As for Closed Systems

In a closed system, mass caimot transfer across the system boundary. We can write Equation (3.11) as  

The change in entropy of the surroundings is identical for reversible heat transfer and for irreversible heat transfer, as long as the magnitude of Q is the same (see the discussion in Example 3.1). If the surroundings are at constant temperature,, we can write the entropy change to the surroundings as  

We must be careful about sign conventions. If heat flows into the system, it must flow out of the surroundings, that is  

Substitution of the previous two equations into Equation (3.11) gives  

As with the first law, the second law can be written in differential form  

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As for closed systems, Da has a function monotonically decreasing along the solutions of eqn. (152)... 

Exact corrosion kinetics must be modeled by solving the second law of Pick for the geometry of the case at hand. However, in some cases a net effect may be calculated from simple thermodynamics, as for closed system conditions in active corrosion [8], For the case of diffusion through scales it has been demonstrated that quasi-steady-state modeling is often a good approximation for an exact solution, at least for conditions tD/x > 2 [9] (where t = time, D = diffusivity, X = layer thickness). Some basic solutions for situations with instant singular corrosion can also be found in the literature [10]. 

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