We have a d38999/20ff35sa connector - http://www.digikey.com/product-detail/en/D38999%2F... that fits in our anodized aluminium enclosure. The PCB sits inside the enclosure and the inside is epoxy'd. We epoxy it by placing the enclosure upside down with the connector screwed to the enclosure and pour epoxy thru the two holes at the bottom of the enclosure. To stop epoxy from leaking between the connector flange and the enclosure, before filling with epoxy we pour some peelable solder mask (2211-8SQ) latex to stop the epoxy from flowing out through the connector and enclosure faying faces. Once we peel of this solder mask the areas where we had the solder mask has turned black. I am pretty sure that its the solder mask and probably thinking of replacing it with peelable silicone. I have a feeling that it is just removing the nickel coat on aluminium connector and leaving a black discolouration mark. I would be interested in knowing what is the effect on the aluminium? could it be corrosive? (we potentially have removed a layer to stop corrosion of aluminium - this is obvious) but will this accelerate corrosion? has the solder mask caused any oxidation? Like to know because there are some units in the field and fix if the issue is very critical as they can be exposed to very humid conditions and salt fogs.
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