"Excessive corrosion" refers to a situation where a material, such as metal, gets damaged or deteriorates too much due to rust or decay. Full definition
«We have observed excessive corrosion in the power steering bolts, though only in very cold climates, particularly those that frequently use calcium or magnesium road salts, rather than sodium chloride (table salt). (businessinsider.com)
In the email, Tesla said it had, «observed excessive corrosion in the power steering bolts,» but that the problem was most prevalent in colder climates where road salt is used. (theverge.com)
Heater control valves can also leak from excessive corrosion from contact with old or contaminated engine coolant. (yourmechanic.com)