How long these boards have been in the field before corrosion started?
Has the corrosion product been analyzed?
How were these boards cleaned?
Are these PIH or SMT or mixed technology boards?
What are the field/use environment conditions - temperature, humidity and gases?
How are boards cooled forced air or natural convection?

You need to establish answers to these questions before a solution can be implemented. YES, there are product and processes that can prevent corrosion on a PCB in a field environment.
 

• Proper solder mask which adheres well to copper conductors - useful for normal office type of environments
• Conformal coatings for use in environments where minute amounts of corrosive gases may be present.
• Conformal coatings like Parylene in an aggressive industrial environment where corrosive gases are present , ex. paper mill, steel mill etc.