Our home, built in 1973, has a 'used' brick chimney, through which the oil burner and fireplace in the den vent exhaust gases. The problem is that the chimney outside seems to be crumbling, as though someone were chipping away at it. I had a mason repair it with mortar, but is breaking up again. No one seems to be able to provide a clear solution to this problem. I would like to repair this so as not to have to completely rebuild the chimney due to the expense. Any suggestions? We live near water, so there is salt air here.