The Fluorocarbon Cleaning Process occurs in three stages. The stages are not observable because they occur sequentially inside the retort under the control of the system process programmer. The process occurs during a controlled temperature rise, and thus time and temperature are correlatable.
Stage I: Crack Cleaning Stage
During the first stage, the ptfe is pyrolized to tetrafluorethylene gas and mixed with hydrogen. The surface oxides and the oxides in the cracks are converted to their fluorides by the following reactions (aluminium is used here as an example)
(1) Al2O3 + 6 HF -> 2 AlF3 + 3 H2O
When the hydrogen is lean the teflon gas cleans cracks by:
(2) 2 Al2O3 + 3 C2F4 -> 4 AlF3 + 6 CO
Most of the fluorides thus are volatile and go off in the gas stream, but CrF3 is refractory.
It will be reduced in Stage III.
Stage II: Surface Depletion Stage
This is the surface depletion stage. It begins at the point where the ptfe is completely pyrolyzed and the fluorine concentration is attenuated by continued introduction of hydrogen. Surface depletion occurs as Al and Ti are drawn to the surface by diffusion and there converted to their volatile fluorides as follows:
(3) Al + 3 HF -> AlF3 + 3/2 H2
Upon completion of Stage II, a layer of cleaned depleted alloy is left on the surface. On top of that is a layer of solid CrF3. The CrF3 is the subject of Stage III.
Stage III: Reduction Stage
This is the final reduction stage. It begins when the process temperature has reached the point where reduction is possible. (This temperature is lower than that of other processes because the carbon constituent). Here, the hydrogen flow is increased to displace the retort gases, and causes the conversion of the solid CrF3 on the surface of chromium.
(4) CrF3 + 3/2 H2 -> Cr + 3 HF
At this point, the cleaning is complete and the system is cooled to accessible temperature by the most expedient means.