Corroded condensing coils has become a common occurrence with modern high efficiency equipment.  The newer coils are thinner to promote better heat transfer and are more susceptible to corrosion damage and failure.  This is one of the limiting factors for modern condensing equipment. Where once we would see equipment last in the range of 15-18 year now 12-15 years or less is much more common.  This is distressing to most homeowners due to the increased costs of high efficiency equipment.