DR 611749843 SIKORSKY S92 A - ATA 6720 TAIL ROTOR CONTROL SYSTEM - RIVET - SHEARED RIVETS. During routine inspection, it was found that one of the tail rotor pulley attachment bracket bolt has failed 100 in-lb torque check. Further investigation found FWD inboard anchor nut rivets had sheared - allowing the bolt to be somewhat tightened, but not able to be torqued. Root cause not established as yet, however the main contributing factor was the finding that the bolts of the attachment were installed dry and light surface corrosion was found on the shank and within the bolt holes. Aircraft is approximately three years old and no major maintenance has occurred in this area. Anchor nut plate replaced, corrosion removed from all areas. All bolts, anchor nuts and mounting bracket treated with corrosion preventative compound. Sikorsky will be informed by established means.