Coverguard is the coating that is designed for protecting graphite from oxidation; it is also stable with molten aluminum, although the surface of Coverguard is wetted by molten aluminum. Thus, it is common to use only Coverguard for graphite surfaces in contact with molten aluminum -- a single type of coating. If it is desired to get non-wetting behavior, such that the molten metal and dross/skin will not adhere, a It is possible to get both the oxidation protection of Coverguard and get good non-wetting by using COMBINATION [or just "COMBO"] coatings -- dual types of coatings.
The use of COMBO coatings requires careful consideration of how best to apply the coatings such that they do not "mingle" and/or interact significantly and will give the best performance. COMBO coatings definitely require extra effort and may not justify the extra trouble in some cases. For Coverguard or MW-50 Paste as the first layer/coating (undercoating), either BN-Lubricoat or BN-Hardcoat can be used as a final coating (topcoating) that will "see" the molten aluminum and thus provide the non-wetting -- with the BN-Lubricoat giving the very best non-wetting whereas the BN-Hardcoat will provide less non-wetting but more abrasion-/wear-resistance along with the non-wetting. Since Coverguard, MW-50 Paste, and BN-Lubricoat are all acidic-pH materials, they are compatible and do not require curing between coatings; since BN-Hardcoat is alkaline pH, if it is used as the topcoating, thorough drying or curing/heating to 150 C is needed to prevent interactions and possible peeling.
The following instructions should help with application of these coatings. COMBO coatings can be considered for graphite rotors/stators used with degassing units. Also, for non-graphite materials with porosity to seal, Coverguard (or MW-50 Paste) will seal the pores, and the COMBO coatings can provide non-wetting with molten aluminum contact.
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COVERGUARD
(Oxidation/Wear Protection)
1. Clean the graphite of oils, dust, etc. Light grit-blasting provides an excellent surface for coating.
2. Apply a thin layer of Coverguard ... preferably using a foam-rubber-brush -- just like house paint.
3. Allow to dry at room temperature and apply a second thin coating of Coverguard.
4. If no Boron Nitride top coating is to be used [i.e., a single type of coating -- Coverguard only], simply allow to thoroughly dry at room temperature and place into use.
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BORON NITRIDE LUBRICOAT
(Non-Wetting Surface)
LUBRICOAT provdies the very best non-wetting surface to molten aluminum and is often used as a topcoating over COVERGUARD.
1. For best results, heat the dried Coverguard-coated graphite part to about 150 C (400 F) for about 15 minutes. Allow to cool to room temperature. This elevated temperature cure is not absolutely necessary ... but, if not performed, will allow the
BN Lubricoat to mix/mingle with the Coverguard coating; this can reduce the non-wetting somewhat but otherwise is OK.
2. Apply one thin layer of
BN Lubricoat using a brush or air-spray -- just like ordinary house paint.
3. Allow to dry at room temperature and place into use.
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BORON NITRIDE HARDCOAT
(Non-Wetting Surface)
For those uses where
BN Lubricoat is proven not to provide sufficient resistance to erosion,
BN HARDCOAT is recommended.
1. It is absolutely necessary to cure the Coverguard before coating with BN-Hardcoat.
TO CURE:
a. Heat to 150 C (400 F) for 15 minutes and cool to room temperature.
OR
b. Allow to dry at room temperature for a minimum of 24 hours.
2. Apply a thin coating of BN-Hardcoat and allow to dry at room temperature.
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These are water-based coatings. Thoroughly dry before use. DO NOT CONTACT WET COATINGS WITH MOLTEN METALS. Consult MSDS before use.
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