Corrosion on fork lowers is not a problem, I have been riding Boxxer forks for 20 years all year long in a country where melting agents are over used 5 months a year. Your problem is a casting defect, but now that you have grinded and exposed the bare metal, it is possible that surface corrosion might start to appear. I don't know if Rock Shox will replace a lower once it's been grinded like that, but you can find a used lower in good condition for cheap. I have replaced scratched Boxxer fork lowers with some used that looked like new for 50$ found on Pinkbike in the winter season.