@brotherrod7,
Slapping "attorney-client privileged" on an e-mail doesn't make it privileged. Courts would look at the nature of the relationship between the sender and recipient and also whether the e-mail was actually seeking legal advice. As HR manager, you may be within the company's "control group," so you would be considered the law department's "client," but I can only guess at that. You may, therefore, want to get a legal opinion clarifying your relationship with the law department before you start sending them e-mails about union activities at the company.
In any event, taking steps to prevent an employee from forming a union is an unfair labor practice and could get your company into serious trouble, so if that's your intent, I'd encourage you to reconsider.