Figure 2: Chemistry of the Process
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.
THE FLUOROCARBON CLEANING PROCESS
Various methods of generating HF were studied, some of which have been adopted by others in the field but in 1980, Dr Jack Chasteen of the University of Dayton patented the Fluorocarbon Cleaning Process (FCP) which uses the pyrolization of polytetrafluoroethylene (ptfe) in a hydrogen atmosphere.
The complex conditions which must be satisfied in order for these reactions to take place correctly are well understood as a result of this research and have been engineered into a package which is both safe and simple to use.
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Chemistry of the Process
The basic problem is that of ridding the surfaces, including the crack surfaces, of the thermodynamically noble oxides of aluminium and titanium, and also depleting the surfaces of these highly reactive elements so that their oxides cannot reform. The problem is solved by creating highly reducing, high temperature (950 C) gaseous atmospheres from mixtures of hydrogen (H2) and gaseous fluorocarbon. The most effective fluorocarbon gas used to date is tetrafluoroethylene (C2F4). Since this gas is hazardous to handle, a true novelty of the Dayton Process lies in the manner in which C2F4 gas is introduced into the cleaning atmosphere. The Dayton Process acquires this gas in a non-toxic, non hazardous form through pyrolysis of its polymer, polytetrafluorothylene, whose best known commercial name is Teflon ®
(Du Ponts registered trademark).
When heated, it is converted to its monomer, namely, tetrafluoroethylene gas: (ptfe) n. C2F4 -> C2F4 (gas)
The monomer reacts with the hydrogen according to the following reaction: C2F4 + 6 H2 -> 4 HF + 2 CH4
The extent to which the above reaction proceeds to the right is dependent upon both the temperature and the preponderance of H2 in the H2: C2F4 mixture.
The reaction produces adequate HF concentrations which are free from impurities. Even small quantities of oxygen (or water) can significant affect the cleaning reactions and a major advantage of the Dayton Process is that the only water in the system is that which is transported in with the hydrogen gas.
Most other processes involve materials which are difficult to produce and to maintain in a completely dry condition.
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.