Evaluation of Heat Stability of Conversion Coatings

Chemical conversion coatings are widely used throughout Boeing, the military, and industry for

corrosion protection and paint adhesion on aluminum alloys. In recent years, non-chrome (non-

Cr) and trivalent chrome (Cr 3) materials have been evaluated or implemented to replace

hexavalent chrome (Cr 6) materials due to worker safety and environmental concerns. It is well

established that hexavalent chrome conversion coatings will dehydrate and crack if heated above

140ºF (60ºC), thereby degrading the corrosion resistance properties. It has been claimed that

non-chrome and trivalent chrome conversion coatings can withstand higher temperatures without

degrading the corrosion resistance properties.

A Boeing test program was designed and conducted to simulate the exposure of aluminum

hardware to elevated temperatures after conversion coating. The objective of the testing was to

compare the heat stability of candidate trivalent chrome pretreatment (TCP) conversion coatings

to the baseline hexavalent chrome (Cr 6) conversion coating. Test panels were prepared with

each conversion coating, some maintained at room temperature and others heated to 160ºF

(71ºC) or 200ºF (93ºC), and then subjected to 5% salt spray per ASTM B117 for 168 hours.

The salt spray test results indicated that none of the candidate non-chrome or trivalent chrome

conversion coatings performed as well as the baseline hexavalent chrome conversion coating at

room temperature. However, exposure to elevated temperatures significantly degraded the

hexavalent chrome conversion coating, but did not significantly affect the trivalent chrome

conversion coatings. These finding are important for Boeing, the military, and industry in

selecting conversion coatings that will best protect aluminum structures or components exposed

to heat processing during assembly or service.