Sep 99 - Primer-Free Inserts

General

Primer-Free (formerly Heli-Guard) Inserts are wire screw-thread inserts manufactured by Emhart-Heli-Coil, a Black & Decker Company. Primer-Free Inserts are designed for use in aluminum and magnesium applications where galvanic corrosion between dissimilar metals is a concern.

History

The aerospace and naval operations industries utilize a great number of stainless steel wire Heli-Coil inserts in their designs. As galvanic corrosion is a major concern, steps must be taken to insure corrosion protection between dissimilar metals. The current protection method involves the use of liquid zinc chromate primer. This primer material, supplied to a Federal Specification (TT-P-1757), provides a barrier between the parent material and the stainless steel insert.

While the primer material itself provides this corrosion protection, it does create several problems if the application procedures and insert installation procedures are not followed closely.

Liquid zinc chromate primer is designed to be applied "sparingly" to the tapped hole. The insert is then to be installed while the primer is still wet. This process can be wrought with problems:

• The term "sparingly" is highly subjective. There is little to no control over the quantity of primer applied. Too little or too much primer could be used.
• The process is messy, slow and very labor intensive.
• Often the installation tool clogs with hardened primer necessitating frequent cleaning.
• Screw-Locking inserts may exhibit high torque characteristics if excessive primer hardens behind the grip coil and the tapped hole. If severe, fastener breakage is also possible.

Primer-Free Inserts

Heli-Coil Primer-Free Inserts are manufactured to the appropriate Military or Metric Aerospace standards and coated with a unique resin bonded fluoropolymer coating . This coating now becomes the barrier between the insert and the parent material impeding galvanic corrosion . Heli-Guard Inserts are manufactured with special corrosion inhibitors to resist fungus growth. Additional benefits realized include lubricity which eases insert installation and provides for a more consistent Screw-Lock insert prevailing torque. Most importantly, no messy primers or additional hole preparation other than drilling and tapping is required.

Testing

Primer-Free Inserts have undergone extensive salt fog testing per ASTM B-117. Both internal and independent tests were conducted using aluminum test blocks. Test samples included:

• Sample set 1 - Bare aluminum with non-coated stainless steel inserts.
• Sample set 2 - Liquid zinc chromate primer coated holes with non-coated stainless steel inserts.
• Sample set 3 - Bare aluminum with Primer-Free Inserts.

After exposure, the samples were sectioned to expose the threaded insert for examination.

Sample set 1 showed corrosion on both the insert coils as well as the aluminum base material. Sample set 2 exhibited no corrosion and Sample set 3, Primer-Free Inserts, also showed no signs of corrosion.

Additional testing has been performed using the same sample sets as above as well as some additional insert coatings applied for comparison. All results have been verified by independent sources. In all cases, the use of Primer-Free Inserts has proven to be at least equally effective and in most cases more effective in the prevention of galvanic corrosion.

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