can someone explain to me what causes pitting?
corrosion caused by dissimilar metals and contaminants. The mix of whatever was swept up off the floors and thrown into the smelter. To restore, each pit must be cleaned out and filled (usually soldiered filled). There is no stopping the reaction to the metals. It is not caused by salt and/or moisture. Restore 'pot metal' is temporary (2-20 years) and will need to be restored again. Best to fix it now if you find a restorer and can afford it. Twenty years from now it will be tough to find a restorer.