1989 Johnson Evinrude "CE" 9.9 thru 30 Service Manual, P/N 507754, Page 41Get this manual
AnodesTesting and Replacement Erosion or disintegration in salt or brackish water indicates the anodes are performing their function The anodes should be inspected andor replaced at intervals or corrosion of the engine will increase
If any anode has been reduced to two-thirds its original size
(one-third eroded) it must be replaced!