pp. 357</p></blockquote><p></p>
[QUOTE="Phil Graham, post: 48544, member: 430"] Re: Water damage... causes Jay, A few (chemistry filled) thoughts:[LIST=1] [*]There are dissimilar metals in this case, as the soft iron core of the driver magnetics system is protected from corrosion by a surface coating of unknown composition. The coating will most likely also offer some measure of passivation and/or cathodic protection. [*]If the local galvanic couple is such that the sacrificial anode is depleted, or the passivation porous, then we enter the zone where O2 potentially matters. [*]Once we are in the zone where O2 matters, the Ksp of O2 in the system has overall little relevance on the corrosion, as there are multiple points of flux for oxygen bearing species, and the local chemical potentials can be far from equilibrium. O2 flows in to the system from the air, and out of the system as a corrosion product. [*]Often, diffusion of O2 through an oxide product layer is the rate limiting step of corrosion process. This is essence of much passivation behavior. [*]For iron or steel, magnetite (FeO*Fe2O3) forms a stable passivation layer in the absence of dissolved oxygen. Dissolved oxygen attacks that layer. [*]Concentrations of ions and neutral chemical species can vary wildly inside localized electrolyte regions on the surface. [*]Differential amounts of aeration of the liquid produce an electrochemical cell between a metal that nominally has the same electrochemical potential everywhere. On the aerated side of the cell, the cathodic reduction reaction is: O2 + 2H2O + 4e- <--> 4OH- [*]The electrochemical potential of this differential aeration cell will drive corrosion of both the new cathode and anode. Over time the chemical reaction in #7 raises the pH and passivates the cathode, resulting in the de-aerated anode corroding faster with time. Again, the base metal is nominally the same everywhere. [/LIST]See [I]Principles and Prevention of Corrosion, 2nd Ed[/I] by Jones Section 6.5, starting on pp. 191 Section 11.1, starting on pp. 357 [/QUOTE]
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